Bob Cordell Interview: BJT vs. MOSFET - Page 53 - diyAudio
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Old 8th January 2007, 04:23 PM   #521
ingrast is offline ingrast  Uruguay
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Quote:
Originally posted by lumanauw
Hi, Ingrast,

Why do you think so many people likes tube amps?

Sure, but I want also to make clear each and everyone deserves my respect as to what likes to listen and how. This is an intensely personal experience where one size cannot possibly fit all.

What I do not accept is someone trying on non-objective grounds to impose an objectively inferior technology as the "correct" or "better" way for everyone else.

Rodolfo
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Old 8th January 2007, 11:02 PM   #522
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Quote:
Originally posted by ingrast
It is simply not acceptable to admit the existence of "unknown" or "unmeasurable" effects placing audio more or less at par with paranormal phenomena or alien abductions.

Neither will I push a naive approach whereby if an amplifier measures better than certain objective limits, then it "must be all right" no matter what others say. This is neither conductive to improvements for that matter.

But what I won't buy is, given a measurement setup including all the signal chain up to the speakers and listening environtment, and in that setup one verifies the acoustic presure fields match accurately the soruce within reasonable tolerances as to what the ear can discriminate, that then someone else may come with a different setup which in principle is less objectively accurate yet is defended as more "close to reality".

This is not black magic, this is not quantum weirdness, this fairly well understood macroscopic physics. If subjective appreciation does not match objective measurement, then something is wrong and can be found, either in the listener or in the complete test setup.
Excuse me, I don't recall making those arguments.

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Old 8th January 2007, 11:59 PM   #523
ingrast is offline ingrast  Uruguay
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Quote:
Originally posted by Nelson Pass


Excuse me, I don't recall making those arguments.


Nelson,

I picked from that remark from yours for it did hint to the line of argument I focused on.

If you do not adhere to the camp of subjectivists who posit there are unmeasurable performance parameters that make for a perceptible (for the better) difference, then just fine, it certainly makes sense with an individual both long experienced and sporting a keen intellect in matters electronic.

Rodolfo
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Old 9th January 2007, 12:25 AM   #524
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I would not argue that the phenomena in question are
unmeasurable, just as you would not argue that we are
already measuring everything of importance.

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Old 9th January 2007, 01:47 AM   #525
ingrast is offline ingrast  Uruguay
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Quote:
Originally posted by Nelson Pass
.... just as you would not argue that we are
already measuring everything of importance.
....

This is interesting.
Most certainly we are not routinely measuring all relevant variables, yet I can bet even if we sere doing it - and it can be done - there will still be a large proportion of audiophiles dismissing the results.

Rodolfo
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Old 9th January 2007, 03:57 AM   #526
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Rod, which relevant variables are we perchance not measuring?
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Old 9th January 2007, 05:06 AM   #527
andy_c is offline andy_c  United States
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Quote:
Originally posted by JPV
Second, I still have question on the proper biasing of a push pull output stage.

I read the paper of Oliver on optimal biasing. It is not easy to read because of inconsistancies in notation and there are some typographic errors in my opinion.

He analysed the output impedance ( open loop) of a push pull amplifier and its variation with load current and biasing.
Hi JPV,

I agree that the Oliver paper is somewhat confusing. He derives some formulas but does not reach a clear conclusion about the optimum bias in my view. If you've read Self's writings on this subject, he reaches some much more specific conclusions. Namely, for several emitter resistor values, he specifies an optimum bias. But he bases that on the variation in the slope of Vout vs. Vin for the EF stage. This in turn depends on the load used. Oliver's approach seems more general, as looking at the output impedance variation removes the load as a variable. If you're using LTSpice and have some good models for the output stage transistors, you can duplicate Oliver's approach numerically by using the undocumented LTSpice d() function that takes the derivative. By applying a DC-swept current source to the output and computing the derivative of Vout with respect to the applied current, you can get a graph of the variation of incremental DC output resistance with current. You'll find that the bias that optimizes the output resistance variation also agrees with Self's optimum bias values. So it's just two different ways of looking at the same thing. This was discussed a while back in this thread. See my web page for the updated models of the MJL3281A and MJL1302A.

With FETs, you won't see an optimum bias. Rather, the output impedance variation gets less and less bad as the bias is increased. But if error correction is used, I suspect the situation would change dramatically for the better. I have not verified this in sim, but there are models of Bob's error correction amp output stage in that particular thread that could be used to easily check this.

I think the results of such an analysis would be much more interesting and informative than Nelson's persistent, yet subtle threadjacking attempts.
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Old 9th January 2007, 12:15 PM   #528
ingrast is offline ingrast  Uruguay
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Quote:
Originally posted by serengetiplains
Rod, which relevant variables are we perchance not measuring?

Measuring amplifier performance parameters alone narrows the analysis to only one part of the system. A few posts back I suggested a much more realistic comparison setup involves at least loudspeakers and listening environment, taking sound presure field comparison with the original program or test signal as performance evaluation.

While this type of test obviously contaminates the amplifier evaluation with foreign distortions, it is much closer to check what is actually expected from an amplifier to perform, i.e. reproduce music.

Rodolfo

PS. If you want an incomplete list of further tests currently not being performed, I should begin with THD across the full range with varying magnitude and phase reactive loads.
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Old 9th January 2007, 12:35 PM   #529
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Quote:
Originally posted by andy_c


Hi JPV,

I agree that the Oliver paper is somewhat confusing. He derives some formulas but does not reach a clear conclusion about the optimum bias in my view. If you've read Self's writings on this subject, he reaches some much more specific conclusions. Namely, for several emitter resistor values, he specifies an optimum bias. But he bases that on the variation in the slope of Vout vs. Vin for the EF stage. This in turn depends on the load used. Oliver's approach seems more general, as looking at the output impedance variation removes the load as a variable. If you're using LTSpice and have some good models for the output stage transistors, you can duplicate Oliver's approach numerically by using the undocumented LTSpice d() function that takes the derivative. By applying a DC-swept current source to the output and computing the derivative of Vout with respect to the applied current, you can get a graph of the variation of incremental DC output resistance with current. You'll find that the bias that optimizes the output resistance variation also agrees with Self's optimum bias values. So it's just two different ways of looking at the same thing. This was discussed a while back in this thread. See my web page for the updated models of the MJL3281A and MJL1302A.

With FETs, you won't see an optimum bias. Rather, the output impedance variation gets less and less bad as the bias is increased. But if error correction is used, I suspect the situation would change dramatically for the better. I have not verified this in sim, but there are models of Bob's error correction amp output stage in that particular thread that could be used to easily check this.

I think the results of such an analysis would be much more interesting and informative than Nelson's persistent, yet subtle threadjacking attempts.
This simulation approach you suggest is a very good idea. One could also do something similar in the lab by back-feeding the output of an amplifier with another amplifier through an 8-ohm resistor. The signal used could be a standard 60:6000 Hz IM signal, and the amplitude variation on the 6000 Hz component could be checked. This would be most useful as an experiment on an open-loop output stage terminated at its input, as in your diagram in your earlier post. This is similar in some ways to the Interface Intermodulation (IIM) test proposed by Otala.

I believe that an even better test for looking specifically at the crossover distortion is to back-feed the amplifier with a 19+20 kHz twin tone and view the voltage there on a spectrum analyzer. That can potentially be a very sensitive test, since the nominal value of the stimulous signal will have been attenuated strongly by the low output impedance of the output stage, making for very good dynamic range in the test. This test makes sense either with an open-loop output stage or with a complete amplifier.

I've also looked at the net output impedance as a function of current using a simple Excel spreadsheet for the calculations.

I have not read the Oliver paper, but should probably go read it. Someone else here I think said that Oliver pointed out a concern about how small the optimim voltage was across RE in consideration of thermal changes that could drive it away from the optimum. I share that same concern. Given the thermal junction variations in the course of playing program material, being at the so-called optimum most of the time is probably an elusive thing. A mere 5C short term junction temperature change will mess things up by about 13 mV, a good percentage of the total optimum value. It would be awful, on a short-term basis, to be left with virtually NO bias, due to dynamic thermal mis-tracking.

Bob
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Old 9th January 2007, 03:19 PM   #530
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Quote:
Originally posted by Bob Cordell

A mere 5C short term junction temperature change will mess things up by about 13 mV, a good percentage of the total optimum value. It would be awful, on a short-term basis, to be left with virtually NO bias, due to dynamic thermal mis-tracking.

Bob
Bob, you are absolutely correct in your assumption. A Sunday or two ago when the RE voltage was tossed around, you may recall that, I played around with bias on an old KSA50 in my workshop.

It was a particularly warm day (41 degrees centigrade). My oscillating airconditioner caused variations in RE between 13mV and 18mV when passing the air over the the open amplifier.

Hence my question to John, at what ambient temperature would one set the bias.

If your thermal compensation is correct then the current will increase when the transistors are cold and decrease when they get hot.

Kind regards

Nico
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