Mod input of 6AU6/6AQ5 amp for iPod - diyAudio
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Old 12th April 2013, 06:59 PM   #1
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Default Mod input of 6AU6/6AQ5 amp for iPod

I have this old voice-of-music amp that I've been trying to mod to use my iPod with. I installed a new preamp and powertube and it passes pretty good signal and sound good. The only concern (besides hum) is that the volume control on it has to be dimed for it to sound ok. This forces me to use the headphone out on the iPod so I can adjust volume on there but ideally I'd like to use line out. How can I mod the input of the circuit to better suit the iPod?

I am thinking of moving the volume pot so that it feeds the input grid of 6AQ5 instead of 6AU6. And then I can setup the input so it has a constant impedance. I am going to add a grid resistor to ground and then a grid stopper after that in series with grid of 6AU6. Does anyone know what values I should use for these resistors. I know the output impedance of the line out on iPod is ~100ohms.
schematic...
http://img811.imageshack.us/img811/4900/vm163.png
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Old 12th April 2013, 07:49 PM   #2
DF96 is offline DF96  England
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Add a resistive attenuator before the volume pot. Resistor in series with the signal, and a resistor to ground after that (in parallel with the volume pot). Values will need adjustment, but try starting with 22k in series and 22k to ground. This will halve the signal voltage.

Your idea of adding a grid resistor to ground is a good one. It will avoid problems if the pot wears out one day, and may even delay that day by reducing DC current through the wiper.
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Old 12th April 2013, 08:24 PM   #3
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Would it not be more beneficial to just move the volume control so it sits in between the pre and power tubes and then the input Z is constant regardless of how the volume pot is set?
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Old 12th April 2013, 08:41 PM   #4
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That should have LOTS of gain... but adding a cathode bypass cap (say 47 uF, 25V across R9) will bump it up a little more. If you bypass the cathode of the 6AU6, the feedback treble cut tone control will no longer work. Adding a termination resistor to the input will reduce gain, if anything. A transformer could be used, though to step up input.
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