LCMP power supply

[stereocyborg]

I bought a Hamptone kit ages ago, and love the way it sounds when it's working.This mic pre and I have been on quite a voyage of self discovery.

Right now it seems the issue is with the power supply. I went with the off the shelf International Power supplies. After the unit warms up there is a high tone (9k) in the signal. It is quieter with the volume down, and in both channels. I am on the verge of repalcing every component on the board, but am wondering if the supplies are hooked up right. Positive voltage giong to the boards and negative voltage going to the common.

I have been in touch with Scott over the years, but hesitate to hassel him what with the move and all.

[brianroth]

Odd...linear power supplies seldom, if ever, introduce a whine.

Do you have access to an oscilliscope?

Bri

[stereocyborg]

No o-scope. Other symptoms are that the bad sound dips momentarily when phantom is engaged. Also I can induce the sound by testing the voltage with my meter at the output of the +225v supply.
Thanks,
Jed

[stereocyborg]

Also wondering if the two power supplies need to be oriented so that the transformer coils are perpendicular to each other.
...robots in disguise...

[brianroth]

Are the power supplies linears, or switchers???

Bri

[stereocyborg]

Sorry, I dissapeared there for a minute.
Both are linear supplies.

I just ordered all new caps for the main boards; I have done considerable reworking of these boards, and may have shorted the circuit once or twice.

Is there a way to test the output of the ps with a lowly digital multimeter. Both are putting out the right amount of DC current.

[mu_amps]

You mentioned that your B+ is 225v, so I take it you have the tube version of these pre's. Did you try changing out the tubes? Microphonic tubes often let out a high pitch tone similar to what you are describing.

One thing you could try is to power up the unit with the cage off and very carefully and gently tap the envelope of the tube and listen to hear if your tapping induces the noise. You could also just change the tubes, one at a time to see if the noise level improves.

I don't think it's your PSU. I don't own these pre's, but i looked at the photos on the website. There's an EMI/RFI line filter in there, and leakage from toroids is minimal. Even if one of the filter caps drifted to the point of near failure, the result at best would form a low pass filter. f = 1/{2(3.14)RC}

I hope that helps

...any thoughts Brian?

[brianroth]

Odd...apparently dual (linear) power supplies, with one supply dedicated per channel?

I HAVE seen cases with a pair of switcher supplies that would create a "beat frequency" if the two supplies were running at different frequencies.

But, AFAIK, International Power builds ONLY linears, unlike Power-One, etc which also sell switchers.

I dunno......hmmm....

Internal "lead dress" between the mic in and preamp out, making the preamps go into oscillation?

What happens if you short pins 2 and 3 of the input XLR?

Bri

[stereocyborg]

I replaced all the tubes one of which was microphonic. All the tubes show no signs of being bad, but the 12AY7s are NOS sylvainias. I might be better off going with all new JJs.

There are two supplies; one to power the circuit and one to supply phantom pwr. I'm glad we're ruling out the PS. Expensive and harder to work on.

Thanks for keeping the chatter up. I was thinking about putting this piece in deep deep storage. Even though it was great when it was working.

[mu_amps]

Just because the The Sylvania's are NOS I wouldn't rule out the possibility of microphonics. It does happen. you might want to try some other tubes. did you let them cook?

Do you have a schematic?

[nclayton]

Is the preamp close to your computer or monitor?

Also, how is the internal ground connected? Is the audio/power supply ground connected to the chassis, or are you letting it float? The reason I ask is because of what you said about phantom power. I've had some issues with equipment whining when the internal ground is floating, although the oscillation seems to depend on what other type equipment is connected. If your phantom supply is chassis earthed and your audio supply is not, then maybe when you switch on the phantom supply things change as the phantom filter caps fill up or something. If you've got a spare .1 uf 400 V capacitor (or something similar) sitting around, maybe try jumping it's leads from the audio ground to the chassis and see if it makes the whine go away or at least if it changes the pitch or quality of the whine in some way. I dunno.

Ned

[brianroth]

Aha...I misread the info about the two supplies, somehow assuming you had two B+ supplies.

I agree that grounding could be an issue, BUT both channels have a common B+ source which may or may not be behaving properly.

As far as system grounding, you should have one "defined" path from the minus sides of both the 250 supply and the 48V supply back to chassis, XLR pin 1, and the "low"/"0V"/"ground" circuit of the preamps themselves.

Damn...wish you could 'scope the 250 V rails....

Bri