Tube DIY Asylum: RE: What should I do with it? by Tre' Do It Yourself (DIY) paradise for tube and SET project builders.

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Posted by Tre' (P ) on August 20, 2012 at 21:31:40

In Reply to: RE: What should I do with it? posted by wheezer on August 20, 2012 at 19:54:27:

There's no AC in the preamp chassis except for the music and the on board MC set up transformers are run balanced (balanced tonearm cables also) so CMR is high.

The unshielded TWPR input wires of the set up transformers run straight to the panel jacks. The unshielded TWPR output wires run straight to the tube socket.

The reason I asked about the volume control and first stage tube has to do with Miller+cable capacitance vs. source impedance and the low pass filter that it creates.

With a 50k pot the worst case output impedance is 12.5k. With the 50pf of the 7308 you don't want to add much cable capacitance but with only one 1 foot it's probably not a problem.

Do you know the capacitance of the cable?

I've seen 250k pots with shielded cable and that's a recipe for HF roll off.

With unshielded cable I can't understand why the noise floor would change with a wire change.

With shielded cable, the cable with the most capacitance might be killing high frequency noise.

Tre'

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