Hi All!

I just pulled out the power supply and transformer out of my multiFX board (which is going in the garbage). I am putting it in a 2U rack space for the new rig. See here:



The wires on the front go to the outputs (in the back of the rack) and the wires in the back to go the LED indicators on the front.

Anyway, a few quick quesitons:

1) True/False: Each PSU should not be grounded, it will obtain ground from the effect once it is connected via a normal DC plug into a normal pedal.

2) True/False: I can use a normal DC jack to carry 12VAC from the transformer to one of my AC powered pedals

3) True/False: The chassis of the transformer should be bolted directly, and therefore make a connection, to the rack chassis, which should also be connected to the AC Power Entry Module's Ground, and Earth Ground

4)True/False: When using these 9V outputs to power pedals that are in another rack (a single 1U spot that holds 4 other basic effects) the effects should NOT use normal Jacks, they should use jacks that do not connect sleeve to chassis, as to avoid ground loops. The PSU ground will connect directly to the effect's ground, but not the the chassis of the rack enclosure which it shares with other effects.

Thanks so much! I tried to make these questions as easy as possible to answer

Quote from: sjaltenb on October 16, 2010, 11:08:38 PM
1) True/False: Each PSU should not be grounded, it will obtain ground from the effect once it is connected via a normal DC plug into a normal pedal.

Correct. If the PSUs are isolated, then they are the rough equivalent of batteries, and do not need individual grounding.

Quote2) True/False: I can use a normal DC jack to carry 12VAC from the transformer to one of my AC powered pedals

Maybe, it depends. The problem with DC jacks carrying AC is that many DC jacks have a mounting scheme that ties one of the electrical connections to the (usually grounded, usually metal) chassis. This makes the AC power grounded on one side, swinging the other side above and below ground. When it reaches the AC powered effect, it's full wave rectified (usually) and then the (-) output of the rectifier is connected to the pedal ground. These two connections short the AC power supply every other half cycle. Bad things result. If the incoming jack does not tie one side of the AC to the grounded chassis, all is well.

Quote3) True/False: The chassis of the transformer should be bolted directly, and therefore make a connection, to the rack chassis, which should also be connected to the AC Power Entry Module's Ground, and Earth Ground

True. The chassis of the transformer can, under some AC power faults, be shorted to the AC power line. This must be set up to blow the AC power fuse. So the metal transformer chassis must be bolted firmly and with good electrical connection to the metal chassis it is enclosed within. The metal chassis must be firmly and with good electrical connection be ties to the incoming AC power safety earth ground. Neither the transformer chassis nor the earth grounding point may be prevented from making good contact with the chassis by paint, so many commercial items either sand a bare metal spot for good contact or mask the connection spots to prevent paint from getting there.

One way which I have seen to do this is to use a bolt on the chassis to secure the incoming AC power ground. This bolt is used for no other purpose - that is, it is NOT one of the bolts which hold the transformer on. The bolt goes through the metal chassis, with the screw thread side inwards, going through the paint-free spot. On the screw, from chassis metal outwards, are stacked a toothed lock washer to bite into the metal above and below it, a crimped-on (not soldered-on) ring terminal, a second lock washer to mechanically secure the connection, and a nut.

Quote4)True/False: When using these 9V outputs to power pedals that are in another rack (a single 1U spot that holds 4 other basic effects) the effects should NOT use normal Jacks, they should use jacks that do not connect sleeve to chassis, as to avoid ground loops. The PSU ground will connect directly to the effect's ground, but not the the chassis of the rack enclosure which it shares with other effects.

Doesn't matter with respect to the 9V outputs (how you ground the effects in the second chassis may matter to them) as long as the 9V outputs are (a) truly floating, not connected to ground and (b) low power, even when shorted, which means 9V and current limited to a small value. (B) prevents the 9V itself from being a current or voltage hazard.

The second chassis, presumably metal, should be set up so it does not induce hum itself or cause a safety hazard. This generally means that the second chassis itself should be tied to the other chassis with a solid ground connection in some way so that if there is an internal short in the AC power transformer that makes the 9V outputs, the second chassis is grounded and cannot carry AC power voltages to the user through the faulted 9V wiring.

I'll add my standard warning - this may not be all that you need to do to wire up your AC power wiring safely, or to be hum-free. This is not just me being obstinate. I did design AC power wiring as part of my power supplies design work, back when. Every time I did this, I incorporated all the tricks I'd found out about safe wiring from the last time. Every time I did this, including poring over the safety standards, the inspectors found something new that I'd left out or, in their opinion, done incorrectly. Every time. The power safety standards change over time; worse yet, the standards themselves do not day "do this and then your wiring is safe". They say "If you don't do at least this, your wiring not safe. It may still be dangerous and we just didn't find it."

Given that the pros in the safety industry take that stance, I'd be foolish to presume I know it all. So - if you ask a question about some particular point of safe wiring, I'll point out what I think is not safe, maybe give an example of how I've seen it done. I will not - I CAN not! - tell you that if you do X, Y, and Z, it's all good. The safety inspection pros don't even do that. I wish I could help more.

Quote from: R.G. on October 17, 2010, 10:15:30 AM
One way which I have seen to do this is to use a bolt on the chassis to secure the incoming AC power ground. This bolt is used for no other purpose - that is, it is NOT one of the bolts which hold the transformer on. The bolt goes through the metal chassis, with the screw thread side inwards, going through the paint-free spot. On the screw, from chassis metal outwards, are stacked a toothed lock washer to bite into the metal above and below it, a crimped-on (not soldered-on) ring terminal, a second lock washer to mechanically secure the connection, and a nut.

I made it standard to do this until I got into audio... I induced an instabilityin a build  because my transformer chassis was already mounted to the case. Not as much of an instability as it increased any humming I was getting in the circuit.

Power supplies are so much simpler when you don't have audio involved! Just gotta watch out for transient spikes, where audio makes everything, well... audible.

Quote from: Earthscum on October 17, 2010, 10:49:43 AM
I made it standard to do this until I got into audio... I induced an instabilityin a build  because my transformer chassis was already mounted to the case. Not as much of an instability as it increased any humming I was getting in the circuit.

That was Mother Nature whispering in your ear that you had ground currents running through the chassis... 

QuotePower supplies are so much simpler when you don't have audio involved! Just gotta watch out for transient spikes, where audio makes everything, well... audible.

Very true. The human ear's 120db (at least) of dynamic range is merciless at bringing out any odd little issues lurking in there.

What I thought was interesting was that I had no humming until I hooked in a pre-amp that had humming. It is always interesting how many ways things are amplified, whether intentional or not. Hum (oscillation) was getting amplified by the otherwise audibly quiet power supply. I still get noise, but now I'm down to heater supply filtering... for now, it's still on the shelf for a snowy day.

Quote4)True/False: When using these 9V outputs to power pedals that are in another rack (a single 1U spot that holds 4 other basic effects) the effects should NOT use normal Jacks, they should use jacks that do not connect sleeve to chassis, as to avoid ground loops. The PSU ground will connect directly to the effect's ground, but not the the chassis of the rack enclosure which it shares with other effects.

Doesn't matter with respect to the 9V outputs (how you ground the effects in the second chassis may matter to them) as long as the 9V outputs are (a) truly floating, not connected to ground and (b) low power, even when shorted, which means 9V and current limited to a small value. (B) prevents the 9V itself from being a current or voltage hazard.

The second chassis, presumably metal, should be set up so it does not induce hum itself or cause a safety hazard. This generally means that the second chassis itself should be tied to the other chassis with a solid ground connection in some way so that if there is an internal short in the AC power transformer that makes the 9V outputs, the second chassis is grounded and cannot carry AC power voltages to the user through the faulted 9V wiring.

[/quote]

Hi RG!

Thanks for the detailed responses! Yes the DC jacks will all be set up completely floating (plastic jacks), just as if they were a battery. I got a little confused on this response, so I just want to make sure I understand.

For simplicity sake, assume I have 3 Boss pedals in a metal rack enclosure. I figured I would use isolated, non-chassis contacting, enclosed Neutrik jacks for input out and 9v. So again, the 9v has not touched any chassis. I then assumed I would simply wire the sleeve of the input to the effect's ground, and also wire it to the output sleeve and the 9V "negative". Just as if it were a stompbox.

My question is regarding the grounding of the effect rack chassis. Since each unit will not be AC powered, it will not have any Earth connection, unless I should somehow bolt on a wire to each unit and somehow earth them. If i use the above wiring scheme, then the chassis is basically just a floating box around the units, with no connection to ground. AFAIK, this prevents it from really stopping any noise interference, so, shouldn't the chassis be grounded to one of the sleeves. Sounds logical for a single effect, but if multiple boxes are in there, will it create a loop? Maybe not..

Ugh its always something. I know you are thinking, why won't he just use a normal pedalboard and quit bothering us?? Because thats no fun! I have a feeling this rig will come together MUCH quicker and much more simply. All I have to do is get these few racks built and working properly and the rest will be left to the Rack guys