Re: Boss DD-7 hum issue

Hi Jon

Hi Nick, thanks for an amazing reply! FYI I contaced Roland and they refused to help as I bought the DD7 from Thomann who are not an "authorized dealer", so I contacted Thomann and they have arranged for a return and repair which I didn't actually ask for but might as well try, as on further inspection the DD7 input socket seems pretty ropey.

Thomann not an authorized dealer?? So I guess Roland/Boss don't want or like the dirty money that Thomann make for them then? Who supplies Thomann I wonder? Playing Devil's Advocate: Are Roland kind of hinting that Thomann are selling fake Boss pedals perhaps as they don't appear to want to take responsibility for kit they've produced in their factories otherwise? Hope the replacement/repair is better in terms of the input jack. I just bought a GT-10. I didn't bother to check if the dealer was a Roland dealer - maybe I should have... ooo-er....


I've spent the day messing with swaping the DD7 with a DD5, swapping the verbzilla with a behringer reverb (which is quieter - the LED on the verbzilla seems to make a noise), and getting mild electric shocks when I touched a loose patch lead and my guitar strings/pickup at the same time. Electrocuted by an acoustic instrument!! Rock n roll.

Seriously, it's probably not a huge amount of current, but it would be worth checking if any voltage potential exists between the strings and the screen connections of any jacks on the board using a digital meter. On electric guitars the metalwork including bridge is usually connected to ground/screen but I've not seen that on an electro-acoustic. It's possible of course. I would be concerned about this just for peace of mind more than anything, because obviously the last thing you want is any hint of mains voltage bridging the input and output of your PSU, although if you are using ground lift at your DI box that may be having an effect.



OK. I might be wrong, but I think you have one or more ground loops. I think that your hum problem may be attributable to several different causes which are possibly summing together into what you are hearing. Treating each cause should at least reduce the problem for you in stages.

There would seem to be several points in your two guitar/FX chains where the screen, connections come together, and this could be causing the hum as what you have are potentially a number of ground/earth loops. I also think from your picture that what appears to be your PSU appears to be too close to the guitar input leads and at least one of the Boss parametrics. I think this is potentially a prime source of hum. Personally I'd probably have placed the PSU as far away from the guitar input leads as possible and probably at the top about 4" above the LS2 (OK - I can see the box you have won't permit that, but even so I think in use I'd rather have the PSU outside the box).

I will move the PSU as far as I can from the input cables - maybe to the far left of the board by the DI box? Is there anything I can wrap the PSU in to shield it? It doesn't get hot.

Is the PSU encased in a plastic box? If it truly doesn't get hot (it probably gets warm), sticking it inside something made of metal and grounded might help, like a suitable sized tin - but in terms of H&S you'd need to make sure that there is absolutely no chance of the mains input cable chafing on any sharp metal edges (I'd suggest a rubber gromit around the cable at the point of entry). I am a bit dubious to be honest of putting the PSU inside an unventilated metal enclosure because of the potential for heat to build up. The PSU is designed as it is to run sufficiently cool in air. It would probably be OK and if it were me I'd probably chance it just for testing purposes. You might find that it would be better to apply the type of foil shielding that you'd find inside the pickup cavities of some guitars. You've got the problem of grounding the shielding whatever you do. I think it will be sufficient to move the PSU from its curent position TBH

The FX chains seem to be a little different in the pic to what you described if I've correctly identified the patch cables - it's a little difficult to see (and obviously the pic was taken pre DD7). I make it: CHAIN 1: guitar pickup A-> top right Parameteric -> LS2 B Return; CHAIN 2: guitar pickup B -> Blues Driver -> Super Octave -> middle top Parametric -> LS2 A Return. Then COMBINED LS2 Output -> DD5 Delay input -> Panning out/Output -> Line 6 Verbzilla L/R stereo inputs -> Verbzilla L/R outputs -> Stereo D.I. L/R inputs. With the sustain pedal providing Tap Tempo for the DD5.

That is 100% correct.

I assume that you have two independently wired pickups from your guitar, and that you use two cables (I think I see two guitar cables tied together with cable ties at the pedal board's top left on the pic). Do these input leads share a common screen/ground connection at the guitar? If so you could cut the screen at the jack plug on one of these as doing so will break the ground loop that will exist between the commonly bonded two ends of the cables and the signals should still get through OK

It is a Fishman Rare Earth blend, which combines a humbucker with an internal mic, and is active. The cable/s you see are a stereo Y-lead, not two entirely separate cables, plugged into the stereo output of the guitar. I can't really get inside the moulded cable plugs.

OK - so you have two screened leads which are connected by the screen at both ends. You only need one lead to be fully connected, and the other can have the screen cut at the jack plug. If you think about it, the screen in two leads (assuming a 10m cable for argument's sake) forms a loop of 20m and can act as an aerial which will potentially pick up any stray RFI and which then can exist as a small voltage AC current. In an unbalanced cable, rather than a balanced cable such as used with most microphones, an equal and opposite reaction can be induced in the inner core - and this can be enough to give rise to hum. I haven't explained that particularly well and the exact physics behind it may not be exactly right, but suffice it to say long unbalanced cables, the longer they are with an unbroken loop in the screen can give rise to problems. It's similar to the phenomenon of people being able to 'steal' electricity from the national grid if they live near overhead power lines, by creating a 'coil' of wire (in their garden under the local pylons) and getting an induced current from the electro magnetic field surrounding the overhead power lines. Sounds like the way a transformer works - well that's because it is exactly that. Running audio input cables alongside mains cables or near devices that contain transformers (like some PSU's) can induce small ripples of AC current in the audio cables and a few milli-volts can give rise to hum. With a balanced cable, there are two cores twisted together inside the cable, each of which independently will be subject to interference from external sources, but each of the wires in a twisted pair carries signals which are 180 degrees out of phase with each other and so any interference is picked up and out of phase and so is cancelled out.

Guitar leads are not normally balanced leads. (You could get a Variax of course - they can use a standard RJ-45 computer network cable to move the signal to an amp via a POD or similar - however having seen video clips of your unique playing style and having read many of your posts, a Variax acoustic wouldn't really work in terms of the percussive stuff you do on the guitar body, and I might be wrong, but I'm not at all sure you'd like the sound a Variax produces anyway. I have a couple of Variax Acoustics and a couple of electrics - they are good for me in a loud rock band environment when an acoustic guitar without feedback at high volume is required, but the bottom line is that a Variax acoustic doesn't offer everything a real one does, even though it's pretty close a lot of the time). You may not be able to modify the guitar leads, but you could easily make a pair of adapters with in line quarter inch jack sockets to quarter inch jack plugs and simply omit to wire the screen in one of the adapters - of course a soldering iron is required for this.

I remember sorting out hum problems due to ground loops in my little home studio a few years ago. I had cut one end of most of the patch leads and elimianted a lot, but some hum still remained. It wouldn't go away. But I noticed that lifting the earth connection to my whole system made stop. The house was old, but had been gutted and fitted with new mains wiring, new water pipes etc.... The earth cable at the point mains enters a domestic property at least, is usually bridged to Neutral in the distribution box. The other end of the earth cable is normally grounded using metal water pipes in the property. The trouble is though that water is delivered mainly through plastic piping to houses these days, so whilst grounding may be sufficient from a safety perspecttive, it doesn't necessarily perform as well as it could from an audio perspective. Obviously I needed all my gear to be earthed, so I ran a separate earth from a copper earthing spike to a central point in my system and attached it to my gear and the hum went away. For interest, I measured the potential difference between the house earth and my separate earth. I found AC current running at about 1.5v between the two, which indicated to me that the close proximity of the earth wire in the house wiring to the other two wires was causing it to pick up induced current.


Then as all the FX pedals would normally share a common ground via the connections to the PSU, with the PSU effectively being at the centre of a star power distribution system, you could feasibly cut the screen wire at one end of each inter-pedal patch lead which would destroy the earth loop but still allow a path for the signal return, or where there are two patch leads feeding the same device like there are with the LS2, you could cut one of the patch leads' screen at one end - and to be honest, that's where I'd experiment first before cutting the screen connections on all your patch leads as you would not be too amused if it didn't cure or at least reduce the problem. I see that in the main, your are using patch cables with moulded jacks. Cutting the screen on these is difficult and not really reversible - the way I've done it when I've had to is to carefully cut about a quarter inch strip around the cable's outer sheath and then I've carefully seaparated the braiding to break the screen connection before sealing it all up with a wrap of electrician's tape. Patch leads with soldered jack plugs are better for doing this type of thing for obvious reasons. Patch leads with moulded jacks tend in my experience to suffer more from internal cable breaks for some reason as well.

From now on I'll buy those George L's home-kit thingies, or actually buy a soldering iron!

I've had several and I've always found it useful to have one in my toolkit, but you need to be able to solder reasonably well and neatly otherwise you can run ito bigger problems than a bit of hum :-)



I have a 5 way 9v PSU for my stomp boxes when I use them (I've got two in fact - but one's quite old, seen better days and not all the original 9v power cables are there any more. I replaced some with cables sourced at Maplin, but the plugs have a larger inner diameter by a fraction than the originals although they physically fit the sockets on the PSU and the stomp boxes. They can occasionally cause a bit of a thump as they are looser than they should be and can occasionally disconnect - which is the main reason I don't use that particular PSU or more than two or three stomp boxes unless absolutely necsssary (which it never is)).

I have an LS2 as well, but I normally use it differently to the way you use yours, however, you'll be pleased to know it doesn't cause hum, so I don't think your problem is actually the LS2, but is rather, ground loop related.

Good hunting. Ground loops are not usually particularly easy to eliminate.

HTH

Nick

Thanks again for the advice, most educational! I will see what I can do.

No problem. Good luck - and be VERY careful with mains wiring or anything that's giving you electric shocks. Don't just trust what I've written. Check it out elsewhere.


Jon

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