|
|
Post by beckerologly on Jul 25, 2007 1:06:32 GMT -5
(EDIT - moved to Effects Units Forum by sumgai, 7/24/07)
Hi all, I have recently aquired a near-new EH Deluxe Electric Mistress. Its a beautiful pedal but the 50-cycle hum is unbearable. Thats right I said 50-cycle, I even picked it as a G-note in a certain octave, and whaddaya know, the G in that octave is 49Hz.
Anyway, this hum is present whether its ON or OFF, but slightly more when on. What makes te difference is if I unplug the power chord.
I live in Australia and it is a USA type pedal so this is how it works:
-mains 340Vpower supply > AUS. to USA general purpose transformer > transformer WITHIN the pedal, unsheilded.
I beleive the pedal requires about 18 V DC that gets 'regulated' down to 12 V by the op-amp. If that makes sense, good, its quoted directly and I dont quite understand it.
I want to either rip out its transformer and replace it with a jack so can use a wallwart situated many meters away, or supply it with 2 9V batteries.
My question is: Is the issue the fact that the transformer is near the circuit board, or can this 'hum' be transferred through the power (i.e. replacing transformers with a NON-AUDIO wallwart general purpose transformer may make it worse)?
EDIT: in Australia our power is at 50hz not 60, also I forgot to mention, this pedal is unusable if this doesn't get fixed, I don't use it with distortion at all, the buzz is bad enough on the cleanest of cleans.
|
|
|
|
Post by sumgai on Jul 25, 2007 2:44:56 GMT -5
beck, Couple of potential trouble spots...... One, I'm sure the step-down transformer is good, but why not try it on another piece of gear, and place it physically close to the EH DEM unit. Don't have anything else that runs on 110 vAC? Hmmm, that could be a problem. Look up a mate and see what he or she can loan you for this test. Two, the circuit board inside the EH DEM has all the power supply components on it, probably excepting the power transformer itself. If you aren't a qualified technician, then find someone who is, and have them check out the power supply's filter capacitors. They shouldn't be a problem, if the unit is truly new or near-new, but one never knows, does one...... Three, along with that thought, one needs to keep in mind that any USA-purposed device that handles 60 Hz just fine may rebel and get downright scurrilous when it has 50 Hz shoved down its throat. In that case, increasing the filter capacitor values may be of help. Not guaranteed to ward off all humming, but it's worth a shot. I see where you're coming from, vis-a-vis the wall wart thingie, but before you start ripping parts out of your new baby, you wanna first make sure that you can get your hands on a wart that can deliver enough power to it. I don't have the exact specs for the EH DEM on hand, but searching EH's website reveals that the battery version of the DEM uses a 24 vDC, 100 mA wall wart. But that very same puppy uses 2 x 9 vDC batteries......  That means that you'll have to double check to make sure you're going for the correct voltage when you're tinkering around in there. Once you have that figure, you'll probably have no trouble coming up with a properly specified wall wart - there aren't many available on the market that put out less than 100 mA. But I do recommend that you have a technician do any conversion of this nature, unless you are well versed in electronics theory and practice. That way, if something goes bump in the night, you don't have to kick yourself, you can kick someone else! ;D HTH sumgai |
|
|
|
Post by beckerologly on Jul 25, 2007 8:23:02 GMT -5
AARGGH every time I think I have this Electronics stuff under my belt something new pops up... I read somewhere that the voltage is regulated 'down to 12 volts so it can take anything up to 24'.... Could using a multimeter to find the voltage drop across the transformer help, so I can drop something equal in it's spot (obviously through a long cable... ) Also, it sounds like my doubts were well... um. doubted. You see, initially I thought that shielding the transformer internally would solve the problem but going by what you've said its the power supply itself? Taking it to someone that actually SHOULD be doing something like this sounds good, but where I live nobody knows anything about anything. Especially about guitars. I've asked around music stores for someone who could replace my pickups, bias my amp for EL34's, replace my floyd rose with a fixed bridge... there USELESS around here! I finish high school this year, and next year im into EE'ing at uni  |
|
|
|
Post by sumgai on Jul 25, 2007 12:52:12 GMT -5
beck, Drop a PM to gumbo and/or JohnH, they're both on your continent. See if one of them knows a good source in your area that can help you with stuff like this. On another note...... In general, power supplies don't accept a range of voltages unless there is a regulator (or more than one of them). In your case, measuring the voltage at the point where the power supply capacitors receive their input (from the diodes) is a good starting point, in terms of knowing how much voltage to supply. To go more than 10% above or below this figure is likely not a good idea, unless you know beyond a doubt that a regulator exists down the line, and that you won't be over-taxing it with your increased voltage. (A regulator knocks off anything over a certain value - where does that extra stuff go? It's discharged as heat!) By the same token, not feeding enough voltage to a circuit will usually result in poor sound or other undesirable performance factors. I've never heard of any components calling it quits for not having enough voltage, so this condition doesn't threaten the life of your device, only the ability to operate correctly. And finally, a regulator needs a certain minimum voltage, or it's just sitting idle. It only clamps down on a excessive voltage, it doesn't raise up an under-voltage. You might wanna dig in on some "light" reading this summer (winter for you!) by perusing Steve Bench's website. In particular, start with his treatise on Power Supplies. That outta keep you busy for a few weeks! ;D But don't hesitate to ask questions, either here or there. You never know, you might learn something that doesn't apply to your current project, but your next one........ HTH sumgai |
|
That means that you'll have to double check to make sure you're going for the correct voltage when you're tinkering around in there. Once you have that figure, you'll probably have no trouble coming up with a properly specified wall wart - there aren't many available on the market that put out less than 100 mA.