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Old 14th May 2008, 09:15 PM   #1
G is offline G  United States
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Default 100K pot in a preamp?

I have noticed that most preamp designs use a 50K or less pot at the input. Is there any reason that a 100K pot should not be used in a preamp? It may seem like a silly question but I think that there may be a reason.
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Old 14th May 2008, 09:32 PM   #2
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It depends on the input capacitance. The potentiometer forms an RC lowpass filter with the input C. Worst-case is at -6dB, where the pot's source resistance looks like somewhere between 1/4 and 1/2 the total potentiometer resistance (depending on the source impedance driving the pot).
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Old 15th May 2008, 03:18 AM   #3
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Default Re: 100K pot in a preamp?

Quote:
Originally posted by G
I have noticed that most preamp designs use a 50K or less pot at the input. Is there any reason that a 100K pot should not be used in a preamp? It may seem like a silly question but I think that there may be a reason.

In other words as I understand it, I believe that using a higher value potentiometer COULD result in a high frequency roll off with valves that have a high input capacitance (high transconductance valves, for instance). OTOH, using a higher value potentiometer CAN increase the input impedance of the amp, good for sources with a highish output impedance.
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Old 15th May 2008, 03:38 AM   #4
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Yes. As others have pointed out, it has to do with interaction against Miller capacitance (and other strays) so the problem is worst with high mu triodes like 7025s or 6SL7s, but presents essentially no problem with pentode input valves like 6267 or 6AU6.
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Old 15th May 2008, 02:28 PM   #5
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Do you think I will run into a problem using a 100K pot with a 396A? The 396A has very low input capacitance.
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Old 15th May 2008, 02:41 PM   #6
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In my Aikido build I have 220K input pot. I tried it right now bypass it with different resistor values to obtain different pot value.

Higher the resistance of pot sounded very open, more bass, but it lacked and higher mids and highs. I bypassed the pot with 100k to obtain 68k pot and sound doesnt have now wide stereo.....it definately sounds different. BUT I DONT KNOW WHAT VALUE IS RIGHT TO MAKE IT SOUND HOW IT SHOULD SOUND - the cleanest hifi way.

I do like sound with both values 220k and 68k.
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