is this the correct way of adding a volume control? - diyAudio
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Old 29th March 2007, 05:24 AM   #1
jarthel is offline jarthel  Australia
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Default is this the correct way of adding a volume control?

Here is a an aikido circuit used as a headphone amp: http://www.tubecad.com/2006/04/27/ai..._amplifier.png

Problem with the circuit is that volume control is missing. A requirement for a headphone amp.

I'm just wondering if this is the right way of connecting a pot to the circuit above: http://img261.imageshack.us/my.php?i...controlsn8.png

thank you for the help
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Old 29th March 2007, 07:50 AM   #2
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Looks OK to me. 100k (audio taper) should be a fine choice.

I also like the way you're planning to retain the 1Meg resistor R1. This will help prevent nasty noises in case the volume control wiper is not a perfect contact to the track. The only thing you might need to add is a coupling cap at the input, but only if the signal source has any DC bias on it.
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Old 30th March 2007, 12:12 AM   #3
jarthel is offline jarthel  Australia
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how do I know if the input has DC bias?

thanks again
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Old 30th March 2007, 12:44 AM   #4
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Hi,
Original post by jarthel
Quote:
how do I know if the input has DC bias?
Since ther is no reply to this question, I will try to contribue my 2 cents worth.

What I will do is to measure the signal's DC voltage across the signal to ground with a multimeter. It's fine if it fall around +/-40mV. (smaller the better.)

But you don't have to worry too much, this DC Voltage is normally very small, and at worst shift the bias of the front stage by a volt or 2, I recon the 2 triodes in your Aikido's front stage will try to balance the effect off by themselfs. and since there is an output coupling cap, this DC signal will not get through to your headphone and ruin it.

I'm still a student in this, so, I hope its the correct.

Cheers
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Old 30th March 2007, 09:32 AM   #5
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As you say, a little DC may not be especially bad as far as the IP tubes are concerned. However, if there is any DC, even if it's not enough to upset the input tubes, will cause the volume control to have a noisy travel. Better to use an input cap only if necessary.
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