transformer isolated outputs

[burningman]

I have seen transformer isolated outputs for things like tuners and splitters. I was wondering why this is the case and what kinds of applications are they useful for?Thanks. Ian

[petemoore]

  I saw a 1:1 audio output transformer at RS yesterday, but didn't get one because...
  I read they're expensive [good ones]...but thinking goodness may be badness when designing say a dirtbox...can't see any reason not to try one really though.

[R.G.]

We live immersed in an ocean of electromagnetic interference. A big one of these is the pervasive AC power line field. Isolation transformers help prevent hum and noise from the AC power line field by breaking the conductive path from one unit to the other.

[burningman]

Where would placing a transformer make sense (an fx box, switcher audio loops, tuner, buffer, splitter). I noticed that the Skrydstrup MR9 has a iso. transformer output for tuner. I'm not quite sure why this is necessary if the guitar signal is leaving the switcher and going straight into the tuner without a tuner output.

[Caferacernoc]

"We live immersed in an ocean of electromagnetic interference. A big one of these is the pervasive AC power line field. Isolation transformers help prevent hum and noise from the AC power line field by breaking the conductive path from one unit to the other. "

So, it would replace the output coupling capacitor?
Thanks.

[Krinor]

Where would placing a transformer make sense (an fx box, switcher audio loops, tuner, buffer, splitter).

Here's an example from my own experience:

I made a fuzz with two outputs. The point was to be able to add more effects after one output and none on the second and then to be able to "blend" in the "wet" signal in the overall sound. I ran the two outputs to separate amps. What happened was that the signal from the first amp with the added effect was present in the second amp too! At first I found it kind of mystical, but someone here told me it was because of some ac interference. In this case a 1:1 transformer between the outputs would probably have isolated the signals from each other and prevented this "backwards leakage". I'm not experienced enough to give you a more scientific explanation, but hopefully this will give you an idea of where such transformers are usefull.

[mojo_hand]

Some other reasons one might use an output or interstage coupling transformer:

Increasing voltage gain, or reducing output impedance, without adding additional active electronics -- e.g., if you have an effect with up to several volts of output, but only need a volt or two, and the output impedance is high enough that it might have problems going into a lowish impedance input, a 4:1 transformer might do the job nicely, and sound better than some of the alternatives.  In some cases this might make an additional volume control (or other resistive discarding of gain) unnecessary.

Turning a single-sided signal into a balanced one, whether to drive an output stage, an XLR output, or whatever.

Getting some tube amp sonic nuances while avoiding the need for capacitive coupling.

[earthtonesaudio]

So, it would replace the output coupling capacitor?
Thanks.

Yep.  For a good example of this, see the Noisy Cricket/Ruby/Little Gem miniamps.  They're power amps which have capacitor-coupled speakers.  They can get away with using a cap because they run off 9 volts, and a 200µF/16V cap is tolerably small.  If you ignore impedance matching, you could theoretically use a capacitor to couple your 100W tube amp to the speakers.  Of course, the capacitor would be as big as a Hummer, but in principle it would work.

1:1 ratios would be used when you don't need to worry about matching impedances, but keep in mind that whenever you change the impedances, you're also changing the voltage ratios. 

[petemoore]

  So .. i wonder what those radio shack 1:1's 'sound' like [supposed to sound like 'nothing', might be some cool rolloff...
  For amp splitting..I'm thinking of getting some and trying them out.

[mojo_hand]

Some of the dinkier ones at RS are only rated as flat between, I believe, 300 Hz and 3 KHz, so the high end rolloff seems pretty useful... all most guitar speakers do above 3K is break up and roll off anyway.  300 Hz is a little higher than I'd want on the other end, but it might work.  It may be a gentle slope, rather than 6 dB/octave.

[earthtonesaudio]

Anyone have an oscilloscope, or something that can measure the frequency response of these little 1:1 transformers?  ...And plot it in a nice handy graph?  Those visual learners among us would be very appreciative!  Thanks.