Bob Cordell Interview: Negative Feedback

[mikeks]

Originally posted by estuart


Hi Mike,

Nope. Neither of the two. I’ll give you one more change to figure it out.
As an expert on this subject, I’m convinced you will find the answer.

Cheers,

PS: Now, I'm going to my local pub, see you later.

I'll defer to you: enlighten us.

[Nelson Pass]

Originally posted by Bob Cordell
First, 0.04% of XO distortion at only 10 kHz is pretty bad, especially when one realises the crest factor of XO distortion. Secondly, this is at only 1 watt! If the XD circuit is working, one should not even be entering the crossover region at only one watt. Something is terribly wrong here

It does seem awfully odd. While I never assume a priori
that .04% is going to sound bad, I don't expect the circuit as
described to have that "feature", certainly no amplifier with a
1 Amp bias.

[PMA]

In case that 0.04% is crossover distortion in nature, it is pretty audible.
And yes, 0.04% is quite high distortion. It would not if it was only 2nd harmonic.

[Tim__x]

I think Estuart is referring to a Cherry-esque second miller loop containing the output stage, with an R in series so that the traditional miller loop becomes dominant at high frequencies to maintain the original stability.

[mikeks]

Originally posted by Tim__x
I think Estuart is referring to a Cherry-esque second miller loop containing the output stage, with an R in series so that the traditional miller loop becomes dominant at high frequencies to maintain the original stability.

Enclosing a power BJT output stage provokes local instability even with a series resistor.

[Graham Maynard]

Hi PMA,

Without the circuit you cannot expect me to guess what might be going on ! I wonder if I am expected to think that plot is satisfactory ?

There is an error in #474 at higher audio frequencies, and this is of greater magnitude than the Symasym for most of the time.

Of course we get greater error voltages with tube and non-feedback designs, and this error voltage variation in time (which is generally far greater than resistively measured THD figures) contributes towards making amplifiers sound different.

Cheers .......... Graham.

[Edmond Stuart]

Originally posted by Tim__x
I think Estuart is referring to a Cherry-esque second miller loop containing the output stage, with an R in series so that the traditional miller loop becomes dominant at high frequencies to maintain the original stability.

Exactly, that’s the crux. Chapeaux!
See below for this simple but valuable trick.
Note: It doesn’t work without a preceding beta enhancement transistor in the VAS stage. Probably, that’s why D. Self misunderstood the potential of this subtle modification.

Cheers,

[Graham Maynard]

Using a resistor from output to Darlington VAS base can also help to reduce output stage crossover distortion and improve NFB response by flattening the open loop bandwidth.
Using an equal value resistor from other half of mirror to ground prevents start-up thump and preserves differential input balance.

This is a Sim demo cct only.

Cheers .......... Graham.

[mikeks]

Originally posted by estuart
Exactly, that’s the crux. Chapeaux!
See below for this simple but valuable trick.
Note: It doesn’t work without a preceding beta enhancement transistor in the VAS stage. Probably, that’s why D. Self misunderstood the potential of this subtle modification.
Cheers,

This accomplishes nothing as far as linearity is concerned: loop transmission at 20KHz is typically 25dB~30dB, while DPC can be persuaded to deliver ~60dB at the same frequency.

[Bob Cordell]

Originally posted by PMA
In case that 0.04% is crossover distortion in nature, it is pretty audible.
And yes, 0.04% is quite high distortion. It would not if it was only 2nd harmonic.

Exactly right.

Bob