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Old 13th January 2012, 04:53 AM   #921
jcx is offline jcx  United States
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Solid State Testing- Tug of war

I really can't see why this is thought to be such a "obscure" - "ignored by conventional engineering" topic

Self, Cordell's books seem like they could be reasonable beginning summarys what is understood by "conventional engineering" - for dealing with "complex" loudspeaker loads, estimates of the peak currents are given, guidance for current "reserve", choosing Q, topology, sizing outputs to avoid "beta droop"

beginning feedback amp courses quickly introduce Bode's sensitivity analysis tools - specifically to address these questions

the simplest view is that you are "measuring" the output Z of the amp, for high loop gain feedback amps this can be ~10 uOhm over the audio frquency range - any "effect" is going to be ~ Zout/Zload ~= -100 dB for typical dynamic loudspeakers where there is enough gain, ouput current capbility

the Cherry paper's conclusion was that distortion for output injected current swing is less than or equal to the distortion of the amp driving the output to the same current levels in "forward" operation

Last edited by jcx; 13th January 2012 at 05:05 AM.
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Old 13th January 2012, 05:27 AM   #922
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Quote:
Originally Posted by Wavebourn View Post
I did that back in 1979 when played with thick film 50W amp ICs designed in our laboratory, when tried to make a bridged amp without frying them.
So my other thought was to a-b the 8 ohm resistor with say a 7inch focal coil mid woofer; could be interesting to see if the " 1/DF nulled" signal & channel B distortion was at all audible.
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Old 13th January 2012, 06:01 AM   #923
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Quote:
Originally Posted by jcx View Post
the Cherry paper's conclusion was that distortion for output injected current swing is less than or equal to the distortion of the amp driving the output to the same current levels in "forward" operation
I don't know about Cherry papers, but I've found then that it is quite convenient to see and measure how non-linear and frequency dependent is output resistance of the amp.
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Old 13th January 2012, 06:03 AM   #924
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Quote:
Originally Posted by vanderpol4 View Post
So my other thought was to a-b the 8 ohm resistor with say a 7inch focal coil mid woofer; could be interesting to see if the " 1/DF nulled" signal & channel B distortion was at all audible.
Good idea, I like it.
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Old 13th January 2012, 06:51 AM   #925
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Hi Folks,

Nearly 200 Posts ago I posed some very precise and topical questions, which if answered directly and to ones best knowledge would clearly illustrate the positions in this thread and would allow these positions to be falsified or accepted based on current knowledge and/or the proofs provided with them:

Sound Quality Vs. Measurements - Thorstens Four Questions post

I have noticed that only two posters have taken the time to even consider these questions in their entirety and then the comments where imprecise with answers which where framed so that in fact no specific position was taken and the questions where not answered at all.

I find this disappointing, as we have enough people here who normally have no problem to express their opinions and positions on what has been distilled into these four questions.

Perhaps it is that everyone expects a trap, by which the unreasoning subjectivist conspiracy is trying to trick them into admitting something they do not wish to admit?

Rest assured, the questions ask precisely what they ask and I already have agreed to modify question #4 to remove the tie in between "good sound" and "purchase decision".

So I would again ask all that have contributed to this thread to take a stand, be counted, make clear their positions and open these positions up to acceptance or falsification.

Otherwise we continue to have the same pointless argument again, all over again and with no resolution or improvement to the situation, meaning we all have wasted our time in this thread. Here again the questions with minor corrections to address criticism from posters about their precision, underlined sections are added later to produce more clarity.

Quote:
Originally Posted by ThorstenL View Post

Given is:

1) An AP2 testset

2) A Resistor Load 4R

3) A Speaker Simulator Load as used by JA in stereophile

4) A Amplifier (DUT in the following)

The DUT is tested using the AP2 with any of the standard tests (SNR, THD&N, IMD, FR, DF) and the tests show that the amplifiers output voltage is essentially an amplified replica of it's input with no measurable (by AP2) distortion and noise added and with a passband sufficiently wide to eliminate issues (phase, amplitude) in the audio band and is not affected by the loads used to test (listed above).

In other words, as far as the AP2 test set is concerned it is essentially not possible to distinguish the amplifier from a direct bypass test. I am fully aware that no such amplifier exists, however it should correlate well with what is mostly stated as the "Ideal"or "Perfect"amplifier

Questions:

Note: Please provide reasoning as to why the answer is as you answer, even though the questions themselves are simple are simple yes/no type and a clear Yes or No is expected. If a question cannot be answered in the affirmative for all reasonable circumstances (and backed by evidence), then a dissenting answer must be recorded.

So if you consider question X to be answerable by a "Yes" please provide either your reasoning as to why the answer is "Yes" or preferably cite independent evidence such as AES Preprints, JAES Articles or other reputable publications of suitable evidence that support your answer.


1) With the AP2 Measurements as given above, do you consider this DUT to be "as near to perfect as feasible now"? (Note, modified to feasible from possible, as it is of course possible, in theory, to improve further)

2) Do you feel any measurements not listed above, that is other than noise, harmonic distortion, intermodulation distortion, frequency response and load tolerance (damping factor); are required to make the determination in Question 1 and if yes, what are they?

3) Do the AP2 Measurements as given above inform us completely about about the actual performance of this amplifier when inserted into a real audio system?

4) Do the AP2 Measurements as given above inform us if the DUT when inserted into a HiFi Audio System will produce what is perceived by the majority of the potential customers "good sound"?
Ciao T
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Old 13th January 2012, 08:31 AM   #926
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Re Thorsten's four questions.

Surely there should be other qualifications such as the choice of weave of the interconnecting leads, composition of conductors and such. Otherwise the test is implying that such items have no effect whatsoever on sound quality. And they very clearly do... don't they? Does the AP2 employ digital sampling, high levels of feedback/high open loop gain, class D type techniques within its ADCs, DACs and measurement circuits? If so, then you'd presumably be happy to use such techniques in building an amplifier. But if you believe that even a mic pre-amp has a 'sound', then surely you could never ever pay even the slightest attention to an AP2 indicating 'perfection'.

Personally, I'd be inclined to believe that the DUT will sound great - unless it looks awful, in which case my mind will tell me that it sounds awful.

(Edit) And there is also the question I asked a few hundred posts ago: Could you design an amplifier to pass all 'conventional' tests with flying colours, but that sounded awful? I'm pretty confident that if I knew all the tests an AP2 is capable of, I could do it.

Last edited by CopperTop; 13th January 2012 at 08:49 AM.
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Old 13th January 2012, 09:07 AM   #927
SY is offline SY  United States
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The extremely vague and restricted choices given mean the question premises are flawed. I can't imagine why someone would want to discuss on those terms.
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Old 13th January 2012, 09:12 AM   #928
SY is offline SY  United States
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Quote:
Originally Posted by benb View Post
I've seen it all (or so it seems, and heard a lot) on rec.audio.pro. Market it as having the "SY signature sound." If someone finds the sound too boring or whatever, say it also works well with a transformer on the input and/or output.
...

I take that back. Make two models, one with higher input impedance than the other. Sell them as two different "flavors."
Ouch. The truth hurts.
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Old 13th January 2012, 10:55 AM   #929
DF96 is offline DF96  England
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Quote:
Originally Posted by jcx
the Cherry paper's conclusion was that distortion for output injected current swing is less than or equal to the distortion of the amp driving the output to the same current levels in "forward" operation
Yes, you are likely to be missing input stage common-mode distortion and VAS voltage swing distortion. Maybe feedback resistor distortion too. So unless there is some cancellation in the normal mode of use, this test would give lower levels of distortion. Output impedance nonlinearity is a useful thing to know.

Quote:
Originally Posted by ThorstenL
Perhaps it is that everyone expects a trap, by which the unreasoning subjectivist conspiracy is trying to trick them into admitting something they do not wish to admit?
From you, Thorsten, with your habit of politely, carefully and accurately restating your opponent's position before begging to differ? Surely not!
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Old 13th January 2012, 11:31 AM   #930
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Thorsten's 4 questions:

1) Not possible to tell with the information given. 'Near to perfect' to me means 'near to perfectly transparent' not 'near to perfect traditional measurements'.

2) No known set of measurements approaches unity correlation with transparency. For suggestions about getting closer see my earlier posting on this thread.

3) Most certainly not - the AP2 only tests in diff mode, not common-mode. (Note to Jan - not thinking CMRR setting on AP2 here ok?)

4) I doubt it very much. But presumably a cloth-eared set of potential customers could be pre-selected to achieve that result if desired.
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