Questions: Balanced & Unbalanced, Opamps & Transform

I'm not quite sure how to go about asking this, so please bear with me if I don't make much sense, as there are some large holes in my terribly small amount of electronics knowledge...This might be the spring board of many more questions

This is something I've conceptually understood/took for face value for a long time but never knew the literal technical details of what goes on under the hood.

In a balanced line driver, the polarity of the balanced input is maintained all the way to the output. If a transformer is used to balance an unbalanced signal, is the following what happens...
Signal into one end of TX primary, the other end is grounded, then at both ends of the secondary you get "two" signals that are out of phase with eachother?

Thanks a bunch!
-Ethan

> Signal into one end of TX primary, the other end is grounded, then at both ends of the secondary you get "two" signals that are out of phase with eachother?

Standard (in 1935) way to drive push-pull grids, as in the third transformer here:

Great, I was hoping you'd answer PRR!

Now for Part 2 of my question...

So, I could easily replace a balanced input summing amp section with a "Direct Box" kind of TX used in reverse, correct?

If so, where do I get the numbers to determine needed resistance before and after the transformer. What I mean is, I understand Ohm's law, but I'm a little lost with the idea of the transformer being there. Is the input impedence the figure I work with?

I know that might sound all over the place, did that question make any sense?

Part 3...
I've seen circuits that have a summing amp to accept a balanced input, yet they still have an input transformer there...WHY BOTH? Why not just the input TX? Or even, dare I say, just the opamps?

Thanks!
-Ethan

[quote author="Admin"]...WHY BOTH? Why not just the input TX? Or even, dare I say, just the opamps?[/quote]

historical

Some people do advocate having a floating diff input and a strong low imp output.
see the JLMx project

If you are in complete control you can go unbal and use opamps. Some mastering houses are like that.

I've seen circuits that have a summing amp to accept a balanced input, yet they still have an input transformer there...WHY BOTH? Why not just the input TX? Or even, dare I say, just the opamps?

Click to expand...

No active circuit will touch a transformer for CMR over long runs. Some "purist" oems (GML, MILLENNIA for example) prefer to obviate input transformers in favour of solid-state solutions as they may feel transformers exhibit phase-shift / leakage inductance / core saturation / insert artifact here, but for a long run (particularly in the PA world) you will not beat the rejection of having a transformer at each end.

One thing to remember if you have a single primary-to-single secondary transformer is that the transformer isn't fussy about what it sees - you can go unbal in - unbal out, leave ground floating and go unbal in - bal out or vice-versa.

Some designs use transformers purely for isolation, a good example would be an interstage transformer in certain vintage designs.

I've even seen input transformers utilised as make-shift mains transformers - i.e. say it's a 1-10 ratio, you could go 220v in - 22v out; providing it can handle the current you should get away with it although I can't imagine it being recommended practice (particularly if you consider the cost of i/p transformers).

Transformers can also be used for blocking DC, this goes both ways - blocking DC in, and also absorbing DC / "loading" an output-stage. I could even think of a design I've seen where you can tap the output from either the primary or secondary of the transformer, as long as the transformer is there to load the o/p-stage it's ok.

(Have been to pub this evening, hope I'm making sense :wink: )

Cheers,
Justin

I'm an electronics novice, but as an AE, one thing I fail to see mentioned is the benefit of a transformer for pure sonics. I've found recently that it's the TX sound I prefer, usually in a class A discrete design, to that of tubes or transistors/opamps. I'm building a rack of tranny i/o stages with neve amps in the middle, just so I can impart that sound on whatever signal I want.

If you work at a job, as I do, where you need to connect together pieces of equipment that are on different floors of a building, or in different buildings altogether, some of the advantages of transformers become crystal-clear, among them galvanic isolation, RFI rejection and CMRR.

oh yes !!

Dave is right.
This is why I have many 1:1 problem solvers in my tool box.