[ashpool]

I had a partially-populated Dynahi PSU board that I decided to adapt to +-15V in order to power a phono amp that I'm building. I finished the PSU this week, but it's outputting a lot of noise:

I've changed the PSU design for +-15V output by replacing the 120 Ohm resistors with 274 Ohms, the 3.5ks with 3.65ks, and the 50k resistor with a 20k resistor. I'm using a 160VAC 2x18V transformer and LT1084s. (overkill, but it's what I had lying around).

The AC from the secondaries going into the PSU measures just over 20.3V AC and if I replaced the the regulator resistors correctly, the regulators are supposed to be feeding the OPA541s +18V and -18V. The 20k resistor should be the right value for +-15V output and that's exactly what I measure coming out of the PSU, but when I look at the unloaded output of the PSU in my scope, I get that noisy mess above.

Any idea on what's causing the noise? The AC in my apartment is pretty dirty, but this level of noise seems inordinate for this PSU design...

 

[ashpool]

Hmm. After more investigation:

REF02 output isn't so steady:

Further up the chain, the LT1084 is also having issues:

This is what the output of the transformer looks like:

And this is what the AC coming out of my wall looks like:

 

[digger945]

This waveform mimics FM.

 

[nikongod]

Quote:

Originally Posted by digger945 /img/forum/go_quote.gif This waveform mimics FM.

Plus 1

Is it possible that the amplifier chips in the dynahai PSU have gone unstable and started to oscilate?

 

[digger945]

Quote:

Originally Posted by nikongod /img/forum/go_quote.gif Plus 1 Is it possible that the amplifier chips in the dynahai PSU have gone unstable and started to oscilate?

The only part changed that I'm unsure of is the 20k resistor substituted for the original 50k at pin 2 of the positive OPA541. I would change this back to 50k and test again.

The data sheet for the LT1084 implies it would work as a substitution for the 338, only the datasheet shows a 10uF tantalum on the output.

Wouldn't the 541's simply "hold" whatever the 338/1084 regs give them?(as per the original design)

Maybe add the tantalum from the 1084 out pin to ground and see what it looks like.

 

[Pars]

In the Dynahi thread (can't recall if it was the one here or on headwize), Dan Gardner developed a spreadsheet that showed the changes and did a thermal analysis for using the Dynahi PSU for a Dynalo. There was discussion of this application in that thread as well, which would be relevant to what you are doing here. This should be a very low noise PSU. See attached.

 

[Nisbeth]

Could be an unstable regulator - the LT1084 is a little more demanding than the LM338 so if you haven't added the tant that's where I would start. If you're using an 18VAC transformer you're likely burning off a lot of power in the 1084, but as long as it's properly heatsinked I don't see any problems in that.

/U.

 

[ashpool]

I've got 10uF tantalum caps installed in the 10uF spots on the Dynahi PCB, so I don't think that's the issue.

Quote:

Originally Posted by digger945 /img/forum/go_quote.gif This waveform mimics FM.

Ugh. This might be related to an FM interference issue that I'm seeing with my preamp. I'll try to bring my scope and the PSU to a friend's house or work and see how it measures there.

 

[Nisbeth]

Misread that, sorry.

/U.

 

[cotdt]

just use an S22 instead. troubleshooting these things tend to be a big headache, and every hour you use up there is an opportunity cost.