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Old 18th June 2013, 06:45 AM   #1
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Default effects powersupply

Currently I'm working on a design for a powersupply with multiple outputs to power a bunch of guitar effects. The output grounds have to be isolated from each other to prevent groundloops.

One solution is to use a transformer with separate secondary winding for every output.
But I was wondering if it is possible to create good ground isolation between the separate output circuits when using a shared transformer and rectifier.

Any tips would be highly appreciated.
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Old 18th June 2013, 07:05 AM   #2
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Yes, you can get a transformer with multiple secondaries, or use many small transformers, such as The Spyder.

Last edited by jazbo8; 18th June 2013 at 07:09 AM.
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Old 18th June 2013, 10:39 AM   #3
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If you use a single power supply, fit a 10R resistor in line with the ground on each cable. That will stop the ground hum.
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Old 18th June 2013, 10:02 PM   #4
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Hi Guys

You should also pick a transformer with a high enough output voltage to allow for double regulation. A single three-terminal regulator won't provide low enough ripple for front-end effects.

If you need 9V, feed the input to the 7809 from a 7812 or 7815. Assure that there is 3V+ difference between the chosen regulators, so the output device does not drop out. Make sure there is at least 10V across the input regulator to accommodate some sag in the mains.

At the final output, use a local capo to keep the regulator stable, then feed the DC through a low-value R, just an ohm or few, then into 470uF or more. 1mF would be better. This helps to give the power supply a frequency response that is not that of the regulator. Regulators are low-bandwidth amplifiers that can modulate the noise and THD profile in quite an ugly and distinct manner.

The best ground isolation scheme is as originally proposed: separate windings for each regulated output. There are tonnes of PC mountable dual-secondary PTs that are quite small and just a few dollars each. However, they exhibit their lowest radiated noise when both windings are loaded similarly.

Have fun
Kevin O'Connor
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Old 20th June 2013, 04:59 PM   #5
Struth is offline Struth  Canada
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Hi Guys

Also remember that the 10R resistor in the ground lead used to isolate grounds applies to signal grounds only.

Adding such a resistor in series with DC currents will definitely place each effect at a different ground potential and exacerbate hum when all are fed from the same supply.

The 10R can still be used, but it must be placed in the signal paths NOT the DC supply paths.

Have fun
Kevin O'Connor
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Old 21st June 2013, 04:32 AM   #6
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I built a supply using the Weber "PDLXFMR-1" with individual rectifiers and 78L09 regulators on each of the 11VAC taps, and it works great with no audible hum or noise.
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Old 21st June 2013, 02:53 PM   #7
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I designed a small pcb of 5x10cm which contains 4 individual LM317 powersupplies with adjustable voltage and battery "sag".

Pitch is 2.54mm so it can also be made on perfboard.
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Old 21st June 2013, 06:32 PM   #8
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Hi Guys

Note that the TO-92 case 78Lxx regs have 10db inferior hum and noise rejection compared to the TO-220s.

The 317s have a slight noise advantage compared to the 78xx provided you include the reference cap.

Note also that with all of these regulators it is important to add the protection diodes across the reg in-to-out and from out to ground. For the sake of <10-cents of parts you can save the reg from destruction at turn-off.

Also note that the app note recommended output and input cap values are for stability only. These do not represent proper real-world values, which as I posted above, should be much larger.

Have fun
Kevin O'Connor
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Old 22nd June 2013, 07:37 AM   #9
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Bulk capacitors in my design are 12,7mm so you could fit 1000uF/25V.
I also used the 2 protection diodes.
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Old 22nd June 2013, 10:06 AM   #10
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Now with silkscreen

Now after some transformers an build the thing
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