Grounding when dealing with linear supply for effect pedals - Page 1
Forum section: electronics / projects

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: August 24, 2016, 12:47:58 pm
content: Hi,A friend and me are going to be building two 9-12V DC adapters for our guitar effect pedals. Since it's pretty hard to get a hold of them (linear ones, not switching) or they cost quite a bit, we thought it'd be a good project to start of with, since my friend is new to practical electronics. It's just in the "let's make this" stage, we haven't done any design at all yet.What I was wondering is how we should do ground/earth and such. There are two things that make me doubt what the right approach is:First, there is the fact that for some (silly? I really don't understand why they did this) reason, the "standard" in the pedal world is to have the 9-12V tied to the outside of the connector, and the ground sits on the inside (barrel connector). Oh, and then there are also some pedals that use jack connectors, which means there is risk of having 9V on the ground while plugging stuff in...In other words, referencing to mains earth would give risk of shorting 9V to earth (through the amplifier, which, iirc, will tied the shield of all the cables to mains earth, or through just having a connector sit on the floor.I would also have to watch out to not do stupid things and cause ground loops, but I think this can't happen if I properly reference to mains earth.There is also the issue of noise: These pedals are extremely sensitive to noise on the power supply. It's the entire reason we are trying to make our own, even the best switching supplies out there cause a lot of noticable hum and/or switching noise. So, tying ground to mains earth seemed nice because it provides another route to the earth for noise "captured" by the shield on the cables. But will this give me any other issues?I hope this ramble makes some sense somewhere, and that I'm not underestimating how hard it is to design a nice linear supply (surely, a rectifier bridge, some 7812-style regulators, and good amounts of capacitance on both ends will do the trick? Maybe just throw in a series power resistor on the transformer side to stop inrush, but at full load the pedals I have together will take a few tens of mA's or so...)

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: August 24, 2016, 06:20:32 pm
content: Have you verified that the source of your hum is on fact the power supply? Does it go away with batteries? Sounds a bit unlikely to me. Never been a problem to me in my 20 years of playing (yes I started at 8 heh).

author: helius
author_rank: Super Contributor
timestamp: August 24, 2016, 07:16:24 pm
content: You should learn about Class II appliances and how they are constructed. This is almost certainly what you want so as to prevent ground loops from each of your pedals.It may be that linear regulators aren't required, if you can select the transformers based on the power needs of your pedals. Unregulated AC to DC adapters are more common than linear regulated ones (and have the advantage of less heat generated in the sealed double insulated housing).