Combining outputs

sserendipity

Well-Known Member
Joined
Jul 3, 2001
Hi,

I need to combine outputs to a single input, and I've read in more than one place that a using a y-cable is a really bad idea, since you are feeding the two outputs into each other.

What's the solution? A mixer would be to cumbersome, and anything powered is going to add noise.

Suggestions?

Thanks,

Jonathan El-Bizri
 

Ethan Winer

Well-Known Member
Joined
Mar 19, 2001
Location
New Milford, CT USA
Jonathan,

All you need is a couple of resistors! The following assumes the sources are low impedance line level outputs, and after combining the cable doesn't need to run more than ten feet or so.

If the sources are unbalanced, you need one 10k resistor per output. If they're balanced you need two per. Simply connect the resistors in series with each output, and then connect all the resistors together to make the new mixed output.

Let me know if you need more clarification.

--Ethan
 

sserendipity

Well-Known Member
Joined
Jul 3, 2001
Thanks Ethan!

I'll try this out.

Help me out with the theory behind this.

By placing the resistor in line, the ampage of signal is reduced so is no longer strong enough to cause any damage to the other amp output, that it is feeding into - especially since it has to travel through another resistor to do so.

Would this cause a change in the tone of the signal - ala impedence loading a guitar amp?


Recording Engineer - thanks - the boxes look nice - but cost and size of the unit is prohibitive. I need about five of these, in the back of modile rack :>


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Ethan Winer

Well-Known Member
Joined
Mar 19, 2001
Location
New Milford, CT USA
Jonathan,

> By placing the resistor in line, the ampage of signal is reduced so is no longer strong enough to cause any damage to the other amp output <

More relevant is that when feeding one output into another, the load impedance is so low that the distortion goes way up and the signal level goes way down. The resistors solve that by isolating the low output impedances from each other. The only downside is the new output impedance - at the mixing junction of the resistors - is now higher. So if the cable from the junction to the next device is too long the capacitance will reduce some of the highs.

> Would this cause a change in the tone of the signal - ala impedence loading a guitar amp? <

Not if the cables are short enough. If you mix two low-impedance sources with 10k resistors, the new output impedance is 5k. Mix three signals and it drops to 3.3k, and so forth. So using up to 10 feet of decent cable should not degrade the signal.

--Ethan
 

sserendipity

Well-Known Member
Joined
Jul 3, 2001
More relevant is that when feeding one output into another, the load impedance is so low that the distortion goes way up and the signal level goes way down.
How does this damage the line output amps?

So far, when combining signals, I've experience no change in the tone at all. However, I've read in two separate places that I was damaging my equipments line output amps by doing this.

The resistors solve that by isolating the low output impedances from each other. The only downside is the new output impedance - at the mixing junction of the resistors - is now higher. So if the cable from the junction to the next device is too long the capacitance will reduce some of the highs.
This probably won't be a problem for me, since I'm simply combining and splitting the outputs from effects units in a rack.

Again, thanks for your advice.

Jon
 
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