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Mixer circuit (Need help on 8 headphone output)

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prwp

New Member
Hi,

I am building a 7 input to 8 output (headphone) mixer circuit. I finish on the design for the mixer circuit. The problem is I want to seperate 1 output to 8 output headphone jack. Can anyone give me some idea on how to do it?

Thank you. The sound is mono
 
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Hero999

Banned
That's not a mixer, it's a splitter which is the other way round.

Use a low power audio amplifier designed to drive a 4Ω speaker such as the LM386.
 

marcbarker

New Member
Hi,

I am building a 7 input to 8 output (headphone) mixer circuit. I finish on the design for the mixer circuit. The problem is I want to seperate 1 output to 8 output headphone jack. Can anyone give me some idea on how to do it?

Thank you. The sound is mono
Is this by chance for something like a Language Studio?

Also the explanation you gave isn't that clear (well to me anyway). "Inputs" and "outputs" are notoriously ambiguous in explanations :)

I know this sounds obvious, but don't forget the reader doesn't know what you know (yet), and the only way available at present is by text!
 

Hero999

Banned
What's the impedance of the headphones?

I assumed 32Ω which is a pretty common standard.

You'll need an amplifier which can drive eight connected in parallel. If their impedance is lower than 32Ω you might need to use two amplifiers or design your own amplifier that can drive a very low impedance.
 

marcbarker

New Member
My 0.02c If it's low volume (i.e. office headphone levels), you don't need to 'match impedances' so much, a 386 will happily drive a fraction of ohm. If it's a recording studio, or for heavy metal listeners or 'break-it consumers', then the total power level will be high. It's the load current that stresses the IC (and causes clicking if there's thermal shutdown present on chip).

My freind's recording studio has a headphone driver box, 8 or 10 sockets I can't remember. The musos plug their cans into it at same time during a big session, called a headphobne loop.
 

Hero999

Banned
My 0.02c If it's low volume (i.e. office headphone levels), you don't need to 'match impedances' so much,
You don't need to match the impedances at all. The output impedance of the amplifier needs to be as low as possible, for a good damping factor.

The amplifier also needs to be able to drive the minimum impedance you plan on connected to it.

a 386 will happily drive a fraction of ohm.
I doubt it, the minimum specified impedance on the datasheet is 4Ω.
 

marcbarker

New Member
The output impedance of the amplifier needs to be as low as possible, for a good damping factor.
Damping factor is well important, if the damping factor is naff, you can "hear the sound of the speaker instead of the music", if you know what I mean.

You'll find the Zout = milliohms. Less than an ohm I'd expect for a 386. I understand Zout is a compromise in commercial HiFi amps, it mustn't get too low for some reason.

The amplifier also needs to be able to drive the minimum impedance you plan on connected to it.

<..> the minimum specified impedance on the datasheet is 4Ω.
A "4 ohm rated" amplifier will drive a lower ohmage, provided the output level is lower than normal. Provided the package power dissipation of the IC is not exceeded, it'll be OK. After all the amplifer is just a power supply with a variable output voltage. In practice you can have a shorted speaker to an amplifier and the amplifier won't blow up until the user turns up the volume high to compensate for the low sound.

Of course using lower than standard load, the IC is not going to perform as standard in all areas, that's the art of engineering isn't it :p

rant mode on: If it was a commercial product, you have to remain inside absmax at all times, for legal reasons, even if one of those absmax ratings is plain absurd. Even if there is no risk whatsoever of stress to the component, no sane design engineer is willing to risk his reputation by programming himself to be a witch-hunt scapegoat, should there ever be a failure totally unrelated to it.
If everybody followed what it says in datasheets to the letter and no one took responsibility for breaking a few rules here and there, there would be no progress and improvements. and we'd probably still have today a man walking in front of every car with a red flag to ensure 'health and safety' is maintained. And if there wasn't acceptable risk managed by a responsible crew, commercial aircraft would be too heavy to get off the ground!! :)
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Damping factor is well important, if the damping factor is naff, you can "hear the sound of the speaker instead of the music", if you know what I mean.
Doesn't matter for headphones, they are designed to be fed from a high impedance source, pretty well every headphone socket on every piece of equipment is simply fed from a resistive attenuator.

Don't confuse speakers with headphones.
 

marcbarker

New Member
Doesn't matter for headphones, they are designed to be fed from a high impedance source, pretty well every headphone socket on every piece of equipment is simply fed from a resistive attenuator.

Don't confuse speakers with headphones.
Good point. Speaker has rather a lot of moving mass needs damping.

It's a historical reason I suspect why amplifier headphones have evolved into become high impedance source and don't require electronic damping. The headphone itself often utilised 8 ohm speakers, the phone socket is wired via a series resistor inside amplifier. Without the resistor, the amplifier blows up the speaker, or causes premature deafness! With headphone of higher resistance means the series resistor can be less important.
 

Hero999

Banned
A resistor isn't required for small amplifiers.

A 32Ω headphone can easily be connected directly to the output of a small amplifier designed to put 250mW RMS into an 8Ω load.
 

prwp

New Member
Thanks for all the reply. How do I design the circuit? I need the circuit before I go and buy the headphone.
 

marcbarker

New Member
Er yes... the topic of the thread, forgot.... :)

I think at the moment the way it's been left, is there's still some outstanding questions need be answered first. I can take a guess but it'd be wrong, do you want to connect 8 headphones to the same output, or is it 1 headphone to 7 inputs, or is it 7 headphones to 8 outputs? Please tell us more...
 

Hero999

Banned
Thanks for all the reply. How do I design the circuit? I need the circuit before I go and buy the headphone.
The circuit is on the datasheet which can be found using Google.
 

scolex

New Member
To marcbarker

do you want to connect 8 headphones to the same output, or is it 1 headphone to 7 inputs, or is it 7 headphones to 8 outputs? Please tell us more..

which is the best, if the circuit will be used in language studio...
and how can I do that... please help.:confused:

im planning to built a language laboratory.. !
 

marcbarker

New Member
which is the best, if the circuit will be used in language studio...
and how can I do that... please help.:confused:

im planning to built a language laboratory.. !
Hiya, at this moment in time I,m here in sunny Gran Canaria (using a different keyboard layout) so please excuse any weird spelling mistakes you see :) I think we have something in common, I teach a bit of english here.

The way I see it: is

1. just measure the ohmage of one of the headphones (32 ohms typical) and divide it by the number of headphoñes, say 32 / 16 = 2Ohms. (don·t forget that 8 headphones have a total of 16 speakers, a L & R for each head) So in my example here, I need an amplifier that is happy driving 2 Ohms.

I will take a gamble that an "higher ohmange output amplifier" will work OK, because intermittent spoken speech at a moderate sound level is nowhere as powerful as music.

2, install these 8? headphone sockets into a box, and wire all the Lefts and the Rights together into a single connection. That is driven by an amplifier.

3· find any audio amplifier, I think I would look for a discarded or unused ´Hifi´amplifier, part of a stereo system. Maybe I would install 8 headphone sockets into the metal case...
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
1. just measure the ohmage of one of the headphones (32 ohms typical) and divide it by the number of headphoñes, say 32 / 16 = 2Ohms. (don·t forget that 8 headphones have a total of 16 speakers, a L & R for each head) So in my example here, I need an amplifier that is happy driving 2 Ohms.
As already discussed way back in this thread, you don't connect the headphones directly to the amplifier - you feed each one via it's own series resistor.

Connecting them direct is likely to blow the headphones, and the users ears!.
 

marcbarker

New Member
What's a good value for the series resistor (with each transducer) Nigel?

Why I ask, is this resistor value can be included in the calculation I'd mentioned in 'step 1'. i.e. if the headphone is 32 Ω and the 'resistor' = 100 Ω, the sums are ((100Ω + 32Ω) * 2 ) / 8 = 33 Ω which will be a doddle to find an amplifer. Make sense?
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
The larger the resistor value, the more sets of headphones you can use, but you also need a higher power amplifier.

100 ohm sounds a reasonable starting point if you're using a small amp.
 
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