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Old 6th April 2012, 02:49 PM   #91
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Quote:
Originally Posted by KSTR View Post
Well, to actually build and verfiy an amp like this on a non-pro DIY-level you'll need a lot of time, experience and access to the finest test equipment available.
So one would need
1) to be a seasoned expert in the field, for the experience part
2) to be retired, to afford the time
3) to have a full-blown lab at hand

Not many people seem to qualify...
Hi KSTR,

And if you find one, most likely, he/she isn't interested in someone else design.

Quote:
Personally I'd find it more appealing to implement a minimalistic version that optimizes performance in relation to parts count and complexity instead of aiming at the ultimate in the sub-ppm realms.
Of course the effort Edmond puts into all this, also the well-done documentation, is probably second to none and highly appreciated.
Thank you!

Quote:
For a manageable DIY project, I could think of a working version of the "barebone" approach plus a standard no-frills lateral output and then see how it compares to designs with similar complexity and parts count etc.
Sure. For a 'super-pmm' amp, you don't need cascodes in the IPS; that save 4 trannies.
The reason a combined the Super TIS with the AB2 OPS is that they are born for each other. Both exhibits low THD and both need a pre-driver*. Another reason is that I get sick of bias stability issues of the OPS. Therefore I opted for an auto bias circuit, which, as a bonus, also provides error feedback. Admittedly, the whole thing is a bit complex, though not as complex as the PGP amp.

But you are right, with laterals you don't need auto bias and the associated complexity. Far more manageable.

Cheers,
E.
edit: * that is, if you want to make TMC effective in reducing OPS distortion.
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Last edited by Edmond Stuart; 6th April 2012 at 03:09 PM.
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Old 7th April 2012, 03:08 PM   #92
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Originally Posted by Edmond Stuart View Post
<snip>
But my question was: Is it new; has anybody seen it before?
<snip>
Mr. Stuart,

What's the story behind all this? Going for patent?

Best,
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Old 7th April 2012, 06:33 PM   #93
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Hi Electroj,

A patent? No! Instead, eternal fame and a statue.
BTW1, you can't apply for a patent if you have made your invention public.
BTW2, on page nr.1 I wrote: I'm curious to learn more about it and how it behaves in real life. That's all.

Cheers,
E.
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Old 8th April 2012, 09:37 AM   #94
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Quote:
Originally Posted by myhrrhleine View Post
hello,

I was thinking of this one:
Distortion Correction Circuits for Audio Amplifiers
J. Audio Eng. Soc., Vol. 29, No. 7/8, 1981 July/August
Fig. 12. Long-tail pair with error correction feedback.
Hi myhrrhleine,

Fig.12 looks the same, but it serves another purpose: error correction.
Thanks, anyhow.

Cheers,
E.
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Old 12th April 2012, 01:34 PM   #95
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This looks very nice indeed. It fits the instinct that a symmetrical circuit is the natural way to produce a symmetrical response. Or that's my preference and it's nice to see some data to confirm it.
Like KSTR I'm interested in a minimal implementation of this, probably to match a Thermaltrak CFP + EF triple OPS since this is established tech. that I may be able to complete successfully within a human lifetime.
So a few questions.
I would like to minimize the noise since it would be directly connected to compression drivers.
How low can we reduce the feedback resistances? To (partly) compensate the IPS current could be increased. 20mA seems close to optimal for NoiseF if I can still find 2SB737+complements.
The Current Mirror emitter resistors would ideally be increased I think?
What is the benefit of the SallenKey input connection? It can increase the Q of the filter but this only affects the response near cut off, is not a low Q acceptable? To reduce the effective source resistance I am tempted to use an LC filter with a small aircore toroid inductor, any comments?
Finally a very minor observation of a few typos in the web text of the circuit description. In section TMC 'C1 and C17' should be 'C16 and C17'. 'C15 & R25' should be 'C15 & R26'. 'R26 & R27' should be 'R27 & R28'
Thanks for the circuit and the brain workout it has caused me.
David

Last edited by Dave Zan; 12th April 2012 at 01:40 PM.
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Old 12th April 2012, 01:54 PM   #96
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Quote:
Originally Posted by Dave Zan View Post
This looks very nice indeed. It fits the instinct that a symmetrical circuit is the natural way to produce a symmetrical response. Or that's my preference and it's nice to see some data to confirm it.
Like KSTR I'm interested in a minimal implementation of this, probably to match a Thermaltrak CFP + EF triple OPS since this is established tech. that I may be able to complete successfully within a human lifetime.
So a few questions.
I would like to minimize the noise since it would be directly connected to compression drivers.
How low can we reduce the feedback resistances? To (partly) compensate the IPS current could be increased. 20mA seems close to optimal for NoiseF if I can still find 2SB737+complements.
The Current Mirror emitter resistors would ideally be increased I think?
What is the benefit of the SallenKey input connection? It can increase the Q of the filter but this only affects the response near cut off, is not a low Q acceptable? To reduce the effective source resistance I am tempted to use an LC filter with a small aircore toroid inductor, any comments?
Finally a very minor observation of a few typos in the web text of the circuit description. In section TMC 'C1 and C17' should be 'C16 and C17'. 'C15 & R25' should be 'C15 & R26'. 'R26 & R27' should be 'R27 & R28'
Thanks for the circuit and the brain workout it has caused me.
David
Hi Dave,

Understood that low amp noise is important when driving a high-efficiency compression driver. A power amplifier that achieves input-referred noise of 5 nV/rt Hz is considered very good. Bear in mind that achieving low thermal noise is only helpful if the noise from hum and its harmonics, and rectifier spikes and noise is REALLY kept low.

When trying to achieve low noise, there is no substitute for good noise simulations, as with LTspice, in my opinion. There are often numerous things about circuit noise that may at first be non-intuitive or easily overlooked. If you are new to noise simulation, there is some good easily-digested discussion of noise and its simulation in my book, "Designing Audio Power Amplifiers".

Good luck.

Cheers,
Bob
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Old 12th April 2012, 02:36 PM   #97
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Quote:
Originally Posted by Dave Zan View Post
This looks very nice indeed.

Thanks, David.

Quote:
It fits the instinct that a symmetrical circuit is the natural way to produce a symmetrical response. Or that's my preference and it's nice to see some data to confirm it.
I'm glad we share the same 'instinct'

Quote:
Like KSTR I'm interested in a minimal implementation of this, probably to match a Thermaltrak CFP + EF triple OPS since this is established tech. that I may be able to complete successfully within a human lifetime.
So a few questions.
I would like to minimize the noise since it would be directly connected to compression drivers.
How low can we reduce the feedback resistances? To (partly) compensate the IPS current could be increased. 20mA seems close to optimal for NoiseF if I can still find 2SB737+complements.
Well, the noise isn't that much. According my sim 313nV, referred to the input and BW = 10kHz. Perhaps you can lower it to 250nV by means of better trannies and lower impedances. Not really a big deal.
BTW, 20mA is a lot of current for an IPS; trannies will get pretty hot.

Quote:
The Current Mirror emitter resistors would ideally be increased I think?
Of course you may increase the emitter resistors of the CM's for more mirror accuracy and lower noise, but at the expense of TIS output swing.
This is because you have to increase the bias voltage of the Baxandall trannies (Q13 & Q14) accordingly.

Quote:
What is the benefit of the SallenKey input connection? It can increase the Q of the filter but this only affects the response near cut off, is not a low Q acceptable?
A lower Q is okay.

Quote:
To reduce the effective source resistance I am tempted to use an LC filter with a small aircore toroid inductor, any comments?
As long as it doesn't add to distortion (or pickup hum) , it's an acceptable alternative or even better.

Quote:
Finally a very minor observation of a few typos in the web text of the circuit description. In section TMC 'C1 and C17' should be 'C16 and C17'. 'C15 & R25' should be 'C15 & R26'. 'R26 & R27' should be 'R27 & R28'
Thanks for the circuit and the brain workout it has caused me.
David
Thanks pointing out these typos. I updated the schematic and forgot to update the text as well, hence the errors.

Cheers,
E.
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Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)
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Old 12th April 2012, 03:18 PM   #98
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Hi David,

BTW, what do mean exactly by "aircore toroid inductor"?
Perhaps a coil around a ring core with a distributed air gap?
If so, I'm not sure if the distortion of such a core is within acceptable limits.

Cheers,
E.
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)
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Old 13th April 2012, 02:28 AM   #99
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Quote:
Originally Posted by Edmond Stuart View Post
Hi David,

BTW, what do mean exactly by "aircore toroid inductor"?
Perhaps a coil around a ring core with a distributed air gap?
If so, I'm not sure if the distortion of such a core is within acceptable limits.

Cheers,
E.
Just air. Would not spoil that distortion!
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Old 13th April 2012, 03:08 AM   #100
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Quote:
Originally Posted by Bob Cordell View Post
Hi Dave,

Understood that low amp noise is important when driving a high-efficiency compression driver. A power amplifier that achieves input-referred noise of 5 nV/rt Hz is considered very good. Bear in mind that achieving low thermal noise is only helpful if the noise from hum and its harmonics, and rectifier spikes and noise is REALLY kept low.

When trying to achieve low noise, there is no substitute for good noise simulations, as with LTspice, in my opinion. There are often numerous things about circuit noise that may at first be non-intuitive or easily overlooked. If you are new to noise simulation, there is some good easily-digested discussion of noise and its simulation in my book, "Designing Audio Power Amplifiers".

Good luck.

Cheers,
Bob
Thanks, I have your book and read it repeatedly! I do find some aspects of circuit noise non-intuitive, like the fact that the current mirror noise decreases as the emitter resistors _increase_. Your circuits values indicate that you know this but it is not mentioned. I know that there's a limit to what you can fit in a book! D. Self also does not mention it but in his case the low values he uses make me wonder if he picked up on this, especially since the overall discussion implies that it's best to minimizes resistances and there is no alert for the trap.
Spice simulation was what alerted me, then I discovered it was already discussed by Samuel Gronner. Still need to _understand_ it better.

As to specifics- 5nV/Hz @ 20X Vin/Vout would still leave about 10dB SPL @1m in the compression drivers. I would like to make it inaudible.
That will take about 2nV/rt Hz. Not easy but should be possible with a 50 ohm source and a 3dB NF.

Best wishes for an even thicker 2nd Edition.
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