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Old 28th October 2007, 09:14 PM   #21
Bob Cordell is offline Bob Cordell  United States
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Default Your amplifier

Edmond and Ovidiu,

This is quite an amplifier you have built and very impressive performance you have achieved. I have still not fully absorbed your fairly complex circuits, but do have some initial observations and questions.

First, combining HEC with NDFL is quite an interesting approach, and I have not seen it anywhere else.

If I understand correctly, you are achieving 0.0001% THD20 at full power into 4 ohms in a measurement bandwidth of 80 kHz. That is very impressive. I think that the 0.0006% I achieved was into 8 ohms (although that was in a 200 kHz BW).

I see you are using a 2.5 uH output coil. This is a bit of a surprize, based on other discussions on the thread. I think that John Curl agrees that 0.5 uH is essentially inaudible, but not 2.5 uH. What are your thoughts on this? Why were you not able to go down to 0.5 uH?

I'm also a little surprized to see that you have a fairly large 10 ohms across the coil, and this gives a bit of an under-damped square wave into the capacitive load at 20 kHz. I would have expected to see a resistor here of no greater than 2.5 ohms.

I was fairly surprized to see 470 ohm gate stopper resistors on the output transistors. This would seem to limit the bandwidth of the output stage a bit. I've usually been under the impression you can safely go down to 200 ohms or less with these lateral devices.

Am I correct in understanding that you have each of the three output pairs biased at 150 mA, for a total idle bias of 450 mA? If so, this helps a lot in getting the output stage distortion down.

It was unclear to me why you chose to synchronously trim both R20 and R81 in the output EC circuit for minimum distortion, rather than, for example, putting a single resistor trim in the path to the common node of R63 and R65.

I have not yet fully understood your input stage bootstrapping circuit. It looks like their rails are modulated with a unity-gain copy of the amplifier's output voltage, so that part or all of these stages floats with the output signal. Could you elaborate on this a bit. This kind of feedback usually scares me a little bit.

Why did you not choose to use a dc servo for offset control? The use of an electrolytic in the shunt path of the NFB loop seems a bit of a no-no.

I'm confused by the THD measurement made by the Amber 5500, and the first scope photo shown in your distortion measurements section, showing the 0.0001% residual. Was the using just the Amber in a conventional way. I did not think the Amber (or virtually any other THD analyzer, unaided) was capable of getting down to 0.0001% and showing such a well-defined distortion. You stated that the distortion shown includes that of the Amber oscillator, but implied that it is largely that of the amplifier. I'm obviously confused here.

You've achieved some remarkable performance here. I only wonder if you would have been able to obtain it with a bit less complex front end. That is quite a tour de force!

Good job,

Bob
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Old 28th October 2007, 10:37 PM   #22
syn08 is offline syn08  Canada
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Default Re: Your amplifier

Bob,

Before anything else, let me thank you for the taking the time and looking at our amp. I will try below and answer as much as possible of your questions. I'm sure Edmond will jump in as well.

If I understand correctly, you are achieving 0.0001% THD20 at full power into 4 ohms in a measurement bandwidth of 80 kHz.

This is correct.

I see you are using a 2.5 uH output coil. This is a bit of a surprize, based on other discussions on the thread. I think that John Curl agrees that 0.5 uH is essentially inaudible, but not 2.5 uH. What are your thoughts on this? Why were you not able to go down to 0.5 uH?

The discussion on the audibility of the output coil was one of the most intriguing I've read here. I still fail to understand why a 2.5uH onboard coil could be audible when the cable to the speakers has an inductance of the same order of magnitude. As of less than 2.5uH, the answer is on the web site. Quote: "The output Zobel network is mandatory for the whole amp stability. For the same reasons, the output coil shoud not be under 2uH air core".

I'm also a little surprized to see that you have a fairly large 10 ohms across the coil, and this gives a bit of an under-damped square wave into the capacitive load at 20 kHz. I would have expected to see a resistor here of no greater than 2.5 ohms.

Honestly, we didn't tried anything less than 10ohm. We may try that ASAP and report back.

I was fairly surprized to see 470 ohm gate stopper resistors on the output transistors. This would seem to limit the bandwidth of the output stage a bit. I've usually been under the impression you can safely go down to 200 ohms or less with these lateral devices.

We have tried 220ohm (my original expectation was identical to yours), unfortunately the OPS was not stable in open loop. The output was oscillating at around 5MHz with an amplitude of about 300mV. One of the issues related to lateral MOSFETs is the presence of a relative large capacitance (measured 420pF) between the output node (before the Zobel) and ground. This capacitance is defined by the heat pad between the MOSFET source and the heatsink (connected to ground). An OPS with vertical MOSFETs would not have this capacitance (the case heat plate is connected to the drain rather than to the source as in laterals). Certainly, this capacitance is not helping the stability of the OPS. We are looking into replacing the Berquist SilPads with 0.08" thick aluminum oxide insulators (having only 10% of the current parasitic capacitance). There are some mechanical issues in using those insulators with our PCB, so this is work in progress. As soon as we'll have some data we will report back.

Am I correct in understanding that you have each of the three output pairs biased at 150 mA, for a total idle bias of 450 mA? If so, this helps a lot in getting the output stage distortion down.

You are correct.

It was unclear to me why you chose to synchronously trim both R20 and R81 in the output EC circuit for minimum distortion, rather than, for example, putting a single resistor trim in the path to the common node of R63 and R65.

You are correct, that would be possible. Originally, when this OPS was on the breadboard, we thought that a THD fair minimum could be reached by using fixed resistors (that is, the component dispersion should not account for any significant THD variance). Based on the same expectation, we did not include a THD adjustment trimpot on the PCB. After building four OPS board we found out the hard way that this is not true. Trimming the Hawkwind balance, on each board, was mandatory to achieve the ultimate performance, so the only option without modifying the PCB layout was to adjust the two resistor simultaneously. If the OPS will get another major revision, then probably we'll place an onboard trimpot exactly as you mentioned.

I have not yet fully understood your input stage bootstrapping circuit. It looks like their rails are modulated with a unity-gain copy of the amplifier's output voltage, so that part or all of these stages floats with the output signal. Could you elaborate on this a bit. This kind of feedback usually scares me a little bit.

Well, you are not alone :-) It took Edmond quite some time to persuade me this is a good way to go and, as usual, the experiment decided that this is an excellent way to fight the evil Early voltage issues. We would love to hear on any pertinent criticism of this approach. As much as the MPSU looks complicated, it is in fact simpler than cascoding all devices in the IPS!

Why did you not choose to use a dc servo for offset control? The use of an electrolytic in the shunt path of the NFB loop seems a bit of a no-no.

We debated this quite a bit. We concluded that a servo offset control is not coming for free and based on our preliminary measurements, a good quality non-polarised high voltage electrolytic seemed to be good enough. This is indeed another point that could be the subject to a future revision if we could get some good reasons (that could be quantified through some measurements) to switch to a DC servo.

I'm confused by the THD measurement made by the Amber 5500, and the first scope photo shown in your distortion measurements section, showing the 0.0001% residual. Was the using just the Amber in a conventional way. I did not think the Amber (or virtually any other THD analyzer, unaided) was capable of getting down to 0.0001% and showing such a well-defined distortion. You stated that the distortion shown includes that of the Amber oscillator, but implied that it is largely that of the amplifier.

You are entirely correct again. The scope photo you mentioned is showing (lower trace) the amp output with the fundamental removed by the Amber 5500 instrument. By no means is that the image of 0.0001% THD20! It was included only to show that while the second harmonic is mainly due to the Amber 5500 internal oscillator distortions, there is a 3rd harmonic component (the small ridge) that is entirely the amp contribution. Otherwise said, in that photo, the amp 3rd harmonic distortions are hidden somewhere in that small ridge. However, as subsequent photos show, the spectral analysis revealed that the amp itself has the 2nd harmonic distortion dominant, and that the 3rd harmonic is about 10dB lower. Therefore, in the Amber 5500 output photo, the second harmonic component is due to the input signal and a little contribution from the amp, while the third harmonic is purely the amp contribution. If you think it will bring more clarity we could easily include in the Measurements page photos with the Amber 5500 output (with the fundamental removed) and the associated spectra.

If you have any further questions or comments we would be happy to address them.
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Old 29th October 2007, 12:32 AM   #23
Edmond Stuart is offline Edmond Stuart  Netherlands
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Default Re: Your amplifier

Quote:
Originally posted by Bob Cordell
Edmond and Ovidiu,

This is quite an amplifier you have built and very impressive performance you have achieved. I have still not fully absorbed your fairly complex circuits, but do have some initial observations and questions.

[snip]

I have not yet fully understood your input stage bootstrapping circuit. It looks like their rails are modulated with a unity-gain copy of the amplifier's output voltage, so that part or all of these stages floats with the output signal. Could you elaborate on this a bit. This kind of feedback usually scares me a little bit.

[snip]

Why did you not choose to use a dc servo for offset control? The use of an electrolytic in the shunt path of the NFB loop seems a bit of a no-no.

[snip]

Good job, Bob
Hi Bob,

First, thank you for your appreciating words.
Although most of the questions has already been answered by Odvidiu, I like to add a few more remarks:
As for bootstrapping the power supply lines of the input stage, probably you mean a unity-gain copy of the amplifier's input voltage. Just a typo I suppose.
Why it "scares you a little bit" ? Because of nobody else has done this before?
Regarding a dc servo, well, you can also do it that way, provided that you are using an ultra low distortion op-amp and an additional power supply etc. Besides, some electrolytic cap's are not that bad. Our approach was also based on the extensive measurement results of Cyril Bateman (Capacitor Sound?, EW, 2002-2003)
As for output coils, wasn't that poisoning discussion closed? Please, stop it, now and forever!

Cheers, Edmond.
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Old 29th October 2007, 01:13 AM   #24
MJL21193 is offline MJL21193  Canada
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Wow! This amp has a component count and complexity that rivals the Human Genome Project.
The circuit board more closely resembles a motherboard than a amplifier.
I am thoroughly intimidated...
Well done.
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Old 29th October 2007, 01:27 AM   #25
GK is offline GK  Australia
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Default Re: Re: Re: PGP

Quote:
Originally posted by Edmond Stuart
[B]Yes and no. It's true that we could also have set Ft at 2MHz. In that case the THD was even three times lower (~0.3ppm), but we opted for a better stability margin (i.e handling of higher capacitive loads). Therefor we set Ft at 700kHz.

No, it is not an order of magnitude greater. If it was, it was impossible to get THD20=6ppm. I guess that Bobs open loop EC-OPS produces only two times more distortion than our EC-OPS.

G'day Edmond.

Hmmm.......
Bob's design managed adequate stability at 2MHz with only a 0.5uH output inductor.
With regards to output stage open loop THD, perhaps Bob could alaborate a bit further here.
In the last paragraph of page 11 in his amp paper he specifies the THD as "less that 0.1%" - this would be a rather modest statement if it was really only ~0.016%. He also did not trim for the lowest THD - just used 5% resistors in the EC circuit.
6ppm THD-20 was done in the 80's with this non-optimized EC output stage, with 80's components and a front end circuit whose VAS linearity can be improved by an order of magintude with Hawksford cascoding and modern ultra low Cob high fT BJT's.



Quote:
Originally posted by Edmond Stuart
I don't think you can't hit the 1ppm mark without NDFL or whatever trick, unless you are willing to heavily sacrifice the stability.

EC on the output stage for sure, but NDFL? I disagree.


Cheers,
Glen
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Old 29th October 2007, 02:23 AM   #26
syn08 is offline syn08  Canada
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Default Re: Re: Re: Re: PGP

Quote:
Originally posted by G.Kleinschmidt


Bob's design managed adequate stability at 2MHz with only a 0.5uH output inductor. <snip> He also did not trim for the lowest THD - just used 5% resistors in the EC circuit.<snip>

6ppm THD-20 was done in the 80's with this non-optimized EC output stage, with 80's components and a front end circuit whose VAS linearity can be improved by an order of magintude with Hawksford cascoding and modern ultra low Cob high fT BJT's.

Glen,

1. We found experimentally that trimming the OPS for the ultimate THD20 number comes to the price of stability margin and I have no doubts that a theoretical analysis can easily confirm this fact. After all, the EC can be (ideally) viewed as a NFB loop wrapped around an amp that has the open gain defined/increased by a PFB loop, and the sweet spot is around the equilibrium point of these two antagonic mechanisms. Which equilibrium could or not be unconditionally stable. But these aspects were already beaten to death in other threads. Bob's ability to use only 0.5uH could be most likely because at that time it was not possible to reach the same level of open loop THD20 in the OPS.

2. Although I have built Bob's front end (I think there's a picture on the web site), I do not have enough data to speak about it's performance in detail. All I can say is that although the difference between 6ppm and 1ppm may not seem huge, in fact it is. And that even with our 2007 EC OPS, Bob's front end topology (also built with new devices) would not enable breaking the 1ppm barrier. Otherwise, you are correct in your evaluation on the new technologies/devices impact.
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Old 29th October 2007, 03:05 AM   #27
GK is offline GK  Australia
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Default Re: Re: Re: Re: Re: PGP

Quote:
Originally posted by syn08



Glen,

1. We found experimentally that trimming the OPS for the ultimate THD20 number comes to the price of stability margin and I have no doubts that a theoretical analysis can easily confirm this fact. After all, the EC can be (ideally) viewed as a NFB loop wrapped around an amp that has the open gain defined/increased by a PFB loop, and the sweet spot is around the equilibrium point of these two antagonic mechanisms. Which equilibrium could or not be unconditionally stable. But these aspects were already beaten to death in other threads. Bob's ability to use only 0.5uH could be most likely because at that time it was not possible to reach the same level of open loop THD20 in the OPS.

2. Although I have built Bob's front end (I think there's a picture on the web site), I do not have enough data to speak about it's performance in detail. All I can say is that although the difference between 6ppm and 1ppm may not seem huge, in fact it is. And that even with our 2007 EC OPS, Bob's front end topology (also built with new devices) would not enable breaking the 1ppm barrier. Otherwise, you are correct in your evaluation on the new technologies/devices impact.


I agree that optimizing the EC circuit for the best THD reduction comes at the cost of phase margin, but I think that your design is limited in this respect a bit (and also in terms of the allowable closed loop bandwidth) by the bandwidth limitation imposed by the large 470 ohm gate stopper resistors used in your OPS.
Have you tried the alternate scheme with a low value gate stopper combined with an RC to ground?.

Cheers,
Glen
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Old 29th October 2007, 03:25 AM   #28
syn08 is offline syn08  Canada
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Default Re: Re: Re: Re: Re: Re: PGP

Quote:
Originally posted by G.Kleinschmidt

Have you tried the alternate scheme with a low value gate stopper combined with an RC to ground?.
n
Yes. Unfortunately it did not help for lateral MOSFETs. The OPS was still unstable with 220 ohm stoppers. Now it is possible that a gate network that would push Ft at 2MHz may exist, but I'm running short of ideas here. Also the simulation models for lateral MOSFETs are very poor. So poor that our Chief Simulator Edmond was unable to get any significant results on the EC OPS.

I'm not sure if the next project will still use laterals but if so we will invest some time in measuring and extracting some good models for the laterals.
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Old 29th October 2007, 10:50 AM   #29
Edmond Stuart is offline Edmond Stuart  Netherlands
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Default Re: Re: Re: Re: PGP

Quote:
Originally posted by G.Kleinschmidt
G'day Edmond.

Hmmm.......
Bob's design managed adequate stability at 2MHz with only a 0.5uH output inductor.
With regards to output stage open loop THD, perhaps Bob could alaborate a bit further here.
In the last paragraph of page 11 in his amp paper he specifies the THD as "less that 0.1%" - this would be a rather modest statement if it was really only ~0.016%. He also did not trim for the lowest THD - just used 5% resistors in the EC circuit.
6ppm THD-20 was done in the 80's with this non-optimized EC output stage, with 80's components and a front end circuit whose VAS linearity can be improved by an order of magintude with Hawksford cascoding and modern ultra low Cob high fT BJT's.


EC on the output stage for sure, but NDFL? I disagree.


Cheers, Glen
Hi Glen,

Regarding 0.1% versus ~0.016%, why you are ignoring my simple math?
Anyhow, let's optimize Bob's amp (I hope you don't mind Bob ), using modern components in the EC-OPS, using an ideal front-end and trimming the EC balance. Guess what my sim say? THD20 = 2.7ppm. Spicing the same OPS with Bob's front-end reveals a THD20 of 4ppm. So, I think the best you can reach in real life is 3ppm.

>He also did not trim for the lowest THD - just used 5% resistors in the EC circuit.
If you read a little bit further, you'll discover why.
"While use of closer-tolerance resistor would improve the correction at lower frequencies, where it is unnecessary, their use would make a smaller improvement at 20kHz because performance there is beginning to be limited by the speed of the error correction loop."

Why the speed of the EC loop is limited, that is, has been deliberately limited, you can read here:
http://www.diyaudio.com/forums/showt...28#post1323328

In case I haven't convinced you, I should say just built an optimized Cordell amp, or at least, spice such an amp.

Cheers, Edmond.
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Old 29th October 2007, 11:37 AM   #30
GK is offline GK  Australia
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Default Re: Re: Re: Re: Re: PGP

Quote:
Originally posted by Edmond Stuart


Hi Glen,

Regarding 0.1% versus ~0.016%, why you are ignoring my simple math?
Anyhow, let's optimize Bob's amp (I hope you don't mind Bob ), using modern components in the EC-OPS, using an ideal front-end and trimming the EC balance. Guess what my sim say? THD20 = 2.7ppm. Spicing the same OPS with Bob's front-end reveals a THD20 of 4ppm. So, I think the best you can reach in real life is 3ppm.

>He also did not trim for the lowest THD - just used 5% resistors in the EC circuit.
If you read a little bit further, you'll discover why.
"While use of closer-tolerance resistor would improve the correction at lower frequencies, where it is unnecessary, their use would make a smaller improvement at 20kHz because performance there is beginning to be limited by the speed of the error correction loop."

Why the speed of the EC loop is limited, that is, has been deliberately limited, you can read here:
http://www.diyaudio.com/forums/showt...28#post1323328

In case I haven't convinced you, I should say just built an optimized Cordell amp, or at least, spice such an amp.

Cheers, Edmond.

I take it that your VMOS models are a lot more reliable that your lateral models then?

I won't build a Cordell amp because I don't like MOSFETs and I like to play with fully symmetrical front ends and I also prefer lots of heat.
But 3ppm is getting pretty close to 1ppm, nu? Crank the bias up for class A and then what? OK, class A is cheating. So how about we build a class AB output stage with lots in parallel of RETís for a starting open loop THD an order of magnitude better than what those icky MOSFETs give us and then apply EC to that?

BTW, my current fully symmetrical TMC amp design has 20 pairs of MLJ3281/MJL1302 running 6.25A bias (+/-65V rails) with EC for 300W into 4 ohms (ordering the same pair of 1500x220x75mm heatsinks as for my other amp).
Do you think I will need a NDFL to hit 1ppm THD-20 with this?

Cheers,
Glen
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