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.
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.
Yes, you can get a transformer with multiple secondaries, or use many small transformers, such as The Spyder.
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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
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
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
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
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
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
Hi Guys
Good to see you are using TO-220s. They have the current capability to support even the hungriest digital effects pedal.
Note that the TO-92 regulators are only rated for 100mA. This happens to be the draw for "vintage" digital delay pedals from Boss - some other brands needed 200mA. Newer pedals may or may not require as much current as low-power techniques have improved quite a bit in the recent decade.
Note also that the tab on the TO-220 is tied to the reference pin (ground for 78xx, ref for 317). The heatsink will be "live" at whichever of these voltages apply. The 78xx and 79xx come as F-paks, fully epoxy encapsulated packages, with a reduction of power rating but an insulated tab. This insulation allows all the tabs to be bolted to the same heat sink or to the chassis without introducing ground loops or upsetting voltage setting.
Your board is nice an neat. The holes for the regulator and diode leads look a bit small. The output caps also look to be sized for the app note value instead of the recommended values above.
Have fun
Kevin O'Connor
Good to see you are using TO-220s. They have the current capability to support even the hungriest digital effects pedal.
Note that the TO-92 regulators are only rated for 100mA. This happens to be the draw for "vintage" digital delay pedals from Boss - some other brands needed 200mA. Newer pedals may or may not require as much current as low-power techniques have improved quite a bit in the recent decade.
Note also that the tab on the TO-220 is tied to the reference pin (ground for 78xx, ref for 317). The heatsink will be "live" at whichever of these voltages apply. The 78xx and 79xx come as F-paks, fully epoxy encapsulated packages, with a reduction of power rating but an insulated tab. This insulation allows all the tabs to be bolted to the same heat sink or to the chassis without introducing ground loops or upsetting voltage setting.
Your board is nice an neat. The holes for the regulator and diode leads look a bit small. The output caps also look to be sized for the app note value instead of the recommended values above.
Have fun
Kevin O'Connor
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