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Old 6th October 2004, 02:26 AM   #1
cowanrg is offline cowanrg  United States
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Default what pot value for a passive pre-amp?

hey guys, while i build my pass bosoz, im wanting to build a passive pre-amp for the time being, but im wondering what pot value should i use? i found a cheap allen-bradley stereo 50k audio mod pot for like $1. i tried it out, but it doesnt go very quiet. even at minimum volume, its pretty loud.

should i do like a 100k or 250k, or should i just put a resistor in line with the pot? thanks guys!
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Old 6th October 2004, 02:52 AM   #2
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Default Re: what pot value for a passive pre-amp?

Quote:
Originally posted by cowanrg
hey guys, while i build my pass bosoz, im wanting to build a passive pre-amp for the time being, but im wondering what pot value should i use? i found a cheap allen-bradley stereo 50k audio mod pot for like $1. i tried it out, but it doesnt go very quiet. even at minimum volume, its pretty loud.
Are you sure you have it wired up right? At minimum, your input should essentially be shorted to ground and you shouldn't hear much of anything to speak of.

se
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Old 6th October 2004, 03:12 AM   #3
cowanrg is offline cowanrg  United States
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hum, maybe it is wrong.

haha, i did forget to connect the grounds together. the input and output grounds arent connected together. meaning, the left and rights are connected, but the in and out grounds arent connected, maybe thats it.

but, a 50kohm stereo pot should be enough?

i have it connected as so: (same for left and right channels of course)

left input > pin on pot
left output > pin on pot.

the resistance across these pins, when the volume knob is all the way down measures 50kohms, as i turn it "up", the resistance goes down. so, as i have it wired, im subtracting resistance from the input to output rca's as i turn it up.

i "think" thats right. at least it seems valid in my stupid little head. but i want to at least know if that pot will work ok.
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Old 6th October 2004, 03:21 AM   #4
rpapps is offline rpapps  Antarctica
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Default Pot connections

Hi cowrang
Your pot has three terminals.
Looking at it from the shaft end with the terminals upward, signal in goes to left terminal, signal out comes from center terminal, ground (both in & out) goes to right terminal.
Cheers
Rob
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Old 6th October 2004, 03:44 AM   #5
rpapps is offline rpapps  Antarctica
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Hi Cowanrg
Sorry for misspelling your handle.
I sometimes suffer from dis lexia or dat chardonay or....
Cheers
Rob
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Old 6th October 2004, 06:12 AM   #6
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Quote:
Originally posted by cowanrg
hum, maybe it is wrong.

haha, i did forget to connect the grounds together. the input and output grounds arent connected together. meaning, the left and rights are connected, but the in and out grounds arent connected, maybe thats it.
Sounds like it. If the "bottom" of the pot isn't tied to ground, all you're doing is increasing the resistance in series with the output impedance of the source component.

Quote:
but, a 50kohm stereo pot should be enough?
Any pot should give you virtually no output when you've got it turned all the way down.

Ultimately you need to strike a balance between the load impedance seen by your source component(s) and the output impedance from the pot.

50k will work, but will have a higher output impedance than say a 10k pot. So if you're not locked into using a 50k pot, a 10k pot would give you a lower output impedance and is a load even most tube sources can handle.

Quote:
i have it connected as so: (same for left and right channels of course)

left input > pin on pot
left output > pin on pot.

the resistance across these pins, when the volume knob is all the way down measures 50kohms, as i turn it "up", the resistance goes down. so, as i have it wired, im subtracting resistance from the input to output rca's as i turn it up.

i "think" thats right. at least it seems valid in my stupid little head. but i want to at least know if that pot will work ok.
Sounds like you have it wired up right except for grounding the "bottom" end of the pot.

se
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Old 6th October 2004, 02:38 PM   #7
rabbitz is offline rabbitz  Australia
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Have a look at figure 5 on this link. http://sound.westhost.com/pots.htm
The paragraph above the picture has the wiring details.

Cheers
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Old 6th October 2004, 06:56 PM   #8
cowanrg is offline cowanrg  United States
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perfect!

ill give that a try today and get back to you.
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Old 6th October 2004, 10:15 PM   #9
cowanrg is offline cowanrg  United States
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seems like it worked. however, the channels arent matched perfectly. it was a dual-concentric pot though, so i just got a knob that works both left and right separately. works fine for now. however, with my low-efficiency speakers, i do need gain. however, this isnt for me, and that person has higher efficiency speakers, so it should work fine for the time-being. thanks guys!
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