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Simple way to combine 2 stereo outputs?

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iXod:

--- Quote from: Terry Bites on December 01, 2021, 12:38:32 pm ---https://www.ranecommercial.com/kb_article.php?article=2106
End of

--- End quote ---
OP here.

Those circuits are for summing channels to drive a mono sub-woofer.

I read this and see:
• unbalanced circuit to sum 2 stereo channels to 1 mono channel
• balanced circuit to sum 2 mono channels to 1 mono channel

What all of these circuits have that confuses me is a resistor between the channels. To my mind this should not be part of a stereo-stereo summing circuit.

What I don’t see is a circuit to sum 2 stereo inputs (4 unbalanced channels) to 1 stereo output (2 unbalanced channels).

Can I infer that to do so, I can use parts of these circuits: the series 475 \$\Omega\$ resistors and add one shunt 20k \$\Omega\$ resistor between T & S and R & S at the output?

AaronD:

--- Quote from: iXod on December 02, 2021, 03:51:07 am ---What all of these circuits have that confuses me is a resistor between the channels. To my mind this should not be part of a stereo-stereo summing circuit.

--- End quote ---

I don't know why it's there either.  I've never used one.  It's just an extra load in parallel with the destination's input impedance.


--- Quote from: iXod on December 02, 2021, 03:51:07 am ---What I don’t see is a circuit to sum 2 stereo inputs (4 unbalanced channels) to 1 stereo output (2 unbalanced channels).

--- End quote ---

You can do like most "traditional" audio engineers, and count each stereo signal as 2 mono's.  Then you have an independent copy of the circuit shown for each output channel.  (2 in your case)

(I don't like it when mixing consoles are advertised as having 16 channels, for example, when the last 4 of the 12 strips are stereo, but that's just a result of the "traditional" counting method.)


--- Quote from: iXod on December 02, 2021, 03:51:07 am ---Can I infer that to do so, I can use parts of these circuits: the series 475 \$\Omega\$ resistors and add one shunt 20k \$\Omega\$ resistor between T & S and R & S at the output?

--- End quote ---

I'm still not comfortable with using anything less than 1k unless you have some special knowledge of the specific things that are gong to feed it, just to guarantee that full volume doesn't trigger the self protection.
(from what I can infer, you probably will use them at full volume because your practical volume control comes later in the chain)

From the text surrounding Figure 1:

--- Quote ---The input impedance is really quite low and requires 600 ohm line-driving capability from the crossover, but this should not create problems for modern active crossover units.

--- End quote ---
I'd even disagree about "not creating problems for a modern thingy".  If you had a pro unit that is designed to drive a 100-ft snake, then yes, it would do that just fine.  But I've also seen quite a few that were only meant to drive an insert return in the same booth, or an amp in the same rack.  Those would not necessarily have a 600-ohm line-driver in them, as that's an explicit consideration beyond just a functional line-out.

And again, I'd leave off the shunt entirely.  I have no idea why it's there.


Essentially, that link is popular to throw around as a big-name-credible example of why you shouldn't just short things together (for people who listen to names more than physics), and for that it's good.  But the actual details and the presentation of caveats could definitely be improved.

iXod:
OP again.

And if you were summing 5 unbalanced stereo inputs to 1 stereo output, would you still recommend a series 475 \$\Omega\$ resistor on each of the 10 input channel?

Thanks.

AaronD:

--- Quote from: iXod on December 02, 2021, 03:44:20 pm ---OP again.

And if you were summing 5 unbalanced stereo inputs to 1 stereo output, would you still recommend a series 475 \$\Omega\$ resistor on each of the 10 input channel?

Thanks.

--- End quote ---

I'd recommend at least 1k for each, regardless of the number of inputs.  475 might be okay for combining long-line drivers, but it's too low for general use.  But I think you've got the arrangement right.

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