Pedal Power Supply - My Design

[Chienmortbb]

I have mentioned in a number of threads about pedal power supplies why many of them are not fit for purpose. The buzzword is isolated, but in many cases especially for the sub £100 products the isolation is only via the voltage and current protection circuits.

 

True isolation in terms of the power supply means that neither the 0 V or ground connection or the positive voltage, normally 9 V is connected to any other pedals 0 V or 9 V line. If either is connected to another pedals supply there is a good chance of Hugh or buzz caused by earth currents or injected Clock signals. 
 

My first idea was to utilise a multi secondary main transformer feeding traditional LDOs. Low Drop Out regulators are integrated circuits that incorporate serious regulator circuits (usually) to obtain a steady and relatively nice free voltage. Well likely looking through the site of an electronic supplier I noticed some compact switch mode supplies that were PCB mounted looking at the data sheets showed that, as with most switch mode supplies, after the initial regulation and smoothing, there was transformer isolation of the mains and secondary sides of the circuit.

 

It was clear that a number of these small circuit blocks could be mounted on a PCB to give a good pedal power supply. After several years of thinking I sat down and designed a PCB to give me four totally isolated 9V supplies. 
 

 

The beauty of these circuit blocks is that the pins are on a standard layout and the original ones from the French company MYRRA and others, including UK’s Vigotronix were interchangeable. In addition, they can be bought in many voltages.  Both types can be seen on the complete module above. The MYRRA one are dark red and the Vigortronix   are orange. The Vigortronix range are slightly more powerful, giving 3W while the MYRRA ones are 2.5 watt. However, MYRRA do a 5watt one on the same footprint.
 

I have just tested them for the first time and all four are working 1% of the stated 9V off load. I will try them with a load later. 

Edited yesterday at 13:15 by Chienmortbb

[Chienmortbb]

The big test will be my Zoom B2 pedal and TC Helicon Mic Mechanic 2. Both are digital pedals and they can cause more problems than analogue pedals as  the clock noise is often left on the power rails.
 

I have had to use the MM2 with batteries for a while to cut down the number of power supplies. My cheapo 10 way made them sing but not in a good way. 

[Chienmortbb]

I am building this in conjunction with me new pedal board, 

 

I did not have this in mind when I designed the PCBs. The PCB is too wide to fit between the ribs of the pedal board and not wide enough to screw directly to the ribs, so I have to add extra wood to allow the PCB to be mounted below the pedal board. I think I will also need a bit more height from add feet. This would allow me to add a case and make it safe (remember 230VAC mains on that PCB).

 

This is one of a number of changes I would make if I redesigned the PCB. As it stands, it does not fit any commercially available die-cast enclosure. I could put the modules 2x2 and that would fit may a Hammond/Gorva/Eddystone style box. This would mean more height required on my pedal board and as I am still in the proof of concept stage, I will work with what I have.  

 

I have ordered more parts for the project so will hopefully make more progress early next week. I still plan a real life test before then, either today or tomorrow.

 

 

[itu]

How low a box could be built? Many units are pretty high, and would be not that easy to assemble under the board... or could the board be the power box as well?

[Chienmortbb]

1 hour ago, itu said:

How low a box could be built? Many units are pretty high, and would be not that easy to assemble under the board... or could the board be the power box as well?

I am planning to build the box around the PSU. I will be making it about 1cm higher at the back. 

[Stub Mandrel]

I've always assumed that 'isolated' PSUs still have a common ground and it's just that each output is a separate regulator fed from the same supply.

 

Mine seems to do a good job of preventing crosstalk from heavy duty digital effects.

[Chienmortbb]

46 minutes ago, Stub Mandrel said:

I've always assumed that 'isolated' PSUs still have a common ground and it's just that each output is a separate regulator fed from the same supply.

 

Mine seems to do a good job of preventing crosstalk from heavy duty digital effects.

True isolation means no connection at the power supply. The problem, is that on the typical 10 way supply such as Donner etc the power supply duplicates the signal grounding and often causes ground loops that  imposes hum or buzz. Many people will never experience it but my two pedals do interact. 
 

Using totally isolated supplies will reduce ground or earth loops.

[Chienmortbb]

Although I am not a rookie as far as electronics goes, however I have made one rookie error. When I tested the unit I had a slight buzz. Very hard to hear but there nonetheless. 
 

Looked at the specs of the two units, the TC Helicon Mic Mechanic 3 needs 300mA at 9V which is fine. The power modules I have are 9V 3 watts so 333mA current supply. In my head that was enough for the Zoom B2 Four too. However I was wrong, it needs 500mA. 
 

Luckily, both Myra and Vigortronix make a pin compatible 5 watt version. That gives me 550mA anf it more than enough. I am annoyed that I could have ordered them with the other bits yesterday but hey.