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Old 15th May 2012, 07:12 PM   #241
Bob Cordell is offline Bob Cordell  United States
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Quote:
Originally Posted by Bonsai View Post
WRT the caps in the feedback network, it's important to remember the distortion issue only arises at LF where the cap impedance increases. So even if you are using lower value feedback resistors, as long as you select the cap value based on the same -3dB corner frequency, your distortion should beno worse than a cap sized for a higher feedback resistance network.
Hi Bonsai,

I shy away from electrolytics in the signal path; analogously, I avoid the use of Mylar capacitors even as coupling capacitors and instead use polypropylene, even when the corner frequency is well below the audio band.

Although I concluded that NP electrolytics produce substantially lower distortion than polarized ones, especially if the NPs are rated at a higher voltage, this was not an endorsement of using electrolytics. I am very cautious about having them in the signal path, and strongly prefer to use DC servos to eliminate electroytics of any kind. I was really saying that if you MUST use an electrolytic in the signal path, you are likely doing less damage to the signal with an NP, based on its better THD performance. Here we are getting into the issues of correlation of measurements with sonic quality, I know.

Cheers,
Bob
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Old 15th May 2012, 07:21 PM   #242
Bob Cordell is offline Bob Cordell  United States
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Originally Posted by owdeo View Post
Hi Bob,

I own your excellent book also, thank you. I found the section on SPICE simulation particularly helpful BTW.

I did read your findings on NP capacitors used in the feedback network, and also recall a referrence somewhere to the input coupling capacitor of a power amp needing to be of the highest possible quality (apologies if I'm misquoting, don't have it handy).

Given that advice, what do you think about the importance of capacitor type in this application (large coupling caps between stages in a preamp)? Are polarised electros harmless provided they are large enough?

Cheers!
Hi Owedo,

Thanks for your kind words about my book.

In answer to your question, see my post above. I avoid electrolytics as coupling capacitors just about always. In saying this, I realize that in such instances there is virtually no difference in measured THD. Although I am a strong advocate of the use of THD as a measurement tool, and of achieving low THD in an amplifier, I'm not always convinced that THD is all there is to it. So I err on the safe side and use quality capacitors (or no capacitors) in coupling applications.

Cheers,
Bob
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Old 15th May 2012, 07:38 PM   #243
Bob Cordell is offline Bob Cordell  United States
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Doug,

If I'm not mistaken, the 5534 used in your design for the RIAA equalizer is in a non-inverting unity-gain configuration at high frequencies, yet you appear to be using a 4.7pF compensation capacitor when the spec sheets for the 5534 recommend a 22pF compensation capacitor for unity-gain non-inverting situations. Did I look at the schematic wrong or misunderstand something here? Can you comment on that?

Cheers,
Bob
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Old 15th May 2012, 08:54 PM   #244
ChristianThomas is offline ChristianThomas  United Kingdom
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Originally Posted by scott wurcer View Post
I agree, too often folks mix and match test conditions to prove their point.
Thank you for that, Scott. Probably needed since I see that Mr Cordell is apparently not making any concessions to them. My excuse is anyway that I can't do without them because my amp runs off a single supply.

But I would rather have a coupling cap any day than my pet hate of a cap to ground in the feedback. Apart from the fact that if there is a place to define the sound of an amplifier, it is the feedback loop, my distaste for it stems from the fact that you don't get a proper high pass function. It simply stops at unity. At least in a power amp it will always be the same (imperfect) function, but in other places - with pots around - it can vary. Enough of them and you'll never know how your amp is going to sound.

I'll post a comparison graph if I can. It really isn't at all clear which you should chose. And a 10uF Wima MKP10 pp foil must be about as good as you can expect to get (it's certainly big enough at 40x45x23, and expensive enough) yet its imperfections start showing up at 5k - and with more pixels that would be lower. By contrast the electrolytics do pretty much what you would expect of a capacitor with a small resistor in series. The loss angle opens up as the resistance gets more significant.
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Old 15th May 2012, 09:16 PM   #245
Bob Cordell is offline Bob Cordell  United States
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Originally Posted by ChristianThomas View Post
Thank you for that, Scott. Probably needed since I see that Mr Cordell is apparently not making any concessions to them. My excuse is anyway that I can't do without them because my amp runs off a single supply.

But I would rather have a coupling cap any day than my pet hate of a cap to ground in the feedback. Apart from the fact that if there is a place to define the sound of an amplifier, it is the feedback loop, my distaste for it stems from the fact that you don't get a proper high pass function. It simply stops at unity. At least in a power amp it will always be the same (imperfect) function, but in other places - with pots around - it can vary. Enough of them and you'll never know how your amp is going to sound.

I'll post a comparison graph if I can. It really isn't at all clear which you should chose. And a 10uF Wima MKP10 pp foil must be about as good as you can expect to get (it's certainly big enough at 40x45x23, and expensive enough) yet its imperfections start showing up at 5k - and with more pixels that would be lower. By contrast the electrolytics do pretty much what you would expect of a capacitor with a small resistor in series. The loss angle opens up as the resistance gets more significant.
Hi Christian,

The whole capacitor thing is an area filled with controversy, so I try not to take a hard position on either side, but as I said, I do usually err on the side of quality capacitors while also keeping the cost and size in mind. I respect your position and just recommend that you try to use an NP crossover capacitor if you are going to use an electrolytic.

Your comment about a coupling capacitor versus a feedback network shunt capacitor is interesting, however, and I'm not sure I concur. It seems to me that if the -3dB LF frequency response point from the use of either capacitor is the same, then any sonic damage from the use of an electrolytic in either position will be about the same. I don't think the fact that the falling gain in the feedback network arrangement stops falling at unity changes the issue at hand. This might make for an interesting discussion, however.

Cheers,
Bob
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Old 15th May 2012, 10:37 PM   #246
redjr is offline redjr  United States
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New Doug Self pre-amp design...
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Originally Posted by Bonsai View Post
... If a custom enclosure is on its way, I may consider building one myself...
I'm hopeful Elektor will provide a custom enclosure. I'm already committed ($$) to this project, and having a nice enclosure will make pulling it altogether much easier not to mention provide a nicer conclusion.
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Old 15th May 2012, 11:39 PM   #247
ChristianThomas is offline ChristianThomas  United Kingdom
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Originally Posted by Bob Cordell View Post
Hi Christian,

It seems to me that if the -3dB LF frequency response point from the use of either capacitor is the same, then any sonic damage from the use of an electrolytic in either position will be about the same. I don't think the fact that the falling gain in the feedback network arrangement stops falling at unity changes the issue at hand.

Bob

My principle objection to the shunt capacitor is almost aesthetic. Having known Kendall for what must be knocking on 30 years, and him having been my favourite tutor throughout, I'm inevitably imbued with a love for filters. And, as you struggle to put your own mark on designs, or alternatively put what you know to work, I instinctively dislike something that fails to meet what should be happening. Whether that has an effect on SQ I actually have no idea, because I have never tried the alternative.

But, if you take the gain down from 20 odd, to something smaller, which I think is the right way to go if you have decent op amps upstream with GBWPs in the 10s of MHz, then the filter shape becomes constrained. Yes, the -3dB point will be almost identical, but it is not much further down that the graph starts getting very different from the theoretical. I'm thinking x10 or x12 as gains - and that sets your platform. To me this is unattractive. It really means that just as you hit your -3dB point the amp is immediately heading off towards it own 0dB.

I don't know if I'm good enough to give an answer to whether the degradation is "about the same" for just the non-perfect aspects of the capacitors. Maybe Jan has an idea on this, since he looks into these sorts of things. The turnover frequencies have to point to the same thing - and if anything with a deficit my way. But I really don't know. Perhaps with a few pointers and suggestions from other people I might get something I could get a grip on, but for the moment its faith.

But if you might think the fact is that my amplifier is single supply and so must have a great big C which inevitably changes its response with load, so making all of this immaterial, it actually doesn't. It does have a great big C, but its response is defined by the bandwidth of the amp alone and it is entirely load invariant. I'm suspicious that there might be more going on than I know about but I have been fairly convinced so far that this load invariance at low frequencies, and my care of the LF transfer function, are significant reasons in it sounding so good. It certainly doesn't sound good because I have loaded it up with expensive components.

ATB

Christian
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Old 16th May 2012, 01:08 AM   #248
owdeo is offline owdeo  Australia
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Originally Posted by Bob Cordell View Post
In answer to your question, see my post above. I avoid electrolytics as coupling capacitors just about always. In saying this, I realize that in such instances there is virtually no difference in measured THD. Although I am a strong advocate of the use of THD as a measurement tool, and of achieving low THD in an amplifier, I'm not always convinced that THD is all there is to it.
Thanks Bob. My ears would tend to agree with you on that. I am an engineer but audio is only a hobby so it puzzles me that many times I have been dissapointed by the sound of gear I've build even when it measures impeccably in terms of the usual measurements. You'd think that with subjective assessment, with psychology being such a big factor, everything I build, especially when it measures well, should sound brilliant to me! Very frustrating that...

A couple of years ago I built a line preamp design published in the local mag Silicon Chip. It uses an OPA2134 in a dual non-inverting amp configuration with the first section set to gain of 10dB, a 10k log pot in the middle, and the other opamp section unity-gain buffering the pot output. Bipolar coupling caps are used throughout incidently... The published THD+N curve was below 0.001% from 20 to 20k at 1V.

Trouble was, I didn't like the sound, smooth but very flat and boring with no sense of acoustic, in short I didn't want to listen to music with it. So I plugged in an NE5532, as it looked as though it should work just as well if not better in the circuit. Very different result, now there was more sense of acoustic and the brass section actually sounded like brass instruments. But, massed loud violins sounded a bit screachy. The Blues Brothers cranked up high sounded harsh too, but much more enjoyable. Clearly I was hearing distortion of some sort that wasn't noticeable with the OPA2134.

At the time I had access to a Neutrik A1 analyser and I measured THD+N vs freq at 0.5 and 2V output with each opamp. It was below the resolution limit in all cases (-100dB) from 20Hz to 20kHz.

The distortion causing screachy violins must have been much less than 100dB below the signal, so it seems that the THD+N vs freq curve was not picking it up. I realise this "distortion" was still a subjective assessment, but I'm quite sure I would still pick it in a double-blind test, and besides, at the time my wife complained that her favourite CD sounded harsh without my having mentioned anything about all this to her (as if she'd be interested ).

In the case of the Precision Preamp '96 the design is such that large coupling electros are essential anyway. Guess I'll build it as designed with polarised caps and can always experiment with bipolars should there be any incentive in terms of the way it sounds... Can't wait to have some good tone controls...
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Old 16th May 2012, 03:50 AM   #249
ChristianThomas is offline ChristianThomas  United Kingdom
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Originally Posted by owdeo View Post
I am an engineer but audio is only a hobby so it puzzles me that many times I have been dissapointed by the sound of gear I've build even when it measures impeccably in terms of the usual measurements.
May I apologize for cutting in between you and Bob Cordell but yours is a story that I have heard so often. There is no earthly reason that we should have amplifiers that ever sound bad. I'm not saying you should beat people who are commercial and have worked hard, but making crap amps should be dead these days.

I do though doubt your measurements at -100dB, but we will leave that to one side. Nonetheless we can make a decent amplifier if you follow a few rules.

The first is to know your reference. Your signal is between one thing and another. And when it comes out it is by comparison to that same reference. Mostly this means a jolly good earth. Without it you are lost. So make sure your output is referenced to the same place. You really do have to look at where the currents go and make absolutely certain nothing has gone wrong. Worry if you see shared paths of any sort. (This actually doesn't mean that you need an elaborate star ground, just something decent and intellectually defensible. But stars are generally good, if in doubt.)

Secondly you need a decent power supply. This will come from a 317 and perhaps its mate. If these are going to be properly referenced supplies the have to go back to the same place. This is your ultimate reference. You will also need to smooth this supply. An RC will usually do, but do not think that the accepted RC on the way in is correct. It will likely not be. Make these choices for the op amps you are going to use.

And have a proper look at the bypassing needed by the op amps 100nf, though standard, is likely not the right value, even if spread around. These capacitors pass signals that are inside the op amps and you had better learn how the rejection ratio happens across their Miller cap. That is why it tails off. (You can perfectly well use any op amp without understanding this, but you will need to when you come to choose them). Knowing how the bypass caps work for different designs is probably key to getting the best out of them.

Then you should look at the loading on the output. It's nice if it can do 600 ohms, but just bloody ignore that. There are about 3 op amps that can manage it. But it had better be able to do 2k, even if it is aiming for 10k.

And within the circuit, have a look out for loads that you don't even notice, like within feedback loops. Those are still loads and didn't get magiced away supposed op amp behaviour,

You probably know all this already, but the fact is that your really ought to be able to put three op amps down on a piece of veroboard, an input, a volume control and an output, and have it sound pretty much near perfect, irrespective of the amplifiers you chose.

CT
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Old 16th May 2012, 06:39 AM   #250
Mooly is offline Mooly  United Kingdom
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In the case of the Precision Preamp '96 the design is such that large coupling electros are essential anyway. Guess I'll build it as designed with polarised caps and can always experiment with bipolars should there be any incentive in terms of the way it sounds... Can't wait to have some good tone controls...
You could always try FET opamps to eliminate DC bias currents (pot wiper currents) which was the main reason for all the electros in the 1996 precision preamp.

My own view on absolute noise figures is that they are one of the least important parameters in a domestic set up. Perhaps the LM4562 with its significanlty better DC specs might be an option. I suspect in practice that there would not be a problemone.

I think it's a superb design and one I would perhaps include in a new amplifier build.
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