EE question: TRS from insert jack to balanced input - why does it work?

[JeffLearman]

Someone said that you can use a TRS cable from a mixer's insert jack to an interface's input, and that'll send the signal back to the mixer. I said no, the typical way is to use TS cable and plug it halfway in (which also works for TRS cable, R conductor isn't used) or else use an adaptor that shorts T and R at the mixer end. I explained what would happen trying to use TRS cable to an unbalanced input. I said I wasn't sure what would happen for a balanced input, but that it sure didn't seem right.

 

He replied back by stating what gear he used Tascam US800) and which inputs worked (the balanced ones).

 

Can anyone explain how this would work? I don't know what the circuit is for a balanced input, but I would have expected a high impedance between T and R contacts -- between "hot" and "cold" inputs (or + and - inputs, if you prefer, which I do.)

 

Take this circuit, for example:

I'd expect a high impedance between the two input pins on an op amp! So how could his setup possibly have worked?

 

I found some helpful stuff at http://www.douglas-self.com/ampins/balanced/balanced.htm which shows a few schematics that might send a (much attenuated) signal back along the path. For example, the variable gain balanced input in Fig #11 might send a signal back, and the strength of that signal would vary as you adjusted the input gain on the recorder (!) The "superbal" configuration in Fig #12 also might send back an attenuated signal. Might that be what's going on?

 

He said he used a Tascam US800; the front (unbalanced) inputs didn't work, but the back (balanced) inputs did.

 

Thanks!

 

[MikeRivers]

Someone said that you can use a TRS cable from a mixer's insert jack to an interface's input, and that'll send the signal back to the mixer. I said no, the typical way is to use TS cable and plug it halfway in (which also works for TRS cable, R conductor isn't used) or else use an adaptor that shorts T and R at the mixer end. I explained what would happen trying to use TRS cable to an unbalanced input. I said I wasn't sure what would happen for a balanced input, but that it sure didn't seem right.

 

He replied back by stating what gear he used Tascam US800) and which inputs worked (the balanced ones).

 

Can anyone explain how this would work? I don't know what the circuit is for a balanced input, but I would have expected a high impedance between T and R contacts

 

It's certainly not conventional, but it could work, sort of, depending on what his definition of "work" is, and the configuration of the interface's balanced input. Just taking a random sample from what's laying on the bench, the DC resistance between pins 2 and 3 on a mic input of a Mackie Onyx mixer is about 14 kΩ.

 

In the diagrams that you posted, you have those two 10 kΩ resistors going between pins 1-2 and 1-3, which puts them in series (20 kΩ) between pins 2-3. What's actually going to be seen between pins 2-3 will be somewhat lower since it has the input of the differential amplifier in parallel with it. So the concept of "it works" isn't preposterous.

 

So let's say that the Mackie on the bench is typical of a balanced input on the interface, and it's not a transformer input, so it'll see a signal between tip and sleeve (the mixer's insert send). But that will go through the 14 kΩ path between tip and ring of the balanced input, and back to the mixer's insert return. You'd probably have 6 to 10 dB of loss due to the voltage divider formed by the interface's impedance between tip and right and the input impedance of the mixer's insert return, but it might be workable.

 

The balanced line inputs of the Mackie are capacitor-coupled so they won't measure DC resistance. I could take this further and measure it with AC if you tell me where to send the consulting bill.

 

By the way, the Cymatic LR-16 recorder/interface gizmo is designed to plug into a mixer's insert send/return with a TRS cable. It has a 1 kΩ resistor between the tip and ring to send the mixer's insert output back to the insert input with minimal loss, and the signal is picked off (unbalanced) from the mixer's output (the tip) to go to the interface/recorrder guts.

 

Hey, if he says it works, it works. Ask him if he knows how well it works. It's probably find for getting the mixer's signal to the interface, and what gets turned around at the interface and goes back to the mixer is probably a little weaker than if you pulled the plug out of the Insert jack and used the normal signal path through the mixer.

[onelife]

Someone said that you can use a TRS cable from a mixer's insert jack to an interface's input, and that'll send the signal back to the mixer.

 

What I do is modify a TRS connector so that the tip and ring (send and return) are connected together. I can then plug it all the way into the Insert jack without disrupting the signal. I get an UN-balanced signal that I can route to another place (such as the input on a recording interface). Perhaps this is what someone was talking about.

 

[JeffLearman]

Thanks, Mike! I was hoping you'd chime in. After all, it's definitely a "half normal" issue. What you say makes perfect sense. You can send me the bill over the internetz.

 

Onelife, I have a couple RCA snakes left over from the bad ol' days of analog, and I use a TRS-RCA adaptor, which does the same thing. There are plenty solutions, but I didn't think that using a TRS-TRS cable was one of them. As it turns out, it is, for some values of "solution".