Quote Originally Posted by gpjt View Post

(As an aside, would a transformer-based power supply produce less RFI than a switching one? Intuitively I would guess it would be much better, as there would be no high-frequency stuff going on -- just the normal 60Hz mains cycle. Not planning on using one in this build, just interested.)
You are very correct there.... Transformer (or analogue) based supplies produce virtually nil RFI.. ever!!..... On my ham radio rig for 7MHz SSB work I resorted to a very old Yeasu 12VDC regulated 20A transformer supply because every suitably sized commercial switch-mode supply created noise hash across all frequencies from broadcast medium wave AM radio right up past 27MHz CB frequencies and beyond. And in certain situations even VHF and UHF were unusable depending on the switch-mode supply used.... They (switch-mode supplies) often produce far more RFI than they ever should, and sadly most people would never notice. Many ham radio operators resort to incorporating batteries for their 12VDC HF/short wave gear simply because of the RFI issue with switch-mode supplies and the increasing unavailability of good regulated 12VDC transformer based power supplies of useful amperage size.

What most people of the general public do notice is those switch-mode brick supplies are compact and light weight and convenient with many voltage options, whereas a transformer supply is typically fixed voltage, large and heavy and cumbersome. And the bigger the output current the more the size and weight become an issue. Under 1A and things are pretty even size and weight wise... get to 10A and the the switch-mode might crack 1kg and the transformer version could easily hit 10kg and still be comparable in size. Step up to 50A and weights shift to 5kg and 50kg respectively.... I have a Regulated 12VDC only 60A (800W) rack mounted 8RU high brute of a transformer based power supply that easily weighs in at over 50kg, and a variable switch-mode 0V to 60VDC 70A 2RU high rack mount unit that would be hard pressed to reach 10kg..... Weight and convenience is an issue to many if not all, and often at the expense and detriment of RFI.

You will notice that any RFI problems will be most apparent in and around the higher impedance and small signal areas of the project. RFI in of itself is only a tiny signal which appear to be big when beside other small signals on high impedance circuits such as those coming from a guitar. Once you get to line level the impedance's of the circuit are often lower and the wanted signal is often higher so RFI still being the same size will have far less overall effect. On output stages that have high currents on lovely low impedance 4, 8 or 16 ohm values then the tiny RFI signals will be near impossible to detect. The net result is having a buffer/small boost pedal on a relatively short lead to the guitar often defeats many RFI problems in most situations due the slightly increased signal size and a change to typically a lower instrument line impedance from the pedal to the amp..