DIY AC power conditioner for theremin

Posted: 3/19/2014 1:34:06 PM

From: Brooklyn,NY

Joined: 12/1/2009

Hi folks,

heres an idea that I have been thinking about for a bit, as I seem to be recently plagued by noise and interference thru the power line...

How useful would using an isolation transformer (on the AC) be in eliminating unwanted noise and 'junk' ? Proper isolation TX are kinda expensive. For years Ive been using two 12v TXs back to back w/ secondaries connected to get isolated  (and lower current) 120v to power tube preamps.  I did a little experiment yesterday,  powering my theremin off such an arrangement, and it seemed to block out some of the noise (clicks & motorboating) I get when any kind of battery charger is also plugged in.       I know, most of the real nasty stuff is going to be coming in thru ground ( like the ring modulation interference when two theremins are in close proximity)... but Im thinking add some caps and some chokes or ferrite rings, put it all in a electrical junction box w/ outlets- may be a pretty good fix for stuff like dimmers, fluorescent lights,  battery chargers...

something like this:



Anyone use the 'good' power strips with RF/ EMI filtering?  How effective are they?

Posted: 3/19/2014 4:22:10 PM
RS Theremin

From: 60 mi. N of San Diego CA

Joined: 2/15/2005

Hello Chobbs,

IMHO (-'

The only way to clean up a noisy ground is to get a better earth ground from somewhere else, a good ground is the only way to have a happy theremin that is pure sounding. Earth ground has more than one purpose in ideal theremin operation.

My guess is your noise first comes in through the Antenna, then Power Supply, and then Earth ground?

Use an AM Radio and check the background noise from the Big Bang right up to your fluorescent lights in the vicinity of your theremin.

The way I have solved this sporadic “EM” noise issue is using a lower impedance connection at the circuit that is feeding the antenna. On my solid state oscillators this is easier to do but on my tube oscillators I am not sure how to do this effectively without more experimenting. Possibly an unwanted Band-Aid is a smaller series capacitor but I would have concern about it affecting your linearity.

When I have tested higher impedance antenna setups I have seen 60 Hz hum EM coupling between my hand and the antenna when it was just breaking out of Null 18” away. My huge body collects energy from the electric field in my lab/room and capacitively transfers it to the antenna.

I believe this front-end impedance issue could be a major problem for digital front ends that rely on high impedance to work effectively. The antenna is a floating input to a very high-gain amplifier.

If you are talking about a solid-state theremin and wanting a cleaner ideal power supply for $5 let me know.

I don't have noise issues today solid state or tube/valve.


All our power lines are underground so I don't get the frying insulators effect on damp nights.


Posted: 3/19/2014 5:28:39 PM

From: Brooklyn,NY

Joined: 12/1/2009

What happen to my image? 

Its my "Hobbs Theremin"    And I have a really good regulated power supply with separate on board regulation for both the pitch and volume boards.   Noise usually isnt an issue, nor are birdies and other interference (well i do have some noise floor hiss that Id love to reduce - regulators?  ...but thats another issue).... but I have discovered that certain lipo battery chargers and 1 specific Boss AC adapter add some noise.    Motorboating at a specific pitch freq in the case of the charger,  and the boss adapter adds a hissy woosh just as the pitch rises from null (I have another identical Boss  that doesnt add noise)

My quick/dirty TX pair (no filtering)  did eliminate the interference from the battery charger but not the boss adapter - which I should add is only happens when connected to either my dlx memory boy or boss loop pedal (both have digital circuitry)   but not  other full analog pedals.  Also it doesnt matter if if the pedals are in the chain or completely unplugged and out of the loop  ( powered but no instrument cables connecting them to anything)

Clearly the easy answer is 'dont use that adapter and unplug the charger' 



Posted: 3/19/2014 5:45:30 PM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as . ...................................

Joined: 12/7/2007

You posted the above while I was typing the reply below..

"Clearly the easy answer is 'dont use that adapter and unplug the charger' "

Get an iron box, oe a box which you surround with several layers of that flexible magnetic sheet used for fride magnets - also cover this box with grounded alu foil or the like.

Put your noisy power supplies into this.

Use clamp-over-cable ferrite filters on input and output supply leads.


Hi Chobbs,

Cannot see the picture! - Also, which theremin are you talking about? (Tube?)

I think the order of noise / interference is most likely to be 1) Antenna 2)Ground 3) Mains.. Mains is converted to DC in the theremin, so checking the cleanness if your DC supply in the theremin should show up any mains bourne stuff, and if there is any, this is probably the best place to get rid of it.

If you can improve the ground connection by reducing resistence and infucktance ;-) in it, this will help - Putting a mains-side filter is a good idea, but you might get a bigger improvement if you have your ground wire running seperately to the L wire si that noise on the L doesnt couple into the mains ground lead..

This however aint a good idea if the ground is needed for safety reasons (as is the case on some tube rheremins)

Antenna - side hum can be reduced with a series cap - say 220pF ... But this will marginally reduce sensitivity and may not help with any HF noise or mains interference from items generating a lot of harmonics.

Wall paper your wall, cieling and floor with alu foil or space blanket, and ground this, and a lot of your problems will dissapear! ;-)




Posted: 3/19/2014 6:04:26 PM
RS Theremin

From: 60 mi. N of San Diego CA

Joined: 2/15/2005


You have given more clues:

... but I have discovered that certain lipo battery chargers and 1 specific Boss AC adapter add some noise.    Motorboating at a specific pitch freq in the case of the charger,  and the boss adapter adds a hissy woosh just as the pitch rises from null (I have another identical Boss  that doesnt add noise)

Do you have a link to the Chobbs Theremin, I live in a box and don't know what that is as I never look at what others are doing?

Solid State or Tube?  Is that it in your avatar picture?  What voltage powers the theremin oscillator circuits?



Posted: 3/19/2014 6:11:07 PM

From: Brooklyn,NY

Joined: 12/1/2009

my avatar is of my RCA replica-

This probably wont help much but heres a photo of the 'hobbs'

no tubes- all SS - jfets-  12vdc regulated wall supply to 9vdc regulators on p & V circuits


just added an photo album- for those interested

Posted: 3/19/2014 6:30:08 PM
RS Theremin

From: 60 mi. N of San Diego CA

Joined: 2/15/2005

You may think you have good filtering but I needed a rock solid clean 12 volts for my optical tuning on my tube hybrid and found this regulator, look in the middle of my webpage. I scrapped the two 7809 setups I was using for pitch and volume and think this will solve your issue using a noisy battery charger. Also these basically develop "no" heat which is ideal to minimize thermal drift. At the 7809 pad's I put a jumper from pin-1 to pin-3.

There may be other things of interest to you on that page which is public to most visitors.

IMHO the noise is coming through the power supply and not the electric field around the antenna.


Posted: 3/19/2014 7:19:12 PM

From: Brooklyn,NY

Joined: 12/1/2009

Christopher- Just ordered the regulators you recommend.  Ill try them out and let you know if there are any improvements.

Not too worried about the charger or adapter noise- in particular... I posted this originally as more of a hypothetical fix-all (or fix most at least) remedy for theremins in general, not necessarily mine.   I take it you guys dont put much stock in the deluxe pro($$) power strips  (furman) and line conditioners... Should of guessed- most of the info I found was on the kind of overzealous, under-informed High-end audio sites that are into ball bearing dampening pads for power amps.

Posted: 3/19/2014 11:10:22 PM

From: Northern NJ, USA

Joined: 2/17/2012

"just added an photo album- for those interested"  - Chobbs

Sweet!  I like the peek-a-boo coils!  I imagine the original intent was to have all 3 vertically oriented?

Posted: 3/20/2014 12:07:32 AM

From: Brooklyn,NY

Joined: 12/1/2009

No- they are RCA replica coils so I borrowed Lev's coil orientation. / spacing.  Yes, visually it be better all vertical, but Im not going to argue with genius.

You must be logged in to post a reply. Please log in or register for a new account.