Selecting the correct common mode choke

[gnif]

Hi All,

The audio bug has bitten me again and I am currently in the process of re-designing my pre-amp power supply. In my office there is a fair amount of common mode noise I would like to filter out, but having never used a common mode choke before and being a little bit lost with the right inductors to use, I would use some advice.

It is a linear +/-15VDC unit, I have already recycled some parts out of a washing machine for the Mains AC side of things (230V), including a chunky common mode choke and so far the output of the supply is 2-3x cleaner then my prior attempt, however I feel that more could be filtered out after the toroidal transformer which is about 22V AC ptp. The choke I pilfered likely isn't right for the job, but it was free .

So my questions are:
  * Should I use a low voltage choke after the transformer also?
  * What value should the choke be to filter as much noise as possible.
  * Please explain the selection of choke so that in future I can figure this out for myself

[tautech]

You shouldn't need a common mode choke on the LV side of a linear supply, period .....if the rectifier and smoothing caps are doing their job. No need to use low ESR caps with a linear supply either but add some faster ceramics near the output to catch any errant spikes.
What regulation are you using ?
With just jellybean LM7815 and 7915 the output should be pretty clean.

[gnif]

Before the changes I was seeing 25-30 mv peak to peak common mode noise.

Yeah, I am using the LM7815 and LM7915 regs. The output is pretty good, but there is common mode noise of roughly 10mv peak to peak at the output. It could however be the scope picking up noise from the room, I am not sure. I have a 4700uf cap before each reg, and a couple of 100n poly film caps, one before and one after each reg.

[tautech]

Before the changes I was seeing 25-30 mv peak to peak common mode noise.

Yeah, I am using the LM7815 and LM7915 regs. The output is pretty good, but there is common mode noise of roughly 10mv peak to peak at the output. It could however be the scope picking up noise from the room, I am not sure. I have a 4700uf cap before each reg, and a couple of 100n poly film caps, one before and one after each reg.

That would be my first suspect and a different scoping method should be able to confirm it.
1x probe with a short reference lead or the little springy clip that you should have in your probe pouch.
Some small load on the output can be necessary to settle the output to 'real world' operation.