12v transformer for relays

[cbarrow7625]

I agree with Enzo. You can get away with it if the impedance of the ground path is low enough in impedance and stable. Lugs bolted to the chassis always worry (over the life of an amp) me in the long run. They will likely oxidize / corrode at some point and make the amp noisier. Of course, they can always vibrate loose and cause problems as well.

I prefer to make only two connections to the chassis if possible: at the input jack and at the safety ground. All other grounds are run independently depending upon the topology of the amp (let's not get into the whole "is star grounding the only good way to ground" discussion - the answer is "no" & "it depends").

I built my most recent amp on double sided copper ground plane PC board blanks and soldered the signal grounds and relay grounds directly to it. Since the ground return impedance is low and stable, I too have a very quiet amp. I did, however, separate my power amp grounds & power supply grounds from the preamp grounds by etching the boards in the right areas to make sure that the ground current flowed in the proper directions.

[lowell]

ok thanks guys!

I'm having an intermittent problem w/ the relay hum. I played my first gig w/ the amp last night... 3 hrs long. The third hour the hum came back as it was before I'd put the pi filter in the supply. Now I got the amp on my bench and it's tres quiet. I'm a bit confused but will poke around and leave it on for a while and see. I may just mover the coil ground to the HT CT point to follow good practice.

[lowell]

ok the hum seemed gone, but again last night during set 2 or 3 it came back... only on OD channel though, which is when the relays are energized. Right now I have a full wave bridge rectifier on the 12v secondaries and then a pi filter w/ 100uf-510ohm-100uf: producing ~18vdc when not energized and ~6vdc when energized. Anything tip anyone off as to why the hum is only there after a couple hours of playing?

[Enzo]

Set your meter to AC volts and measure that supply loaded down to 6v and see what you get. WHy do you have 500 ohms in series with it?

usually when you get hum associated with relays, it is not because the DC supply to them is rippled. it is because riple currents share ground paths with the signal grounds.

[lowell]

Ok I'll check the AC volts while it's 6v tomorrow and report back.

I put a pot in b/t the 2 caps and moved it around until the hum was minimized and then measured the pot at 500ohms. Should I not have that? Or maybe a different value?

Oh and I moved the coil grounds to where the DC relay supply ground is, which is also where the first B+ filter ground is.

[lowell]

Enzo I'm getting 1vac on top of the coil when energized. FYI the amp is not humming on the OD channel right now.

[lowell]

enzo does the 1vac throw up any red flags? At this point I'm thinking of regulating the relay supply because this hum issue only arises after 2-3 hours of playing, leading me to think the relay voltage is not remaining steady.

[defaced]

I've got two thoughts. Do you get any sort of buzzing/hum when you float the relay supply? Meaning, removing the ground wire that connects the relay supply from the rest of the amp's circuitry.

My other thought is are you pulling too much current from the relay supply's transformer causing it to over heat and lose stability?

[Enzo]

Yeah, your 1VAC means the poor supply can;t keep up with the relay demand. Lose the 500 ohm resistor.

[lowell]

Ok, the only issue there is no more pi filter and the hum may come back. I did buy a 12v regulator so I'll throw that in there and use this schem. I'm also curious why some power supplies have a small cap in parallel w/ a large cap. This schem doesn't have that but thought I'd ask. What's the purpose of this? Are the smaller caps faster at filtering small ripple, or delivering current in small fast burst and then larger current demands are fed by the larger cap after the small cap drains?

[Enzo]

Your 1000uf cap or whatever is there to make smooth DC. The little film cap, .1 or whatever< is there to shunt off high freq noise the large cap can't react to. And if you are talking about on the regulated output of a regulator, it is there to help prevent the regulator from oscillating.

What can happen is that a regulator can sense the output changing, so it adjusts itself to compensate, but might overshoot just a hair. The thing is lightning fast, so it might sense that overshoot and try to correct THAT. which causes it to go the other way and it tries to recover, etc etc etc. SO a little cap on the output of the regulator doesn;t allow any high frequency wiggling on that otuput for the regulator to try to respond to. That is why the data sheets shows a small cap on the output with a note that it should be physically as close as possible to the regulator itself.

I think, just my opinion, that you are to focused too much on the power supply to the relay COIL as a source of hum. The coil is not in the signal path, and if the ground return for the coil (through a footswitch or whatever) doesn;t share any conductor with the signal grounds, then it won;t inject any hum. Just a reasonable filter on the little power supply will get a nice smooth relay action - that is the relay coil will mechanically activate the switching contacts.

Defaced has the idea, float the thing. Make up your 12v supply, just basic filtration, and connect it to the relay coil and a switch ONLY. Don;'t ground ANY part of it. Still get hum when it is on?

Here is another experiement. Make up a battery pack of the right voltage. if the thing is sensitive enough, a 9v battery might energize your relay. Now use that to switch your relay. If it still hums when on, you know it sure ain;t the battery.

I suggest that because, what if the hum is something about the channel that switches in and not the relay at all? The battery test would reveal that. The relay would be just doing its job, and the hum was part of whatever circuit it switched in.



YOu originally added the pi filter and your hum went away, but then later it came back. Well it came back but you didn;t remove the pi filter, which tells me that something else is the problem, like a intermittant ground somewhere or something.

[lowell]

ok right on. if I float the coil and it hums then there HAS to be AC on it? Is that what that test is after?

I'll redo this circuit and report back.

[defaced]

That's the thought. If it doesn't hum floating, then you've got a grounding problem when you do ground it. If it does hum, then we've got to look elsewhere. AC on the coil is a possible culprit. If you want to test for that, I'd see if you've got a laptop or printer supply laying around (switch mode power supply), and see what your setup does with that. I've been finding them at the local Goodwill for next to nothing and using them for for various DC applications.

[Enzo]

Try the battery trick too.

[defaced]

Forgot you mentioned that, yes batteries are the absolute best.