Input potentiometer selection

koolatron · 15th April 2008, 01:43 AM

Hi guys,

Just a quick question that I couldn't readily answer for myself after a few minutes of persistent Googling; I'm getting very close to being done with my very first Aikido line stage, and the time has come for me to select the proper input potentiometer for use as a volume control.

This led me to the following question: What guides input potentiometer selection, in terms of total resistance? I see designs that call for 50k, 100k, 150k, 220k, et cetera; and I'm not sure I fully understand what constitutes a "good" choice for input attenuator.

I realize that the input attenuator sets up a voltage divider relationship, and that the precise total resistance ought not to matter when it comes to % attenuation.. so why all the widely different values? Is this to account for different source output impedances? Is source output impedance even a factor?

--k

ray_moth · 15th April 2008, 01:48 AM

You want the smallest value that you can use without posing too much of a load on the source. 10k is good if the source is a CD player or other SS device. If it's a tube output device, then 100k might be better.

koolatron · 15th April 2008, 02:26 AM

I'll split the difference. 50k!

--k

Geek · 15th April 2008, 04:29 AM

Hi,

All mine are 100K (50K for my leftover CTS bin). People bring in SS for repair/mod first thing I do is rip out the 10K pots and put in 100K. They find it plays so much better with their tube pre's

Cheers!

hermanv · 15th April 2008, 06:44 AM

If the top of the volume pot is the first thing the signal sees then your 50K compromise is dead on. 47K Ohms is the traditional input impedance for pre-amps or amps.

I have some doubts that CTS is the best choice for sound quality. Purpose made audio attenuators are available from several sources (such as Michael Percy ) they can run into real money. If you buy, don't forget to get LOG taper.

ray_moth · 15th April 2008, 09:41 PM

I don't know what CTS stands for, but I agree that a switched attenuator is a good idea, if you have space for it and don't mind the extra cost. This is especially true for a stereo volume control, because tracking of ganged potentiometers leaves a lot to be desired.

koolatron · 15th April 2008, 10:00 PM

Aye, I'm 100% aware of the consensus that stepped attenuators are superior to the carbon-track-scraper potentiometers I'm used to (That's probably not what CTS stands for. As far as my cursory Googling has led me to believe, CTS is a brand name).

Since I'm a cheapo, and this is my first real foray into the world of valve audio, I'm attempting to keep things sane and in-budget. As compromises go, I bought a fako-ALPS 50k stereo pot on eBay; I figure this is probably a measure better than whatever I find in my own junkpile. Even so, there's room in the case for a proper stepped attenuator should I decide that's where I want to go.

ThSpeakerDude88 · 16th April 2008, 02:49 AM

A lot of people seem to have good results with the 100k ALPS pot from RatShack. Its decent quality and cheap. I have two in my tube amps and their not noisy yet...

I guess the main problem with cheap pots is the balance between the two sections, which mine seems to be pretty decent.

puginfo · 16th April 2008, 02:51 AM

Quote:

Originally posted by Geek
Hi,

All mine are 100K (50K for my leftover CTS bin). People bring in SS for repair/mod first thing I do is rip out the 10K pots and put in 100K. They find it plays so much better with their tube pre's

Cheers!

i use a 100k in the PV12 design. for my use, i need to turn it only very little ie. 10% of the total wiper. how do i get it to sound where i need it, say around 50% of wiper?

should it be lower than 100k or higher?

Geek · 16th April 2008, 07:12 AM

Hi,

Yes, CTS is a brand name.

Quote:

Originally posted by puginfo

i use a 100k in the PV12 design. for my use, i need to turn it only very little ie. 10% of the total wiper. how do i get it to sound where i need it, say around 50% of wiper?

Use a logarithmic (audio) taper instead of linear.

Quote:

should it be lower than 100k or higher?

100K is fine.

Cheers!

15th April 2008, 01:43 AM	#1
koolatron diyAudio Member Join Date: Jul 2006	Input potentiometer selection Hi guys, Just a quick question that I couldn't readily answer for myself after a few minutes of persistent Googling; I'm getting very close to being done with my very first Aikido line stage, and the time has come for me to select the proper input potentiometer for use as a volume control. This led me to the following question: What guides input potentiometer selection, in terms of total resistance? I see designs that call for 50k, 100k, 150k, 220k, et cetera; and I'm not sure I fully understand what constitutes a "good" choice for input attenuator. I realize that the input attenuator sets up a voltage divider relationship, and that the precise total resistance ought not to matter when it comes to % attenuation.. so why all the widely different values? Is this to account for different source output impedances? Is source output impedance even a factor? --k

16th April 2008, 02:49 AM	#8
ThSpeakerDude88 diyAudio Member Join Date: Jun 2005 Location: USA	A lot of people seem to have good results with the 100k ALPS pot from RatShack. Its decent quality and cheap. I have two in my tube amps and their not noisy yet... I guess the main problem with cheap pots is the balance between the two sections, which mine seems to be pretty decent. __________________ always preach the gospel- and when necessary use words.

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15th April 2008, 01:48 AM	#2
ray_moth diyAudio Moderator Emeritus Join Date: Jan 2004 Location: Jakarta	You want the smallest value that you can use without posing too much of a load on the source. 10k is good if the source is a CD player or other SS device. If it's a tube output device, then 100k might be better.

15th April 2008, 02:26 AM	#3
koolatron diyAudio Member Join Date: Jul 2006	I'll split the difference. 50k! --k

15th April 2008, 04:29 AM	#4
Geek diyAudio Member Join Date: Sep 2004	Hi, All mine are 100K (50K for my leftover CTS bin). People bring in SS for repair/mod first thing I do is rip out the 10K pots and put in 100K. They find it plays so much better with their tube pre's Cheers!

15th April 2008, 06:44 AM	#5
hermanv diyAudio Member Join Date: Apr 2004 Location: Northern California	If the top of the volume pot is the first thing the signal sees then your 50K compromise is dead on. 47K Ohms is the traditional input impedance for pre-amps or amps. I have some doubts that CTS is the best choice for sound quality. Purpose made audio attenuators are available from several sources (such as Michael Percy ) they can run into real money. If you buy, don't forget to get LOG taper.

15th April 2008, 09:41 PM	#6
ray_moth diyAudio Moderator Emeritus Join Date: Jan 2004 Location: Jakarta	I don't know what CTS stands for, but I agree that a switched attenuator is a good idea, if you have space for it and don't mind the extra cost. This is especially true for a stereo volume control, because tracking of ganged potentiometers leaves a lot to be desired.

15th April 2008, 10:00 PM	#7
koolatron diyAudio Member Join Date: Jul 2006	Aye, I'm 100% aware of the consensus that stepped attenuators are superior to the carbon-track-scraper potentiometers I'm used to (That's probably not what CTS stands for. As far as my cursory Googling has led me to believe, CTS is a brand name). Since I'm a cheapo, and this is my first real foray into the world of valve audio, I'm attempting to keep things sane and in-budget. As compromises go, I bought a fako-ALPS 50k stereo pot on eBay; I figure this is probably a measure better than whatever I find in my own junkpile. Even so, there's room in the case for a proper stepped attenuator should I decide that's where I want to go.