How to select the right impedance for volume control

Lavend

Member
2013-02-18 7:54 pm
London
Dear All,

I'm currently putting together a 12v sound system and I need a volume control for the setup, but I'm unsure how to calculate the impedance of the potentiometer I would need.

The system consists of 4 JBL GTO 2000 amps each running in bridged mode powering 2 x 12" woofers and 2 x midrange and treble speakers. The signal input will mostly be coming from a sound card with an output impedance of 150 Ohms, but there will be instances where a mobile phone or other portable device will be plugged in as the source (for which I have no idea of output impedance).

I've not found any specific info on the input impedance for the JBL amps, but looking at similar amps, it seems to be in the region of 20k.

The signal will be split into 4 stereo signals using Y RCA connectors. How does that affect the the overall impedance of the amp inputs? 1/4?

As I understand it I should be aiming to have 1/10 of the input impedance of the amps as the impedance for the pot? That would mean I need a 2k pot for one amp and 500 Ohm for all 4 amps? To me this seems very low? Typical volume control pots seem to be more in the 10k or 50k range?

Can anyone shed some light on this as it's really the last thing I need before the project is complete.

Many thanks

Lavend
 
feeding 4 inputs each of which is 20k leaves the pot trying to drive a 5k load in parallel to all the cable capacitance in those 4 cables.
A 2k volume pot with a maximum output impedance of 500r will probably just about manage that if all the cables are not too long.

Can your source drive a 2k vol pot?

The alternative is to buffer the vol pot output to make it capable of driving all those cables.
 
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Lavend

Member
2013-02-18 7:54 pm
London
feeding 4 inputs each of which is 20k leaves the pot trying to drive a 5k load in parallel to all the cable capacitance in those 4 cables.
A 2k volume pot with a maximum output impedance of 500r will probably just about manage that if all the cables are not too long.

Can your source drive a 2k vol pot?

The alternative is to buffer the vol pot output to make it capable of driving all those cables.

Thanks for this. Given that this system will be an outdoor party machine where people can plug in their own device, I think this sounds risky. I don't want to ruin the headphone output of someones phone. I now realise that I need a more 'proper' preamp ,than just a pot on the signal to run this system. The sticky thread you pointed to was very useful for understanding this stuff, but I haven't grasped the concept of buffering the volume pot output. Will do my research and come back. Any links or info would be much appreciated.

Many thanks

Lavend
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
A headphone output is intended to drive low impedances (typically 10 to 30 ohms for hphones) and is in all probability short circuit proof anyway.

(A headphone output isn't considered a "preamp output" by which we usually mean something that is designed for 600 ohms and above loading)

I don't see a problem at all.
 
The alternative is to buffer the vol pot output to make it capable of driving all those cables.

If you take this route, you can use whatever value pot is convenient. I recommend anything from 10K to 100K.

Without a buffer, the output impedance of the pot coupled with the capacitance of the cables will create a low pass filter. The bigger the pot, the more pronounced the effect will be.

For best results, put the buffer right on a little circuit board with the pot.

but I haven't grasped the concept of buffering the volume pot output

A simple unity gain buffer will suffice. A TL072 (dual op amp jfet inputs) fits the bill perfectly.
 

Lavend

Member
2013-02-18 7:54 pm
London
This is starting to make sense now. A buffer would make sense in this case as I'll be using various sources ranging from headphone output to normal line RCA outputs from an external USB sound card. I'll search the forum for a diagram of how to wire the TL072 and pot, but any links would be appreciated.

Are there any potential drawbacks of using a OP amp like the TL072 and a 10k pot compared to just putting a 2k pot in the signal path as mentioned above? (as the components are quite cheap I guess I could try both setups and see what works best)

Many thanks

Lavend
 
Are there any potential drawbacks of using a OP amp like the TL072 and a 10k pot compared to just putting a 2k pot in the signal path as mentioned above? (as the components are quite cheap I guess I could try both setups and see what works best)

The only drawback is added complexity.

The benefits are

-You can drive multiple amplifiers as long as the parallel impedance isn't too low (I would try to stay above 2K; 10K or higher would be ideal)

- It will allow you more flexibility in layout. You can drive multiple long leads to the amplifier's inputs without the capacitance of the wires affecting your frequency response.

- You can use a higher impedance pot (10K minimum is what I suggest) which will allow you to use line level input (component CD player etc) with no problem, as well as Ipod etc.

- The volume control will perform exactly like it was intended and how you want. 10K or 100K, it will perform exactly the same. Low input impedances (in proportion to the pot) will "pull down" the voltage divider and alter the control of the pot. The buffer entirely mitigates this effect, providing predictable and consistent performance.

If you want to try it without the buffer, use as low a value pot as practical (2K like you suggested would be max); the lower the better. Of course this will limit the usefulness of your project. Also make the output leads to the amplifiers as short as practical.

I did a google search and did not find a single supply buffer to my liking (some of them wouldn't even work :joker: so beware of copying someone else's work). I'm old school and don't have all the fancy software to draw schematics on line, so maybe somebody could provide a simple, single supply non-inverting buffer circuit for you.
 
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http://www.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=tl072&fileType=pdf

Figure 25 is the circuit you want. Although it does not specify, this is a single supply distribution circuit.

For unity gain, all resistors should be 100K. You can scale them down accordingly ( within reason) and scale the capacitors up. The input impedance is 100K as shown. The 100 uF capacitor provides a "soft start" circuit as well as decoupling the power supply from the non inverting input. 47 uF might work fine for your application.

For simplicity you can dispense with buffers "A, B, C" and just use the input amplifier stage. You will need an output capacitor, a bleeder resistor for it, and local power supply decoupling as well. Also put a 0.1 uF cap (cheap ceramic is fine) across the power supply pins as close as practically possible. I solder them directly to the pins on the bottom of the board.

I hope that helps.

Edit- for unity gain, the 1 Meg resistor and the 100K resistor from the inverting input to the 100 uF cap are eliminated and the output is connected directly to the inverting input. I wouldn't want to steer you wrong.
 
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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 :)
 
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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.

Dan.
 
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
 
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.

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.
 
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 :D , which makes suggesting *any* pot value a futile exercise :shhh:

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

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.
 
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