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[Wireline]

Anyone have a good schematic for a headphone breakout box with individual volume controls for each of the 4 or 5 H outs?  I would like to use a spare BGW amp that puts out 70w at side, but obviously padded down for most users.  Sometimes the wall-wart things just don't have enough clean output for drummers.

Most of the tracking area cans are the More Me style...soon to be replaced or augmented with AT or Fostex (50-60 ohm)


Thanks for any help

[amorris]

the problem is you need to soak 70watts through your volume pot. this can mean fire. at least smoke. (thats always a fun service call, "the headphone box is smoking"). whirlwind foolishly has a replacement policy, so, well, you can see where this is going. we use whirlwind boxes.

[johnR]

I use a 75W/ch. power amp for headphone distribution, but I'm using 400 ohm DT100s. It's about the right level for those. I still see studios over here using multiple Quad 303s for headphone duties, but again they're usually high impedance.

I put 100 ohm 10W resistors in series with each 400 ohm output to protect the amp against accidental shorting and to prevent low Z headphones from catching fire if plugged in by mistake. I also added a couple of outputs with an extra 330 ohm 3W resistor in series and they work fine with 80 ohm headphones (not quite as loud but OK).

I probably wouldn't use an amp with such a high output voltage for lower impedance headphones. The problem is you have to drop the signal voltage to something that won't break the cans (or eardrums). In theory you could use high powered wire wound pots, but there's a lot of heat generated for a relatively low output power into the headphones. You'd need a lot of air circulation around them, and IME wire wound pots are nowhere near as reliable as fixed wirewound resistors when run near their power limit.

[drknob]

Assuming your amp is rated at 75W into 8 ohms, that works out to 24Vrms available signal voltage. That's not too big. To be clear - the power rating of the amp is only a description of how much IV it can deliver before it protects itself or goes kablooey. If you're driving a lot of headphones in parallel, the load impedance will drop correspondingly, drawing more power from the amp as needed. It's just Ohm's law.

So - two issues:

1) to protect the amp, I recommend 100W 4 ohm resistors in series with the L/R amp outputs. This way, if someone or something shorts your headphone lines, you'll avoid the nuisance of blowing fuses or smoke in your amp. It has the added benefit of isolating the unpredictable line impedances of a zillion headphones through hundreds of feet of cable.

2) to protect the headphones, I agree with JohnR - a 10W 100 ohm resistor before any volume pot will do some useful current limiting. This is how the Whirlwind Sat-1 boxes are designed.

I used the 4 ohm resistors and the Sat-1 boxes for hundreds of orchestra sessions with dozens of headsets driven by Bryston 4Bs or Hafler 5000s (IIRC) without a problem. The only problem was that the Sat-1s had 2W 100 ohm resistors that would smoke. Once we put in 10W (or was it 5W?), all was good.

Oh yeah, I learned this lesson the hard way..... Nothing like losing headphones on a 50 piece scoring date to sharpen the mind.

[sodderboy]

Distributed cue systems were all there was for decades.  They worked great with the 600 ohm impedance driver headphones for which they were designed.  They hate the modern 60 ohm cans that are a result of the "headphone wars".  Just try to buy new 600 ohm cans- they are no longer available.  So you have a tenfold increase in juice baking boxes and blowing drivers.

I recommend cue mixing systems or small powered mixers, even Rolls 4 ch, with headphone outs.  Distributed is only good for 10 plus quantity situations.

I have a circuit or 2 that I will find, scan and post.  I went through a bunch of boxes and drew some back in the day.

As far as boxes go, my fave is the Simons.  Never had a blown box, so I say it is worth the price.  Second is Redco, but the trick with them, and any other box is to keep volume knobs in use as high as possible, and all the way UP if unused.  A pot turned down just acts like a short circuit to the guts inside, and a 100W power amp is happy to supply the damaging energy.  That is why so many boxes burn pots on channels 2-4. They are not used much and the volume pots are kept down.  Third fave is ProCo, but that is after I have replaced the baked 1W pots with 3W.  I think the Whirlwinds are bogus with that wimpy dual concentric pot, it usually gets bent anyway or loses the mini-nob, so I just rebuild blown boxes with the ProCo circuit.
Mike

[Wireline]

Starting to look like an off the shelf solution may be best, with a 5W/ch amp (padded from the drummer's headphone amp out to line level) to give him/her a little extra push if needed.

I used some AKG k160s for quite some time in the CR until they finally died...they were 600 ohm.  Don't know if they are still available.

Sidebar - most of the time the standard Rolls things we have seem to work OK (except for the drummers), but like last night, we had a 22 voice choir, almost all of whom were donning earbuds (sharing leads from the mono mix) - distribution was somewhat daunting to say the least.

[sodderboy]

If you are doing over 10 people once a month or more, then you should have a distributed cue system.  If they are performing their own pieces, then they are under direction and blend themselves, but if they are performing along to a track, then cans are important, and you want to be professional.

Here are two circuits:
The Simons circuit is quite simple with the input connected to the CW side of a 1K audio taper pot through a series 330 ohm resistor.  CCW is common and the wiper feeds the cans.  They use a 2W resistor and 3W pot.

A classic studio in NYC used the following circuit in their custom boxes:
Input feeds the CW side of a 5K linear pot, CCW to common, wiper feeds the cans through a series 120 ohm 2W resistor, and there is a 620 ohm 2W resistor between each side of the cans and common.  That circuit provides a load even with nothing plugged, but the pot takes all the juice when turned down.  That is why I keep all pots high or full CW.

You can up the wattage ratings of everything, although it is tougher to find higher rated dual pots, so perhaps you go like the ProCo box where there are two mono pots feeding two outputs.  There is a switch for stereo or mono.

Cost it out for parts, and you might do better making some customs.  The big hit is the headphones anyway.  Also look at the Furman or other simple self-powered headphone boxes.  That 4CH Rolls costs the same as a ProCo passive I think, and a Simons is $300 for 4 channels.

What I advise studios if they HAVE to have a distributed system is to buy 1 or 2 Simons for the bulk of the sessions, and a bunch of Redco or ProCo for the large gigs.  Keep all volume pots UP!

Mike

[Geoff Doane]

Sorry I'm late to the party on this one. I had trouble finding the photos.

Here's what we built.  We had various L-pad or pot versions over the years, but they tended to wear out or burn up faster than we liked.  They're fed from Bryston 2Bs, so about 50 W/ch, and will work for high or low impedance headphones (the 680Ω resistor on the output of the switch takes care of that).



For legacy reasons, we used 7-pin distribution for headphones in the studios, but I wouldn't expect anybody to copy that nowadays.  The unattenuated signal from the power amp loops through on the input pair of XLRs, goes to the switch and resistors, and then to the output XLR.  The 1/4" jack was added later.

The resistor values were calculated to give a roughly 3 dB per step attenuation.

GTD

[compasspnt]

What were those big transformers that Peavey used to make?

[Geoff Doane]

compasspnt wrote on Sun, 05 December 2010 09:41

What were those big transformers that Peavey used to make?

Peavey Automatch?

Basically an autotransformer (only one winding, no isolation), with taps for 2, 4, and 8 ohms (perhaps more).  Not sure what their application would be for headphone cue systems.

GTD