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Old 8th February 2005, 12:05 AM   #1
py is offline py  Canada
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Join Date: Oct 2004
Default Question about preamp feeding multiple outputs

I want to build mylself a preamp for my amp project. I think it would be nice to have two outputs.... one for my amp and the other to record things down to my pc (converting LPs or just playing with microphones). I also think it would be great to hear it with headphones.

My actual problem is to know if it is possible to drive all these outputs with a single line amplifier (with pots after the line amplifier (one for each function)). Well how this affect the output impedance seen by the preamp and how this could affect the sound delivered. Amplifier input and souncard input can be expected to be somewhat high impedance but the headphones are much lower. I also migth think that the heaphones will need a little signal adaptation to deliver the best sound but I dont know how to do this.

I wanted to know if it is logical to do this since I dont want to build 3x line/heaphone amplifier because more of these = more $$$ and less enclosure space!!!

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Old 8th February 2005, 07:20 AM   #2
AndrewT is online now AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
normally a preamp has some fixed output voltages eg. record output and a single/double variable output.
The volume control will occur before the final line out amplifier to ensure a low output impedance.
If you put volume controls in after the line out amp you will push out a signal with a high output impedance whose impedance varies with volume knob position. In other words you end up with a variable treble filter on the outputs.
Have a rethink and consider line out amplifiers(buffers) on every output.
regards Andrew T.
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Old 8th February 2005, 12:01 PM   #3
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Join Date: Jul 2003
Location: Newcastle, Australia
You will definitely need a separate headphone amp to drive the headphones, or alternatively, hook up a headphone ouput to the output of the power amp (through dropping resistors).

A recording output is normally taken just before the master volume control, so that varying the volume does not effect the recording level. The output impedance at this point should be fairly low, so loading will not be a problem. Don't put a separate volume pot on the recording output otherwise you get the effect Andrew mentioned.

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Old 9th February 2005, 12:49 AM   #4
py is offline py  Canada
diyAudio Member
Join Date: Oct 2004
Thanks a lot for your advise! Now I know this kind of thing is not possible and best of all I know why! I will now find another way doing all this, maybe a bit more complex but fonctionnal !

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