Volume Control between amp and headphones

[DNA2251]

Hi I'm like super new to all this but I need help with a small situation I need a volume control unit between a Behringer Powerplay Pro XL-4700 Amp and a bunch of people on earbuds. Since everyone is on different earbuds and everyone has different volume preferences I need a solution that allows each person to adjust their volume accordingly. (The powerplay has 4 channels but they can support up to 3 people on each channel).

I've seen a few offerings like the Koss VC-20 and stuff but after hearing bad reviews on it I'm thinking of building a custom one so that I have a quality unit that lasts longer in the long run. I have very limited knowledge on choosing pots for this application other than the fact that I need a dual gang pot.

So mostly I need help choosing a pot for this application. Also do I need a pot that can handle higher power stuff since it'll be attenuating a powered signal? Or am I over all just wasting my time and I should just get a retail offering on an inline volume control?

Also great forums, learning a lot!!!

 

[n_maher]

My 2¢ is that what you are proposing is a bad idea. Why use a power amp at all since the earbuds need virtually nothing to drive them? I'd propose that a better use of your efforts would be to build 7 small headphone amps to attenuate the signal to the users desired listening level and omit the power amp entirely. You might also need an active stage (buffer?) between your source and all that attenuation since it could load the source in weird ways but I haven't give it enough thought to really know for sure.

 

[Steve Eddy]

The problem you face here is that if you're wanting to add a volume control inline with the headphones, you want the pot to be of a relatively low value to keep the output impedance to the headphones as low as possible.

And about the lowest you're going to find in a dual gang, audio taper pot is about 1k ohms. What that translates into is a worst case output impedance of about 250 ohms, if we assume that the PowerPlay has a very low output impedance of its own (they don't say in the spec sheet).

This may work out just fine if you're not worried about absolute audio quality.

Check out Mouser part number 313-2420F-1K.

se

 

[n_maher]

Steve,

I just wonder, since you're going to be using the pot at almost maximum attenuation presumably, what kind of channel matching one could expect from a cheap pot like that? I know the data sheet says +/- 3db but I think most of us have experienced the bottom end of similarly rated pots being off by a lot more than that.

 

[Steve Eddy]

Quote:

Originally Posted by n_maher /img/forum/go_quote.gif I just wonder, since you're going to be using the pot at almost maximum attenuation presumably, what kind of channel matching one could expect from a cheap pot like that? I know the data sheet says +/- 3db but I think most of us have experienced the bottom end of similarly rated pots being off by a lot more than that.

The way around that would be to keep the output level of the Behringer as low as possible so that the individual volume settings with the pots will tend to be toward the least amount of attenuation, which also has the added benefit of keeping the output impedances as low as possible.

se

 

[n_maher]

Quote:

Originally Posted by Koyaan I. Sqatsi /img/forum/go_quote.gif The way around that would be to keep the output level of the Behringer as low as possible so that the individual volume settings with the pots will tend to be toward the least amount of attenuation, which also has the added benefit of keeping the output impedances as low as possible. se

Ahh, when he said powered signal I somehow assumed he was using a power amplifier. D'oh! Now that I've looked up what the powerplay is I think you've got a workable solution described above. Of course if it were me I'd probably just buy a second Powerplay unit since they appear to only be ~$100 and look like they offer separate controls for each of the 4 stereo channels.

 

[Steve Eddy]

Quote:

Originally Posted by n_maher /img/forum/go_quote.gif Ahh, when he said powered signal I somehow assumed he was using a power amplifier. D'oh! Now that I've looked up what the powerplay is I think you've got a workable solution described above. Of course if it were me I'd probably just buy a second Powerplay unit since they appear to only be ~$100 and look like they offer separate controls for each of the 4 stereo channels.

Yeah, I was rather surprised at the price of it as well.

And for not a whole lot more, he could get two more units and have as many outputs as he would using the one unit to drive three headphones per output and they'd be much better optimized as far as volume control goes.

However not knowing exactly what he's wanting to do, that may not be the best solution. I mean, if he wants the headphone jacks and volume controls to be located in different locations, say, in a coffee shop or something, then having everything in a central location wouldn't work so well.

But if that's not the case, then a couple more units would really do the trick.

se

 

[DNA2251]

Thanks for the feedback guys,

I think the powerplay can do I think down to 8 ohms impedance per channel and i think it arranges the headphones in a parallel circuit.

And as much as I'd love to get another unit and spread them between 2 groups the biggest reason why i want a small volume control that's accessible to the person in their immediacy is because I'm using this as a crappy wired monitor system for a small band that I PA for and while during practice It might be ok for them to go over and change the volume. When in an actual performance I'd like for them to be able to change their volume from where they are "most of them don't move around a lot."

So like wireless or wired IEM systems I'd like to be able to make something like a small beltpack unit that they can adjust their headphone volume with. To resolve the details on what I plan to use this for and why I just don't get another powerplay.

Also I guess if its better for the situation that i wouldn't mind getting 2 single gang pots for the job if its a bit more suitable.

 

[Steve Eddy]

Then absolute audio quality isn't an issue. They're just monitoring.

Go with the dual gang, 1k audio taper pots then. Simple simple.

se

 

[DNA2251]

Gotcha, I'll go with these then and as one last question... I've never bought audio pots before but i was reading that the level matching between the two pots on a dual gang pot can sometimes not be calibrated very well so i'm guessing i should buy more than i need and just pick the best ones?

 

[Steve Eddy]

Quote:

Originally Posted by DNA2251 /img/forum/go_quote.gif Gotcha, I'll go with these then and as one last question... I've never bought audio pots before but i was reading that the level matching between the two pots on a dual gang pot can sometimes not be calibrated very well so i'm guessing i should buy more than i need and just pick the best ones?

You could do that, or you could just buy a few and check 'em out and see if it'll be any problem in the first place. If it's a problem, then buy a pile of 'em.

se