Bob Cordell Interview: Error Correction

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#2,541
traderbam said:

This is THE most complicated amplifier design I have ever seen. Engineering achievement comes "before anything else"? That objective may have been achieved.
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The "engineering objective" was not to create a complicated amplifier, but to reach an ambitious target perfomance. If anybody has an idea on how this could be done simpler, then we would be happy to hear about.

The real question is if such a level of performance makes any difference for a listener. We do not have an answer to this and that's why we will conduct audition tests as soon as possible. While at this level the THD number means probably nothing, there's much more to listen in such an amp than harmonic distortions. Does the low level of TIM and DIM make a difference? Is the LF overshoot acceptable? And many other such questions needing an answer that instruments can't provide.
 
#2,542
Re: Re: Re: PGP (Pretty Good Poweramp)

Edmond Stuart said:


Thank you Jan. Don't forget Ovidiu (syn08). He has built it, designed the PCB and did all the measurements. An awful lot of work.

Take your time to grab all the details and feel free to contact me in case of any questions.

Cheers, Edmond.
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Edmond and Ovidiu, this looks like a great piece of work, and very well-documented. As Jan said, it will take a bit of time to digest. Thanks for putting this together and letting us see it and learn from it.

Cheers,
Bob
 
#2,543
andy_c said:


Hi Bob,

You can show with a block diagram and calculations that if you put a voltage divider in this path with a "gain" of, say, K1 (where K1 < 1), then the EC diff amp collector resistors must be increased by a factor of 1/K1 to get the optimum error cancellation.

Let's say you have an accurate sim of the output stage, and you measure the large-signal gain of the output stage to be K1. Then if the voltage divider "gain" is also K1, and the EC diff amp gain is set up correctly, you can show that the error signal consists almost entirely of distortion. This is a good thing, as it optimizes the dynamic range of the EC circuitry itself. You've probably observed in sim or on the bench that when the EC dynamic range is exceeded, the distortion becomes quite bad - much worse than if there is no EC. So it's a good idea to optimize the dynamic range of the EC.

But since the output stage gain is dependent on the load, what load impedance should the EC dynamic range be optimized for? I thought about this for a while, and started thinking about the Stereophile amplifier tests. They do power and distortion measurements down to a 2 Ohm load. If you optimize EC dynamic range for a 2 Ohm load, then for higher and lower impedances than 2 Ohms, the error signal amplitude increases due to the gain correction. This can give a surprisingly high error signal with an open circuit load. So I decided to optimize the EC such that with a 2 Ohm load and an open circuit load, the amplitude of the error signals would be equal (but opposite polarity, because in one case the EC is decreasing the gain, and in the other it's increasing it). This is roughly equivalent to optimizing it for a 4 Ohm load, which seems reasonable. Forcing the gain to exactly 1 is nearly the same as optimizing the EC dynamic range for an open circuit load. We can do better.

If one is trying to design an EC amp with output currents approaching what John Curl's JC-1 is capable of, one finds that it's necessary to both increase the DC voltage drop across the EC collector resistors as much as possible, and to do some kind of EC dynamic range optimization as well. Otherwise, with the very low impedances used in output current tests, the EC error signal will become unmanageably large - at least with a MOSFET output stage. The required voltage divider ends up being a very good place to put the distortion nulling pot as well.
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Hi Andy,

I got to thinking some more about this matter of optimizing the EC stage dynamic range. Obviously the choice of nominal attenuation to shoot for is an issue, since the nominal <1 gain of the output stage depends on the load, and some compromise is necessary.

However, unless I'm wrong (always a distinct possibility), this whole approach has much less value in the context of realistic reactive loads. The attenuator approach implicitly makes the assumption that the gain of the output stage is governed by a resistive load. But the gate drive actually required by the output devices depends on the current demanded by the load, not the voltage being put into the load. So with a reactive load, the phases of the output stage gain and the attenuator gain will be out of phase. It seems in the worst case of, say a purely reactive inductive load, this technique would have little or no value in reducing needed EC circuit voltage swing. Am I missing something here?

Cheers,
Bob
 
#2,544
Re: Re: Re: Re: PGP (Pretty Good Poweramp)

Bob Cordell said:

Edmond and Ovidiu, this looks like a great piece of work, and very well-documented. As Jan said, it will take a bit of time to digest. Thanks for putting this together and letting us see it and learn from it.
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Thanks Bob!

Looking forward to hearing your comments and suggestions. After all, this design and implementation can be seen as a bridge over time, having your original design as one of the pillars.
 
#2,546
Originally posted by anatech
Some day I would very much like to hear these on my Stratus Golds. I have the new Cyrus Mono X amps and a Marantz 300DC to compare if you are interested.
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Sure Chris, I'll let you know as soon as I'll have the amp ready to hit the road here in Ontario. The more ears we got, the better 🙂 Send me an email with your contact.

Good speakers, BTW 🙂
 
#2,547
Hi syn08,
No rush on that. But whenever you find it convenient.

I don't get out much due to accident injuries, but I would absolutely want to see and play with your setup. I lust after an HP 3562, or similar. I also have a service bench in my basement if you want to play. We can pop the cover off a Cyrus if you are curious about it.

So, getting together at some point is something I would look forward to.

I'll PM you the details.

-Chris
 
#2,553
Class AB topology, 100W into 8 ohm and 200W into 4 ohm
• Bandwidth of minimum 20Hz to 20KHz +/-0.1dB
•· Under 1 ppm (80KHz bandwidth) THD (that is, 0.0001%)
at all frequencies between 20Hz and 20KHz and output
powers between 1W and 200W
• Under 1 ppm 19+20KHz CCIF IMD (that is, 0.0001%) at all
output powers between 1W and 200W
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Edmond, is this what you are refering to by your helpful "RTFW"? I saw this but I didn't think it was complete.
 
#2,556
Re: Re: PGP (Pretty Good Poweramp)

Bob Cordell said:
Edmond and Ovidiu, this looks like a great piece of work, and very well-documented. As Jan said, it will take a bit of time to digest. Thanks for putting this together and letting us see it and learn from it.

Cheers,
Bob
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Nelson Pass said:
Congratulations. It would appear that the name is quite modest.

😎
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Bob and Nelson,

Thank you!

Cheers, Edmond.
 
#2,557
traderbam said:
Edmond, is this what you are refering to by your helpful "RTFW"? I saw this but I didn't think it was complete.
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Hi Brian,

True, it is not fully complete. There were more performance targets, like low SNR, high slew rate, stability (i.e. handling of capacitive loads), overload protection (i.e. clean recovery from clipping), group delay etc. Not all these matters are covered on the first pages of our website, some of them are scattered on other pages and some of them are covered not at all (that is, not yet), because of lack of time.

Cheers, Edmond.
 
#2,558
traderbam said:

No. :bigeyes: Can you email me the circuit? I'd love to see it. What does it sound like?
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A channel (no supply) fits on two "E" size plots, and thus is
difficult to scan. Also, it clearly is the property of Madrigal.

What does it sound like? I can't make a fair assessment as I
have not had a pair in my system, and of course it's not my
place to comment on the sound of a competitor.

😎
 
#2,559
PGPA

Good design, congratulations Edmond and Ovidiu, and warm appreciation for sharing.

I would not have agreed with the ultra low distortion model a couple of years back, preferring to make a silk purse out of a sow's ear, but I suspect this is a golden ear from the start!

It is interesting that for all the engineering parameters people still say 'What's it sound like?', implying that they would be surprised if, for all the superb measurements, it sounded good. This identifies a certain, um, ambivalence in the measurements stakes but perhaps reflects only high end market realities.

I don't feel it's too complicated, Brian. It covers a lot of ground, this amp, and a few TO92s/126s extra is really not too expensive, particularly if some hapless, driven technocrat has already laid out the pcb. No one enjoys needless complexity, but I don't believe there's anything redundant here.

Great to see such faith in the analog arts. I thought it was dying with Class D, this is a real shot in the arm!

Cheers,

Hugh
 
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