Bob Cordell Interview: Negative Feedback

[Fanuc]

Originally posted by PMA


I agree. This kind of signal shows time response of the system, though the system can be perfectly linear. Every sudden change, even if it is straight line of zero level suddenly changed to sine starting at zero, not maximum, needs infinite bandwith to be transferred without linear distortion. It is all about understanding of linear and non-linear distortions and time response to any signal.

Yep. I've seen Graham mention this arguement before etc and it seems to lack common sense (irrational). Loudspeakers do not really have back EMF - nothing serious anyway.

[jan.didden]

REGISTER ENTRY FOR GB2424137

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Title AMPLIFIER

Applicant/Proprietor
AUDIO PARTNERSHIP PLC, Incorporated in the United Kingdom, Gallery Court,
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Inventor
DOUGLAS SELF, AUDIO PARTNERSHIP PLC, Gallery Court, Hankey Place, LONDON,
SE1 4BB, United Kingdom [ADP No. 09246794001]

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MAGUIRE BOSS, 5 Crown Street, ST IVES, Cambridgeshire, PE27 5EB, United
Kingdom [ADP No. 07188725001]

Publication No GB2424137 dated 13.09.2006

Examination requested 11.03.2005

Grant of Patent (Notification under Section 18(4)) 16.01.2007
Publication of notice in the Patents and Designs Journal (Section 25(1))
14.02.2007
Title of Granted Patent AMPLIFIER

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15.06.2006 Notification of change of Address For Service address of
MAGUIRE BOSS, 5 Crown Street, ST IVES, Cambridgeshire, PE27 5EB,
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[Graham Maynard]

Hi PMA,

10kHz is approx 60mV/uS/V, input or output, nothing approaching infinite about that.
For an amplifier to be accurate to the limits of an audio source it must of course have a much greater bandwidth and short propagation delay if it is to be expected to damp real world loudspeaker back-EMFs at such frequencies - as can be generated by loudspeaker system crossovers.
This is why it is essential to preserve output stage damping linearity throughout the AF range for class-AB operation, or to use some form of class-A augmentation.

If a class-AB amplifier tests out better with a suddenly starting 10kHz sine (no matter whether this might be thought unrealistic) then it is also likely to be capable of establishing a more stable stereo image of an instrument like a cymball or violin due to either or both amplifiers being less disturbed by back-EMF.

[Bob Cordell]

Originally posted by Graham Maynard
Hi Bob and Nelson,

Re Douglas' Patent. I surmise he advanced upon earlier designs by claiming novelty for an *active* voltage controlled current source which he uses to displace the class-AB crossover beyond the range of loudspeaker generated back-EMF, such that his 'XD' amplifiers run in class-A at normal listening levels.

Re back-EMF induced distortion, see the attached illustration from my 2004 EW article.

This shows class-AB amplifier output simulations of a generic type similar to 'Blameless'
All outputs are 15V sine, with each trace *fundamental nulled* to examine individual distortion residuals.

The black trace illustrates the -40dB level of the Blue output.
Blue with choke and Miller C.dom into R load
Red with choke and Miller into 'Ariel' equivalent.
Mauve with choke but no Miller C.dom into Ariel
Green no choke but with Miller C.dom into Ariel
Yellow no choke and no Miller C.dom into Ariel (same architecture)

Yellow circuit not constructed, so might not be stable.

Those who state it is wrong to study how an amplifier responds to a suddenly starting wave, may follow the repeated error residuals from 150uS onwards.
Note that individual amplifier component variations overlaid upon the same semiconductor layout have an influence upon group delay, and thus crossover timing within each voltage output waveform.
Note how 'loudspeaker' loading exacerbates emitter follower output stage crossover distortion when a Miller C.dom is used at the VAS within a closed NFB loop.

Thanks, Graham. Can we get a copy of your EW article? I'd also like to see the schematics of the amplifiers you refer to in your simulation so I can better understand what's being done.

In regard to Self displacing the crossover point away from where back EMF has an influence, would that perhaps not be the same as running the amplifier in Class AAB, where it is in Class A up to between 1 and 10 watts, or so? Or is Self trying to achieve the same effect without the consequent increased idle bias power dissipation?

Cheers,
Bob

[jan.didden]

Originally posted by Bob Cordell
[snip]In regard to Self displacing the crossover point away from where back EMF has an influence, would that perhaps not be the same as running the amplifier in Class AAB, where it is in Class A up to between 1 and 10 watts, or so? Or is Self trying to achieve the same effect without the consequent increased idle bias power dissipation?

Cheers,
Bob

Bob,

That's part of it. In the whitepaper I refer to above, there is a table comparing efficiencies of A, AB, and XD in several incarnations.

http://www.cambridgeaudio.com/assets...r8-2-06web.pdf


Jan Didden

[Bob Cordell]

Originally posted by Graham Maynard
Hi PMA,

10kHz is approx 60mV/uS/V, input or output, nothing approaching infinite about that.
For an amplifier to be accurate to the limits of an audio source it must of course have a much greater bandwidth and short propagation delay if it is to be expected to damp real world loudspeaker back-EMFs at such frequencies - as can be generated by loudspeaker system crossovers.
This is why it is essential to preserve output stage damping linearity throughout the AF range for class-AB operation, or to use some form of class-A augmentation.

If a class-AB amplifier tests out better with a suddenly starting 10kHz sine (no matter whether this might be thought unrealistic) then it is also likely to be capable of establishing a more stable stereo image of an instrument like a cymball or violin due to either or both amplifiers being less disturbed by back-EMF.

I can't argue with this, but I think it is not much different than saying that the small-signal bandwidth must be greater than 100 kHz or so, and that the amplifier have adequate slew rate, which, in the extreme case here, might be enough slew rate to handle a full-power 100 khz sinewave, in very rough terms, if the 10 kHz sudden tone burst were bandlimited to 100 kHz. I think that only comes out to about 24 V/us for a 100 watt amplifier.

Neither of these requirements is asking a lot for a decent solid state amplifier.

Bob

[PMA]

Originally posted by Graham Maynard
Hi PMA,

10kHz is approx 60mV/uS/V, input or output, nothing approaching infinite about that.

Only in steady state (settled). When turned on, from zero line, even at at phi = 0 deg, the corner (transition) from line to sine has infinite frequency spectrum. Every RC lopass, even 100MHz, will introduce some delay (1.59 nanoseconds in this case) and will "round" transition between line and sine.

[Graham Maynard]

Hi Bob,

In his '90s texts Doug showed how there would be a transconductance change going from class-A to AB (not recommendable).
He also achieved highly linear circuits when driving resistors, but there were comments about distortion when driving real world (back-EMF generating) loudspeakers.
His class-A XD arrangement allows him to use his established highly linear class-AB circuits for driving loudspeakers because the back-EMF induced current crossover non-linearities are shifted away from normal listening voltage outputs.

[Graham Maynard]

Hi PMA.
Yes, every circuit introduces a delay.
The same delay that can fail to prevent a NFB loop controlled class-AB amplifier output stage from damping loudspeaker back-EMF due to an inability for a VAS to reverse output drive.
Output stages store charges and these must be actively removed, but the control response is finite, thus the development of 'crossover distortion'.

[PMA]

So called "back EMF" issue can be completely described by speaker electrical equivalent circuit. "Back EMF" is just exchange of energy between amplifier and parallel resonant circuit, representing acoustical impedance of speaker in the enclosure transferred to speaker terminals by electro-acoustic transformation.