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Old 3rd July 2012, 07:51 PM   #591
Steven is offline Steven  Netherlands
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Quote:
Originally Posted by Edmond Stuart View Post
>Are you going to use relay speaker protection on the output.
Yes, by means of a relay or low Rdson MOSFETs. Don't know yet which of the two.

Cheers,
E.
Chocoholic once posted a circuit for a MOSFET relay:
http://www.diyaudio.com/forums/solid...ml#post1997422

Distortion seemed to be virtually unmeasurable in practice.

Steven
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Old 3rd July 2012, 07:58 PM   #592
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Hello Edmond,

Y G M

Cheers
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Old 3rd July 2012, 09:30 PM   #593
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Originally Posted by Edmond Stuart View Post
Thanks Damir.
I will have a look at it later on, because at the moment I'm finishing the adaption* of the schematic to the PCB version (a lot of work). Besides, LS protection will be put on a separate PCB and will be one of the next steps.

* That also means the incorporation of a DC servo, making the amp even more complex. As a matter of fact, I'm doing it with huge aversion: adding (read: wasting) needlessly a dozen extra components, simply because some gurus say that any electrolytic cap (no matter what type or brand) in the NFB loop is bad.

Cheers,
E.
Hi Edmond,

I plead guilty. I like DC servos and hate electrolytics in the signal path. A decent DC sero takes only a few components and often less board space than a good electrolytic. BUT, I will admit that, as I pointed out in my book, that a non-polar electrolytic of generous voltage rating, like those used in a loudspeaker crossover, can perform (at least measure) extremely well.

Cheers,
Bob
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Old 3rd July 2012, 09:40 PM   #594
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Edmond, doesn't the PGP have an electrolytic in the feedback path? Doesn't the PGP get at least -120 dB distortion across the audio band at all output powers from 1 watt to 100 watts into 8 ohms? What exactly then is the problem?
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Old 3rd July 2012, 09:48 PM   #595
wahab is offline wahab  Algeria
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Quote:
Originally Posted by Bob Cordell View Post

A decent DC sero takes only a few components and often less board space than a good electrolytic.
Bob
A 220uF to 1mF capacitor is that huge.?...
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Old 3rd July 2012, 10:31 PM   #596
Bonsai is offline Bonsai  Taiwan
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Caps are not that bad. . . ;-)

I think this is more of a personal choice thing since the AP shows no distortion.
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Old 4th July 2012, 12:06 AM   #597
Bonsai is offline Bonsai  Taiwan
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Quote:
Originally Posted by Steven View Post
Chocoholic once posted a circuit for a MOSFET relay:
http://www.diyaudio.com/forums/solid...ml#post1997422

Distortion seemed to be virtually unmeasurable in practice.

Steven
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.
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Old 4th July 2012, 01:19 AM   #598
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Originally Posted by wahab View Post
A 220uF to 1mF capacitor is that huge.?...
I said a GOOD electrolytic :-).

Cheers,
Bob
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Old 4th July 2012, 08:18 AM   #599
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Quote:
Originally Posted by Steven View Post
Hi Edmond,

You could keep the elcap in the servo, but still isolate the servo almost completely from the amplifier, just using thermal coupling between servo an amplifier. Tandberg did that with some of their amplifiers in the past. Example: Tandberg 3000A, shown here. Q102 is the servo LTP driving two resistors R111 and R110, thermally coupled to the diodes CR101...104 of the input bias current sources.
I assume most audiophiles will not argue a servo with no electrical connections to sensitive input circuits. Also, the loading of the amplifier output with an RC filter to drive the servo will not harm any half decent amplifier.
Furthermore it is an open loop amplifier (i.e. no global feedback) with Hawksford EC for the power stage with MOSFETs.
Attachment 290036
Cheers, Steven
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.
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Old 4th July 2012, 09:40 AM   #600
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Quote:
Originally Posted by HarryDymond View Post
Edmond, doesn't the PGP have an electrolytic in the feedback path? Doesn't the PGP get at least -120 dB distortion across the audio band at all output powers from 1 watt to 100 watts into 8 ohms? What exactly then is the problem?
Hi Harry,

The problem is that other people make a problem of it.

Cheers,
E.

Quote:
Originally Posted by Bob Cordell View Post
Hi Edmond,

I plead guilty. I like DC servos and hate electrolytics in the signal path. A decent DC sero takes only a few components and often less board space than a good electrolytic.
Hi Bob,

I'm afraid I have to disagree with your definition of a 'few'. The DC servo on page 165 of your book consists of 10 components. Add to this number 12 components for a stabilized +/- 15V PSU. So in total, 22 extra components.

As for board space, in the PGP amp a Panasonic bipolar cap was used, 470uF, 16V, diameter: 16mm, protected by two DO-35 diodes.


Quote:
BUT, I will admit that, as I pointed out in my book, that a non-polar electrolytic of generous voltage rating, like those used in a loudspeaker crossover, can perform (at least measure) extremely well.

Cheers,
Bob
My aim is trying to build an amp that can withstand any kind of critic. So if I'm using an electrolytic cap in the signal path, even in case of a proven distortionless one, there will always be people who have serious objections to its use.
Now the question is: should I raise a middle finger to those people*, or should I give in to their objections and add (read: waste) 22 extra components? I would be much obliged if you help me to overcome this dilemma.

* people who also advocate using polystyrene caps for decoupling the supply rails.

Cheers,
E.
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