"keithr" <keithr@[EMAIL PROTECTED]
> wrote in message
news:4815ac51@[EMAIL PROTECTED]
>
> "Trevor Wilson" <trevor@[EMAIL PROTECTED]
> wrote in message
> news:67l564F2os07sU1@[EMAIL PROTECTED]
>>
>> "keithr" <keithr@[EMAIL PROTECTED]
> wrote in message
>> news:48155794@[EMAIL PROTECTED]
>>>
>>> "Trevor Wilson" <trevor@[EMAIL PROTECTED]
> wrote in message
>>> news:67k7ilF2onhd7U1@[EMAIL PROTECTED]
>>>>
>>>>> Good transistor amps function perfectly well with wider tolerances
>>>>> than
>>>>> you say because of the abundance of
>>>>> local and other NFB.
>>>>
>>>> **"Good" is your term. not mine. I happen to feel that amplifiers
which
>>>> use lots of Golbal NFB are not "good". They are average.
>>>>
>>>> Trevor Wilson
>>>
>>> Speaking as one who has never seen, heard, or even smelled an ME amp
>>> perhaps you can clear something up for me. As I understand it the ME
is
>>> made up of several modules in the signal path. Each of these modules
has
>>> it's own feedback loop.
>>
>> **Not quite. The amplifier is not all that complex. It is pretty
>> standard, full complementary symmetry thoughout, dual differential
>> inputs, with the obligatory active current sources. The Voltage
amplifier
>> stage uses around 15dB of Global NFB to stabilise Voltage gain to the
>> desired level. The Voltage amp feeds the current amplifier, which uses
>> local feedback only (no loop, from output back to input). The output
>> devices are coupled to the load via the collectors, rather than the
more
>> usual emitter follower configuration. So, no, each module does not have
>> it's own feedback loop. Only the Voltage amp has a feedback loop.
>>
>> Assuming that the module and it's feedback path are correctly
>>> designed and thus pass an undistorted signal to the next downstream
>>> module, why does this require such tight tolerancing in the output
>>> stage?
>>
>> **Because there is no loop feedback around the output stage.
>> Additionally, the feedback that does operate, is lightly applied.
>>
>> I do
>>> assume that the ouput stage module has it's own feedback loop and is
>>> being fed an undistorted signal.
>>
>> **The output stage uses no feedback loop. All feedback is local only.
>> IOW: Degenerative feedback only.
>
> Maybe a little feedback around the output stage would eliminate the need
> for such close matching without changing the sound, but I suppose that
in
> audio, everybody has to have their own gimmick to differentiate their
> product.
**There's the rub. Peter Stein did build some sample amps which used some
Global NFB. Distortion was reduced (from what was already an inaudible
level) and it would have reduced the dependence on matched devices.
HOWEVER,
blind testing showed that most listeners preferred the zero Global NFB
approach. I was one of those listeners. I received two, otherwise
identical,
amplifiers. I was asked to judge which I preferred over a period of
several
weeks. I was not told what differences there were, nor what to expect.
Whilst the differences were not earth-shattering, they were obvious.
Trevor Wilson
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