Thoughts Concerning Cordell, Otala, and Gilbert papers

[PB2]

The "What's your reasoning?" and not "What's your belief?" thread got my interest to skim the Otala, Cordell, and Gilbert papers and I thought I'd offer a few comments for discussion:

It's interesting that Otala's assumptions 1.1 - 1.3 do not include frequency dependent nonlinearities, or even worse, time variant nonlinearities in the open loop model. This would be fine if it were even approximately true, but it usually is not. It seems that Cordell's findings also suggest not, and this is why most real world amplifiers have PIM in the forward path, and as is seen in his article feedback reduces PIM when it is present in the forward path.

Very interesting that Cordell only tests OP amps in the inverting configuration since it is well known that their high frequency performance is much better than in the non-inverting case. The non-inverting configuration is much more widely used and there's no excuse for him choosing inverting. The inverting configuration has both diff amp inputs at ground, or virtual ground, and thus there is no common mode signal at these inputs. The worst case unity gain non-inverting configuration has both diff amp inputs swing the same range as the output. 10V out means a 10V diff amp common mode signal which modulates Vcb and the Miller capacitance. Refer to "Distortion in low-noise amplifiers" by Eric F. Taylor, Wireless World August, 1977 for a detailed discussion.

Seems if Otala expected to get a paper published he should have done some verification of his assumptions to determine if they offered a "good enough" model of what we commonly see in typical amplifier topologies. Cordell seems to be determined to prove the other side and weakens his position by using the less common inverting configuration.

It's interesting that often a certain configuration, such as amplifers with a Cdom cap (or Diff amp driving the VAS as an integrator) are assumed in a paper and we have to keep in mind that the analysis and conclusions are only valid for that configuration. This is seen in the Gilbert paper and Self's analysis with regard to Cdom. Some amplifiers such as the Bryston and Tigers do not fit this topology.

I always liked the Hafler XL power amp null test for those who claim there is an "unknown" distortion or design issue where one can use real source material, real loads, and hear or view the unmasked distortion(s), even types that have no name. Obviously, linear errors such as frequency response and phase shift result in the presence of non-distortion components in the subtraction but it is still worthwhile.

Comments please?
Pete B.

[ingrast]

Originally posted by PB2
....I always liked the Hafler XL power amp null test for those who claim there is an "unknown" distortion or design issue where one can use real source material, real loads, and hear or view the unmasked distortion(s), even types that have no name. Obviously, linear errors such as frequency response and phase shift result in the presence of non-distortion components in the subtraction but it is still worthwhile.
....

I agree this should be an ultimate means of determining and characterizing deviations from the proverbial wire with gain. It seems though there is no much interest in pursuing the issue further, check here and here, and given the lack of interest, I will be doing some more research on the subject in connection with a very high performance project I am currently pursuing.

If results warrant, I will be posting details.

Rodolfo

[Cortez]

Could you please write the links about these papers to me here ? Thx a lot !

[Tube_Dude]

Originally posted by PB2
I always liked the Hafler XL power amp null test for those who claim there is an "unknown" distortion or design issue where one can use real source material, real loads, and hear or view the unmasked distortion(s), even types that have no name.

Hi Pete B.

Wellcome, to the club of the "null test fans" !

For me, is the only measure ,that coincide with "subjective " sound quality of amplifiers and is a invaluable toll in amplifier design.

See my post # 17 in

Blind Listening Tests & Amplifiers

And my post # 8 in

Blind Listening Tests & Amplifiers

Obviously, linear errors such as frequency response and phase shift result in the presence of non-distortion components in the subtraction but it is still worthwhile.

Yes ...but the opposite is also true . If the null residual is only thermal noise (as in one of my amp design ) we have a amplifier that is the proverbial "wire with gain".

Cheers

[Cortez]

What is your nulling test method ? I dont understood it.
/The two link, that you gived are the same./

[Tube_Dude]

Originally posted by Cortez
What is your nulling test method ? I dont understood it.
/The two link, that you gived are the same./

Hi Cortez

Sorry!!...The second link must be post #8 at


Null Difference Testing

Cheers

[PB2]

Originally posted by Cortez
Could you please write the links about these papers to me here ? Thx a lot !

Hi Cortez,

I don't think the papers are online, I got them from another member. I'll forward them if you email me with your email address as it seems you don't accept email from this site.


Hi Rodolfo,
I agree about the wire with gain, but I do prefer some filtering just to eliminate out of band garbage.


Hi Jorge,
Oh I didn't know there was a club, ... thanks! Yes it's possible to minimize freq response and phase errors as Hafler did in the XL by peaking up the ultrasonic response to improve the inband response. Did you use another method? Is your amp, bipolar, FET, tube, or?

Pete

[Tube_Dude]

Originally posted by PB2
Hi Jorge, Yes it's possible to minimize freq response and phase errors as Hafler did in the XL by peaking up the ultrasonic response to improve the inband response. Did you use another method?

Hi Peter.

No I don't use any form of peaking . For a good null you must use the maximum overall feedback that you can .
The more the feedback the better the null.

Is your amp, bipolar, FET, tube, or?

All bipolar DC coupled.

No way to make a tube amplifier with a good null!!...To much phase shift components,as capacitors in the signal path and the output transformer...

[mikeks]

Originally posted by PB2
........ Yes it's possible to minimize freq response and phase errors as Hafler did in the XL by peaking up the ultrasonic response to improve the inband response.... ..... ........Pete

Bad idea! ultrasonic peaking merely excerbates sensitivity to RF...

[noname]

some words to support null test. Seems the best technique – to find the objective parameters of well designed amplifier. Accuracy which is enough for all purpose and with arbitrary signal - not a close loop trajectory as in harmonic analyses (very important). I've made a successful improvement of such a measurement and even with 16 bit onboard ADC high accuracy level (not more need:-) of transfer characteristic measurement is reachable. It is on my sight http://amator2001.nm.ru/Voltage_meas...easurement.htm
( in Russian yet though
Even better result will be with the delay line or programmable delay of input signal, but not for all cases it is need.
This was about HOW and WHAT to measure - my opinion is a GedLee approach
http://www.gedlee.com/distortion_perception.htm
Reducing a thermal distortions is also possible - http://amator2001.nm.ru/Voltage_Amp_...ge_Amp_pII.htm - is a corrected Hawksford correction:-).