AudioCircle
Other Stuff => Archived Manufacturer Circles => Hagerman Technology => Topic started by: cengclimbing on 31 May 2013, 09:12 pm
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(http://www.audiocircle.com/image.php?id=81461)
You probably are saying, "Wait a second... The Bugle is a small SS design and I see tubes!" Well this is a story of a Bugle that started modestly and through a series of upgrades, due to a little mission creep, has morphed into what is above.
A little history...
I needed a nice and cheap phono stage to use while my K&K audio pre was going through a few upgrades and an exterior rework. I had built a Cornet 2 a long time ago and was confident that if Jim designed the circuit it was a good bet that the sound would be up to standard so, I ordered the PC boards for the Bugle and matching power supply and had plans to put the units in an old Hammond chassis I had laying around. I built them stock and set for 60db of gain. The sound was OK but compared to the K&K it was replacing it was a bit flat, hazy and one demential. To be fair, it is being compared to a phono preamp that costs 10x as much.
But, the bones were there. It got me thinking.
First upgrade... I had a spare set of S&B step-up transformers sitting in the parts box. To run them I would need to change a few resistors so I figured while I was desoldering anyway I would take the opportunity to change the rest of the resistors and the caps. Mundorf for the caps, PRP and Kiwame for the resistors. A couple of days hooked to Jim"s reverse RIAA...The sound opened up and the hazy presentation was gone! In it's place was pinpoint imaging and a nice wide soundstage. Vocals were a bit forward. Still, there was a little upper midrange glare but it now was heads above the stock unit.
I was thinking this was it. Good enough. But...
Second upgrade... I had a blue LED going from the power supply while it was on the project board and figured out quickly that when the lights were low that blue light would drive me crazy. I like the way tubes look at night. Maybe I could kill two birds with one stone and get rid of the LED and address the upper mid glare by giving this thing a tubed output? A few quick clicks and an Aikido cathode follower and power supply board were on their way.
We are now getting away from nice and cheap and moving to really nice and not so cheap. (Mission Creep has set in!)
The sound with the S&B TX103's into an upgraded Bugle into an Aikido cathode follower set up for 12au7's... Excellent. Great mids, airy highs, wide soundstage and above all wonderful musical flow!
Still sitting on a piece of plywood...
Third upgrade... This thing sounds so good I can't just stick it into an old used Hammond chassis now could I? Again, a few quick emails and Mike at KM Audio Chassis had the aluminum bent. I had some Ebony in the shop so he did the metal work and I the wood work. The metal comes brushed but I took a little time and brought it to a full polish. (We are now past mission creep and into the stage where my wife is asking me sarcastically when will this "quick" project be done?)
All together and it sounds good but... What about some NOS Telefunkens or Mullards from the tube stash? Yep. They both sound excellent. Mullards for the mids, Tele's for the top and bottom punch. And while I'm at it maybe I'll stuff some Mundorf silver/oil caps in the output stage. (I had the caps in the parts bin.)
It now looks and sounds as good as anything I have had in the system. I've had it running a Dennon 103r, Shelter 501 mk2 and a Benz Ruby. All sound wonderful and the Bugle shows off each cartridge's personality!
Here are a couple of pics of the build.
Lots of holes.
(http://www.audiocircle.com/image.php?id=81462)
Shiny is good.
(http://www.audiocircle.com/image.php?id=81463)
Tube Power Supply and Transformer.
(http://www.audiocircle.com/image.php?id=81464)
The layout. It all fits in a 10x12x2 chassis.
(http://www.audiocircle.com/image.php?id=81465)
Jim's Bugle under the Mudorf caps.
(http://www.audiocircle.com/image.php?id=81469)
Finished!
(http://www.audiocircle.com/image.php?id=81484)
(http://www.audiocircle.com/image.php?id=81486)
It's too bad that S&B no longer supports the DIY community. The TX-103's are really special.
(http://www.audiocircle.com/image.php?id=81466)
(http://www.audiocircle.com/image.php?id=81468)
Thanks to Jim for designing such a great circuit. One so good it lead to a month of extra work and a bunch of boutique parts. It's now a keeper for the long haul!
Cheers,
C-
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Wow - that's some amazing woodwork!
jh
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Thanks Jim. You, and the other designers, make stuff like this possible by offering your designs up to others in such a user friendly form. I don't think I would have tried anything like this without printed PC boards and great support.
Thanks.
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Simply brilliant!
Jack
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Thanks! It's burning in and sounding great. Just a few more tweaks to do when time permits. I'd like to try some different opamps in position 3 and change out the input caps on the tube cath follower to mundorf silver/oils.
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Wow! :notworthy: From one diy'er to another. Great attention to detail, parts execution, topology, etc...
Enjoy the fruits of your labor!
Best,
Anand.
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Got my new kit from Jim. You have set a pretty high standard!
Cheers,
Geary
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Beautiful work, love it. :thumb:
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Thanks guys for all the kind words. This was a fun project all the way around. As for your new Hag kit... Upgrade those resistors and cAps if the budget permits! They make a ton of difference and Jim's circuit design really benifits from them. Also, if the aesthetics matter to you look up Mike at KM audio chassis. With his help you can have a piece of gear that shines as well as sounds great for not much more than the cost of a simple Hammond box.
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...look up Mike at KM audio chassis.
Google wasn't my friend on this one. Do you have contact info?
Cheers,
Geary
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Absolutely! Mike just took over building these after his longtime friend Keith Kirby pasted a few months ago. The design is Keith's and I have to say, having Mike building these and making them available to the DIY community gives us all a great reasonably priced option. There is nothing wrong with a stock Hammond chassis but these just bring our projects to another level. Anyway... Here is his email and I also included the ebay listing. It's an example of what he can do and has available now. (Farming is his "real" job.) He can build custom sizes and is doing one for a headphone amp project that I have cooking.
Oh ya... Just a quick note I feel I need to make... I have no financial connection to Mike's company. I'm just a satisfied customer.
I've learned so much and discovered great suppliers though this forum and others, I hope that some DIYers benefit from the info.
kmaudiochassis@gmail.com
http://www.ebay.com/itm/281081716230?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649
Cheers,
Chris
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Thanks. I strongly believe a well engineered chassis is as important as the build components, especially with low level signals. I built my Cornet2 chassis from scratch. I wanted lots of real estate and protection from outside influences. I got it, but the time and expense, (not having all the tooling and experience), was challenging.
I will take a look!
Cheers,
Geary
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It looks so nice! Mine is just in a grey box without any labels, on/off switch, and a big chunk-chunk feeling input selector knob (no labels :) )
I recommend better input resistors. I'd say something about the signal caps but you already replaced them, and the none signal caps!
I was using these (http://www.partsconnexion.com/resistors_caddock_tf.html), considered these (http://www.partsconnexion.com/resistors_vishay_var.html), but have switched to these (http://www.buildanamp.com/Resistor-Replacer-RR.htm) (haven't heard them yet though).
The signal is a little fragile to subject to the sonic signature of a Kiwame, at the input. I think the signature is appealing for phono preamp, but maybe not as heavily laden as on the input.
The stock choice of opamp is good, but sometimes these (http://www.partsconnexion.com/SEMI-76010.html) can be had cheaply on Ebay, and look to be maybe the best candidate short of discrete (expensive, and worried about RF a little bit on the first opamp in the chain).
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Thanks for the input. I guess some explanation on my parts selection would help others so here it goes.
To start: I have used the Vishay Nudes in my integrated amp when I changed the resistors but at $12-$17 a piece the cost started to rise a little to quickly. I do agree with you that they are terrific. At this price point though, I think the best bang for the buck was in the caps. In fact I would have used the silver/oil supremes instead of the standard supremes had I thought that the circuit was up to it at the time. I know now that it is but didn't have that experience when placing the first cap order. I like the silver/oils and their smooth yet detailed presentation and think they would be great in this application. I did have a few V-caps in the parts bin but not all that would have been required. Just like the Vishays I thought the cost was a little high for the design. As far as the Kiwame resistor choice on the input... The part of the stock sonic signature that I was trying to quell was an unrelenting brightness. This was evident with the Shelter and the Benz in an all tube system using 12au7's into el-34's. My system, for good or bad, leans towards the romantic side of things . Having never used a piece of gear based on opamps I figured, maybe incorrectly, that the opamps and the inexpensive metal film resistors were largely to blame for this condition. I had a reasonable assortment of Kiwames on hand and I enjoy the sonic signature they provide. YMMV. Ask someone what their favorite resistor is and you will get a bunch of different opinions. Add to that, asking what your favorite resistor is in what position and you will get another bunch of opinions.
If I were to do it again: Mundorf silver/oil for the caps, and Riken resistors (NOS sourced from overseas) and I would solder the first two opamps in place.
I would be super interested in your opamp recommendations. It sounds like you have a lot of experience with different types and it is the last part of the puzzle that I haven't worked out. It seams like it is just as much, if not more, of a black art as tube signature so any guidance would be great.
Lastly... As with all things audio, system synergy is the most important thing so I caution everyone to take my recommendations with a grain of salt. What I have built works well in my system and while the K&K does most things better, I haven't enjoyed listening to albums any less with the upgraded bugle in the mix. I could happily live with it for the long haul.
Cheers,
Chris
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Is the tube circuit used as a buffer stage on the output of the Bugle or does it replace one of the Bugle's op amp gain stages?
Russ
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Currently I am running all three bugle opamp stages. The tube stage is a follower after the last opamp. Your question got me thinking if it is possible to bypass the last opamp and have it running straight to the cathode follower tubed stage. Unfortunately, I have no idea if it would work. Maybe Jim could chime in here? I would be willing, if not anxious, to build another Bugle using the different parts specced earlier and having it go straight into the Aikido Cathode follower circuit. If someone can look at the schematic and tell me where to pull the signal from on the PCB I would be happy to give it chance and report back.
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You can get the signal from the top of R15. It's very likely you have an input capacitor and the grid leakage resistor in your tube buffer - you should remove both of them. I'd also look at the voltage ratings for C1 - the capacitor is unlikely to be presented by high voltage, but in the tube circuits it's better to be on safe side. I think 200V or more is enough.
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Thanks Poty! I think I will give it a try. This is the schematic of the cathode follower I am using. I think I need to pull C1 but can you confirm and guide me as to what resistor needs to be removed?
(http://www.audiocircle.com/image.php?id=84526)
I'm still in the infant stages of schematic reading.
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Jim,
I was wondering if the opamps in positions 1 and 2 can be substituted with LME 49720HA's as suggested by Salis Audio?
Thanks,
Chris
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Chris,
I did not see your cathode follower schematic and I should admit that you'll have to make more changes than I suggested.
First of all, the input tube grid is tied to B+ (through R5/R2 divider). I'd use the C1&C5 in Bugle rated more than B+ for safety.
Then you should remove R10, C1 from the tube circuit.
I don't know your B+ value, but it seems the power ratings of R15 and R33 in the Bugle is not suitable for the R2 replacement of the cathode follower. Then the R15 and R33 should be removed from the Bugle. The R2 will play the role of the removed R15&R33 and must be 47k5 to save RIAA characteristics of the Bugle. I don't know what the actual value of the R2 is. The main considerations here: new R2=47k5, new R5= old R5 * 47k5 / old R2. But if the old R2 is considerably bigger in value than 47k5, than there could be some problems (the dividers will draw more current - questions to power supply power and the resistors wattage).
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You still need the 3rd opamp stage in the BUGLE, as it provides the last 15dB or so of gain. The CF won't have any voltage gain. Also, the lower tube in the Aikido is a triode? If so, then it's plate impedance is probably not a lot higher than just using a power resistor for a pulldown (which would also provide more stability). A pentode would be far better in this spot. The alternative would be using the triode in a grounded grid configuration (like the TRUMPET).
jh
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Thanks Jim. This sounds like a project that is way out of my ability level. Maybe someone else wants to take it on? It would be interesting to hear the results.
Cheers,
Chris
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The last stage is definitely provides some gain, but the gain can be obtainable from the previous two stages just by altering the resistors.
Second consideration is not the cathode follower, but tube gain stage as the last stage. There are a lot of examples of that: SRPP, mu-follower, Allen Wrights gain stages... The way opens more possibilities like transformation the Bugle + tubed output to full-fledged preamp with integrated phono stage. :)
IMHO it is better to made this more advanced (like new trumpet, based on JFET input stages) than to continue to use opamps, but who knows...
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One of the beauties of the BUGLE / BUGLE2 design is that of gain management. I do NOT ask too much from a single stage, but rather spread out the gain duties as much as possible. This does several things. It prevents overloading of the next stage (critical when using passive EQ!). It maximizes the bandwidth of each stage. It minimizes distortion. It improves stability.
There are LOT of these little things going on in the BUGLE2 design that are NOT obvious. Those extra 316/332 ohm resistors that seem to make no sense at all are actually some of the most important in the design.
Go ahead and make the changes, but don't be surprised if some of that magic goes missing...
jh
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One of the beauties of the BUGLE / BUGLE2 design is that of gain management. I do NOT ask too much from a single stage, but rather spread out the gain duties as much as possible. This does several things. It prevents overloading of the next stage (critical when using passive EQ!). It maximizes the bandwidth of each stage. It minimizes distortion. It improves stability.
Jim, as you may already noted I differ from you in recognition of some "magic" things and put my power solely into technical abilities of the schematic and listening practices. Then I don't agree with you about almost everything you wrote above.
First of all, you designed the usage of the (both) Bugles for a 20dB more gain and ironically saying - the gain is spread across the first two stages. Then - use the stages in this field is completely approved by you, the only question is about overloading, because MM carts have more than 10 times more output than MC.
Overloading: Passive EQ does not have any relations to overloading. Initially the overall output of most of MM catridges is 5mV (amplitude is around 7.2mV). Lets use twice as much of the amplitude - 15mV. Output of the first opamp (before the first filter) is 165mV (MM version) and 512mV (MC version). Well in the range of the ouput voltages even for 9V power supply (max 9V-2.5V=6.5V). After the filter there is: almost 0dB in low frequencies to -20dB in high frequencies. Then the most prominent part of the spectrum (due to RIAA it is high-freq) is -20dB lower: 16.5mV (MM), 51.2mV (MC). At the output of the second opamp there is: 181,5mV (MM) and 1.75V (MC). There is no overloading either.
Bandwidth: 20dB has around 600kHz (-3dB), 30dB - around 350kHz, but it seems -40dB at 20kHz of reverse RIAA says more about the bandwidth...
Distortion: According to the datasheets there is no significant changes in THD+Noise in the output voltage region first of all. Second consideration - the voltage amplitudes exist in the 3-rd stage in the original design. More input voltage is better for signal/noise. The only possible problem - the input opamp gain, but IMHO 3 stages has more THD+noise than two.
There are LOT of these little things going on in the BUGLE2 design that are NOT obvious. Those extra 316/332 ohm resistors that seem to make no sense at all are actually some of the most important in the design.
It is one of the "magic" thing in which I do not believe. The resistors is for "mythical 3.18ms corner in the RIAA response" which is someones opinion on the cutting head limitation. Not very trustworthy opinion sort to say. Lengthy reasoning and no argumentation. I tried several times to incorporate the pole in a design and never hear any difference. So the matter of importance of the resistors is: believing or not.
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Well, first of all I meant "magic" of the sound, not implying that there was any magic going on in the circuitry...
Yes, MM carts are indeed more of an issue for overloading than MC. And 5mV is merely a "typical" value at 4.5cm/s velocity. In practice you can get transient peaks 16dB higher than this. And note those numbers are at 1kHz! The values at 20kHz are 20dB higher, which is where you need to do your calculations. Not only that, but splitting the EQ into two sections means you do NOT get all the treble cut in the first section (it takes the combination of both). As a result the gain at 20kHz is not greatly reduced heading into the second stage opamp. You can easily see this by measuring the response on an oscilloscope (especially using square waves via the iRIAA filter). This again was on purpose, to keep noise to a minimum. Obviously putting the EQ in a feedback network will result in a lower noise floor, but I do not like the sonic penalties incurred by such a topology. Good design is about balancing a number of compromises. As is, the BUGLE does not overload given real world conditions. Increasing the gain of either the first two stages can compromise that - and lead to adverse affects in the sound. You'll hear it as a shrillness on transients and/or merely a fatiguing of your ears over time. I mis-spoke if I had said the "gain is spread across the first two stages". It's actually across all three.
And there are several more 300 ohm resistors in the circuit besides the 50kHz corner ones. I should have specified they were the important ones. And yes, not everyone can hear the effects of the 3.18us corner. I can't detect it directly, but more as how it impacts the "air" and ambiance of a recording. Here's a good test: Can you hear a Nissan Leaf coming? I can. The electronics give off quite a whine. And it's not even that high up in frequency.
Bandwidth of the opamps in this circuit is not related to EQ.
jh
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...MM carts are indeed more of an issue for overloading than MC... 5mV is merely a "typical" value at 4.5cm/s velocity. In practice you can get transient peaks 16dB higher than this. And note those numbers are at 1kHz! The values at 20kHz are 20dB higher, which is where you need to do your calculations. Not only that, but splitting the EQ into two sections means you do NOT get all the treble cut in the first section (it takes the combination of both). As a result the gain at 20kHz is not greatly reduced heading into the second stage opamp...
I was thinking where to stop the quoting. Decided at this place. You've said about AFC of cutting gear (I mean +20dB at 20kHz), but this is not directly related to the absolute amplitude at this frequencies. The total power of harmonics at this frequencies is usually 10-30dB less than at 1kHz. You can see this dependencies in many professional and Hi-Fi/Hi-End devices: multiband amplifiers, modern acoustic systems and so on. Even in the vinyl era the RIAA +20dB at 20kHz was chosen intentionally - it doesn't greatly affect the width of cutted track which allows to increase the density of tracks per a record surface.
Another consideration: I got the nominal value of the record which already includes the boosted hi-frequencies.
I understand splitting EQ and estimated only the first part of the EQ as -20dB @ 20kHz. Don't you agree with this estimation? Then - count this by yourselves - you'll get the same numbers.
Let's count your example of full power 20kHz as a "musical signal": (5mV(RMS)+16dB)*sqrt(2)=45mV in amplitude. MC-version output of the first opamp would be around 1.5V, after the first part of the filter - 150mV. MC-version output of the second opamp - 5.2V - still under the opamp limit (9V supply) of 6.5V. We are speaking here about 15V supply for which the limits are 12,5V - way more than this never happen amplitude. This signal will definitely fry many ASs twitters and make a listener deaf.
Obviously putting the EQ in a feedback network will result in a lower noise floor, but I do not like the sonic penalties incurred by such a topology.
I didn't say a word about EQ in the feedback network, but if we speak about feedbacks: I do not like any feedback: frequency-dependent or not.
Both Bugle versions have also a mismatched (and also frequency-dependent) source impedances (as described on page 9 of the opamp datasheet) for the second and the third stage. "The effect is increased distortion due to the varying capacitance for unmatched source impedances greater than 2kOhm."
You have not mentioned also my second suggestion - use a tube amplifier circuit instead of cathode follower and save "MM-values" for the first two stages.
I mis-spoke if I had said the "gain is spread across the first two stages". It's actually across all three.
You mis-readed my explanations:
you designed the usage of the (both) Bugles for a 20dB more gain and ironically saying - the gain is spread across the first two stages.
And there are several more 300 ohm resistors in the circuit besides the 50kHz corner ones. I should have specified they were the important ones.
No, you meant the resistors I answered you about, because the only more around 300Ohm resistor in the original Bugle is at the output. Resistors at the output are nor novel not very important ones. Very low output impedance of the opamp nulls all line instabilities. Certainly the precautions are a good things, but not important ones. The (input to opamp) resistors in the Bugle 2 also (I think) are the part of the decoupling circuits, but (surely lowering frequency dependence from the preceding filters) adds to impedance mismatching.
And yes, not everyone can hear the effects of the 3.18us corner. I can't detect it directly, but more as how it impacts the "air" and ambiance of a recording.
I wonder - how people with such "ears" differentiate things? For example, in the Bugle there are rather prominent frequency-to-phase dependence in the hearing area, but Hi-End persons hear only some (always positive!) changing way over the hearing ability of any person and don't hear the obvious.
Here's a good test: Can you hear a Nissan Leaf coming? I can.
Haven't ever come across the car, so can't say surely.
Bandwidth of the opamps in this circuit is not related to EQ.
Yes, but artificially lowering the signal to 40dB@20kHz says more than 3dB@ >300kHz.
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Actually, the 300 ohm resistors on the outputs are the most important. It's just not obvious what they do. You have to take the big picture into account - and by that I mean interconnect cables and the load of the linestage / volume control / amplifier being driven.
At audio frequencies the interconnects present essentially a capacitive load to the output stage. Without the 300 ohm decoupling, this capacitance is sitting right on the output of the opamp. Adding such a capacitance leads to instability, ultrasonic ringing, and worst of all, slew rate limiting which breaks the feedback loop (and hence no relation between input and output until it catches up). Decoupling the load from the feedback network prevents such issues. The opamp can do its thing without having to deal with some unknown load.
These things are usually ignored by most designers, as they assume the capacitance to be negligible. Well, audio is about things that are practically unmeasurable (I still can't define how to measure good soundstaging). And it is the little things that add up. That's what I mean when I say all the little things in the BUGLE add up.
Similarly, the input 300 ohm resistors do two things. They provide a little bit of ESD protection by working with the input diodes on the opamp to limit current. They also form a parasitic RC low pass filter with the input capacitance of the opamp. This helps to cut RF energy above 10MHz, where radio signals can still be demodulated by the junctions in the input JFET pair. Again, none of this is obvious to the lay person and many people who copy my circuits leave them out not understanding their intended function.
I'll give you another example. We were talking about the difference between 20dB and 30dB gain per stage. The differences are easy to neglect, no? Well, let's do a thought experiment. What if I had five 20dB stages cascaded. That would give me 100dB of reasonably wide bandwidth and low distortion. Now what if we tried to do the same thing with one stage? 100dB with a single opamp presents a number of issues. The open loop gain is not high enough to result in much negative feedback leading to high distortion. The feedback resistor will be ridiculously high, and bandwidth will necessarily be low. Much lower than with the five stages. Ok, that's a pretty extreme case, but I use it because it makes things more obvious. The same issues exist in the 20dB / 30dB example, just to a lesser degree. My point is that they do exist and you should not always ignore them.
jh
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FYI - I did do a sound check on my ears. Made it to 12kHz easily, but then it dropped off super quick and 13kHz was gone. My 11 year old daughter made it to 18kHz with the same equipment.
And having done this calibration, I'd guess the Nissan Leaf to be in the 8kHz to 10kHz range...
jh
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Sorry for not answering quickly - on vacation now. Trying to estimate the example you've given.
I did the (frequency) check 2-3 years ago. In my case the result greatly depended on my conditions. Sometimes I recognized 16-17kHz, sometimes even 12kHz was in doubt. I don't think it is much important, because at the frequencies a person can mainly only detect "something audible" and use it (maybe) for recognizing the direction to the source, nothing more.
According to the interconnects... I think the Bugle is not positioned as Hi-End gear and users will use "matched class" interconnects. While the resistor at the output definitely solves (at least eases) the problem of stability for the opamp I can't agree with your estimation of slew rate. Instead of "active circuit problems" we'll have "passive filter problems". The 330 Ohm resistor connected to - say - 1500pF cable (remember matched class) gives us a filter with 320kHz pole. I agree that in this case you may decide for the less difficult problem, but in some other cases - it may be preferable to use some other output stage layout, supplying successfully to higher class interconnects without additional parts.
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Regarding slew rate - it helps to think in extremes, then the problems become apparent. For example, let's place a 1uF film capacitor on the output of that opamp. I can pretty much guarantee the slew rate will go to hell. Once the input signal demands a fast transient, the output can only go so fast, and slews at the current limit of the output stage, resulting in a ramp waveform. During this time, any other musical content contained in the input signal does NOT show up at the output. Only the ramp. We have gone open loop. Eventually, the output catches up to where it should be and the loop closes and acts like an amplifier again. Well, unless of course there is some integration component in the feedback which will cause something called "windup" (but that's for another day).
Change the load capacitance to 1pF and the problem pretty much goes away. The BUGLE resides somewhere in the middle. We DON"T know exactly what the loading capacitance is going to be. I play it safe and add inherent protection, such that it becomes very difficult to cause problems. Most other designs do not have this protection.
Oh, one other thing. The stability of that opamp depends on how it was designed, it's output drive capability, and the gain it is operated at. Low gains (such as +1) are the most unstable of conditions. That is why I shifted a bunch of the BUGLEs distributed gain to the output stage. Not only does it improve stability, it also ends up increasing overload capability (as discussed previously).
Lots and lots of little things adding up to make one big difference. :)
jh
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Thank you for this most educational discussion, Gentlemen. I love reading the posts of those who know way more than I do...........I always manage to learn a little something.
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Well, let's do a thought experiment. What if I had five 20dB stages cascaded. That would give me 100dB of reasonably wide bandwidth and low distortion. Now what if we tried to do the same thing with one stage? 100dB with a single opamp presents a number of issues. The open loop gain is not high enough to result in much negative feedback leading to high distortion. The feedback resistor will be ridiculously high, and bandwidth will necessarily be low. Much lower than with the five stages.
Well, better late than never. 100dB is a very high gain. Of course it can't be achieved by only one stage, but let's be more practic. Let's take OPA2134 as the base opamp powered by +/-15V and compare two-stage device with 50dB each to five-stages device with 20dB each. I used the official datasheet of Burr-Brown OPA134, OPA2134, OPA4134 and counted s/n, IMD and bandwidth for both versions:
Version........S/N..........IMD............BW(-3dB)........dGain@20kHz..........S/N@20kHz
2 stage........37dB........0.03%........23kHz.............-2.34dB...................17.7dB
5 stage........36dB........0.78%........490kHz...........0dB.........................16.5dB
As you can see even at this extreme case the results are comparable and bearable. The OPA2134 is not very good at GBWP and slew rate which adds to difference in the bandwidth.
Regarding slew rate - it helps to think in extremes, then the problems become apparent. For example, let's place a 1uF film capacitor on the output of that opamp. I can pretty much guarantee the slew rate will go to hell. Once the input signal demands a fast transient, the output can only go so fast, and slews at the current limit of the output stage, resulting in a ramp waveform. During this time, any other musical content contained in the input signal does NOT show up at the output. Only the ramp. We have gone open loop. Eventually, the output catches up to where it should be and the loop closes and acts like an amplifier again.
It is difficult to count this params, so I used a simulation program to measure the things. There are the results:
1. Directly connected 1uF capacitor to the output of +/-15V powered OPA134, connected as 20dB gain, 0.25V input signal:
Difference in Gain @20kHz: +0.4dB
BW (-3dB) on the capacitor: 147kHz
90grad phase at the output of the opamp @ 94kHz
Slew rate on the capacitor: 42mV/us
Result: there is a possibility to oscillate at the frequencies more than 94kHz, but the bandwidth is OK. Slew rate is small.
2. 1uF capacitor conected through 332 Ohm to the output of +/-15V powered OPA134, connected as 20dB gain, 0.25V input signal:
Difference in Gain @20kHz: -32.4dB
BW (-3dB) on the capacitor: 477Hz
90grad phase at the output of the opamp @ 3.2MHz
Slew rate on the capacitor: 6.5mV/us
Result: there is a lot of stability, but the bandwidth is completely unusable. Slew rate is even smaller than in the first example.