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Old 14th January 2005, 06:35 AM   #1
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Default Tech help needed Threshold S/300 MkII optical bias

Hi guys, i need help with this amp it has 77k input impeadance and i need to up that to 200k, is it possible by just changing the resistor that goes across C3 i think it`s r18 or 10, without upseting the dc offset or bias or anything?

Also can anyone tell me what the switch is to the right of that resistor is for?

The reason for uping the input impeadance is for the high source impeadance that i`m driving it with, and it`s also in a bi-amped setup.

Nelson if you read this can you help please, after all you designed it.

And thanks to Dave (planet 10) for shrinking the circuit to the right size.

Cheers George
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Old 16th January 2005, 10:48 PM   #2
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This post has been put in Solid State forum and has a lot more disscusion on it.
And if anyone is interested a different kind of volume control that blows away any brand of poteniometer.

Tech help needed Threshold S/300 MkII optical bias

Cheers George
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Old 17th January 2005, 09:22 PM   #3
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You can set R18 at just about any value that you like.
Try a 200K - the JFET's input impedance is so high that
it should have little or no effect.
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Old 17th January 2005, 10:09 PM   #4
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Nelson thanks, did it to 330k, no chage in dc offset or bias, i`m a happy man now. ( change to solid state forum, lot more intrest there) Tech help needed Threshold S/300 MkII optical bias

Is it possible to increase the gain a few db simply with a resistor change to r17 or what looks to be r6or8?

Also can you enter the circuit that i painfully obtained into your circuit data bank http://www.passlabs.com/np/ as it`s about the only one that you did`nt have listed.

Cheers George
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Old 18th January 2005, 06:49 PM   #5
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Quote:
Originally posted by georgehifi
Is it possible to increase the gain a few db simply with a resistor change to r17 or what looks to be r6or8?
Yes.
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