Bob Cordell Interview: Negative Feedback

[mefistofelez]

Dear Pavel,

It is PM-AB1, I think you know the amplifier quite well ;-)

Indeed, several of them. ;-)

Dear Jacco,

High time to post a picture of that fat Dudek guy to make an end to this perpetual confusion.

just to confuse you more, I am not Upupa Epops, more of an Asio otus. LOL

M

[Edmond Stuart]

Hi Mike,

Let’s go back to alternatives for class-XD.
I wonder why D. Self hasn’t opted for a far more simple solution, consisting of a slight modification of the Miller loop. Only one additional C and R are needed, giving a thd reduction of 80% or more. (No, it’s NOT a two pole compensation). It’s a very old trick, as far as I know, first pioneered by Baxandall. Curiously, D. Self does know about it, but for some reasons, he has rejected it. Why?! That is my point.

Regards,

[lumanauw]

Hi, Estuart,

Only one additional C and R are needed, giving a thd reduction of 80% or more

What is it? Do you have drawing of it?

[Edmond Stuart]

Originally posted by lumanauw
Hi, Estuart,
What is it? Do you have drawing of it?

Yes, of course, but at the moment, I'm waiting for a reply from Mike.
After that, you will get it.

Cheers,

[Bob Cordell]

Originally posted by Graham Maynard
Hi Nelson,

Quite so, but it will produce an individual sound that customers can -and do- like, and with the Patent, no one else can copy that 'sound'. Hi-Fi World did a review of an XD amplifier in their May 2006 issue. Of course their early sample could have been wrongly set up or their test bench different to what Cambridge and D Self uses, but the crossover distortion and its offset were clearly visible.

They did stress however, that there is much more to reproduced sound than crossover distortion and amplifier types, with which I must agree.

Will be back shortly to examine PMA's interesting reverse testing responses.

Cheers ........ Graham.

Hi Graham,

Thanks for posting this picture of 10 KHz 1W crossover distortion of the XD amplifier. This picture is horrid on a number of levels. 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 (although I usually don't buy the argument that "this was justa bad sample from the factory").

Bob

[mikeks]

Originally posted by estuart
Hi Mike,

Let’s go back to alternatives for class-XD.
I wonder why D. Self hasn’t opted for a far more simple solution, consisting of a slight modification of the Miller loop. Only one additional C and R are needed, giving a thd reduction of 80% or more. (No, it’s NOT a two pole compensation). It’s a very old trick, as far as I know, first pioneered by Baxandall. Curiously, D. Self does know about it, but for some reasons, he has rejected it. Why?! That is my point.

Regards,

Hi Edmond,

I assume you're referring to a C-R connection between the output and the inverting input of the amp.?

If so, then it is lead compensation, and it was not, as far as I know, first developed by Baxandall.

Alternatively, if it is assumed you're referring to the connection of a small resistor in series with the Miller capacitor, then you'll accomplish exactly nothing in respect of linearity, since this resistor's only raison d'etre is to provide nominal cancellation of the RHP zero occasioned by the HF feedforward path through the Miller capacitor. This, incidentally, is of no consequence in an amplifier with a BJT transimpedance stage.

[Edmond Stuart]

Originally posted by mikeks
I assume you're referring to a C-R connection between the output and the inverting input of the amp.?
Alternatively, if it is assumed you're referring to the connection of a small resistor in series with the Miller capacitor...

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.

[Graham Maynard]

Hi PMA,

Looking at the reverse driven scans.

#474. The traces are horizontal so ostensibly the phase response of the output stage is flat to external stimulus, however there is that lack of return to zero when the applied current (back-EMF wrt input) returns to zero.
Also the error increases with frequency like a half cycle slew energised offset, so I cannot begin to imagine what this might sound like because the error is greatest when the leading current returns/passes through zero.
If this trace was along a linear axis the zero voltage crossovers would be fractionally shifted in time yet the peak amplitude points would be linear, so this could be some sort of quadrature slew induced amplitude modulation.

#475. The symasym shows phase shifted damping, thus the error component would be greatest at peak reverse energisation currents wrt input. There is an attempt to return to zero, but here it shows a transconductance problem due to what I know to be caused by the reflection point of differenced currents between first and second stages caused by the nature of the circuit failing to prevent a reference current shift at higher frequencies.

[Graham Maynard]

Just a thought PMA,

Would the wrong value of parallel capacitor on the NFB sensing resistor cause this zero error ?

Cheers ........ Graham.

[PMA]

Hi Graham,

the narrower is the bandwidth, the wider is the Zout(f) ellipse. Think it over. Also, try to realize the real interpretation of plots, concerning they reflect frequency response and phase shift as well. This is not a DC plot. Take into account dV(f) at the output (less than 10mV) in ratio to output voltage. For tube or non-NFB design, you would obtain volts or hundreds of milivolts respectively.

Regards,
Pavel