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zemlin 24 March 2007

In my quite compact home studio, my rackmount headphone amp works fine, however I'd like to put a more versatile setup together for location work. It looks like I'll have a big gig in the near future which will involve headphones widely distributed on a stage.

I have an old stereo power amp I can use for the feed. I figure I'll make up a bunch of little boxes with (3) TRS jacks each and a pot for volume control. Two of the TRS jacks will be wired parallel (input/pass thru) and the third will be wired through the pot.

I checked out a little Radio Shack headphone volume control - they use a 500 ohm audio taper pot wired up voltage divider style (hot to ground across the resistor - the tap goes to the output). Is this a reasonable design to follow?

My thought is that I can feed one box and daisy-chain others with TRS patch cables. Off the top of my head, I don't know what kind of voltage a power amp will put out - thus I don't know if I need to be careful about the power rating on the 500 ohm pots, or if I should go with higher resistance to keep the power draw down.

Any tips ?

Comments

bwmac Sat, 03/24/2007 - 17:26

zemlin wrote: I already have the cans - I'll probably only need 5 or 6 people with headphones - they'll just be in different places and should have their own volume control.

Cool zemlin,
No disrespect ment.
It was hard to read into why you were using head phones on stage, but I know you have your reasons.
To answer your question, It looks like you have a good idea of what you are doing. The pots should have know trouble, but; If you run your line in parallel so that the levels at each phone jack is equal, then the total resistance is added, is that not correct (i maybe wrong).
My point is; to much total resistance is no good for the amp and you can't run the chain in series because the last one wont get much power.
I'm sure you already thought of this but we don't know each other so I thought I would just through that out there

Boswell Mon, 03/26/2007 - 07:49

Karl,

Your scheme should work just fine, but the value of the pot depends on the impedance of the headphones. You should aim for a value 5 - 10 times the phone impedance. You didn't say this, but I assume this is a stereo feed, so you will need 2-gang pots. The pots should be 1 Watt or more. This is not an easy spec to fill.

For 32 Ohm phones, a 250 Ohm or 470/500 Ohm pot is suitable. For higher impedance phones, use a proportionately higher value.

Daisy-chaining the inputs on TRS jacks and powering from your stereo amp should be OK. I would put the in/through connectors on one end of the box and the out (via knob) on the other.

zemlin Tue, 03/27/2007 - 07:07

Boswell wrote: This is not an easy spec to fill.

:-?
The closest I've found is 5K audio taper.
:?
I haven't tried to figure out the details yet, but I've seen/heard some sort of arrangement with a linear pot and a fixed resistor that reportedly yields a somewhat appropriate nonlinear output. I might need to go that route.

Boswell Tue, 03/27/2007 - 07:56

zemlin wrote: I haven't tried to figure out the details yet, but I've seen/heard some sort of arrangement with a linear pot and a fixed resistor that reportedly yields a somewhat appropriate nonlinear output. I might need to go that route.

Don't bother with the fixed resistor - the headphones fill that role. The pot law is not too important in this case either, as it isn't as though you were going to calibrate the knob in dB. If you go for a pot value that is 5 - 10 times the headphone impedance, you'll get a result that is usable rather than an all-or-nothing adjustment.

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