Hi,

I’ve been struggling with this for a while now, and I thought some of you could probably help me.
Most of you will probably think that I’m waisting my time with this project and that I’d better go buy an equivalent system that’s available on the market. However, I’m pretty sure that this thing will work, and even though there are a few limitations to it, I guess it could function well if a few rules are taken into account.
Feel free to participate, as any comments will help me optimize this thing.
Here’s the idea:
Much like the Furman system, I would like to build myself a 4 channel headphone mixer distribution system but PASSIVE!
Each mixer would be driven by two identical stereo power amplifier (4 channels) attenuated by four individual pots and summed (mono) before the headphones. I would like to make four small mixers capable of handling two headphones each.
In other words, imagine that the headphones are treated just as speakers, which they are ! and so makes me think that this project is worth giving a try.
Here’s a diagram:

Console
aux sends 1 & 2 3 & 4
l l l l
stereo amp 1 stereo amp 2
l l l l
l l l l
_______________________
l BUFFER/DIVIDER x4 l
l_______________________l
llll llll llll llll
llll llll llll llll
box1 box2 box3 box4


Because I would like to make four mixers, I was thinking of building a divider/buffer box into a rack enclosure with a resistor network for dividing each signal into four. Now here comes the “math” and the questions that come with it. If my thinking is right, the goal here is to NOT fall below the load impedance rated by the power amp.
Also, the input impedance of the mixers will inevitably vary when one manipulates the attenuator pots on one of the mixers. Therefore I’m thinking each little mixer is going to also need what I call a “buffer”.
I’m not sure yet how efficient a "buffer" can be, and if i can totally prevent the amplifier from "seeing" the headphone impedance load at the very end of the chain by using resistors at the input of each little mixer. of course the answer to this will be crucial as it will determine if this project is a good idea afterall ooohhhoooooo and this is why I need your help! feel free to criticize or clear up my thoughts.
I talked to some people about this project, and they reacted very differently when i told them about this idea, some of them told me i was waisting my time and better be buying a proper headphone thing like the furman stuff, others thought it was a good idea, others barely even reacted.
I also thought of starting an advantage/disadvantage comparison between the standard “powered" headphone mixer and this passive 4 channels headphone mixer project.
I came up with this:

disadvantages:
- having to use the same type of headphones/impedance-wise to minimise volume shifts between the headphones within the same mixer.
- the cost of the power amps.
- the fact that one might have his average mix affected when another person manipulates the knobs of another box. actually this is a question ? help !
- Spending some time “tuning” the system (resistor values, rating, power amp type, volume position etc…)

advantages:
-the satisfaction of making such a project.
-passive so no powering required
-less cables on the studio floor
-choosing the aspect/aesthetic of the boxes which i think is cool.
-lots of headroom as i'm sure the power amps won't be fully cranked
-lots of gain available if needed (i found the furman stuff very weak in terms of gain when i experienced them as a drummer the first time)
-the ability to choose a single strong/solid multipin connector to feed the mixer compared to the very fragile shitty ethernet/telephone-style RJ45 connector used in the furman stations. (mabe 8 pin speakon or tuchel?)

i'm sure that this exact project could function very well by using beautiful transformers instead of using resistors, but in this case they would have to be custom-built and it would cost a fortune !
on the other hand, i was reading some stuff about wire wound potentiometers, more precisely "rheostat with fixed impedance", some 8ohms version are available but they're BIG!
anyways, i'm going to start experimenting soon with just a cheap hifi 60W stereo amplifier (2channels mixer) to see how well it works.
in the meantime, what do you think ??
thanks for sharing your opinion,
J-

oops the "diagram" didn't come up well, but i'm sure you comprehend it

Since you are using a modrn stereo amp for the drive, you should have no problem with either the load the amp sees or with varying impedances caused by changing the pot settings. Make your system dissipate power into a load that 1/10 of the impedance of the 4 sets (at max) of phones in parallel suchthat you get enough voltage (100mw) this is then fed by a divider/step down resistor that keeps the amp seeing a happy load ... which will always be about the same because it is dominated by the step down resistor, not the "load" of your headphones.

Then drive your phones from this stepped-down voltage with no varying loads or sources.

Without doing any math I can say it would work fine.

thanks !
do you think 2watts resistors would do the job for the "divider"?
if i understant right, this means i don't have to use the big 8ohms speaker attenuattors cos the "divider" resistors is already doing the job ?
what type of pots should i use? 50Kohm linear dual pots ? are you sure they won't fry ?
thank you

Kindly do the math. I don't know if you'll end up with an eight ohm load but your power amps will be stable from probably 2-ohm up to 1k ohm or more. So you'll be fine.

Make the high resistor in your step down divider such that you have enough voltage to drive 100mW across each pot. Then remember you'll have all 4 pots in parallel, driven by one step down resistor.

Figure your step down resistor value by P=v^2/R where R is the total resistance of the step-down pot network and the V is what you want to run the amp at ... say 1/2 power or more to keep the noise down.

You may decide to put blocking caps in the outputs since your power amps may both idle with some DC going through the output load, and since at turn on, turn off, and unplugging/replugging hot ... there may be bigger voltages that you want to drop into your 'phones.

Actually, what I said above is good for getting the levels right at the headphones and the power amp output, but I haven't really considered the mixing aspect of what you're doing and I don't know what the circuit would be.