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Mod input of 6AU6/6AQ5 amp for iPod


2013-04-12 7:47 pm
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.
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.