Hello all, this is my first post, I hope this is not entirely threaded water (I searched and saw related questions, but not exactly the same).
I have a B3 instrument that I have used a few times with lent pro amps (on stage, and on studio). There it sounds crystal clear, so I don't think the theremin is defective.
However, in my many other attempts at several places, with simpler equipment, I always get a ghost tone and also the sound seems more "aggressive" to me (I guess, more noisy). There is not a fixed frequency hum, as I read in other cases.
I've read about the critical importance of grounding. Curiously, the AC adapter that came with the theremin has no ground pin. Also, I see no way of identifying if it is linear or commuted.
This bad behavior happens, of course, when directly using headphones. However, it also happens with an entry-level grounded keyboard amplifier from Behringer. It could well be that the amplifier is ignoring the ground, but with keyboards it works OK.
I've also tried to ground the audio output with a single cable from the metal ring to the ground pin that protrudes in EU sockets. It somewhat diminishes the ghosting, but it's still unconfortable.
TL;DR: In my limited experience as keyboardist I never had such noise problems. The end of my expertise is using an isolation box. So, I was wondering if something along these lines (e.g.: http://thehub.musiciansfriend.com/live-sound-buying-guides/how-to-choose-the-right-direct-box ) could be the final solution for the grounding problem (I'm betting here that interference isn't the problem, since in several places it sounds the same), and in that case what would be the right choice.
I intend to also check with a linear AC/DC adapter I saw recommended in another thread, but since I'm using the original one I have low expectations for that.
I hope my question makes some sense. Thanks for any advice!