How to select the right impedance for volume control - diyAudio
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Old 19th March 2013, 05:05 PM   #1
Lavend is offline Lavend  United Kingdom
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Default How to select the right impedance for volume control

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
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Old 19th March 2013, 05:23 PM   #2
AndrewT is offline AndrewT  Scotland
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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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Last edited by AndrewT; 19th March 2013 at 05:33 PM.
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Old 19th March 2013, 05:24 PM   #3
AndrewT is offline AndrewT  Scotland
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BTW, there is a Thread dedicated to vol pots. I think it is a sticky.
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Old 19th March 2013, 05:27 PM   #4
AndrewT is offline AndrewT  Scotland
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Help in choosing a potentiometer as a "Passive preamplifier"
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Old 20th March 2013, 01:00 PM   #5
Lavend is offline Lavend  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
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
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Old 20th March 2013, 01:08 PM   #6
Mooly is offline Mooly  United Kingdom
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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.
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Old 20th March 2013, 04:59 PM   #7
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Originally Posted by AndrewT View Post
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.

Quote:
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.
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Old 20th March 2013, 09:41 PM   #8
Lavend is offline Lavend  United Kingdom
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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
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Old 21st March 2013, 05:12 AM   #9
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Originally Posted by Lavend View Post
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 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.

Last edited by Fast Eddie D; 21st March 2013 at 05:15 AM.
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Old 21st March 2013, 06:28 AM   #10
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http://www.ti.com/general/docs/lit/g...2&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.

Last edited by Fast Eddie D; 21st March 2013 at 06:52 AM.
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