John Curl's Blowtorch preamplifier part II

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I wouldn't even be so strict as John is being. Sure it would be nice if you had a dozen or so different designs available for independent evaluation. But I would be just as satisfied if you had built a dozen or so designs, some with feedback and some without and listened for yourself if you noticed any sort of trend as to sonic differences between the two.

Then at least you would have an informed opinion. But at this point you are just some guy posting on the internet. That doesn't mean you're wrong. But it also doesn't mean that you're right.
 
As an engineer, Abrax... what kind of engineer are you?

A semi-retired one. I'm not employed by any consumer or professional electronics company at the moment. Just a hobbyist with a passion for pleasant sound.

How many audio amps and preamps have you designed and are available for independent evaluation?

If you count all the amps I've designed I think none are in production at present, as I left the audio business in 2001. This means whether or not they're available for independent evaluation is debatable. But sure, if you want product names and approximate dates for their introduction I can supply them to you in a PM - I don't want to clog up your thread with irrelevancies. Just let me know (by PM).

I think this is a fair question, as you infer that we are not engineers, and therefore are somehow flawed in our reasoning.

That's an example of flawed reasoning in itself. To me whether or not a person is an engineer is merely syntactics, irrelevant. Both your and Charles reasoning is flawed in various places but that has no bearing on whether you're engineers. In my book, if you say you are, then I accept you are. Being an engineer is no immunity against unreasonable arguments I hasten to add.:D

Of course, MY definition of 'engineer' might be different than yours.

Indeed, I agree. So that's one reason why its irrelevant. Why introduce it?

Does a piece of paper saying 'Engineer' make you an expert in this specific area?

Of course not, did you for one moment consider that it did?
 
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Hi John,
Oh, there is plenty of evidence, it is just buried by the op amp enthusiasts. ;-)
I have attached a file, data sheet actually, from New Japan Radio Company that should look a bit familiar to you. It seems that you are now an op amp enthusiast! The horror!

This may be the best way to satisfy the matching requirements inherent in your design. I wonder if they will come out with a similar design with a J-FET front end?

Are you an electronics engineer? This has been investigated earlier in time. Nope, you ain't. The definition is pretty clear on that count. Why is this so important to you?

Hi Charles,
Self and the distortion analyzers have all the answers.
I don't think you have to knock the guy, he doesn't knock you. You disagree with his approach and he with yours. It's that simple. No biggie.

I guess I have a question for you though, if I may? How much measuring do you do while designing a circuit? I can understand that you don't design to satisfy measured specifications, nothing wrong with that. I can also accept that you've designed enough circuitry so that you know know what works and what doesn't. Your need for measurements has dropped as your familiarity with audio circuits increased in other words, but I would find it odd if you didn't use some equipment to actually see what your designs are doing.

My take on what makes a good designer is someone who accepts information from many sources, including the use of test equipment. So using accepted tests for audio design doesn't mean that you have to go in the direction of lower numbers for their sake. In your case, it's obvious that you aren't interested in the common pursuit of lowest THD (for example), but just how much attention do you pay to these various tests?

-Chris
 

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Actually, Charles Hansen has a lot of the 'answers'. After all his products are world class in their acceptance by the sophisticated audio community, you know, the rich guys, who find audio quality to be one of their obsessions. I have a few in my 'camp' too. It takes more than some standard measurements to get this sort of acceptance. Charles and I feel that we are on the 'right track' in audio design. Your opinion may differ.
 
Anatech, I have perhaps $50,000 worth of test equipment if it was purchased new. I can measure to -120dB pretty easily with the addition of noise averaging, and I can measure this out to 100KHz in harmonics. Charles Hansen has even better equipment than I do, even though it is surprisingly similar. To imply that we don't measure our designs is just not part of the real world.
I'm glad to see that my 1968 design has finally made it to a new IC. It took long enough.
 
Abrax... I use plenty of feedback, just not as much as some op amp designers. Look at my Parasound JC-1 and JC-2 products. They have been measured in 'Stereophile' for all to see. The Parasound JC-3 will soon be introduced, and it is ALL IC op amps. It is the best that I can do, given what I have to work with. I would prefer a higher open loop bandwidth in my op amps, but the video amps are too noisy for MC input. A pity, as they sound pretty good. For the record, both Charles Hansen and I have degrees in Physics rather than Engineering, and I think this is the ONLY fact that we can be criticized for. However, I went back to university 4 years after finishing, in order to take the undergraduate and graduate analog design courses that I missed when getting my physics degree. I don't feel that I missed out on much.
Looking up your record, apparently you have designed some audio products. I hope that you will also allow that we have designed a number of successful products ourselves, and we continue in the direction that we feel works for us.
 
no news

Hi John,

I have attached a file, data sheet actually, from New Japan Radio Company that should look a bit familiar to you. It seems that you are now an op amp enthusiast! The horror!

This may be the best way to satisfy the matching requirements inherent in your design. I wonder if they will come out with a similar design with a J-FET front end?

....
-Chris

The NJM2116 is late.
Harris already had the complementary input pairs way back in time.
See for example the HA-2540 datasheet:
http://www.intersil.com/data/fn/fn2897.pdf
That was in 1993 in my databook! The metallization picture shows the year 1989.
:cool:
 
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Hi Bob

Wow, thanks for the detailed reply!

I go along with most of what you say, and think your point 5 about IM products being reflected down to lower frequencies than the original signal components is particularly worth taking seriously.

I'd like to play devil's advocate and take issue with your points 3 & 4 though. :D

I agree that amplifiers with high and low OLBW will have about the same amount of feedback at high frequencies, and thus about the same size of error signal at the input to the first stage. However I don't agree that this implies that the input stage distortion will be the same.

Let's take as an example an amplifier of similar topology to Doug Self's Blameless, and try to improve it by doubling the loop gain while reducing the open-loop bandwidth from 1kHz to 500Hz.

The easiest (and perhaps only) way to double the open-loop gain is to halve the amount of emitter degeneration in the first stage LTP. At the same time, the compensation capacitor will have to be doubled.

The input stage now has to deliver double the current, with half the local feedback compared to the original. We can thus expect the input-stage distortion above 1kHz to be about 8 times worse, while the amplifier's maximum slew rate will be reduced to half it's previous value.

I will concede that as long as the input-stage distortion remains much lower than the output-stage distortion, the overall measured distortion of the "improved" amp won't be significantly worse than the original at high frequencies, and will be about 6dB better below about 500Hz.

Cheers - Godfrey

Hi Godfrey,

Yes, much of the question revolves around how the open-loop gain got larger and the open-loop bandwidth got smaller. Doug Self's blameless design already has high open-loop gain, so the example of doubling it by going from 1 kHz OLBW down to 500 Hz OLBW is not a great one. However, even if we go with that, doing it the way you suggest is not the only way and is definitely not a good way. Reducing input stage degeneration and increasing Cdom is virtually always a bad thing to do, as it tends to reduce slew rate and make the input stage work harder to drive the larger Cdom (a recipe for TIM).

However, here's a better example of how you would do it if you really wanted to. First, go from Self's output stage double to a triple, greatly lightening the load on the VAS. This immediately increases low-frequency open-loop gain and decreases the open loop corner frequency. This is a perfect example of where improving the amplifier naturally increases feedback at low frequencies and decreases open-loop bandwidth. Secondly, to go further, you cascode the VAS, which further linearizes it and will often further increase the LF gain by near elimination of Early effect in the VAS.

However, my main point was that deliberate efforts to increase open-loop bandwidth by decreasing VAS voltage gain usually make the amplifier worse, because the usually necessary resistive loading of the VAS that must be applied necessarily increases the VAS current swings, adding to VAS distortion. BTW, some designs have a modestly high open-loop bandwidth due to VAS gain that is fairly low even without the imposition of VAS output shunt resistors. Such a VAS with naturally lower output impedance will often not be good for distortion, since the natural output impedance of a VAS may be fairly non-linear.

Bear in mind that we are not TRYING to get more feedback at LF - there is usually plenty anyway. Instead, we are trying to avoid messing up the natural operation of the amplifier by limiting its open-loop gain at low frequencies.

Finally, bear in mind that the shunt feedback of the Miller capacitor around the VAS has a strong and valuable linearizing effect that extends from surprizingly low frequencies to very high frequencies. The frequency down to which the Miller capacitor effectively linearizes the VAS depends largely on the input impedance of the VAS. A Darlington VAS fed by a current mirror IPS load has a very high input impedance for Cdom to work against, meaning less attenuation through Cdom and control of the gain by Cdom down to lower frequencies. By the same token, it is important to realize that Cdom makes the output impedance of the VAS very low at high frequencies, mitigating nonlinear loading effects of the output stage.

Cheers,
Bob
 
I don't think you have to knock the guy, he doesn't knock you. You disagree with his approach and he with yours. It's that simple. No biggie.

I'm not knocking Doug Self. I have at least three or four editions of his book and just ordered his book on small-signal design. I've learned a few things from his books and find him to be a thorough and methodical author.

It was Abraxalito who brought up Self as if he were the holy grail of amplifier designers ("Bob Cordell even agrees with - say it in a reverent tone - ***Doug Self***"), which I'm sure that even DS himself would disagree with.

I guess I have a question for you though, if I may? How much measuring do you do while designing a circuit?

None.

In your case, it's obvious that you aren't interested in the common pursuit of lowest THD

Well, how low is "low"?

Our latest power amp has 0.015% THD at 100 watts into 8 ohms. That's with no feedback.

How low do you think you need to go?

Do you think that if an amp were 0.0015% instead of 0.015% that you could hear any difference?

When the distortion starts masking the noise on that amp at about 2.5 watts (a typical listening level), the THD is 0.005%. Do you think an amp would sound better if the THD were 0.0005%? Or should it be 0.000000000005%?

How low is "low"? How low do you need to go?

Or maybe you just have a pre-conceived notion about our products instead of actual facts?

but just how much attention do you pay to these various tests?

We never measure a design until it is finished. I think one time we designed a product and it sounded great but had THD higher than we felt the marketplace would accept. So we worked on it some more to bring the THD down. Sounded the same, but now it wouldn't offend the "measurement" brigade.

Other than that we only use our test equipment as a QC control and/or diagnostic aid. Every product we build goes through a suite of tests on an AP analyzer. We set up pass/fail limits on all of them. If something was assembled incorrectly (usually a wrong resistor value), it will show up on these tests.

If a transistor is in backwards or something like that, it won't even get that far. Everything has to be set up, rail voltages checked, DC offset checked, and so forth before it even gets to the AP. Looking at the residual from a distortion analyzer is one of the best ways to troubleshoot an amp that is working, but doesn't meet spec.

That's where these tools come in handy. I'll be the first to admit that I wouldn't be able to troubleshoot a defective amp solely by ear. But JGH was right when he founded Stereophile magazine in 1962 -- a product should be judged on that basis by which it is used.

So if you use your amp to calibrate distortion analyzers, then it darned well better measure well. But if it is used to listen to music, it should by judged by -- (drum roll, please) -- listening to music.

It would actually be more useful for the magazines to test heatsink temperature versus time than distortion. Then you could get an idea about the operating temperature and thermal stability and therefore the potential longevity of the amp. But to judge an amp by measuring distortion is like judging a woman by the length of her large intestine. Sure, you could measure it, but what does that have to do with anything?
 
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it is important to realize that Cdom makes the output impedance of the VAS very low at high frequencies, mitigating nonlinear loading effects of the output stage.

Why is it a good idea to lower the output impedance with Cdom, but a bad idea to lower it with resistors?

Cdom only helps at high (out-of-band?) frequencies, while resistors work equally well at all frequencies. Do you think that the output stage only presents a non-linear load at high frequencies?

Maybe that is one reason that low and/or zero feedback amps sound better -- they have a constant effect on the signal, from below the audio band to above the audio band. Whereas a feedback amp will (almost without exception) have an inflection point right in the middle of the audio band. Below that point it operates one way and above that point, another.

Sounds like a recipe for a sonic disaster to me. Like the time I tried bi-amping with a solid state amp on the woofer and a tube a amp on the mid + tweeter. When I turned the system on I just about fell out of my chair, I was laughing so hard. What a completely unnatural sound! But some people actually like it. Maybe it's just their speakers, I don't know...
 
Well, most probably YES. The question is what would be the real reason of sound difference ;)

I notice that you didn't say that the difference would be an improvement. Most people reading this thread think that the difference would be an improvement. What do you think? Why?

And what do think is the lower limit for a change in distortion level being audible?
 
I notice that you didn't say that the difference would be an improvement. Most people reading this thread think that the difference would be an improvement. What do you think? Why?

And what do think is the lower limit for a change in distortion level being audible?

First, I think it is almost impossible to change one and only non-linear parameter (0.015%THD to 0.0015%THD) without affecting anything else.

Second, you have not mentioned spectral content. If the 0.015%THD was only 2nd order and 3rd order components, all else below -120dB, it would be inaudible. If the 'improvement' from 0.015% to 0.0015% THD created high order components between -110dB and -100dB, they would probably be audible and sound 'worse'.

Regards,
 
It would actually be more useful for the magazines to test heatsink temperature versus time than distortion.

The German "Audio" magazine did, in the '80s-'90s, series of infra-red images.
Unfortunately, the East bunch went "surround and homeys theatre", along with the rest of the hurd.
 

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