Driving multiple headphones

[AussieBruce]

I’ve landed the job of setting up a headphone facility for a temporary recording setup, I need to feed the same mono signal to up to 12 sets of standard stereo headphones (usually 32 ohms). Everyone has to have their own volume control, which needs to have maybe a 20dB range.

Initially it seemed a simple task but a bit of thought has shown that it isn’t. The signal will be mono, so the cans will be paralleled, given a Z of 16 ohms per set. Parallelling 12 sets results in a bus load of 1.3 ohms, which is pretty low for a typical audio source, and adding series resistance complicates the individual volume control.

I did consider running the 2 cans in series, that changes the situation but it’s still a bit gnarly.

The other factor that needs to be considered is aural safety, the max voltage that will be fed to any set of phones needs to be limited to provide auditory discomfort or damage.

All this has to be in place within a week and at low cost, so nice solutions like individual amps (eg. LM386) are not on.

Any ideas??

[bob91343]

Use a stereo amplifier with one channel each driving half the headphones.  That should bring the load impedance into the ballpark.  Of course, drive both amplifiers with the same signal.  Or use two stereo amplifiers.  Or divide the headphones into four groups, series one group with another in one channel, etc.

[Circlotron]

100 ohm pot across the line at each listening position. Put a resistor of maybe several hundred ohms in line between the pot wiper the 'phones. Assuming you are using a single power amplifier of several watts output to drive the whole show, the resistor will drop the max level down to something usable as well as keep the total impedance up to a reasonable figure.

[engineer_in_shorts]

I would make a little unity buffer board with phonos in and out and power in and out. I would probably keep all of it in stereo in case someone changes there mind later... then connect left and right together at source. This is what opamps are good at, convert low impedance to high impedance.

[engineer_in_shorts]

Just seen you have a time limit. Fit all of them on stripboard and sockets and order lots of headphone extension cables. Cost for the whole project about $150, including next day delivery.

[t1d]

I recall that there used to be adapter boxes to accomplish this very thing and that they were not so very expensive. So, I searched "headphone splitter mixer" on Amazon.
https://www.amazon.com/s?k=headphone+splitter+mixer&ref=nb_sb_noss_1
Yep, they are still available. And, I don't think that you can build anything for less money.

As far as the budget goes, it is not your responsibility to make it happen without realistic funds. (I don't like bosses that ask for the impossible.) And, the sorter the time to accomplish the task, the greater the expense. HTH.

[Zero999]

Why are you ruling out the LM386? Just buy 12 pre-built LM386 boards. One for each headphone.

[coromonadalix]

And the lm386 are easy to work on, dont cost a fortune and you can easily adjust their gain  ??    20 db  is like wispering ??

or like theses  =  more costly ?? 

https://www.amazon.com/Headphone-Distribution-Amplifier/s?k=Headphone+Distribution+Amplifier

[NiHaoMike]

Keep in mind that the signal to a pair of headphones is very small, even 0dBm would translate to on the order of 100dBA from the headphones. I would say to get a bunch of inline volume controls for the headphones (so that they'll be on the remote end of the cables, a consideration for better user experience made even more important by the pandemic) then make a simple splitter box with 220 ohm or so resistors. Make it stereo so that it will be futureproof in case requirements change, the extra cost is insignificant.

Actually making sure the amplifier you're planning to use is single ended could be a problem since a lot of modern amplifiers are differential output. Probably the easiest would be to get an amplifier board instead of an enclosed module, then you can look to make sure that one end of the output goes to ground. For maximum flexibility, add binding posts on the outside of the box directly connected to the amplifier output so that it can also be used with speakers.

[Zero999]

Here's an example of an LM386 board. It has a gain of 200 and a lower cut-off of frequency of 160Hz. You can modify it for a lower gain and cut-off, if you want.

https://www.aliexpress.com/item/32582926153.html?spm=a2g0o.search0304.0.0.3bf85aebD90I65&algo_pvid=d8d72ab5-0522-4c6b-8e5b-522ceb8825c4&algo_exp_id=d8d72ab5-0522-4c6b-8e5b-522ceb8825c4-41

[floobydust]

You best have isolation resistors from the power amp, like the 100R plus potentiometer or so in the case there is a short-circuit on one set of headphones and it does not take out the entire network.
ESD is a common occurance at plug in, so add clamp diodes for the amplifier's output.
RF ingress also occurs due to the miles of wire, so an output inductor and Zobel are needed.

I've used an op-amp with couple discrete output transistors, or the TDA2040 family of IC's for driving many headphones in studios.
I would not use the LM386 as it has high distortion and noise for the almost 50 year old IC.

edit: Rod Elliot Sound Headphone Amplifier he has pcb's in Australia maybe

[SiliconWizard]

I would also implement one amp per headphone. Tons of proper headphones amp ICs out there, even with proper protection and all, so no need to reinvent the wheel.
The cost will be marginal compared to the cost of reasonable quality headphone jacks anyway, not even talking about the headphones themselves.

[2N3055]

Just buy 3 pieces of these and feed them same signal:

https://www.thomann.de/intl/behringer_ha400.htm

[John B]

What kind of physical layout are we talking about?

I would just buy an off the shelf headphone amp. I have a 6 channel 1RU unit, that's probably as large as they will typically be designed.

Then you would simply run a 1/4' stereo extension to every person. But then 12 people would have trouble all wanting to adjust their individual levels from the 1 rack unit.

A typical solution for individual headphone foldback/monitoring with individual volume control (that's in easy reach of the user) would be an audio over IP solution like Dante. But I assume that's a little complex for a quick an temporary solution.

If you want an analogue solution, you could try 12 headphone amplifier modules, all connected to a common signal source and probably a common DC power bus as well (probably split rails +/-)

I have no experience with this module, but it's as reasonably priced as you'll get, and they're in Australia.

https://www.jlmaudio.com/shop/amp-head-ii-phone-kit.html?display_tax_prices=1

Looks like a bullet proof design though, each channel powered by 2x2 parallel NE5532 op amps.

Also, forget the notion of making the headphones mono, make it stereo and just feed the signal to both channels. That way you won't be pulling your hair out when someone changes their mind and wants to feed a stereo signal in. 

[John B]

Just buy 3 pieces of these and feed them same signal:

https://www.thomann.de/intl/behringer_ha400.htm

I have one of those, and I find it to have an unacceptable amount of white noise.

[Zero999]

I've used an op-amp with couple discrete output transistors, or the TDA2040 family of IC's for driving many headphones in studios.
I would not use the LM386 as it has high distortion and noise for the almost 50 year old IC.

The TDA2040 is no longer made and the LM386 really isn't that bad, if the output level isn't too high and the gain is set to the minimum of 20. Anyway, the original poster didn't say it needed to be Hi-Fi, just that it needs to be in place within a week and low cost, which he believed precluded the LM386, but isn't the case, as it's the quickest and cheapest way to do this.

The TDA2822 is also obsolete but the Chinese must make copies, because I found this cheap module. It probably doesn't sound great, but it's cheap.
https://www.aliexpress.com/item/33048701082.html

[2N3055]

Just buy 3 pieces of these and feed them same signal:

https://www.thomann.de/intl/behringer_ha400.htm

I have one of those, and I find it to have an unacceptable amount of white noise.

Mine doesn't produce "unacceptable" amount of noise.

He wants it quick, only mono sound, noise and distortion are not critical (people are talking about LM386 here).
It is ready made, very inexpensive solution to his problem that can be bought tomorrow and be done with it.
It's in a sturdy metallic box, it has individual pots and amplifier per headphone output, comes with a power supply.
You just make few cables and off you go...

It's a no brainer really. You would be crazy to spend time and effort to make something.
Waste of time.

If there were some special specifications that had to be met or something then yes. But this way, just buy this.
You can't make anything better or faster for that kind of money.

[SiliconWizard]

Oh yeah. Apparently the OP wants something cheap and wants it fast.
Behringer makes decent gear for cheap. You're probably not going to make anything better, as safe an properly tested within one week. People are giving design ideas because this is what this forum is all about, but here going for something off-the-shelf looks like the better idea indeed.

If you want something better, you can go for a Presonus HP4. Not the same budget, but we have no clue about the OP's budget here, so...

[John B]

I would agree that an off the shelf headphone preamp will do the job, as long as it isn't crucial for each user to have easy reach of the volume. Although don't forget the expense of 1/4' TRS extension leads x12.

[EPAIII]

You can get what are called "distribution amplifiers". They are made for and sold to the broadcast and professional people. Getting one with 12 independent outputs is no problem and generally they will have a very low output impedance so you can just add a pair of series resistors (they will have a balanced output) to achieve an impedance match for your particular head phones. Many of them will have level controls on each of the outputs but this may be in the form of trimmer pots so you may need to extend them to panel mounted ones. A high quality unit along with the power supply can be purchased for well under $1000. Average (for the pros) quality units can be under $200.

Talk to someone like B&H.

https://www.bhphotovideo.com/