[Phil]

Patrick Turner wrote:

[snip]
Well, remember, the maximum slew rate found in audio signals is much
greater than what a theoretical 20 KHz signal is going to supply, and
not all power amps, with their big, slow, output transistors, are going
to be as fast as even a 741.


Not all output bjts are big and slow.
Some do however dislike turning OFF quickly and some display the truly
horrible habit of cross conduction at HF, ie, the two bjts in a typical complementary
pair
are BOTH turned on during a wave cycle during large signal excursions at above 10kHz,
and the power supply has to supply a lot more current that is simply passing from rail
to rail
and its hang onto your hat time for the ride.


Plus, the point of Matti's work is that
problems begin to appear at all levels below the theoretical
"breakthrough" point of TIM/SID. In any case, the topic here is not
whether most amps have sufficient slew rate -- I assume that *most* good
amps do -- but rather about Otala's proof that a feedback amp's
"correction" of an amplitude distortion of the open loop phase shifts
the high frequency components in the closed loop.


Both amplitude distortions and phase distortions of the open loop
response are BOTH corrected by the NFB.
Typical open loop phase lag in open loop at 20kHz is 90 degrees, and the 40dB of
applied global NFB
at 20kHz reduces this typically to less than 5 degrees.

Patrick, for god's sake, if you disagree with me, fine, but how many
times do I have to say that what Matti was talking about was not the
usual lag at 20 KHz, but rather phase-SHIFTING, as in MOVEMENT, not lead
or lag, that as you yourself correctly guessed, is a function of low
frequency amplitude, not overall frequency. Yes, feedback allows greater
bandwidth, thereby improving "phase distortion," where distortion is
defined as CONSTANT phase shift as a function of frequency. Otala, the
technical director of the Finnish Institute for whatever, was not that
stupid! Come on, you know that "dynamic phase distortion" refers to
something else, you said so yourself.


I am currently
discussing this in the "Negative Feedback in Triodes: The Logical and
Experimental Proof" thread from 8/15, so if you're interested, look
there (articles posted on 9/6). Phil Allison had a "response" here --
his usual slams and blams with no supporting evidence -- but I actually
would like to see his simple test that can show whether low frequency
signals in a feedback amp do or do not cause high frequency phase
shifting, as it would be useful test, and I'm having a hard time coming
up with a simple way to test that myself.


Just apply 70Hz and 5kHz signals to the input of an amp
in a 4:1 ratio.
Filter out all below 1kHz from the output signal.
Then you will see what the effect of the 70Hz large signal is upon the
fidelity of the 5kHz signal and whether there is any phase modulation
in addition to the expected intermodulation.
With most well made SS high NFB amps, the IMD is not visible on the CRO
and a careful peak detector must be used to measure amplitude variations in the 5kHz,
or else filter out the IMD products at 4,930Hz and 5,070Hz.

Thank you, but there may be more to it than that. For one, let's see
what happens when the 70Hz signal is 80 dB higher than the 5KHz signal,
and let's make certain that our equipment is sensitive to rapid forward
and backward shifts in time of the 5KHz signal. A 'scope would almost
certainly catch that IF triggered with a constant timer, not the 5KHz
signal.





Apparently, Otala incorporated
a lot of ideas/solutions into his Citation XX power amp, and maybe, if I
can find papers by him on that amp, there will be some useful
information and tests there, but if PA can come up with something in the
meantime, hell that's fine by me! He'll probably think of something
really simple and easy, and then refuse to tell me, the ****head ...


But all these investigations have been done many times before.

Could you name one?

What exactly do you hope to gain by goading the ungoadables on the group
to find out what you should be willing to find out for yourself?

Who said I was trying to "hire for free" the ungoadables? I am
discussing a subject, including possible problems and solutions.
Ideally, we all have a complete workbench, but I think it's a bit
bigoted to imply to those of us who do not that we should get our own
bench and do all investigations ourselves, before we are permitted to
discuss a subject here.

Do you suspect to find some hitherto unused uninvented techniques of making
amplifiers perform better?

I can almost say, "Duh, of course!" However, I do not necessarily EXPECT
to do anything, because I AM discussing a subject, which I have a
right to do on this list, as it is basically what the list is all about!
Besides, you have yet to give an honest answer to a method I suggested
that should improve things, my higher resolution double-blind
suggestion. I gave you a PROOF -- which is no more and no less reliable
than the premises -- that if you use that method, you can find
differences between components that the normal double blind cannot
reveal. That will not automatically enable you to make a better amp and
make more money, but it should help! You're welcome, by the way ...



Anyway, what PA was saying/yelling is that feedback amps DO NOT EITHER
CAUSE PHASE SHIFTING OF THE HIGH FREQUENCIES LIKE THAT DUMMY DR. OTALA
SAID! My response was simply to ask whether (1) he knew of references
that would back up his claim, that Otala's analysis was flawed, and (2)
whether he knew of a good, simple test that can be used to test whether
LF signals in a feedback amp cause phase shifting of the HF signals,
like Otala said they do.


Be like me, find out by building one's own test gear and testing.
It took me months to do it all but after reading all the conflicting opinions about
all this in
Electronics World copies from the 1970s to 1980s BEFORE the internet was mainstream,
I decided to look myself at what happened in amps that i should be worried about.

And I am impressed by that. You are not, like Graham or Arny, either
useless or a pussy. But not everyone can do that, and it is unfair of
you to suggest otherwise.

Phil

Patrick Turner.



Phil

To email me directly, cut off my head