100K pot in a preamp?

G · 14th May 2008, 09:15 PM

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.

SY · 14th May 2008, 09:32 PM

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).

zobsky · 15th May 2008, 03:18 AM

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.

tubewade · 15th May 2008, 03:38 AM

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.

G · 15th May 2008, 02:28 PM

Do you think I will run into a problem using a 100K pot with a 396A? The 396A has very low input capacitance.

kacernator · 15th May 2008, 02:41 PM

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.

14th May 2008, 09:15 PM	#1
G diyAudio Member Join Date: Apr 2002 Location: Belleville, IL.	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. __________________ Gavin

14th May 2008, 09:32 PM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	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). __________________ If there's a sucker born every minute, where do the rest of them come from?

15th May 2008, 02:28 PM	#5
G diyAudio Member Join Date: Apr 2002 Location: Belleville, IL.	Do you think I will run into a problem using a 100K pot with a 396A? The 396A has very low input capacitance. __________________ Gavin

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15th May 2008, 03:38 AM	#4
tubewade diyAudio Member Join Date: Jun 2007	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.

15th May 2008, 02:41 PM	#6
kacernator diyAudio Member Join Date: Jan 2008	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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