Is it safe to make a dual supply out of two wall adapters?

Question

I am trying to design a guitar distortion pedal circuit and I need +9 / -9 V dual supply in my application. I have two 9 V adapters:

https://www.phihong.com/assets/pdf/PSM03A-XXX.pdf

Would it be a proper way to create a virtual ground and dual supply if I connect the outputs of these two wall adapters in the following way?

What are the drawbacks? Would ripple be out of space? Or would it explode in some circumstances because I haven't seen any application as such?

Answer 1

There are two possible problems.

Figure 1. The PSU has high ripple.

1. Ripple on the DC may make the circuit noisy. The datasheet doesn't say what the frequency of the ripple is. The PSU is switched mode as, judging by the shape, there isn't enough room for a regular transformer in it. (Almost all of these PSUs are SMPS these days.) Since these switch at high frequencies, > 20 kHz, the ripple may be inaudible.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 2. (a) Two 9 V SMPS supplies represented by 7809 9 V regulators. This should work. (b) Trying to generate a split rail supply using an 18 V supply and a 9 V regulator doesn't always work.

2. Be aware that your power supplies can only source current from the positive terminal Your proposal looks OK but to show a potential problem that has caught some people out, have a look at 1b. In this case an 18 V supply has been used and a single 9 V regulator to generate the mid-supply "GND" rail. Now if we try to connect R1 as shown U4 can't sink the current and strange things will happen.

Your circuit should be OK. If ripple proves to be a problem a couple of hundred µF of capacitance across each rail should help.

Answer 2

I see nothing particularly unsafe about attaching these two specific wall-warts together to produce a split supply. However, there may be some issues related to their different switching frequencies that might cause audible problems when used with an effects pedal. To counter these I would use some form of filtering to reduce the output noise of each wall-wart.

Difficult to say what form the filtering would take but I expect that 100 uH inductors in series with both wires and a decent 10 uF capacitor after the inductors would be a good start. Each wall-wart would require this filtering and then the common wire can be joined. I might also consider using linear voltage regulators for positive and negative supplies before feeding to the pedal: -