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Old 15th April 2008, 01:43 AM   #1
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Default 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
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Old 15th April 2008, 01:48 AM   #2
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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.
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Old 15th April 2008, 02:26 AM   #3
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I'll split the difference. 50k!

--k
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Old 15th April 2008, 04:29 AM   #4
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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!
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Old 15th April 2008, 06:44 AM   #5
hermanv is offline hermanv  United States
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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.
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Old 15th April 2008, 09:41 PM   #6
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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.
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Old 15th April 2008, 10:00 PM   #7
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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.
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Old 16th April 2008, 02:49 AM   #8
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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.
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Old 16th April 2008, 02:51 AM   #9
puginfo is offline puginfo  Malaysia
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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?
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Old 16th April 2008, 07:12 AM   #10
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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!
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