48V Relay issue...

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Doesn't look like it whould be a problem. Check out shorts or a bad phantom power cap. Do you have diodes across your Relays?

What wlse are you powering?

There is 3K4 usually between input XLR socket and phantom power source.

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[quote author="drpat"]Wavebourn,

I'm reading 3K51...[/quote]

Does not matter, 6.8K to each leg usually.

I like the idea to power relays from a seperate source so no transients will be heard in phantom power. Also, a seperate ground wire will help.

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[quote author="drpat"]I've tried it without the 1/2W resistor too... no go...[/quote]

Leave "phantom only" source alone and power relays and LEDs from separate sources. Phantom power for mics must be as clean as possible. Also, no relay current must go through ground wires on XLR sockets.

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[quote author="drpat"]Maybe that's the issue. The preamp's audio ground and power ground are touching (it's a 312 circuit). Then we're back to finding a transformer that has a +15/-15 and two 48V taps with 2 separate grounds. Does this exist, or is there another way to isolate the relay grounds from the audio ground?[/quote]

If you have 2 separate 48V sources you may connect 2 separate grounds in the single point, close to phantom voltage regulators.

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[quote author="drpat"]OK, so I simplified the whole situation, and completely eliminated the phantom power and polarity relay circuitry as well as the audio connections to the board, and I still have the same, original problem. A 28V drop when the pad relay is engaged.[/quote]

Ok, did you measure rectified voltage after your "double-doubler", before the regulator? If it drops it means you need to increase capacitances from transformer to the rectifier in order to power your relay. What is DC resistance of the coil of your relay?

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Pat you do have a good heatsink on the 48v LM317 or it will thermal shut down at about 80mA without one? You will also need higher than 49v unreg feeding the reg to get 48v regulated.

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[quote author="drpat"]rectified voltage between the rectifier and the LM317 goes from 49.9 to 49.7 when the pad relay is engaged...

relay DCR is 4K[/quote]
49.9V is not enough headroom for normal 48V PP operation, let alone for driving relays that need 12mA each. The regulator doesn't stand a chance of maintaining a constant output. Voltage-doubled input feeds have a relatively high output resistance.

My advice would be:
(1) Increase the headroom for normal PP operation. You should be seeing 50-53V at the LM317 input. This may mean going to a transformer with a higher voltage secondary.
(2) Run the relays from a separate rectifier. Another fullwave bridge connected like BR1 would be a good place to start, especially if you are switching the power to the relay coils with mechanical switches. This allows you to power the relays from +/-24V, resulting in improved balance on the PSU.

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get a 24-0-24 trafo and hang an lm317 from it as well as the +-15v regs. the 15v regs will run hot but your 48v will be stiff.

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