Determining desk connections

[Sookie]

Hi all,

Welcome to my first post.

An old desk that has been modified a lot has me puzzled about what is actually soldered internally on the connectors and how.
It's not an easy task to get inside because stuff is in a wired studio unfortunately.
The schematic said some things which already turned out to be different.
Since I don't know if I maybe overlook some of the amplifier design possibilities, and therefore possibly shorting and damaging IC's, I'd like to ask you.

I would like to know if there are ways to check for different types of connections on the inputs and outputs.
Balanced, unbalanced, electronically/transformer balanced, maybe Pin3 hot.
Without opening it up, can I determine it somehow with a MM and testtone?
I know the TRS an XLR pinout rules, just have knowledge holes about amplifier design and possibly damaging things.

eg. Hypothesis transformer balanced:
I should measure continuity between pin 2 and 3. And probably between all transformer in's a certain equal resistance, and between all tx balanced out's another equal resistance for all. (could have been wired with mixed tx's but maybe this helps determining).

Thanks

[Nick Sevilla]

"An old desk" = ???

Maybe letting us know what brand / model might help...

[Sookie]

But I'm trying to find a way to do that on all desks. Determining actual soldering without opening a unit up and just trusting a manual which prooved wrong on some connections already. Broadcast Chilton QM3

[Nick Sevilla]

But I'm trying to find a way to do that on all desks. Determining actual soldering without opening a unit up and just trusting a manual which prooved wrong on some connections already. Broadcast Chilton QM3

Hi Sookie,

Based on your original post, and this latest answer, I am going to venture and say that you have some E.E. Knowledge, but only a little experience in the field. Is this correct?

Usually, the best way to figure out how to connect an audio console is according to mfr. Specifications. Especially really old consoles, as back then standards differed mor than today. Knowing this, I can answer your question, as to being able to connect up any console, is that there is not standard way, but you can gain experience and more knowledge in the field and through reading manuals.

As to being able to determine if a console's inputs are balanced, unbalanced, transformer coupled, etc, again the easier way is to read the schematics preferably while looking at the console in question. An ohm meter will only tell you so much.

As you found out, your console has probably been modified, so even knowing the schematic does not guarantee accuracy. And no one can "know" without looking
unles it is a newer model console without any modifications.

Cheers

[Sookie]

That is correct.

How about I post some hypothectical ways to test like the one I started with and you tell if it is the correct way to determine a specific connection?

[The Scum]

This isn't a trivial task. There are just too many circuits that get used, with many variations of each. For any simple test, I can probably come up with a circuit that disguises it's identity...in your proposed continuity between 2 and 3 test, what would you see if the transformer secondary were also cap-coupled?

With a 2 channel oscilloscope, and good test signal generator, you can learn some things. You can probe pins 2 and 3, and if you see the same signal, but inverted at one, you've probably got a balanced output. But you might not be able to tell if it's a transformer or solid-state stage. With a transformer, grounding one side should cause the signal on the other leg to double - but one of the most common opamp circuits also does this!

For scope polarity tests, I like to use narrow pulse waves or sawtooths, because you can see polarity easily, where triangles, squares and sines aren't so obvious...you could mistake phase shift from an AC coupling stage for a polarity flip.

Squares can be a good, quick judge of frequency response...they get pointy or rounded at the corners for HF changes, and the plateaus get sloped for LF changes.

You may or may not be able to infer whether it's pin 2 or 3 hot...if you've got an unbalanced insert point in between, you can possibly judge relative polarity there.

One way to discern might be to turn the DUT off, and measure inductance between pins 2 and 3. If it's modestly high, it's probably a transformer. If it's got inductance, but not a lot, it might have a common mode choke.

If it's impedance balanced, or an insert point on a TRS, it might be even harder to judge.

I'd be careful measuring continuity between pins with the device turned on, because most Ohmmeters put a small DC voltage between the probes...it's kinda rough to learn that a circuit is DC coupled by smoking a speaker. There are also some folks who get nervous about applying that DC to a transformer, because it could possibly magnetize the transformer core (a theoretical possibility, but practically rather remote.).

If you need a reference for audio circuit design, the new Doug Self Small Signal Audio Design is really good. The relevant chapters of the Handbook for Sound Engineers might also be helpful.

[Nick Sevilla]

That is correct.

How about I post some hypothectical ways to test like the one I started with and you tell if it is the correct way to determine a specific connection?

How about I go back to mixing... And you keep hypothesizing on how things work?

Cheers

[Sookie]

That is correct.

How about I post some hypothectical ways to test like the one I started with and you tell if it is the correct way to determine a specific connection?

How about I go back to mixing... And you keep hypothesizing on how things work?

Cheers

How about I offer my apologies for offending you with my crude english. It was not my intention to hurt you.
You don't have to help people if you have better things to do of course!

[Sookie]

This isn't a trivial task. There are just too many circuits that get used, with many variations of each. For any simple test, I can probably come up with a circuit that disguises it's identity...in your proposed continuity between 2 and 3 test, what would you see if the transformer secondary were also cap-coupled?

With a 2 channel oscilloscope, and good test signal generator, you can learn some things. You can probe pins 2 and 3, and if you see the same signal, but inverted at one, you've probably got a balanced output. But you might not be able to tell if it's a transformer or solid-state stage. With a transformer, grounding one side should cause the signal on the other leg to double - but one of the most common opamp circuits also does this!

For scope polarity tests, I like to use narrow pulse waves or sawtooths, because you can see polarity easily, where triangles, squares and sines aren't so obvious...you could mistake phase shift from an AC coupling stage for a polarity flip.

Squares can be a good, quick judge of frequency response...they get pointy or rounded at the corners for HF changes, and the plateaus get sloped for LF changes.

You may or may not be able to infer whether it's pin 2 or 3 hot...if you've got an unbalanced insert point in between, you can possibly judge relative polarity there.

One way to discern might be to turn the DUT off, and measure inductance between pins 2 and 3. If it's modestly high, it's probably a transformer. If it's got inductance, but not a lot, it might have a common mode choke.

If it's impedance balanced, or an insert point on a TRS, it might be even harder to judge.

I'd be careful measuring continuity between pins with the device turned on, because most Ohmmeters put a small DC voltage between the probes...it's kinda rough to learn that a circuit is DC coupled by smoking a speaker. There are also some folks who get nervous about applying that DC to a transformer, because it could possibly magnetize the transformer core (a theoretical possibility, but practically rather remote.).

If you need a reference for audio circuit design, the new Doug Self Small Signal Audio Design is really good. The relevant chapters of the Handbook for Sound Engineers might also be helpful.

Thanks Scum for the clear answer!
I have to chew some more on this.