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 21st April 2007, 09:20 AM #1 diyAudio Member   Join Date: Apr 2007 High Quality Audio Preamp(PCB Soon). want to take audio signal from PC sound card.. It's gain should be flat from 30Hz To 22KHz.. Low noise.. Neeed improvement?? Thread on other WEB Thanks.
 21st April 2007, 10:44 AM #2 diyAudio Member     Join Date: May 2002 Location: The great city of Turnhout, BE Blog Entries: 8 R2 is superfluous. Divide R3, R4 by 10 for lower noise. Divide pot by 10 for lower noise. I think your gain is too high for sound card into power amp. What's the sound card output level? Jan Didden __________________ Music is dither to the brain; lets me think below the usual chaos - me Linear Audio Vol 13 is out! Check out my Autoranger and SilentSwitcher
 21st April 2007, 11:27 AM #3 diyAudio Member   Join Date: Apr 2007 If i divide all resistor by 10 then VR1 = 10k R1 = 1K R2 = 100K R3 = 1K R4 = 6.2K AND C1 should be equal to 4.7uF for flat responce down to 30 Hz. Which type of capacitor(C1 = 4.7uF) i should use for goodresult?? I am assuming Output of sound card is 200mVolts and this circuit have Gain of 7. Thanks for reply.
 21st April 2007, 11:53 AM #4 Electrons are yellow and more is better! diyAudio Member     Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4 Have investigated how much the output voltage is? Probably more than 200 mV. __________________ /Per-Anders (my first name) or P-A as my friends call me PA03 LM4780 amplifier group buy, SIGN UP HERE for the group buy 0 boards left. 118 paid.
Electrons are yellow and more is better!
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Join Date: Apr 2002
Location: Göteborg, Sweden
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Quote:
 Originally posted by janneman R2 is superfluous. Jan Didden
Jan, this resistor can be wise to have sometimes if you have a scratchy pot and in particular an opamp with bipolar transistor input stage. If the pot is bad and you don't get any base current you will get a quite nasty output signal. JFET inputs are less sensitive to floating inputs (in short term).
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/Per-Anders (my first name) or P-A as my friends call me

diyAudio Member

Join Date: May 2002
Location: The great city of Turnhout, BE
Blog Entries: 8
Quote:
 Originally posted by peranders Jan, this resistor can be wise to have sometimes if you have a scratchy pot and in particular an opamp with bipolar transistor input stage. If the pot is bad and you don't get any base current you will get a quite nasty output signal. JFET inputs are less sensitive to floating inputs (in short term).

OK I see what you mean, but in my book, the only right thing is to replace the pot . No scratchy pots in my hifi!

Jan Didden
__________________
Music is dither to the brain; lets me think below the usual chaos - me
Linear Audio Vol 13 is out! Check out my Autoranger and SilentSwitcher

diyAudio Member

Join Date: May 2002
Location: The great city of Turnhout, BE
Blog Entries: 8
Quote:
 Originally posted by Ayne If i divide all resistor by 10 then VR1 = 10k R1 = 1K R2 = 100K R3 = 1K R4 = 6.2K AND C1 should be equal to 4.7uF for flat responce down to 30 Hz. Which type of capacitor(C1 = 4.7uF) i should use for goodresult?? I am assuming Output of sound card is 200mVolts and this circuit have Gain of 7. Thanks for reply.

R1 was ok as it was.
Yes 4.7u seems OK. a good foil cap would do. Many will advise 100\$ designer caps, but that's your taste.

I would think sound card output is 1 to 2 V. Google the brand/type and look it up to be sure.

Jan Didden
__________________
Music is dither to the brain; lets me think below the usual chaos - me
Linear Audio Vol 13 is out! Check out my Autoranger and SilentSwitcher

 21st April 2007, 05:32 PM #8 diyAudio Member   Join Date: Jun 2004 Location: Minnesota If the source is a soundcard, you know that it will be capacitively coupled already, so there is no need at all for C1. I can think of about a 100 better op amps than a TL071. The size of the resistors probably doesn't matter that much in this circuit since the input level is high. Johnson noise is probably irrelevant. I would still scale down the resistors however as recommended. If nothing else, it minimizes the effects of stray C. If the resistors are relatively small, you can consider a BJT input op amp as an alternative. You may not need an output C either if the input offsets are reasonably low. You don't show power supply details. Make sure you decouple the op amp properly. Al Clark
 22nd April 2007, 09:30 AM #9 diyAudio Member   Join Date: Apr 2007 PCB compeleted PCB done. C1, C2, C3, C4, C6 are added for decouple the op amp properly. Suggest me any thing more to improvement. What u say input Coupling capacitor should be ommited?? Thanks.

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