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Old 16th February 2006, 10:49 PM   #1
KT is offline KT  United States
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Default Effect of using 50k pot instead of 100k pot in tube preamp?

Hello,

What would the effect of using a 50k pot instead of a 100k pot at the input of a tube preamp be? Is this a bad idea?

The preamps I'm thinking of use either a 5687 or 6sn7 after the pot.

Thanks,
KT
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Old 17th February 2006, 02:05 AM   #2
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Quote:
What would the effect of using a 50k pot instead of a 100k pot at the input of a tube preamp be?
Unless the stuff upstream has extremely poor drive capability, you will be FINE.

Quote:
Is this a bad idea?
It's a good idea, as cable capacitance in combination with a large I/P resistance can roll off high frequency information.
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Old 17th February 2006, 02:32 PM   #3
Joel is offline Joel  United States
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Quote:
"It's a good idea, as cable capacitance in combination with a large I/P resistance can roll off high frequency information."
Eli,

That would only be true if the entire "transmission line" were at a high impedance. But, in most audio lines, the low source impedance shunts down the line, so the stray capacitance is shunting under 1k ohms or so usually, pushing the corner frequency up into the MHz region.

The real risk of lowering the Zin of an amp is loading down the source, causing increased distortion and lowered signal voltage. But I agree that in most cases, 50k is not going to do harm.

Joel
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Old 17th February 2006, 04:35 PM   #4
kevinkr is offline kevinkr  United States
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Using a 50K pot is a very good idea provided that the source can drive it comfortably, and presumably most can. (My worst designs could drive 10K)

Using a 50K pot will improve high frequency response at all positions of the pot because the thevenin equivalent source Z into the grid of your pre-amplifier tube will be halved, that in conjunction with the existing miller capacitance of the tube (it's a triode remember) will effectively push the high frequency pole (-3dB roll off point) further above the audio band.

In addition should there be any grid current flowing the lower resistance will generate less current noise.

Kevin

edit: fix typo, add clarification
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Old 17th February 2006, 08:12 PM   #5
cerrem is offline cerrem  United States
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One other thing to consider is the low frequency response...
This would depend on the size/value of the output cap from the source, which in this case is your pre-amp.... Chaning input resistance to 50K will have an affect, how much of an affect depends on what output resistance and capacitor value are..
I usually put the -3dB point way belown 20Hz...somewhere in the mid to lower single digits...

Chris
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Old 17th February 2006, 09:56 PM   #6
KT is offline KT  United States
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Thanks, guys. After reading your comments I think it won't be a problem.

Btw, the preamps I'm considering using this pot in are both output transformer preamps. One is the Diego Nardi WOT featured in Sound Practices #10 and the other is the Electa-Print Ultrapath from Jack Elliano.

The sources will be a non-OS dac, like the Monica II or DacKit. I think these are comfortable driving an 50k load.

Best,
KT
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