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Old 21st March 2013, 07:46 AM   #11
JMFahey is offline JMFahey  Argentina
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A 10K pot will be both an easy load for most signal sources and low impedance enough so you will not have cable capacitance problems .... unless the pot and amps are , say, 50 meters away; but for reasonable distances, a few meters, no problem.

EDIT: just to throw some numbers in:
> a 10K pot behaves *worst case* as a 2K5 source impedance generator .
> run of the mill audio cable has 100pF per meter capacitance or less, let's use 100pF .
So a 10K pot (2K5 impedance) driving a 20 meter long cheap cable (2000pF capacitance) has a -3dB point around 32 KHz .
Incredible, huh?

Now if you need more than 20 meter distances, yes, you'll need a buffer.
Just yesterday we were commenting on a Greek Carnival live sound and music installation, along a 1000 meter street, with 40 distributed active boxes .
*That* is a tough Job, yet it was successfully solved by the installer

Last edited by JMFahey; 21st March 2013 at 08:03 AM.
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Old 21st March 2013, 10:20 AM   #12
AndrewT is offline AndrewT  Scotland
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JMF, you have not referred to the fact that the poster has told us he needs to drive 4 cables feeding 4 amplifiers.
4 cables = 4 times as much capacitance. Or am I wrong?
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Old 21st March 2013, 12:21 PM   #13
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Quote:
Are there any potential drawbacks of using a OP amp like the TL072...
Yes, TL072 is not really suitable to drive low load impedences....better to use a NE5532 but include build out resistors (resistors in series with outputs) of 100 ohms or so.
You could just try passive with 10k pots (linears pots will work fine enough in this application) and see what you get....if you need gain go the active route.

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Old 21st March 2013, 01:51 PM   #14
Lavend is offline Lavend  United Kingdom
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Thanks all for the input so far.

I would definitely prefer a passive solution as it's so much simpler to design, but there are still a few question marks.

There will be 4 amps with approx 20k input impedance each and the cables from the volume control to the amps will be 50cm each. The source will vary between headphone output from mobile phone and RCA line output from USB sound card.

AndrewT suggested a 2k pot would work in principle, with a potential problem that it might put a lot of stress on the source, whilst others are suggesting that a 10k pot would be fine?

Given that the tweeters in this system aren't hi-end by any means, I guess a small amount of high frequency roll off would be acceptable (within reason of course)

At this stage I'm tempted to order a few different pots and see what works best; 2k, 5k and 10k. To keep things simple I'm thinking Bourns dual gang with audio taper, rather than the setup with a linear pot and an extra resistor.

What are your thoughts on this? Any final input into this would be much appreciated before I go ahead and order the components and give this a try.

Thanks All!

Lavend
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Old 21st March 2013, 02:09 PM   #15
Mooly is online now Mooly  United Kingdom
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10K pots are fine. You won't overload or stress anything. Use something like the cheap (but perfectly good) Omeg pots,

PC2G16ECO10KB - OMEG - POT, DUAL GANG, LOG, 10K | CPC From This Range
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Old 21st March 2013, 02:54 PM   #16
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Quote:
Originally Posted by Lavend View Post
AndrewT suggested a 2k pot would work in principle, with a potential problem that it might put a lot of stress on the source, whilst others are suggesting that a 10k pot would be fine?
2K pot would be fine for Ipod or cellphone headphone output. It might or might not be too low for a line level output.

If you keep everything short and neat, you might get away with a 10K pot.

Quote:
At this stage I'm tempted to order a few different pots and see what works best; 2k, 5k and 10k
That's probably a good idea.
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Old 21st March 2013, 03:05 PM   #17
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by AndrewT View Post
JMF, you have not referred to the fact that the poster has told us he needs to drive 4 cables feeding 4 amplifiers.
4 cables = 4 times as much capacitance. Or am I wrong?
No, you are absolutely right.

What I suggested should be read as "total cable capacitance".

It's the same whether it's a "star" or "cross" with, say, 4 x 5 meter cables, or what I envisioned, a side-of-stage (or ballroom) music source, with 1 pair amps close by, 1 pair on the far edge 20M away, or intermediate layouts.

In fact the OP didn't mention any distance at all , which makes suggesting *any* pot value a futile exercise

I always try to introduce some real numbers in such estimations
, or it becomes an adivination contest, better solved with a crystal ball

Thanks for the clarification

EDIT: now I read a later post, the cable lengths will be mere 50 cm !!!
So total capacitance for 4 of them will be 200pF!!!!!!!!!! Nothing!!!
A 10K pot will allow for hundreds of KHz bandwidth.

So just order them, no need for lower value ones.
Try to get Log/Audio taper.

Last edited by JMFahey; 21st March 2013 at 03:12 PM.
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Old 21st March 2013, 03:20 PM   #18
Lavend is offline Lavend  United Kingdom
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Great stuff! All my questions and concerns answered - this is truly a great forum! I'll go for the 10k log pot suggested above.

I hope to report back in a week or so about the fabulous volume control for this system!

Thanks All!

Lavend
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Old 15th May 2013, 12:32 AM   #19
Lavend is offline Lavend  United Kingdom
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Default Works great!

I've finally managed to put this together, and it works perfectly! Many thanks to all people contributing to this thread and answering my questions.



Best

Lavend
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