Hoffman relay question.

[Captain chunkulus]

So in the attached picture is shoes the relay power supply connected to ground at the 100 ohm artificial Center tap. Is that correct? I have mine floating. Would I benefit noise wise from grounding it at the ground for the artificial Center tap? Would that fry the bridge rectfier? Thanks in advance for your help.

[sluckey]

Leave it floating.

[hesamadman]

Do you have noise?

[Captain chunkulus]

Yes. There is a lot of buzz. If I take the relay circuit out the noise goes away.

Do you have noise?

[Captain chunkulus]

Okay, so I have tried a lot of stuff. Here is the issue I'm dealing with. I am using the hoffman relay circuit. When i hook up the relays they hum like crazy. When I unhook power to the relays, silent. I have an artificial Center tap with elevated dc around 70v. My transformer is non center tapped. I'm using a fender stand alone reverb transformer  for this preamp build. It is just a preamp. What I'm curious about. Is in one of Dougs diagrams, it shows the bridge rectifier being grounded at the 100ohm artificial Center tap and the filter cap not hooked up to the bridge. On the regulated supply with the voltage regulator it is not. It is left floating. I have also tried a seperate transformer  just for the relays and get the same hum/buzz. I have to be missing something. I thought you weren't supposed to hook the bridge rectifier to ground but the diagram show that you do. Can anyone clarify what I might be missing?  The hum sounds like massive ground hum and it's definitely 120hz. It goes away when the relays are unhooked from power. Thanks guys.

[sluckey]

If you use a separate 6.3VAC transformer and build that circuit that has a bridge, big cap, and 7805 regulator it will work just fine.

[Captain chunkulus]

I have a 12 volt Center tapped transformer I could use. Would I still float it or use the center tap to ground?

If you use a separate 6.3VAC transformer and build that circuit that has a bridge, big cap, and 7805 regulator it will work just fine.

[sluckey]

I would build it like this using that PT...

[Captain chunkulus]

So, I would put the negative side of the relay to ground with a switch to make or break the ground connection to switch the relays?

I would build it like this using that PT...

[sluckey]

yes

[Captain chunkulus]

Thanks!

[hesamadman]

Your relay power needs a ground reference. If it’s floating it will introduce noise. You can’t ground it though if you’re center tap of the 6.3 volt winding is connected to ground. It will short the bridge rectifier in the Hoffman power supply. If you use an artificial tap, it will work. But that relay power needs a ground reference.

[sluckey]

Your relay power needs a ground reference. If it’s floating it will introduce noise.

Not true.

[Captain chunkulus]

I have found that in my build, running the relays off of the heaters with an artificial Center tap hummed like crazy. Using a seperate transformer with ground reference killed the noise. I guess it just depends on the circuit.

[hesamadman]

Your relay power needs a ground reference. If it’s floating it will introduce noise.

Not true.

I don’t have a theoretical explanation. Just one from experience. Floating ground has always cause noise for me. How I learned the problem is this. I’m obviously switching the grounds on and off to operate relays. Using a footswitch. Amp would hum like crazy. As I reached around to unplug footswitch, as I grabbed the metal connector of the footswitch the noise went away. 100% gone. When I let go it would come back. Odd. So I used alligator clip from ground to chassis. Completely removed. I brought this up on one of the forums and someone said “you just experienced an Unreferenced ground”


Since I’m not an electrical engineer, I just go with that. The method removes all hum for me. Now days I use a separate transformer for the relay power but anything previous, this is what I experienced.