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Boss Se-70 hum

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Overtone85:
Since I've got my MP-1 in October I've been working on my first rack and obviously I am encountering noise or ground loop issues.

I am using 2 Boss SE-70 units for post distortion effects. I noticed that they have quite some hum even when the effects are totally bypassed.

At first I thought it was the toroidal transformer I mounted at the back of the rack. I am using it to power both units at the same time because I don't have space for the huge wall warts the SE-70 comes with.

I also tried to isolate them by removing any other units from the signal chain and connecting the SE-70s directly to the power amp. And powered them with the original wall warts from outside the rack. And they definitely have some hum of their own.

I haven't tried to move the SE-70s out of the rack yet though.

I found this thread where the OP mentions he modified the SE-70 to accept a standard connector and he powers 2 SE-70 with a single 65w 12v AC adapter and he said he got rid of the noise.
http://adadepot.com/index.php?topic=1771.msg19909#msg19909

I get the same amount of noise with wall warts and the single toroidal transformer, maybe even slightly less with the toroidal. Certainly a lot more hum than the MP-1 (noise modded :) )
I am wondering if this depends more on the internal filtering of the units? These things run quite hot so maybe the electrolytics should be changed, at least around the power supply section...

Is anybody using the SE-70 and had this issue?

Cheers
R

vansinn:
Do you have any hum with the MP-1 stand-alone, i.e. with no cables connected to the SE-70?
Likewise, any hum with the SE-70's alone?

If yes to both questions, you likely have a ground loop issue, in which case you could try to fully isolate the two SE-70's outside your rig, but with all signal cables attached, to see, erm, listen if there's still hum.

Overtone85:
The MP-1 alone has very little hum. It has the DMRT, noise mod and I just replaced all the plate and grid resistors to 1/2w metal film on the tube board. I can only hear it when I crank the power amp fully and it's barely noticeable under the normal hiss. OD1 3.8, OD2 6.0, master 5.0,  guitar unplugged. I never tried to change the tubes though, still has the GrooveTubes 12ax7 in it.

The SE-70s alone have more hum. When I take them outside the rack the hum doesn't change in intensity. And I mean only those connected to the power amp and nothing else.

The power amp by itself, when cranked at full volume, has a tiny bit of hum but you hear it only with your head against the speaker. As soon as I connect the SE-70 output to it the hum increases. its's still quiet but it's more.

When I add the MP-1 to the chain I get a slight further increase of hum. So today I insulated the rack ears on all the units with electrical tape and insulated the screws with plastic washers and heatshrink tubing. The situation did not change, but I added a ART hum eliminator between the MP-1 and the SE-70 which fixed the increase in hum, so now I am only left with the hum generated by the SE-70 themselves.

The SE-70s are powered with wallwarts. The adapters have a ground pin but at the other end the barrel connector has only 2 contacts, tip and sleeve. So I wonder how they get grounded power wise, is the whole circuit floating? The transformer I am using to power both in the rack is a 240v triad electronics toroidal transformer, with 2x 12v taps, 2A each tap, it's bolted to a rack tray, does the job.

Both the SE-70s get quite hot during operation and the have a bunch of electrolytic caps in them, so maybe they got old due to the temperature. I wonder if replacing at least the ones around the voltage regulators and power supply could make a difference in the hum.

EDIT: I did try to put the ART Hum Eliminator between the SE-70s and power amp, but that didn't remove any hum from the SE-70.

vansinn:
Good, your isolation tests cleared out those issues.

Well, as your wallwarts have no grounding, the circuit essentially is floating.
However, WW's mostly doesn't generate much hum themselves, and, AFAICT, you've replaced those with your own toroidal supply - but is the output used as ac or dc?  What about rectifiers and caps...?
If ac, your toroid could generate hum, especially as it sound like it's free-mounted. 

Sounds like you've likely identified the problem yourself: Could likely be those old caps within the SE-70's ;)
Plus maybe something related to your toroid setup..

(EDIT: Fixed God for Good ;) )

Overtone85:
The original Boss WW are AC-to-AC, no caps and no rectifiers in them. The rectification and filtering happens in the rack units.

So I sourced a 240v 4A transformer with 0-12vac-0-12vac taps and I found some high current barrel connectors that fit the SE-70s. I mountet the toroidal with the bolt provided on a rack tray just above behind the SE-70s. I keep it on the left side and the signal wires are all on the right side.

The transformer itself is a bit noisy/buzzy but it's actually causing a bit less hum than the WW, at least that's what i thought when testing outside of the rack.

Anyway, tonight I was playing with the rig and tried to listen to the hum again. With the hum eliminator in place, and this time I used both channels, one for MP-1 -> SE-70 and the other SE-70 -> PA. I cranked the PA at full volume and under the hiss the hum was noticeable but just not as loud as the hiss. it's very unlikely I'm gonna crank it that loud anyway.

I am definitely happier now, since I have also recapped the parametric eq I run in front of the MP-1 and everything is much cleaner than before.  8)

I think I'm gonna leave it like this and then try to replace the caps in the SE-70s when I buy my next order of components.

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