[eico1]

dvv, regarding your earlier comments on damping factor, isn't the series resistance of a driver ultimately going to determine the maximum 'damping' of the speaker?

steve

[audiojerry]

DVV, you are succeeding in drawing international interest to this forum, with contributers interacting (almost) real time around the globe. This is so cool.

[DVV]

dvv, regarding your earlier comments on damping factor, isn't the series resistance of a driver ultimately going to determine the maximum 'damping' of the speaker?

steve

Of course it will. In return, it will decouple the amp from the load, so it isn't necessarily a bad thing. However, I haven't tried it yet, so I can't really comment on it.

Also, I must add there are other variations on similar themes, so it's not a touch and go subject, it needs careful investigation.

Cheers,
DVV

[DVV]

DVV, you are succeeding in drawing international interest to this forum, with contributers interacting (almost) real time around the globe. This is so cool.

Well Jerry, that was the point of it all, wasn't it? As far as I can see, this is evolving VERY nicely, with Irish friends kicking in, Ferdi from Holland being an old timer here, now Guan from Hong Kong is in.

You know, I left TNT a year and a half ago (incidentally, about 6 months after Thorsten Leasch left - a coincidence? Not at all, unfortunately for TNT), and I still get people asking me questions about texts I posted there. My own site is up from around 66 visits per day to over 110 per day.

Remember what I told you - our fate is in creating a circle of independent sites around the globe, which can and should take over from the "official" audio press, and will do so eventually, as long as they stay honest. Sooner or later, we will start to specialize, delving deep into parts of audio each, but together, covering it all. And for free, why not?

If people want quality, they have to haul some ass, nothing in life just drops into you lap (anbd what does, you don't want it, it's called guano). Not that I have any reason to complain about this site, quite to the contrary. Why else would I be hauling my ass?

Cheers,
DVV

[DVV]

DVV. I was trying to pull your leg, and now I think maybe you are pulling mine. In case you were not, my tease meant that I wanted your discourse on signal travel to be presented with audio/video, THX 7 channel format.  

With all this leg pulling, people will start talking, and sooner or later, somebody's leg will get dislocated.

Hey, what's wrong with some fun? I do clips every now and then, like with our Irish residents, but I never tease anybody I don't like, if that's any consolation. And I don't see myself as an untouchable holy cow, just a plain, old brownie bull, producing, er, bull.

Ciao,
DVV

[DVV]

You mention that the amp and speaker are very much intertwined as far as the damping factor and speaker load.  I realize that it depends largely on a designer who has the ability to design both, but would you recommend purchasing both from the same designer?  I wonder if most designers build things as systems or as the best they can do with each individual item?  It would be nice if they looked at them as systems, but I have no idea.  It's worked for me in my case, Symphonic Line, but I just go by the sound, not the specs.  It was a big leap of faith but I haven't been disappointed.  From what I've read it sounds to me like the most useful aspect of specs is determining synergy, but damned if I know enough to do that.  Has anyone ever put together a chart of the relationship of various specs between components and described what each value does in relations to the others.  That would be a hell of a project but I think it would be pretty useful, provided you could get the useful, and accurate, specs from the manufacturers.  Sorry if I don't make sense, like others have said, this is mostly over my head, but it does make me think, and that's good...thanks all!

cheers,

Dick

Not at all, Dick, your question is perfectly sensible. Dan d'Agostino of Krell believes in this logic, and now Krell makes speakers too. I haven't heard them so i can't comment, but I will bet my last penny they simply take the his philosophy to its pinnacle. But whether that will be good enough for you or not, I really can't tell.

For myself, when I wanted a speaker of my dreams, I sat down and wrote the design brief - what type of speaker, what configuration, what drivers, etc, and then asked a good friend to make them. He did this professionally, and did it damn well. Read about them on my site at http://www.zero-distortion.com , the text on B&M 1041 monitor, that's what I'm listening to as I write this.

I believe in specialization, no one man can know it all supremely. I know for a fact I could never design anything half as good that my 1041, I simply don't know enough. But then, Mirko, my friend who did it, comes to me for amplification.

Synergy is what we are after. But getting there is long, hard and full of pitfalls.

Cheers,
DVV

[DVV]

The saga continues....

Most power amplifiers consist of, roughly speaking, three sections at the very least, often four, and possibly more, basic blocks.

The first block is the input stage. Its task is to accept the incoming signal, mix it with a part of the output signal, the negative feedback, so as to correct for deformations of the output signal, and as a result of these two actions, to amplify in net terms the input signal. It must do all this with as little noise as possible, as little distortion as possible, as little DC drift as possible, over as wide a bandwidth as possible, and with as much signal rise speed as possible, but also with as short a settling time as possible. Oh yes, and it should smile all the way, taking it all in its stride.

If you thought your life was hell, how would you like to be an input stage? Compared to what we audio freakouts want of it, our wives' demands of us are peanuts, a cinch.

While this looks like the original installment of "Mission Impossible", actually it's not too hard at all. By far the most popular form of the input stage is what we call a differential amp. This consists of two transitors in its basic form. One, the non-iverting input or "+", accepts the incoming signal on its base and lets it out on its collector. The other, the inverting input, or "-", accepts the negative feedback signal (or rather, a part of it, as it has been divided previously) at its base, lets it out on its emittor, and since it has been turned around (firuratively speaking, what was the positive part of the signal is now negative) and since the emittors of both transistors are connected together, it send this signal to the "+" transistor. The net gain of the stage is its absolute gain minus the inverted signal gain.

These transistors also must have, if they are to work at all, what is known as a current source. In other workds, they must be made to conduct all of the time, signal or no signal, which puts them squarely into the pure class A operating mode. This current generator could be almost anything, from a simple resistor, to a resistor, zener diode (which locks the voltage to its nominal zener breakdown voltage) with a parallelled capacitor and another resistor, to a full discrete transistor or two circuit. In all cases it will work, but it will work better with some arrangements than with others, and you won't be surprised to learn that more elaborate arrangements will yield greater linearity and lower distortion, but at an increased price (which is why the mainstream commercial industry doesn't do it).

There are so many variations on this basic theme it's hard to even list them all, but suffice it to say they are all intended to make the input stage differential pair work better, with more linearity, less drift and distortion, etc.

There are a few points worth remembering about the input differential pair. The first is gain - if it's too small, then your subsequent stages have to work all the harder for it, which cuts their bandwidth and increases distortion, so it can't be too small. If it's too big an amplification factor, the bandwidth decreases, distortion rises sharply because feedback is applied over a lower badwidth. So what's "small" and "large"? Generally, it is accepted that the input stage gain should be set at 4...8, neither lower, nor higher. 50 years of design have shown this range to be optimal as a compromise between many conflicting requirements.

The signal is then passed on to the voltage gain stage. This is a second stage of voltage amplification which does what's required to get the desired overall gain. This stage could be a simple single transistor (as on, for example, some Naim, Pioneer, etc amps), or two or more transistors connected in a variety of ways. These ways include a Darlington connection, a cascode connection, an emmittor-follower connection, another differential amplifier, and so forth. Whatever it is, its job is to finalize the voltage amplification of the signal.

It works into what is again a current generator, which also polarizes the signal, so the current amplification can be performed in a push-pull manner. This can be a simple circuit, but it's been years since I saw anything but a proper active stage with at least a single transistor.

The second voltage gain stage and this generator are connected by a bias tracking network (also called a simulated zener diode). This is typically a transistor whose job is to monitor the temperature rise of the output stage and adjust the bias (quiescent) current as per the requirements. It can also be used to set and/or change the idle current of the amplifier; this is the current it will draw signal or no signal. The more current it draws, the more it is biased in class A, but the less efficient it is, as it heats up more and more. For more of this, see my text "Put a Tiger in Your amp" on http://www.tnt-audio.com . Nice pictures, too.

Lastly, we come to the current gain stage. This stage's job is to amplify the current of the signal so that it can be passed on to the speakers, but its voltage gain is unity, i.e. 1. In other words, it does not amplify the voltage, only the current.

This can be a pair (NPN/PNP) of Darlington transistors, which have tremendous current gains of 1,000:1 and upwards, as much as 5,000:1 being rather normal. With such a gain, there's no need for any drivers. Or these could be MOSFET transistors, which also have similar tremendous gains, and consequently don't need drivers either.

Before you jump to the conclusion that this is perfect, let me pour some water over your hot head. Yes, it is very convenient, and while there are some applications where this is indeed a God-send (e.g. virtual battery supplies, relay drivers, etc), in audio this is of doubtful value. For a start, single transistors of either type will exhibit higher output impedances that those with drivers. Next, your voltage gain stage has to be able to deliver fairly constant levels of signal both in voltage and current terms, and this can be a problem, as it's partly conflicting, forcing you to run the preceding stage in at least partly current mode. It can be done, it is being done, but it's not easy to get right. Lastly, it's never wise to put all your eggs in the same basket; if true elsewhere, then why not in audio as well? Your entire amp-speaker interface is reduced to just one pair of transistors, so if they run into some evil load, they may also run into a lot of trouble, as it may turn out that the designer's anticipated current level of the voltage gain stage is simply not enough to cope.

Also, Darlington transistors are rather severly bandwidth limited at greater power levels, which is a minus in this context. MOSFETs aren't, but their problem is exactly the opposite, their bandwidth is far too large, so you could find your audio amp happily oscillating at say 30 MHz, and your audio measuring equipment doesn't go anywhere near those wuthering heights. Believe me, I survived one such case.

The most typical setup is to have a pair of drivers. These are an interface between the voltage gain stage and the output stage, and since they add a level of current amplification of their own, the voltage gain stage can be kept much more in the voltage gain mode and less in the current gain mode. On the other hand, the output stage can count on some powerful drive if required, and the net result of this configuration is lower output impedance - which we also want badly. Lastly, distortion is reduced, which we also want.

On better amps, there are not two, but three current gain stages - the predriver, driver and output stage. This simply adds another current gain stage, making output impedance still lower, reducing distortion still more (since the required work is now divided into more stages, each stage is less stressed). By the way, in any half-hearted design attempt, everything but the output stage works in pure class A, only the output stage proper works in class AB.

If we are dealing with larger power output levels, and are anticipating nasty speaker loads, we will need more current capability. We can either use more powerful output devices, or we can use more pairs of output devices, thus dividing the work among them, reducing distortion and increasing overall power handling.

Typical power amplifier circuits use a single set of +/- supplies for the entire amp. This is cheap to implement, all you need is a low value resistor (typically 2...10 ohms) and a capacitor per supply line to decouple the sensitive voltage gain stages from the high power stages, but this is the horse manure way of doing it (and it's only done for the low price of it). A much better way is to use separate power supply lines for the voltage stages and separate for the current gain stages. This way, one can have highly stabilized, fully regulated power supplies for the voltage gain stages, while keeping loose lines for the current section, which will improve transient power delivery (impulse power). This will also produce a more coherent, better detailed amplifier.

One could also fully electronically regulate the current gain stage supplies as well, which is very hard to do properly, and is rather expensive as it often costs more than the power amp itself, but it does have its good sides. This produces what is known as stiff power supplies. These will offer almost no transient extra power, but will keep the supplies locked on when the load impedance starts to vary, thus improving load tolerance and enabling prodigious power delivery into difficult loads.

Lastly, we come to various ancilliary circuits. I would list three as what I consider to be essential for any decent power amp. The first is the DC servo. This is a circuit which keeps DC drift, a most unwanted phenomenon, to extremely low levels under any and all conditions. This circuit has been flamed much over the last couple of years, but I feel very unjustly so. True, it can produce undesirable side effects if not designed and executed properly, but then, failing to do that will produce a bad amp anyway. I feel this is another high end cruisade to make their products still cheaper to make, and be able to add still more on the price tag because hey, lookee here, we eliminated the DC servo.

These circuits keep the speaker safe (what else can it do with DC but turn it into heat?) and keep the amp locked in place and mode. They can be designed and executed fairly easily if you are willing to work on it, and the price they add is no more than say $10-12 per channel.

Next come the protection circuits. These can be divided into two groups, those which protect against overheating and excessive DC (which could appear as a result of a breakdown somewhere within the amp, and which cannot be compensated for by the DC servo), and those which protect the output devices against overload.

I find too many amps in the DIY sector which completely lack DC and overheat protection. I find this irresponsible and deplorable, this is in fact gambling with your reader's audio equipment, using the type of logic like "hey, what's a few kilobucks between friends, as long as they are your kilobucks?". Worse, some "high end" companies are also treading this path, all in the name of simplicity (horse manure! These circuits have NOTHING to do with the sound!), but I notice they haven't dropped their prices for it. By at least those $5-8 they cost to make.

Regarding output stage transistor protection circuits, the current vogue is that they are bad for the sound, and are best replaced by simple fuses. More horse manure. Yes, as applied from simplistic text books, they will limit current and will level off the sound when they shouldn't be, but what's to stop serious designers from investigating new circuits which don't do that? If I can do it, so can they, hence no excuse is acceptable. I have seen quite a few power amps with burnt out power stages and still in place fuses. The fuse bit can work on lower powered stuff, but above say 60W or so, the fuse (quick blow) can stay put and the stage can burn out. In short, fuses are simply not safe enough as the only protection.

What we need is a "tracking" protection (one which as closely as possible follows the safe operating area of the specific transistors used in the output stage), one which will not act for a set time interval so as to pass transients untouched, one which will act progressively (slowly at first, then faster and faster if the abnormal operating conditions persist) and one which will cut out almost immediately after the causes of abnormal operation have been removed. It can be done, others did it, I did it, but you have to want to do it. Thus, any amp lacking some sort of this protection I regard as lacking overall, despite its good sound - incomplete.

Lastly, the overload indicator. This is a very simple circuit, its only intention being to warn you with a LED that you are approaching or have come to the nominal power threshold of your amp. This enables you to turn down the volume and thus avoid possible severe clipping, which is the fastest and safest way known to man to destroy your speakers. Total cost of this circuit is below $1 retail prices per channel. More if you insist on high end blue LEDs.

There are other issues regarding amplifiers I have not even tried to cover here. For example, go for fully complementary, or not? Both approaches have their pros and cons, and I don't believe either to be inherently better, and/or to guarantee anything, it all comes down to specific implementation. Then the single ended versus multiple output device argument - again, both have pros and cons, and again, I don't believe either to be inherently superior, but over a certain power level, one simply has to go for multiple pairs.

Not to even mention the tube versus solid state - now, there's an argument from here to eternity, without the famous Burt Lancaster and Deborah Kerr long kiss on the beach. My personal view is that tubes are great for low current applications, like preamps, but are not a good solution for high current applications, such as power amplifiers. But essentially, I don't really care how it's made, as long as it sounds good.

I generally agree with Dan's view that a good amp needs to have a sane and balanced design and execution throughout. No one part is more important than any other, one small slip up anywhere, and it all goes down the drain. If I tend to overdo it anywhere, it's in the power supplies, somehow I have a nagging feeling that I should have added more no matter how much I already sunk into them. That's my known and documented quirk.

Cheers,
DVV

[Dan Banquer]

I am going to address damping factor for the moment. Damping factor is just a fancy name for output impedance. The following is what can determine that. Many SS amps will typically have a resistor, or a resistor paralleled with a small choke, at the output. The resistor is to provide some amount of resistance to reduce ringing on transients for a capacitive load, and the choke for any number of reasons.  What else affects damping factor? Dejan made some excellent points. Amplifier output impedance is dependent on frequency, ( power amplifiers output impedance will go up with frequency, but just how much is dependent on a number of things.) it is also dependent on biasing of the outputs, the output impedance of the power supply, and, get this one folks, the output impedance can vary with level!!!
If the amp has a choke at the output the output impedance by definition will rise as the frequency rises. If the amp's power supply transformers are saturating at higher output currents, the impedance rises, When the output exceeds the DC bias at some point the output impedance will rise also.
The LNPA 150 and it's implementation addresses those problems by implenting the following. The output has 0.05 ohms of resistance at the output, the output transistors are not statically biased but the bias is "sliding", the higher the voltage swing, the harder the DC bias. The power supply is fully regulated, and has an output impedance we can measure in milliohms. There is no choke at the output.
Sliding bias can be very difficult to implement but the rewards are worth it.
The advantage here is that when the output level is high the output transistors are biased harder to reduce output impedance and give better stability and performance into reactive loads. J.L.L. Hood didn't particuarly go for this, but I think he might have missed a few things when applying this technique. One of the real advantages here is at low power, the amp is not a space heater, most folks have not been able to get the LNPA 150's above warm. Remember, our average power to the loudspeaker is typically 1 to 2 watts. In retrospect, it's too bad most "objective" reviewers don't measure "damping Factor" over different power levels and frequency.
The draw back is you need a big honking power supply, and you can't skimp on the heatsinking.
  All for now folks.

[DVV]

I am going to address damping factor for the moment. Damping factor is just a fancy name for output impedance. The following is what can determine that. Many SS amps will typically have a resistor, or a resistor paralleled with a small choke, at the output. The resistor is to provide some amount of resistance to reduce ringing on transients for a capacitive load, and the choke for any number of reasons.  What else affects damping factor? Dejan made some excellent points. Amplifier output impedance is dependent on frequency, ( power amplifiers output impedance will go up with frequency, but just how much is dependent on a number of things.) it is also dependent on biasing of the outputs, the output impedance of the power supply, and, get this one folks, the output impedance can vary with level!!!
If the amp has a choke at the output the output impedance by definition will rise as the frequency rises. If the amp's power supply transformers are saturating at higher output currents, the impedance rises, When the output exceeds the DC bias at some point the output impedance will rise also.

Which is why I'm all too happy to avoid them. Get your stability elsewhere, not by using this as a crutch.

The LNPA 150 and it's implementation addresses those problems by implenting the following. The output has 0.05 ohms of resistance at the output, the output transistors are not statically biased but the bias is "sliding", the higher the voltage swing, the harder the DC bias. The power supply is fully regulated, and has an output impedance we can measure in milliohms. There is no choke at the output.
Sliding bias can be very difficult to implement but the rewards are worth it.

It's always hard to implement, Dan. Oh sure, there are plenty of quick'n'dirty ciruits out there, but most are really no good at all. Like everyhting else, this takes much care and effort.

The advantage here is that when the output level is high the output transistors are biased harder to reduce output impedance and give better stability and performance into reactive loads. J.L.L. Hood didn't particuarly go for this, but I think he might have missed a few things when applying this technique. One of the real advantages here is at low power, the amp is not a space heater, most folks have not been able to get the LNPA 150's above warm. Remember, our average power to the loudspeaker is typically 1 to 2 watts. In retrospect, it's too bad most "objective" reviewers don't measure "damping Factor" over different power levels and frequency.
The draw back is you need a big honking power supply, and you can't skimp on the heatsinking.
  All for now folks.

Precisely where the industry just loves to "save".

Cheers,
DVV

[tmd]

So, from someone really just starting to understand amplifier design. Let's see if I have this right;
The signal path can be kept fairly simple with few enough components in it, but the 'control' electronics which 'manage' the components that do the real work should be very elaborate to keep it all in check. Not for the sake of being elaborate but because they have to perform miracles basically.
Also, you can't build too big a power supply. In fact, three separate power supplies, one for each of the three stages of the amplifier would be even better. It would make for a huge and heavy amp and add a lot of expense though.
More devices in the output stage means a better damping factor, which is desirable. However, more devices also brings problems. Matching the devices is very important and keeping their thermal characteristics matched is also important.

So, the secret to a great amplifier is to get a killer design and incorporate as many implementation tips as possible, including things like output device placement for thermal matching and extra large power supplies with lots of capacitance. Also solder a very high quailty power cord onto the transformer directly.
I guess another no brainer would probably be a monoblock design rather than dual mono in one chassis.

Is any of this accurate? I will edit it out if not as I don't want to confuse others trying as I am to get a handle on all this.

Neil.

[DVV]

Hi Neil, all,

So, from someone really just starting to understand amplifier design. Let's see if I have this right;
The signal path can be kept fairly simple with few enough components in it, but the 'control' electronics which 'manage' the components that do the real work should be very elaborate to keep it all in check. Not for the sake of being elaborate but because they have to perform miracles basically.

Well, not really. I mean, you can build an outstanding current generator with 2 transistors, 1 capacitor and 4 resistors (2 of which are optional, but I like having them). In effect, this is a repetative circuit, get it right once, then just change usually one resistor value only to get what you want in any particular case.

Or build one with 1 transistor, 2 diodes, 1 capacitor and 2 resistors. It will work just fine, and you can define the desired current level simply by changing the value of one resistor.

Regarding ancilliary electronics, they are not all that complicated. Yes, there's work to be done, but again, much of it is repetative. For example, once you get the protection circuits right, you can adjust them to any voltage and any transistor I ever saw by recalculating the values of just two resistors, or 2x2 for both sides (NPN and PNP). And of course, what's good for one channel, is also good for the other.

Also, you can't build too big a power supply. In fact, three separate power supplies, one for each of the three stages of the amplifier would be even better. It would make for a huge and heavy amp and add a lot of expense though.

I feel just two per channel is quite enough. One is fully electronically regulated for the voltage gain stages, while the current gain stages are allowed to "breathe", meaning they use capacitor banks.

All other voltages are derived from those two. For example, the input stage voltage is sub-regulated from the regulated voltage, as are the voltages for the DC servo, etc. My protection circuits have their own, completely separate regulation, but it is fed off current amp supplies, as it requires only 12V to operate.

[/quote]
More devices in the output stage means a better damping factor, which is desirable. However, more devices also brings problems. Matching the devices is very important and keeping their thermal characteristics matched is also important. [/quote]

Exactly. Of course, there are ways to match them in-circuit, but this still assumes you have matched them before soldering them in.

So, the secret to a great amplifier is to get a killer design and incorporate as many implementation tips as possible, including things like output device placement for thermal matching and extra large power supplies with lots of capacitance. Also solder a very high quailty power cord onto the transformer directly.

Quite so - that and a few other things besdies.

I guess another no brainer would probably be a monoblock design rather than dual mono in one chassis.

Not necessarily. A dual mono in one single chassis will be just fine.

Is any of this accurate? I will edit it out if not as I don't want to confuse others trying as I am to get a handle on all this.

Neil.

Very accurate.

Cheers,
DVV

[DVV]

What?

What is this silence, we playing Paul Simon's "Sounds of Silence"?

Or have I gone overboard again?

Somebody stop me ...

Cheers,
DVV

[Ferdi]

Eeeeh Booo?

Waiting for the next installment...

A few topics that would interest me:

Different types of distortion (total harmonic distortion/Intermodulation distortion) What are they and what do they mean to you?

Phase-linearity and impact on an amp's character

We've had some discussions on the differences between Toobs and Sand.    How about comparing this with digital amps. There have been some comments on these but probably not enough. Maybe not enough is known yet.

Some sort of discussion of the link between technical features/measured behaviour and observed performance. (There has been some already)
Clear, warm, analytical, musical, PRAT, harsh, smooth (who adds more?) and rise time, power, damping factor, distortion, weight  (again, who adds more?)

I have to say this topic is reading more and more like a crashcourse in amplifier design. Great even though almost all is over my head.    I mean, I know what many of the terms mean from highschool and university physics but they still have to "connect" to form some sort of overall picture. I'll reread this thread a few more times.

[tmd]

Speaking of rereading it, it would be a bit of a pain even now to slog through the whole thing from the start. I'm glad I read it when it was a few posts. Is there any way for future readers of these very valuable but very long posts to get through them more quickly and get the most out of them?
This has been the single most interesting post I have read in a very long time on any forum. I guess it is where I am at the moment anyway to some extent as I dick around with mods and innards in general. Almost everyone could really learn from it though. A better appreciation for the workings of an amp probably never hurt anyone trying to buy one. It is also harder for the slick sales guy to baffle you with bullsh1t

[audiojerry]

This may be off topic a little, but I recently picked up a pair of B&W Series 80 802's, whcih are vintage mid 1980's I believe. I have read that B&W creates a diifficult load for an amp with varying impedence and phase shifts, and with 24db slope crossovers.  This is the first pair of speakers I've had that have not worked with my Audio Research VT200 tube amp. The results of this combination are less than enjoyable. I notice a lot of ringing and what seems like overshoot. I would surmise that this might be related to damping factor, and the fact that the ARC tube amp just doesn't have it in this category. Using the much less expensive solid state Odyssey mono's results in a more enjoyable combination without the ringing and with tighter bass. I have never had a negative experience with the ARC before. Can anyone explain why?

[DVV]

This may be off topic a little, but I recently picked up a pair of B&W Series 80 802's, whcih are vintage mid 1980's I believe. I have read that B&W creates a diifficult load for an amp with varying impedence and phase shifts, and with 24db slope crossovers.  This is the first pair of speakers I've had that have not worked with my Audio Research VT200 tube amp. The results of this combination are less than enjoyable. I notice a lot of ringing and what seems like overshoot. I would surmise that this might be related to damping factor, and the fact that the ARC tube amp just doesn't have it in this category. Using the much less expensive solid state Odyssey mono's results in a more enjoyable combination without the ringing and with tighter bass. I have never had a negative experience with the ARC before. Can anyone explain why?

No synergy, Jerry. Why is something that would need to be investigated. Very possibly the load the B&W presents to the amp is varying one, with impedance dips and possibly nasty phase shifts.

Just in case you haven't tried this already, try connecting the speakers to your 4 ohm taps rather than 8 ohm taps, or vice versa. It works sometimes.

On the other hand, given their age, it could be that the speakers are in bad need of capacitor exchange.

Cheers,
DVV

[DVV]

Ferdi says "next installment" - fine, here goes.

Power amp power supplies

Before anyone asks, this is the second and last part of the power amp issue. Going beyond would incur using pictures, and ultimately, writing a monography if not a book. For chossing power devices, see appropriate text on http://www.zero-distortion.com .

In typical modern amplifiers, we have separate plus and minus power supply lines. We could use just one, the plus, but this would require us to use a decoupling capacitor between the amp's output and the speaker, and this is always a bad idea, because capacitors are junk in general audio terms, and should be avoided whenever possible.

The most common arrangement is to have a power transformer, which takes the power from the grid and your wall outlet, say 120 VAC, and converts this to say 30-0-30 volts. Mind you, the voltage after the transformer is still alternating, so we need to rectify it, meaning to turn it from alternating current (AC) to direct current (DC), which does not have the 50 Hz alternating component.

This is achieved by using full wave bridge rectifiers. There are half wave rectifiers, but in audio, full wave rectifiers are used, and I can't even remember when did I see anything less last. A full wave bridge rectifier is essentially four diodes aranged so that the AC signal is cut by halves and then joined at the output, resulting in two supply lines. In this process, our incoming AC voltage from the transformer will be rectified and multiplied by the square root of 2, or by 1.41. Therefore, our incoming 30-0-30 VAC will become +/- 42.3 volts DC.

But this rectification is incomplete, it hasn't removed much of what we want removed, so we must additionally filter it. To that end, we use large filter capacitors, which in simplified terms pick up the junk riding along with our voltages and shunt it to the ground. Make no mistake, they do a good job of it, but the question is how good a job? More of that anon.

By default, large capacitors are parallelled by small capacitors of different material type, typically in the 100...330 nF range. They do what the big caps don't do well at all, they filter the very high frequencies well into the megahertz range, ridding us of unwanted RF intereference.

And that would be our typical power supply in commerecial units.

A step above this is to use exactly double of the above, one for each channel. This would give us a dual mono concept, its main benefit being that channel crosstalk is greatly reduced and there is independence between channels. When one is called upon to deliver large power, the other should ideally be totally unaware of it altogether. This helps keep a stable stereo image.

Another step above this is to use not one, but two full wave rectifiers per channel, i.e. two for each channel. One rectifies only the plus side, the other only the minus side. This has many benefits - better rectification, faster response due to offloaded rectifiers (work is split in half with two), far greater obtainable currents (double, in fact), better cooling of the rectifiers (offloaded and heating split up in two), etc.

Next, we can split up the supplies for the voltage gain stages from the current gain stages. This is a VERY logical decision for many reasons. First and foremost, voltage gain stages work with small currents, so full regulation is not a problem, nor is it prohibitively expensive to make. Second, since each transistor in the signal path will naturally produce a voltage drop of 0.65V, we can increase the small power rails to compensate for these drops and to ensure that even if clipping should occur, it will happen in the current stages first, due to their lower supply rails (lesser of two evils, though clipping of any kind should be avoided at all costs). Next, this would allow us to run the current gain section at lower voltages, which in turn means we would be keeping our transistors more within their safe operating area and could draw more current from them.

This greatly improves every amplifier I have ever heard. In general terms, it produces a more stable sound stage, more depth, more definition and better overall balance. I combine this with dual rectifiers per channel for best effect.

Lastly, we can do a full regulation of the whole amp. This means electronic regulation of both the voltage gain stages, and of the current stages. This will put our entire amp in a most stable environment, which will improve its performance if properly done, but at a price roughly twice that of the initial amplifier. This is because you effectively need another amplifier to supply the original amplifier, and if you want to be safe, the power supply amp has to be faster than the audio amp if it is not to start strangling the audio amp. All this takes much time, much development work and much money, and is consequently rather expensive.

I have tried it a few times, but was never really happy with this arrangement. I don't use it, but I don't knock it either. To each his own.

OK, so that how it works in general, now let's see how to dimension it all.

The first question we have to answer is how much power do we want, with what load tolerance, and under which conditions?

There are many compromises routinely made in this, because power supplies are not seen from the outside, but the customers do see shiny LEDs, and in the end, money ends up in LEDs, courtesy of the marketing departments, while engineers tend towards hara-kiri.

But the only rational way is to work backwards - start with what you want to leave the amp, and then design the power supplies for the juice you need. To illustrate this, let's take two examples, let's assume we want to design power supplies for a 40W/8 ohm amp and for a 100W/8 ohm amp. Follow this, and that 40W per side amp will subjectively sound like a commercial 100W per side amp.

We are all audiophiles here, a nice way of saying we're by and large mad as hatters, perfectionists in audio to the marrow of our bones. Thus, we would want our amplifiers to behave as ideal voltage sources, meaning to deliver an output voltage no matter what the load is, because we want to be free to buy the speakers we like, not those we must.

Let's also assume we want our amps to deliver a little more than their nominal power rating under dynamic conditions, in short term boosts. This is called dynamic power, and while distortion may rise, your amp will not compress and will not clip.

Let's take the small guy first. Let's say we'll be happy with a 40% extra margin over our nominal rating, or just over 55W under dynamic conditions. Let's say we will tolerate a 5% power loss every time our load impedance halvs, BUT in terms of our dynamic power. Lastly, let's say we will be happy with 2 ohms as our lowest impedance for normal operation.

So, if 55W is what we want for 8 ohms, then our required voltage will be sq.root ((2x55), or 29.66V peak, call it 30V. We must now accommodate for inherent voltage drops, say 3V, and must add some reserve, say another 2 V, so we need +/- 35V supplies for the current gain stages. Now, every transformer when pushed will sag, meaning it will deliver less voltage, so what we need are transformers made and sold with nominal voltages being declared UNDER FULL LOAD, not in the usual off load conditions. The differences can reach 7-8% otherwise. In short, we need power transformers rated at 25-0, 25-0 volts (twin secondaries, twin neutral wires) under full load.

Since 30V peak into 2 ohms represents - hang on now! - 226W into 2 ohms, we obviously need at least a 200 VA transformer PER CHANNEL. We could go for less, say 120-150 VA, if we decided we weren't interested in continuous operation into 2 ohms. Remember that every transformer will deliver approximately 1.5-2 times its nominal rating in impulses, so even lower values will actually do fine. Also, don't forget the filter capacitors, which will also help ride out any nasty transients.

Speaking of which, we need to consider them as well. Now, all this jazz about power supplies is really talking about energy, not only voltage, or only current. We need them bound together, not piece by piece.

Empirical experience shows that we need 1-2 joules of energy per every 10W of dissipated power. The reason why it's a range is that it depends on just how reactive a load is; if it's mostly resistive, 1 joule will do, but if it has nasty phase shifts, we will need 2 joules for the same power. Since 226W is our maximum power, we will obviously need 22.6-45.2 joules of energy. We know what our supply voltage is, we know how much energy we want, all we have to do now is calculate how large capacitors do we need. There's a very simple formula for this, and while not terribly precise or mathematically corrct, it does the required job just wonderfully:

1/2C (V squared) = x joules,

where 1/2C is the capacitance on either one of two symmterical supply rails and assumes the same value on the other rail in Farads, and V is the supply voltage of that same rail. For a start, let's take 20,000uF as an initial value (for a whopper value of 40,000uF per channel, or 80,000uF for the whole amp - not many 40 watters around like that). We have:

0.02 (35x35) = 24.5 joules.

A bit over our lower level, but with quality capacitors, and using two 10,000uF caps in parallel for each of the four supply lines, more than enough in real life. I would therefore use this and call it a day, I know for a fact that this would be more than enough. Since music pulsates, it changes in level, "continuous sine wave" power is hardly a meritory way of measuring power, though it is accepted (just as watts are accepted, when they are a measure of heating and when we really need to talk about volts).

You could use 40V capacitors, but I would advise against it. A 40V rating is too near our actual rails, I'd go for a 50V rating, I like being safe. Since they are parallelled, you don't need ultimate quality, go for good quality, such as say Nichicon, Panasonic, etc.

Regarding the 100W amp, let's say we are happy with +/-50V, as this would give us about 144W of peak power into 8 ohms. But this is 288 watts into 4 ohms, or 576W into 2 ohms. Really rockin' here. We need 57.6-115.2 joules of energy.

Using parallelled 10,000uF caps per channel rail, or keeping what we had in the previous example, we'd get 50 joules of energy storage.

But let's get ambitious here, hey, we don't do this every day. Let's say we want to go to the hilt, so let's keep the two 10,000uF caps for good filtering, but let's also add one 4,700uF cap for speed. Our energy reserves rise to 61.75 joules. As in the previous example, while this is below what we might nominally require, in real life this is more then plenty assuming we use double rectifiers per channel and sufficiently large power transformers. Speaking of which, I would recommend no less than 400 VA per channel, and 500 VA is better yet. More would be a waste of time, I think, for everything but benchmark pure sine wave testing. That's 1,000 VA for the whole amp - how many do that? And in between, you have the third, say 100VA toroid, feeding the voltage gain stages and protection circuits.

The best place for the filter capacitors would be as near to the power devices as possible; this is a good point to remember, proper placing makes a lot of difference. Unfortunately, this assumes you make your own printed circuit board layout, which most can't.

Please remember, the above is an example only, and should not be taken as conclusive.

One last item - voltage gain stage regulation. As everything else, so this too can be done in many ways. For example, you could use our trusted LM 317/337 regulators; while they nominally go up to 38V only, you could always put their ground pin under a voltage from a simple zener diode, which would raise the adjustment level by 38 plus the zener voltage. Or you could build a discrete regulator, which takes time, but can provide excellent results. Or, you could use a single power MOSFET, such as say IRF 540/9540, to produce a "virtual battery", which is essentially a voltage regulated power MOSFET circuit followed by a high quality capacitor of some size, say 1,000uF or larger, so between the two of them, they deliver something very near a battery qiality of supply (but not quite battery).

Based on still inconclusive tests I am conducting, a line filter which actually works, in my case a DeZorel, will make your power supplies far more efficient. This is an interesting aspect, because it makes me think that instead of pouring inordinate amounts of money into massive power supplies, I would be better advised to invest into a line filter, and then downsize my power supplies simply because I will get the same effects with more reasonable components, which after filtering have a much easier job to do. But I still have quite a few things to try before I'm sure of this.

The problem is that my initial results show that I cannot duplicate the effects after the filter with any reasonable number of ultra high quality capacitors. Putting 6 4,700uF Siemens Sikorel caps per supply line on a +/-33V circuit is what I'd call already quite unreasonable, yet those six cannot do what just two in parallel with a filter preceding them can. And just one channel pays for that filter in less capacitors, so I get the second channel for free. But as I say, I have some work to do on this subject yet.

Cheers,
DVV

[DVV]

Speaking of rereading it, it would be a bit of a pain even now to slog through the whole thing from the start. I'm glad I read it when it was a few posts. Is there any way for future readers of these very valuable but very long posts to get through them more quickly and get the most out of them?
This has been the single most interesting post I have read in a very long time on any forum. I guess it is where I am at the moment anyway to some extent as I dick around with mods and innards in general. Almost everyone could really learn from it though. A better appreciation for the workings of an amp probably never hurt anyone trying to buy one. It is also harder for the slick sales guy to baffle you with bullsh1t

Well, we could do it differently.

Since this is a discussion forum, perhaps the lot could be rearranged and some schematics added, and then put on my site as a whole, with a PDF version for easy download.

How about that?

Cheers,
DVV

[DVV]

Silence again ...

In the dark, not a whisper, not a breath of wind. Darkness everywhere. Nothing moves, as if to say that all life has been abandoned. What could it be, this silence? The calm before a storm? Or is the world caught unawares, unprepared for what struck it? Has some unknown power been unleashed upon all of us, leaveing us speechless, unable to respond, devoid of all life's functions?







I've no idea, but if I find out, I'll let you know.

Seriously, have I overdone it again?

Cheers,
DVV[/b]

[hairofthedawg]

Not in my book, but I was lost awhile ago   If you ever find yourself in Cyprus, you're welcome to analyze my SL equipment.  I love the sound and have no regrets as far as the cost but I do wonder what makes it tick.  There's not much written about Symphonic Line that I've found, aside from my own babblings, and I feel there should be.  

cheers,

Dick