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beginner forum: for novice and developing blues harp players >
Amp hum
Amp hum
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Fil
78 posts
Dec 02, 2015
11:23 AM
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I bought a Fender Pro Jr a couple of years ago, my first significant purchase and while I was still in the throw money at it stage. It was a feedback headache until I swapped an AXfor an AY tube. I like the clean sound with my lo z BMA 57 from Greg. I had put it away for about a year after I got a VOX modeling amp for practice. Pulled it out a few days ago and this is my issue...with the 57 plugged in thru an impedance transformer (IT) I get a hum that varies with volume. The IT is one of those with the 1/4 plug and a 6" or so cable to the transformer. When the IT rests on or over the amp, it hums. When I raise the IT above straight above the amp, no hum. An IT with no cable hums. When I plug my hi z bottle-o-blues in, no hum. No mic plugged in, no hum. I tried shielding the IT with foil (ala tin hat theory, I guess) with no effect. Is there a quick answer? I could build a stand to suspend the IT above the amp.... BTW, I am a dinosaur late to the game.
Thanks.
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Phil Pennington
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MindTheGap
816 posts
Dec 02, 2015
11:43 AM
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Maybe some magnetic induction going on between amp and the impedance matching transformer. Aluminium foil can shield for electric field effects but not really magnetic effects. And anyway the IT is already shielded electrically. I thought they might be shielded magnetically too with some mu-metal (very high permeability metal used for this kind of thing) but I don't know.
Are you sure the amp is OK electrically? i.e. in good shape earthed correctly? Any doubts on that score and I should get someone competent to test it.
But from the symptoms you describe (i.e. hum changes as you move the IMT around) that is consistent with it being some magnetic effect.
For the sake of it, I would check your XLR cable from mic to IMT. Try another one.
If the XLR cable isn't the cause, then you could buy a 1/4" cable extension which would allow you to get distance between amp and IMT. You can either get a cable with a 1/4" socket on one end and jack on the other, or get a short barrel-type unit that has 1/4 sockets at each end. The IT goes in one end, a conventional guitar cable goes from the other end and into the amp.
Last Edited by MindTheGap on Dec 02, 2015 11:59 AM
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SuperBee
3019 posts
Dec 02, 2015
1:28 PM
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Yep, I reckon moving the IMT away with a short extension would be the go. Or don't use that loZ mic with the amp. I've got amps that hum in one room of my house, but worse with certain cables. I worried for a while but that didn't seem to help. It was just about all my amps and both guitars...I've heard fluorescent lights can cause problems that way...
I really don't know the specific issue though but it's gonna be about the interaction of magnetic fields...amplifier has a lot going on and just the layout of the amp can cause humming...for instance, when I built my champ I had the lead from preamp stage one plate to volume pot crossing the AC to power switch. Resulted in exactly the symptom you describe. I moved that wire about 15 mm and hum disappeared.
If your amp functions normally with a guitar and with a hiz mic, and loZ without imt, I wouldn't be concerned about the amp.
If you use a pedal at all you should plug the imt into the 'in' side of the pedal. I have a similar device which I think I just let it dangle if I use it plugged directly in the amp. Which I rarely do.
Last Edited by SuperBee on Dec 02, 2015 1:32 PM
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MindTheGap
817 posts
Dec 03, 2015
1:07 AM
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"I worried for a while but that didn't seem to help" :)
This is the kind of thing (link below), a 1/4" adapter. I have a couple of these so I can make a long cable runs out of two guitar cables when needed. Not actually this one, but look it's 75 pence!
Electrovision-635mm-Mono-Female-635mm-Mono-Female-Adaptor/NCI
But even better is SuperBee's suggestion of using a pedal e.g. if you have a delay pedal in the chain you can go mic -> IMT -> pedal -> amp and then the IMT is kept away from the amp.
While I think of it, the IMT's available don't have a great design IMO. Either they go straight into the amp and you have this big lump of metal sticking out - vulnerable to being knocked and bent, or stuck out the side of a pedal ready to be stepped on and bent. Or dangling off a 6" lead. I keep meaning to put one in a little RS box with XLR socket and 1/4" socket. But other thing are always more important.
The theory is that the hi-Z side of the IMT should be as close to the amp input as possible, but it's taken to extremes with this design I think. And I couldn't find any different designs.
Last Edited by MindTheGap on Dec 03, 2015 3:13 AM
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Fil
79 posts
Dec 03, 2015
9:01 AM
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Thank you, gentlemen. $4.23 solved my problem. I bought the adapter and used a 6' 1/4" cable I already had to get distance between the amp and the IMT. No hum. You also provided me with a rationale for the delay pedal I've always wanted (redundancy is good, re the hum), so in somewhat of a good conscience I can offer this suggestion to my wife as a Christmas present I could use.
Phil
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Phil Pennington
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