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Old 4th July 2012, 09:58 AM   #601
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Originally Posted by Bonsai View Post
You can also use non isolated drive on the mosfets as well. If you switch the speaker ground return line, the drive is very simple: Audio Amplifier Design and Circuits | hifisonix.com a topics down from the top of the page.
Hi Andrew,

That is a very neat and clever solution.
>distortion [..] and the figure is less than 1ppm
Do you know how much 'less'?

Cheers,
E.
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Old 4th July 2012, 01:05 PM   #602
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Hello Edmond.

Only in simulation from my side. However, there was a guy on the forum that ran one of their SSLR's through an AP and the distortion was basically buried in the noise floor ie 1ppm. Sorry, I cannot remember who it was. However, since you are shooting for <100ppb, you can always put the SSLR inside the feedback loop. Of course, then you will need to switch the speaker hot wire, and not the ground return. Bob Cordell showed a solution in his book on how to insert a conventional loudspeaker relay inside the feedback loop.

Of course, I can also imagine a solution in the ground return line where you use an opamp to measure the volt drop across th relay and then modulate the gate drive to keep the voltage drop constant. However, I am ok with the levels as discussed in my document.

Looking forward to seeing pictures of your amp btw!
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Old 4th July 2012, 01:09 PM   #603
Bonsai is offline Bonsai  Taiwan
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I said a GOOD electrolytic :-).

Cheers,
Bob
LOL!

And watch out for those ceramics too btw! The only good ceramic is one that not on your amp or preamp board.
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Old 4th July 2012, 01:16 PM   #604
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Hi Steven,

Thanks for providing the schematic. Indeed, a thermally isolated servo is a very clever and original solution. I've never seen this before.
BUT.... it has the same drawback as a conventional servo: over twenty additional components. It is again a trade-off between complexity and performance; that's precisely my dilemma.

BTW, maybe using optocouplers for isolation also provide a viable solution.


>Chocoholic once posted a circuit for a MOSFET relay:
Thx for the hint!

Cheers,
E.
Hi Edmond,

Am I correct in assuming that you are talking about a conventional power amplifier DC servo?

The one I prefer, and described in my book, uses an inverting integrator and an inverter, using a single JFET dual op amp and an integrator capacitor. It is completed with 2 protection diodes and 4 resistors, for a total count of 8 small cheap components.

If I add an optional additional RC noise filter after the integrator, it adds one small capacitor and maybe one resistor.

Where do you get 20 components?

Cheers,
Bob
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Old 4th July 2012, 01:27 PM   #605
jcx is online now jcx  United States
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And watch out for those ceramics too btw! The only good ceramic is one that not on your amp or preamp board.
no reason for this level of ignorance today - NP0/C0G ceramics are quite good on all specs, even DA which some claim is the explainer of "audio quality"

higher K X7R may not be the best for signal apps but the higher C density can be useful in PS bypass, local C, low inductance are needed for today’s faster op amps, some suitable for audio reaching 50 MHz GBW

DBT test of audio circuits with film vs X7R bypass have gone the "wrong" way - the ceramic bypassed boards rated as "sounding better" as long as the evaluation was blind

Last edited by jcx; 4th July 2012 at 01:36 PM.
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Old 4th July 2012, 01:36 PM   #606
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Yes, no need for this level of ignorance. Agree hat NPO/COG are very good. Beyond that, they are mostly problematic. Temperature dependancy and Voltage dependancy are two hings that concern me most.

If you are talking about output filter caps in an SMPS, then they are ok. Bottom line is for decoupling and time constant setting, there are better solutions (NPO/COG the exception).
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Old 4th July 2012, 01:40 PM   #607
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Default 22

>Where do you get 20 components?

Hi Bod,

On page 165 of your book I really do see 10 components:
U1, C3, R4, R8, R7, R6, C2, R5, D1, D2.

Then we have the PSU:
2 Regulator ICs (+ and -15V)
2 Resistors and 2 Zeners to lower Vin for the regulators.
4 Electrolytic caps, put at both sides of the regulators.
2 Ceramic caps to by-pass Vdd and Vss of the op-amp.
In total 12 components and together with the servo that makes 22.

Cheers,
E.
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Old 4th July 2012, 01:48 PM   #608
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In every respect (high voltage) NP0's are better, even compared to styroflex, which have a higher ESR, can't stand my solder iron and dissolves in many cleaning solvents.
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Old 4th July 2012, 01:55 PM   #609
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[...]
DBT test of audio circuits with film vs X7R bypass have gone the "wrong" way - the ceramic bypassed boards rated as "sounding better" as long as the evaluation was blind
I'm not surprised at all.

Cheers,
E.
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Old 4th July 2012, 02:46 PM   #610
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Take a look at the temp coefficient and even more importantly the voltage coefficient

Basics of Ceramic Chip Capacitors

As to the sound tests and ceramic sounding better or worse, well, 0.5% distortion amps regularly score higher in golden ears reviews, so I am really not going comment on the impact on sound quality.

My concerns are really more around the use of ceramics in other areas of amplifier circuits. During testing on my e-amp, the ceramic snubber caps that I originally used sang at the test frequency when the output got to above 30V pk. In a separate issue, I used MLCC devices to decouple the cascode bases and also had problems. Use poly caps or silver mica and you have none of these issues.
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Last edited by Bonsai; 4th July 2012 at 02:49 PM.
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