marshall 8080 low level hum

[Mars Amp Repair]

Hello folks,
I am servicing a Marshall 8080 valvestate 1x12 combo that has a low level hum/buzz with all controls fully ccw, send/rcv loop open, as well as the pretube removed.

Filters good & power supply clean. You'll get this hum/buzz from the line-out with all as described above, too.

It is really only an issue as the guy uses it for recording, otherwise I would think most wouldn't even notice it.

My question is does anyone have one of these to compare or possibly recall if they just inherently have a low level hum/buzz. My take is that it is an inherent ground loop issue that we'll have to mod to get rid of.

Any ideas...thanx, glen

[Robert M. Martinelli]

Hi Glen,
I once repaired one of these amps belonging to a friend, and, though I didn't like the way it sounded, I don't remember it as being terribly noisy....

I agree that the amp doesn't have a "real" problem, and what little hum is there, it's an issue only because the amp is being used for studio recording.

I think the "problem" could be due to poor/cheap/insufficient preamp's power supply voltage stabilizing/filtering ( good for "amateur" use but not good enough for "professional" use ) rather than a ground loop though.

I'd advise you to mod the preamp's power supply by removing the 15V zeners, cutting the positive/negative PCB rail and throw in a couple of 7815/7915, as they are better stabilizers than any zener diode could ever be, and they also have an excellent ripple rejection ratio. I'd also improve the filtering for the preamp section by "souping up" the filter caps. A couple of 100 nF poly caps between each stabilizer's in/out terminals and GND are not out of question. At this point the problem should be at least greatly reduced, if not eliminated. I have sometimes added small caps (100 to 220 nF) between each op-amp positive/negative supply and GND (as close as possible to the relevant op-amp) and this has helped with some very "hard to die" hum issues.

I did all of the above on various SS amps in the past with very good results.

JM2CW

Hope this helps

Best regards

Bob

[Mars Amp Repair]

thanx bob,
All good suggestions. I'll have to see how serious this cust is regarding cost to mod this amp.
I was thinking because I can interrupt the signal path at the return jacks & still have the exact same level hum, that the issue might be more related to ground loops than something in the higher gain stages (which would be out of the circuit with the effects loop open). I do see where they are using the +-15V for the diff amp, tho.

I'll check it out & let you know. Thanx, glen

[Lee_ranaldo]

i've had the same problem with ampeg designs

that means if the jacks and ouputs are isolated any thance that the input jack or another control touches the chassis will hum horribly even when the controls are set to 0.

my suggestion is to replace at least the fillter caps and the ones in the regulator section if there is any.

also coupling caps (electrolitics) mostly will improve the transparency of the sound, giving a bit of more bottom end and less sag.

good luck!

[Robert M. Martinelli]

Hi Again Glen,
I thought about the "problem" being related to the preamp because you stated that the hum is already present at the "send" socket....

As to the customer, as you said, preparing a quotation for the mods could make for a good "filter" - many hair-splitters or cork-sniffers instantly become far more accommodating when they discover that cork-sniffing is expensive - he might instantly discover that, after all, he can live with a little hum

Cheers

Bob

[Mars Amp Repair]

hello folks,
thanx for all the great info. I did manage to trace down the 'hum' issue to what basically amounts to one stage drawing current through the ground trace & causing that hum to be injected into the final op-amp stage (diff amp). Not really a 'ground loop issue' but similar.

This reduced the hum by more than 1/2, which was then completely acceptable for this amp.

Find attached pix to show the solution...thanx all, glen

[Mars Amp Repair]

Follow up,
The cust was very pleased with the resultant reduced hum.

Bob; yeah I can see where the info about the hum being present at the 'line out' jack would be confusing. Fact is with an audio tracer, the hum seemed present just about everywhere depending on where you put your ground lead.

BTW, I find most of these weird ground loop issues with my home brew high gain audio tracer. It's not really a 'home brew' as in 'I built it', but it's an old small radioshack amp from the 80's or so with 'mag-ceramic' and 'Aux' inputs.

I've built the same thing for my other techs with a mag phono preamp & powered computer speakers switching between the 'mag' preamp & bypassing the preamp for lower gain. I do this mostly because I'm cheap & refuse to pay $140.00 per bench for the prefab crappy low freq response-tiny speaker pre-built ones you see in CE Dist, for example.

I end up finding the powered computer speakers for about $5-10 & the preamp for about $10-20 in MCM unless I stumble upon one in the thrift stores.

I change the input coupling caps from electrolytics to values around 0.01uf to keep the shock of connecting it to hi volt like plates of tubes from knocking out the 1st stage.

I switch between Mag Phono & line in for the different gain settings. I have a simple audio switcher that allows me to also switch my probe between the 'aux' & 'mag' inputs with more ease than switching the jacks.

I hook the ground lead where the circuit in question gets it's ground & then start probing around that circuit or even just touching the chassis from one side to the other to determine where the current draw is creating the hum.

Works pretty good. It is difficult to accomplish this with a scope as you really don't know if you're actually seeing the hum you hear or just looking at some errant anomaly...an audible indication is much more telling and you can always use the scope if the situation calls for it

Pix are of 'Realisitic amp & switcher' for audio signal tracing. I have this hooked up to a small rat-shack 3" speaker in a metal box that provides great freq response in a small space.

The second is the computer monitor/rat-shack preamp, both from thrift stores for under $15. The third is of course the Eico audio signal tracer with 'magic eye'. Gotta love that.

thanx, glen