Electro Harmonix 12AY7 Mic HUM problem – $33 solution for newbies
Have you got an EH 12AY7 Mic pre with hum problems?
Well, I did and this is how you can nail that annoying noise:
1) buy a short XLR to TRS (“stereo” for us, newbies) Jack cable (it should have a XLR female and a TRS male jack) – around $3. If you find the TS (“mono”) DON NOT buy that one, is useless;
2) buy a Behringer HD 400 hum destroyer (do not pay attention to the fact that is Beh*, is just a box with two transformers and some resistors/caps in it, you will smash your head against the wall how good it sounds) – around $29.
You have saved one buck for the road
Now, connect your microphone to your valve preamp, connect the wall wart to the preamp and then stick it into the wall (if you are doing it the other way, you may see nice sparks when the power plug touches the preamp case, which is nice though ).
Connect your short cable to the XLR valve microphone preamp output and the jack to the “INPUT 1”. It is important to use the “INPUT 1″ port, since this is the only one that provides grounding for the case (clever design, to avoid any ground loops, btw).
Now connect another balanced/unbalanced jack cable to your computer interface, mixer or your guitar combo.
Miracle, THE HUM IS GONE !