Isolation transformer

[hesamadman]

I have a 120v isolation transformer. Would this be useful on my bench? Could it help with any interference from my circuitry in which my amps are plugged into?

[eleventeen]

The specific purpose is to 1: avoid potential shock hazards from transformerless "hot chassis" items...such as the old "All American 5" tube radios. Of course, you can still easily get shocked from *internal* voltages created by the power supply of anything that runs on tubes (except old battery-operated tube radios) There are very low-end cheapo guitar amps from the 50's-60's that lack a power transformer. How can you tell? Easy. When you see tubes such a 50L6, 35W4, 25L6, that is your indication that the heaters are being operated from the AC line, and for that to happen, generally one will find one side of the incoming AC line connected to the chassis. There is an attempt to connect the neutral (wide blade) side of the AC line to the chassis, but of course it's always possible that your electrical outlet is/was wired incorrectly. That produces a shock hazard. If you don't work on those older amps nor 5-tube radios, there is almost no need from this particular cause. 2: More saliently, an isolation tranny is good to have to avoid line-shock hazards when using a variac, which is after all an "autotransformer" where one side of the secondary is connected to one side of the primary. Again, if the common primary-secondary connection attaches to the "hot" side of the AC line, it could present a shock hazard relative to pipes or perhaps the metal frame of a workbench that is sitting on a concrete floor.


I doubt it would have effect at all on say fluorescent light or dimmer interference.

[jjasilli]

Here's a helpful write-up:  http://www2.schneider-electric.com/resources/sites/SCHNEIDER_ELECTRIC/content/live/FAQS/123000/FA123947/en_US/Isolation.pdf


The iso tranny can provide both isolation & common mode noise rejection if it has a grounded shield.  The thing is, all transformers provide isolation, except Autotransformers, which IMHO are poorly named because they are not really transformers, more like a variable inductor.  But they look like transformers and may give a false sense security, leading to carelessness.  The addition of an iso tranny is a safety practice.  Note that for iso alone (not noise rejection) any 1:1 tranny will do. 


Arguably, for us, noise rejection is not needed, because we need to know how a guitar amp under test will perform in the real world with dirty power.


A tranny's magnetic field might affect other electronics.  If so, you can put it in a shielded cage.

[2deaf]

How many amps is the transformer rated at?

I have a couple of isolation transformers that I have used many times for experimental circuits.  The one that I have used the most is a 115V:120V CT @ 50 ma in a small stomp-box chassis with a fuse, switch, and indicator lamp.  It was particularly useful a few months ago when I was playing with BJT and MOSFET controlled bias circuits.

 

[PRR]

> find one side of the incoming AC line connected to the chassis. There is an attempt to connect the neutral (wide blade) side of the AC line to the chassis

Except the wide-blade plug/outlet didn't happen until after most hot-chassis guitar amps were effectively banned. I remember tons of equal-blade outlets well into/past the 1980s, even the 21st century.

> I have a 120v isolation transformer.
> help with any interference

You got it? You try it.

However as that informative yet off-point "write-up" says, isolation transformers pass normal-mode not just 50/60Hz but pretty much the whole audio band, even with add-on filters. Your buzz-light is likely to come right through. Or not, depending how it puts buzz on the wires.

[eleventeen]

"Arguably, for us, noise rejection is not needed, because we need to know how a guitar amp under test will perform in the real world with dirty power."


Well...until you go to fix something in your amp and your bench has a noisy fluorescent fixture hanging above it. If you KNOW that this is the cause, OK. If not, you could drive yourself nuts trying to find a noise problem inside the amp when the cause is outside.


Plus, if you happen to play in crappy bars, you could get crappy lights/power/dimmers.


This happened maybe ten years ago, but I used to go see Scott Henderson at a small club in Studio City (LA) CA which was a very comfortable and good sounding but wiring-wise, crappy bar. The whole band had to rotate 90 degrees and play sideways on the stage to reduce the noise induced from the lights.


In a bad situation, bad AC noise can ruin your night. I would not discount the issue to nothing. If you play in a quiet band versus thrashing in metaldom, I would pay more attention.