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Old 18th December 2011, 02:06 AM   #1
timhum is offline timhum  United Kingdom
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Join Date: Nov 2010
Default x5 preamp circuit

I need to make a X5 preamp to boost a signal after some volume and balance controls and send it off at a nice low impedance to a power amp. It needs to be happy running off a single 12 volt supply.
I thought google or ebay would throw up something but so far I have not had any success.
Does anyone have a circuit to share, it need not be fancy, transistor or IC, inverting or non inverting will do. Variable gain would also be fine as long as it included around 5 times in its range.
I can make stuff but not confident enough to start from scratch.
Cheers, Tim
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Old 18th December 2011, 03:26 AM   #2
jaycee is offline jaycee  United Kingdom
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Location: Norwich, UK
Something like this would work. Change C3 to 100u and reduce R4 to 2K4 for 5x gain.

However note that if your input signal is more than 700mV p-p it will clip due to the limited swing of the opamp (about +/-4v with the NE5532 shown on 12V supplies). To get more headroom you would need a higher supply.

You should be able to do this on stripboard easily.

edit: Oh, if you like bass (this for a car amp?) then make C2 at least 33u.
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Last edited by jaycee; 18th December 2011 at 03:49 AM.
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Old 18th December 2011, 06:03 AM   #3
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It will make a little more than 8Vp-p on the output at clip. That's more like 1.6 Vp-p on the input with gain at 5. Remember that's only a little .5V rms.

Personally I'd run a little higher feedback components to place less load on the output. 7K5 for R4 and 30K for R3 will get you the gain of 5 and still be reasonably low impedance.

G
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Old 18th December 2011, 09:35 AM   #4
timhum is offline timhum  United Kingdom
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Thanks guys, just what I was looking for. It is nearly for a car system jaycee, I use a car amp for a portable PA and have to use the -10dB outputs from the mixer. Creating a suitable output with controlls has cut down the output enough to limit the audio to the power amp so it needs this boost. A gain of 5 may even be excessive but controllable.
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Old 18th December 2011, 03:50 PM   #5
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Quote:
Originally Posted by timhum View Post
Thanks guys, just what I was looking for. It is nearly for a car system jaycee, I use a car amp for a portable PA and have to use the -10dB outputs from the mixer. Creating a suitable output with controlls has cut down the output enough to limit the audio to the power amp so it needs this boost. A gain of 5 may even be excessive but controllable.
For use in a car I would use a differential input and ties the '-' input to the ground of the source. Any ground loop noise gets subtracted out of the signal as the signal is actually 'signal + ground noise'. In stereo you'd simply tie the 2 '-' inputs together and connect to the source ground. It would be the same general circuit but with a few more resistors and a cap.

G
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