[Paul L]

Marbles,

That is quick.  I think you misunderstood my point.  

I stayed away from the comparison of the Bent to the LW-1.  I am more interested to raise what Ozzy said the difference between two versions of the Bent.  I think both Ozzy's are factory made.

[Marbles]

Jon,

It's hard for me to know which one he thought was original as there are/ were at least 3 units done.

http://www.harmonicdiscord.com/forums/viewtopic.php?t=6941

Mine are the Silver MKII's with the 2 sets of primaries.

I understand that you are saying that since they sound different they must have a sound.

No arguement from me, just that they have less of a sound than any other component I have tried.

[Paul L]

Well Marbles, at least we agree on the same thing.  Passive has less sonic signature than any active.  
If you consider how many components involved in building an active and each of one has its own sonic signature, we need to have a very powerful computer to calculate all the permutation and try out everyone of them in many systems to find the most neutral sound, how long it takes to R & D a most neutral active!!!  Don't forget the circuit board layout is a factor too.

[avahifi]

With a passive preamp, you are making your source (CD, Tuner, RIAA phono preamp, etc.) drive the entire distributed capacitive load from its outputs to the power amplifier inputs.  This includes the cable capacitance between the source and the passive preamp, the distributed capacitance of the wiring and controls in the preamp, the cable capacitance between the preamp and the power amp (which can be really high if you use the "long interconnects, short speaker wires" approach), and the input capacitance of the power amp.

The questions audiophiles never ask is:

1.  What is the total distributed capactive load my sources have to drive, and how much peak current is required to change and discharge this load on every duty cycle?

2.  How much peak drive current is my source capabile of?

Do the math!

When you find out your source can only deliver (for example) 5 milliamps or so, and that the load needs 50 milliamps to drive at high frequencies, then you know that your source is going into saturation and cutoff on every duty cycle (100% distortion for part of every duty cycle, plus recovery time and stray oscillations generated during recovery!)

Or why do you think that scope photos of most equipment driving capacitive loads look so horrible?

It may very well be that what you are really liking is not music, but distortion.

Do the measurements, and the math.  Of course that is not nearly as much fun as just listening and saying "but it sounds just great."

Wouldn't it be nice if the proponents of passive preamps would do the measurements and publish them to show that their equipment is not causing difficulties for the music sources.  I would like to be proved wrong. If the math does not work, the system does not work.  If the math does work, you can be pretty confident that what you are hearing is the music, not components under excess stress.

I would suggest that  a simple non-mathimatical test would be to set up the system with really short and very low capacitance cables between the source and the preamp, and between the preamp and the power amp.  If this changes the sound at all, you know you have capacitive load problems.

Frank Van Alstine

[Paul L]

I won't use the term "passive preamp" in my message.  I only use passive controller or passive pre and avoiding the amp terminology.  A passive controller is doing what it meant to do, nothing more and nothing less.  If the source is not capable of driving the poweramp or the poweramp and cable has high capacitance, it is not the passive controller's flaws.  It is the design flaws of the source output stage.  I can say every preamp has a built-in passive controller.  The passive controller is only an interface device like cables between two active components.  It is how well the interface is done that separate a great controller from a ok one.  I am not a great believer in wrong fix wrong and in my opinion the active line stage approach is wrong fix wrong approach that developed many decade ago when phonostage is a must in the preamp.  With the high output of CD source, the way of building an audio system need to be changed.  More and more of integrated amp are having passive pre section now and that is the indication of that direction.  That is also a good indication of the 1 stage of the poweramp section is well-thought.

[dado5]

Frank,

If you could, would you clear up a little confusion that I may have.  Speaking of tube amps only, isn't the input stage of a pre amp and a power amp more or less identical?  Most examples of both use some version of a common cathode circuit as the initial voltage gain stage.  Why would a source have a more difficult time with a power amp vs. a pre?  Is is it solely the proximity of the volume pot to the load?  If so than isn't the entire issue centered on cable reactance?

I generally prefer the sound of actives over passives (although I have not used a high quality passive or a xfmr based unit) but I don't think this is due to overload distortion. Actives, if anything, have a noisier sound than passives, my preference lies in the more dynamic nature of the sound that an active pre provides.  Could it be maybe that the demands placed on a source using a passive result in some sort of compression type effect? Is this just the LP filter problem of capacitive cables?

Lots of questions I know, but whatever info you can pass on would be greatly appreciated.

Thanks much in advance,
Rob

[dado5]

Paul,

I see you hit on more or less the same point I did.  Is there something inherently wrong with the voltage gain stage of a power amp vs a pre that would make it more of a task for the source to handle?  I can't see how this could be the case, at least theoretically.  On paper, there is no need for the extra voltage gain a pre provides for 99 and 44/100% of all amps.  At most the need would reduce to a buffer if the issue is the current drive of the source outputs. And would not this be resolved with low capacitance cables and/or a sufficently high input impedance at the amp? Am I misunderstanding anything here?

Thanks,
Rob

[Paul L]

Rob,

You actually get all the point.  If the poweramp has high enough impedance and voltage gain, it is a good candidate for passive controller.  

Another challenge is that if you are resistive base volume control, when using traditional implementation method, the sonic character will change depends on different volume setting.  This also appears on active pre but the different will easily mask by the sonic signature of the line stage itself.  If you surf AA as well, you will notice a lot of postings said the preamp is most important in terms of sonic character and passive controller is very system dependent.  But not many care to explain why.  Here I am, the active line stage is trying to fix the faults of other components like source and cables etc.  Once it does it job, the source and cable difference will not be showing up that easily because it has been "fixed".  Since the preamp is a distribution device, that makes it dictate the sound of the sources.  You are hearing more the sound of the active line stage than the source and cable.  However, when changing from one active to another active, you are hearing from one fault fixing machine to another.  With all the variables that I said before that can contribute sonic signature, it will be very difficult for two active preamps sound the same.

For a properly designed passive, it supposes not adding or subtracting anything (and I know that is only dreaming), what you heard the people saying passive is more system depending is true but actually they are hearing more difference of their system.  A good system will sound fantastic and it also won't make a poorly match system sound good.

[Steve]

I think both Frank and Paul have valid points. I will refer my comments to resistive passive, and Amplifiers to Tube type amps.

Almost all actives have sonic signatures and create problems. Only the best minimize these problems. However, Passives also have sonic signatures, but can be of a different nature.

Whether passives or actives are employed, at least two LF filters, and probably one highpass filter need to be dealt with. I think this is handled differently in both topologies.

Passives have problems in that they must not load the source too heavily, yet must present a low output Z to the amp. But this causes some problems. For instance, if a 25k "pot" is used, it may not load the source, but at mid point (worst HF response as measured on a scope), the output Z is many times that of a descent active. Thus the IC, stray, and the input capacitance of the amp must be very low to minimize HF rolloff. If too low an input Z is used in the passive to help the HF, then problems in the bass region could easily develop, especially if the output of the source is capacitor coupled. (Of course the LF problem can also occur with an active preamp, and the output of the preamp to the input of the amp. But one doesn't normally need any lower input Z than 25k.)

Actives can use the same 25k input Z and not load the source nearly as much (HF region) as the output Z of the volume control isn't directly driving the second IC, and thus the capacitance isn't as high, since even the best ICs have substantial amounts of capacitance. Actives, having an output Z many times lower than the passives output Z, with 25k "pot" used, the second HF pole is much higher, thus less HF rolloff.

As mentioned before, the active uses many parts, and most actives do leave a sonic signature, lots of distortion if not designed very very very carefully.

Another point. Since most amps seem to have an abundance of gain, why duplicate this gain with another active stage, and active preamp? Distortion will only be increased and the gain isn't necessary.

As a suggestion, why not outboard the first stage of the amp?

1) An active pre, with unnecessary gain, would be eliminated. Some might say the first stage of the amp is eliminated, but I think there is a difference between those two comments.

2) The volume control would not have to drive a second IC, which is the same scenario as with the active pre discussed above.
 
3) Hum problems could be reduced as the power tranny wouldn't necessarily be in close proximity. As a side note, the number of ICs would remain the same.

4) Frequency dependent sonic feedback through the power supply would also be eliminated, which is not the case if the first stage remains in the amplifier (all one has to do is examine all the amp schematics one can download on the internet. This is the case against integrateds. However, they reduce the IC requirements by one, which is a plus. IF the integrated uses a separate power supply for the first stage, the hum problem could  still pose a problem.)

The only problems I quickly see with my suggestions would be extra cost, any DC present on the output Z of the preamp (if DC out) and possibly dealing with any parasitic oscillations, which should be easily dealt with.

In closing, I would submit a possible solution would be to export the first stage of the amp, with a volume control, as this would give the necessary gain one needs, another useless stage would not be added, hum would be minimized, and frequency dependent sonic feedback would be minimized.

.

[eico1]

As a suggestion, why not outboard the first stage of the amp?

Why not put the volume control in the amp? All the active components in one place would make for less redundancy, ie chances for something to screw up the sound.

Then only a source selector if any would be needed.

steve

[Paul L]

ecio1's suggestion is actually valid but will eventually going back to my recommendation.  47Lab Gaincard has bulit-in volume control and input chooser as separate box.

But with this arrangement, it may better off to buy a well designed integrated and with passive pre section.  This will save another I/C between the input chooser and the amp.  Of course we need high quality volume control and selector as well.  For example, our Little Wonder sounds better than the built-in step attenuator of the Gaincard if the I/C connected is good enough.  So the argument is why buy the input chooser if the LW-1 can do the job for the input chooser and improve the volume control performance.

[Steve]

Reread my previous post again Eico1. However, I understand the budget does dictate.

1) Hum pickup can become a problem, unless very very good shielding is employed. If you can do it, to your satisfaction, in your DIY project, I would say go for it VS a separate passive.

By the way, just because the amp has a high input Z doesn't mean anything in regards to what passive Z can be used. The amps input Z at 1khz will never be the Z at 20khz (variable differences between brands etc.) and reactive. The lower active Z will provide better high frequency stability.  Actives are far superior than passives in this regard.

The passives  will usually beat most preamps, except the top tier.

2) If you must have an integrated, separate the stages from a common power supply. That is why I suggested removing the first stage from the amp (has its own PS). It costs more, but is much better sounding. Cheap is cheap.
ICs can be tested for sonic purity in a special test jig.

I realize the budget is important Eico1, we all so as well as we can.

 

[Bill Baker]

Why not put the volume control in the amp? All the active components in one place would make for less redundancy, ie chances for something to screw up the sound.

Then only a source selector if any would be needed.

  This is done quite often. Many of todays "integrated amps" are actually power amps with a passive input stage. In other words, they are power amps that contain a source selector and volume control prior to the amplifier circuit. This is a passive preamp in the same chassis as a power amp.
  I do like this idea as it does save on the problem of more interconnect cables that could pose problems. What I have been doing is installing a bypass switch in such units that will bypass the passive control stage and allow the hookup of an external preamp, passive or active, to offer more flexibility of the unit.

  Checking these units on the scope usually (not always) shows a better signal when using the passive built in stage as compared to [some] external preamp / interconnect combinations. As mentioned elsehwere in this thread, the quality of the volume control itself plays a big role. Not always for better or worse but definitely different results. Using an external preamp, especially a passive, does require careful thought into choosing cables.

  I am a firm believer that any one of these designs can provide exceptional results but like anything audio, experimentation, personal taste, listening habits and system synergy all come into play.