[Tone Deaf]

Thought it was something that only happened to other people, but I have my first ground loop!  Two monoblock power amps (DIY mullard 5-20's) that work perfectly in isolation, hum annoyingly when both connected. And more annoyingly, worked fine together in a different location.  Grrr..

These were one of the first projects I built & I can see some mistakes (& untidiness) to improve on - mainly the connection to chassis ground is at the 1st PSU capacitor rather than at the signal input terminal.  Hopefully that'll fix it, but I'll add a switchable ground lift for good measure. 

Any tips on grounding gratefully received (and is there any special things to watch for with monoblocks?) or any hum-related sob-stories?  All of a sudden I empathise..

TD

[Niteshade]

Ah.. Try grounding the RCA input to different places under the chassis. Try grounding it first near the tube you're using as a voltage amplifier. (Your input voltage amp.)

If your RCA input is NOT isolated form the chassis (where it bolts on to the chassis)- I recommend doing that first.

[vett93]

Do you have the hum when connected to the preamp? Or the monoblocks just hum by themselves?

[Speedskater]

You might find this Dave Davenport paper helpful.

http://www.raleighaudio.com/Audio%20Component%20Grounding%20and%20Interconnection.pdf

[jtwrace]

You might find this Dave Davenport paper helpful.

http://www.raleighaudio.com/Audio%20Component%20Grounding%20and%20Interconnection.pdf

Thanks for that!  What a great paper.

[ratso]

http://www.audiocircle.com/index.php?topic=90251.0

lots of stuff discussed. ground loops = PIA

edit: as i mentioned in that thread, i cannot urge you strongly enough to follow the well laid out troubleshooting steps as described at jensen transformers (under white papers - troubleshooting guide. you can skip down to step 4 if you like. you can make the test adapters as shown or buy them for a few dollars from jensen). it can save you a very long time and a lot of hair in the long run (from not tearing it out). please DO NOT randomly start buying products without figuring out what/where the problem is as your chance of success is not good.

[Tone Deaf]

Thanks all.  I'm reading through the Davenport article & getting some ideas.   My power supply is probably not the clean one he describes, just remains to be seen if I can clean it up since it is a mixture of PCB & point to point components.  Will have a look.  Also looked at the Rod Eliot loop interruptor circuit, but I like the idea of fixing the problem instead of just coping with it.  I reserve the right to change my mind if it gets the better of me!
 
Vet93 - the hum disappears completely if i disconnect either one of the input cables OR either one of the power cables, ie there is no hum if any 3 of the 4 cables are in place - the 4th wire brings the hummmmmmm

TD

[Tone Deaf]

Some success.

Separated signal ground from chassis and attached a croccy lead to sig ground at the input.  I measure a fair voltage between floating ground and earth but little current. 

Following Niteshade's suggestion & connecting it to various points on the chassis, I get more or less hum, with the loudest place being the bolt securing safety earth to chassis.  The quietest place (perfect in fact NO hum) is one of the mounting bolts holding the mains transformer.  My suspicion is that this is because the resistance from this bolt to the earth pin on the IEC socket is 18r, about 10r more than chassis to earth.

So I can fix it, but still not sure I've cracked it!

I've got some ideas & a picture that, if I can work out how to upload, I'll talk about in a while.


td

[Tone Deaf]

I've read around the issue a fair bit now - problem is I can't wok out if my grounding scheme is star-like or bus-like.

Safety earth and electrostatic screen from mains transformer are bolted to chassis near mains inlet.  Then I made a star ground at the terminal of the 1st filter capacitor with the centre taps from HT & heater, & from there to the PCB where all other components are mounted.  Here is the circuit board - it has 2 ground planes connected by a wire link.  The star just mentioned connects at the point labelled "Earth".

http://www.audiocircle.com/index.php?action=gallery;area=browse;album=622

I'm unsure what point on the board was intended to connect to the chassis ("Earth"?  Most guidance I've read would suggest the cleaner of the 2 grounds is preferable) and where to ground the off-board components.  Any suggestions?

What I'm tempted to do is implement a bus bar earth from 1st PSU cap to ground at the input socket, disconnect the link between the 2 ground planes & treat them as 2 stars to connect to the bus. Does this seem like a good move? 

There are some more pics of this amp in my gallery for anyone interested in taking a look - DL7-35 replicas.

Ta folks
TD