Equal split of a stereo audio signal 8 ways

[phill.cann]

Hello. I have a small project that I am working on. I play in a band and we are trying to find a suitable way of rehersing with making as little noise as possible in our neighbourhood. As such, I have come up with the following idea.

All electronic instruments line out to our mixer, mixer line out to a power amp, power amp headphone / line out to a signal splitter (8 ways) which has 8 1/4" stereo headphone jacks in it.

I have built a box which takes a stereo input (two phono plugs) and just equally splits this input to 8 outputs (1/4" headphone jacks). The problem i am having is now the more headphones / devices plugged into the splitter box, the more I need to increase the output from the power amp so that the headphones signal is loud enough to be heard.

Now I understand why this is happening. The signal is being split, as such, there is a proportionate loss in signal strength for each headphone device attached to the box. What i'd like to do is incorporate somekind of circuitry in my signal splitter box that matches EACH output to the INPUT signal.

Apologies but I am not sure exactly the terminology that I should be using to describe this any better but I can certainly try to answer any questions if someone might be able to suggest some possible options open to me?

Thanks very much.
Phill.

 

[Nigel Goodwin]

Don't feed the splitter at line level - feed it from a speaker output - and feed each headphone socket via a resistor.

The resistor values depend on the impedance of the headphones, and how large the feeding power amp is.

Each headphone is fed from it's own resistor, so adding extra ones doesn't affect the volume of the others.

 

[MikeMl]

What impedance are the headphones you plugging in? 8Ω is typical for stereophile type headphones. After paralleling 8 of them, you are down to ~1Ω. If the power amp is designed for 4Ω speakers, this should work, even though it is driving only 1Ω. Since the amplifier will be driving only the headphones, the level will be low, and the amplifier won't care about the low impedance.

A better solution is to put an ~22Ω 1/2W resistor between the common amplifier feed point, and each set of headphones. You will have to crank the amplifier just a little higher to overcome the loss in the resistors, but the total impedance seen by the amplifier will (22+8)/8 = 3.75Ω, which the amplifier will happily drive.

 

[MikeMl]

deleted deleted

 

[phill.cann]

Thanks Mike and Nigel for your prompt replies. Bit more info for you...

We haven't as yet sourced the complete set of headphones we'll be using. The pair that I have (SONY MDR-XD200) which looking at the sony site reveals the headphones Impedance value is 70.0 Ohms. That's taken straight from the website. I'm not sure if this would be the appropriate way of measuring this but taking an ohm-meter (right name?) and measuring the resistance across the ground and signal lines reveals a value of approx 70.0 (67.5 to be exact). So i'm assuming this is correct. The rest of the headphones when acquired will be of similar specification.

I have checked the rating on the speaker output(s) on the back of the amp. Front speakers 8-16 ohms, Rear speakers 8-16 ohms and centre speaker 8-16ohms. I'm not sure on the headphone / line out rating as i have no manual and the poweramp is pretty old so tough to find detail online (Marantz 74pm700/01b). Is there a way that I can check this myself with my handy little ohm-meter?
Cranking the amplifier up shouldn't be a problem as I have still got a huge amount of line input gain and volume to play with, however I didn't want to do that "too" much as it also generates a lot of line hiss... which I can live with if there's no solution around that.

With the scenario of using the speaker outputs rather than headphone / line out, the power amp (i believe) is a 700 watt amplifier. But as I mentioned above, no manual, little detail online and not much info on / around / under the case of the amp. So i'm taking this value from the model number as a guesstimate. It seems reasonable to me cause its a hellishly loud amp when cranked !

Questions (please forgive ignorance) :
- With the inclusion of a resistor in the headphone(s) feed from the splitter box, would this be in the signal line? and if so, would that be for both left and right channels for EACH headphone output.. so 2 x 8 = 16 resistors?
- Likewise, if I was to include a resistor between the common amplifier feed point and the signal splitter, would this just be including said resistors in the signal path of both left and right channels? (2 resistors)

Thanks again for your advice.

 

[Nigel Goodwin]

You need 16 resistors for stereo - but for a start try just one resistor feeding just one side of a pair of headphones, and see how loud it is, and where the volume control is set. Adjust the volume until you get a decent level at about quarter to half way on the volume control. Then buy 16 resistors of that value.

 

[MikeMl]

70Ω is not what I expected. Audiophile headphones are usually 4, 8 or 16Ω. Earbuds and light duty headsets that come with MP3 and IPods are usually like 35 to 70Ω. Aircraft headsets are like 500Ω.

If you parallel 8ea 70Ω headsets, you have about 9Ω, which is within the range the amp will like, so you don't need the resistors. Just set the master amp level to get sufficient loudness. If some of the listeners are hard of hearing (not uncommon amongst musicians and pilots, ask me how I know), you may want to get 8 stereo L-Pads (fancy ~ 10Ω pots) so that you can custom set the levels for the musician that haven't lost their hearing yet.

 

[Nigel Goodwin]

70Ω is not what I expected. Audiophile headphones are usually 4, 8 or 16Ω. Earbuds and light duty headsets that come with MP3 and IPods are usually like 35 to 70Ω. Aircraft headsets are like 500Ω.

If you parallel 8ea 70Ω headsets, you have about 9Ω, which is within the range the amp will like, so you don't need the resistors. Just set the master amp level to get sufficient loudness. If some of the listeners are hard of hearing (not uncommon amongst musicians and pilots, ask me how I know), you may want to get 8 stereo L-Pads (fancy ~ 10Ω pots) so that you can custom set the levels for the musician that haven't lost their hearing yet.

I suspect you've missed that he's using a 700W amplifier - personally I'd MUCH prefer to have resistors - otherwise no headphones, and no ears

 

[phill.cann]

Hey guys. Thanks for the information. Sorry about the delay in replying, I've been away for the weekend.
Well, as it turns out, all the guys in the band have decided to get completely different headphones, ranging from 8ohm to 70ohm !! Its nice to know the sound advice I gave them was duely ignored...

I have however now taken the output from the actual speaker outputs from the back of the Power amp and we had a play around the other day and managed to get a reasonably workable scenario so unless i'm actually doing any physical damage to the power amp by using it like this, I think we might be ok. Do you think that this is likely / possible ?

A couple of the lads were saying that they were hearing different mixes of instruments on different headphones though. I'm fairly certain this will be because they are all using different headphones with different headphone speakers and they are rated differently. I suspect that putting the resistors into the signal line on the splitter for each of the headphone sockets *should* help with that though would you agree?

Again, thanks very much your thoughts on this. It's already helped us reduce the overall volume emanating from our garage substantially. I just hope the neighbours decide not to complain to the council again !! Next letter we get will be an abatement order !!! Doh !!!

Thanks.
Phill.

 

[Nigel Goodwin]

I wouldn't consider NOT using resistors, as you're likely to blow the amp, the headphones, and your EARS.

Presumably you have a drummer in your band?, you just know a drummer is going to do something completely stupid!

 

[phill.cann]

Hey Nigel.

We do indeed have a drummer and I have already had to intervene on a couple of occasions to prevent him blowing up everything in our garage. Why is it always drummers that cause the most trouble??

Anyway, thanks for all your input so far and really I suppose there is just the last stage of this that I would like to get some advice on. Now that its coming to the point where the resistors should be included in the circuit, I am assuming that implementing Ohms law is going to be required to do this. Remembering back to my (long ago) days of school, V = I*R is about right isn't it?
That being the case, I am a little unsure exactly how I would go about gathering all of the currently *unknowns* that I will need to work out the resistor values.

- I know that the power amp will output anything between 0 - 700 watts
- I know that the power amp's output impedance is 8 - 16 ohms
- I know how many headphones I've currently got (6) and how to acquire the impedance ratings for each of these (not as yet done), but am not totally sure of the implications as the headphones are wired in Parallel. will each set of headphones require different rated resistors based on its impedance value? or can I rely on using the same resistor for each set of headphones if I put a resistor in the signal line to the splitter box?
- I am not sure quite how to measure the strength of the signal that is output from the power amp as my understanding is that it changes as the waveform changes? is this even relevant?

I appreciate that I am asking quite a lot of you and if there are some basics that I should really read before trying anything like this, I would be grateful if you could point me in the direction of said material. I'm just trying to get this up and running a.s.a.p as we have a competition on the 23rd of this month which we still need to practise for and currently this little headphone box is what's going to allow us to practise without being taken to court by the E.P.A.

I thought that I would also include a little diagram of what i've put together so far (forgive the sketchy nature of it, only got Paint to use in work) to give a bit of a better idea of what i've got so far and about where I think that my resistors would go.

**broken link removed**


The Ohm values i've put on the images are examples to indicate the diversity of headphones connected at any one time.
I think the resistors will go in the signal lines of the L and R inputs (2 resistors) before the copper board and in the L and R signal lines of EACH output (2 x 8 resistors)

If I've misunderstood anything so far, please let me know.

Thanks again for all your help.
Phill.

 

[Hero999]

What about using the same value resistor for each earphone, say 470R and have a 5k pot. for each volume control, that way they can all adjust the volume to what they're comfortable with?

I think having different sockets for different earphones is just going to confuse people.

 

[Nigel Goodwin]

Like I said a long time back, stick a resistor in series with one side of a pair of headphones (or two to drive sides both if you like), and see how loud it is. Try something like a 1K for a start, try the different types of headsets, and see what each one sounds like. Once you've found the optimum level, then you can make the box up using all resistors of that value, or even use different values on each side (so a side of the box for quiet, and a side for loud).

 

[phill.cann]

Thanks very much. I shall give it a go at the weekend when I can get down to Maplins.
I will post up the results of what I do and what happens.

Cheers again.
Phill.

 

[Hero999]

I was just about to say don't go to Maplin they're more expensive, until I found that the cheapest dual pot. I could find is at Maplin!

**broken link removed**

I'd recommend the 10k log pot. JM83E you'll need 8, one for each headphone.

 

[Nigel Goodwin]

I was just about to say don't go to Maplin they're more expensive, until I found that the cheapest dual pot. I could find is at Maplin!

**broken link removed**

I'd recommend the 10k log pot. JM83E you'll need 8, one for each headphone.

I'm doubful those would be any good, for independent volume controls I would suggest wirewound pots about 100 ohms, connected as a potentiometer between the resistor and chassis, with the headphones tapped off the slider.

 

[Hero999]

Why would you say that?

Don't tell me it's because carbon is noisy?

It shouldn't make any difference because the signal is so strong, it'll drown out any noise. I've used equipment with carbon pots before and they're not that bad.

 

[Mishael]

Monitor Matrix

i dont know if this has been said but they do make headphone matricies. i dont know about now, but a few years ago i was looking through a sound magazine and i saw a purpose-built audio-monitor matrix like the **broken link removed**. now i know that the mackie is only 6 channels but they probably make larger systems. these are nice too because you can tweak how much of each input goes to each output.

I hope this helps!