RS Needs a Passive 50/60 Hz Audio Notch Filter

Posted: 3/25/2014 10:40:06 PM
RS Theremin

From: 60 mi. N of San Diego CA

Joined: 2/15/2005

I would like to go to Amazon and just add a couple of these Notch Filters to my weekly order.

Sometimes I can see in my sound faint 60 hz residue using a spectrum analyzer which comes in through the antenna and surroundings. In authentic theremin work there are tradeoffs in the balance of the scheme of things to achieve that classic sound.

I have made my own passive and active filters before but there must be something commercially available $5 to put between the theremin output and the power amp. The level of my 60 Hz interference is probably undetectable to normal hearing.

I am not talking about ground-loop-hum, that originates from a completely different method.


Edit: I imagine most would use a 10 Band Equalizer for this purpose but I want to travel light.

Posted: 3/25/2014 11:35:56 PM

From: Brooklyn,NY

Joined: 12/1/2009

could you post a link?


Posted: 3/26/2014 12:47:43 AM

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

What I say below isnt an "answer" to the question you are asking - I dont know the answer to your question, of any pre-made passive notch filters - I dont know - they may well exist, and they may be good - but I dont know.

I give the following in the hope of helping - but I will happily remove this posting if it isnt helpful.

---- NOT about passive filtering ! about modifications / design / active filtering / Volume CV filtering / Harmonics / Other technical stuff ---  and therefore OT ----------------->

If you have hum coming from the theremin (as in, induced by some antenna or other pickup, is not the theremins power supply, and you are certain its not ground-loop related) and you cannot eliminate this other than with a notch filter, here is my advice:

Passive filters between the theremin and the power amp are, IMO, rarely worth the effort - but if you did fit one, IMO it needs to be an LC rather than an RC filter.

If the thereminist plays down at the bass end, any filter could attenuate an intended 60Hz 'note'.

You will have some form of active output stage in your theremin, and IMO this is where any filtering should be applied if at all possible - With the right R's and C's in this stages input and feedback paths, one can usually manage to add quite complex filters using only some additional passive components.. Its usually possible to limit, for example, any HF from the mixer here - and should be possible to have a notch filter - ideally adjustable, so you can trim the 'notching' to the required level that isnt too severe when bass notes are played.

Its quite likely that the hum is only a bother when played signal is really at low volume levels, so you may just find that a simple RC notch filter preceding your output stage, does the job.. A lot comes down to where the hum signal is coming from - if its in the pitch path, then it will be attenuated at low volume levels - if its from the volume antenna / circuit, it could be heard all the time and probably noticed more at low volume levels.

If its from the volume circuit, you may just find that the easiest fix is to filter (or increase the filtering) on the volume CV - But beware that whatever you do here (like increasing the volume integrating capacitor) could make the volume response unacceptably slow - a sharp 60Hz notch filter (or 40/50Hz LPF) on the CV might fix the problem - but I HAVE NOT tried this.

Its also worth keeping in mind that if theres 60Hz, its possible that under other conditions when mains harmonics are higher, you will get higher harmonics (120Hz, 180Hz etc) - If hum is from the volume circuit, and you can tolerate about 25ms of 'lag' on the volume response, then giving the volume integrator a longer time constant (increasing the "holding" capacitor is probably easiest)  may eliminate 60Hz and any higher harmonics.


Posted: 3/26/2014 1:15:13 AM
RS Theremin

From: 60 mi. N of San Diego CA

Joined: 2/15/2005

Hi All,

Fred your input was good. I tried to make a sample sound file that demonstrates classic tube texture as I call it but my Altermen will not misbehave with 60 Hz right now. The spirit is alive in them there tubes. I have two power transformers right next to my RF oscillators, I am amazed a classic theremin works at all.

Chobb this sample is for you, it is not the sound I am after at the moment but it is not that bad either. It is theremin cable to sound card direct. This reveals all with no band aids.

Altermen2.mp3  This is what I mean about unique texture that is not that modern day whistle buried in reverb. I have grave concerns that a generation may never know what a classic sound was the way everyone processes sound.

I am in the final fine tuning stage before a pro Thereminist will try out my original work. Then I can retire.


Edit: I use to have a homemade filter on the line from my hand held optical pickup to the amp, room lights had a 120 Hz flicker I needed to attenuate.

Posted: 3/26/2014 1:46:50 AM

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

Christopher -

That core waveform is quite good, IMO - It has potential in that it is about right for formant shaping by filters etc.. Some audio output transformers capacitors etc with resonances in the right places and you could have a "classic".

But its not there yet IMO - There is an "undertone" but I dont know what it is.. It might be a 60Hz frequency modulation of the pitch oscillator producing sidebands, but thats just a guess... It might even be "unreal" - as in, something from theremin -> soundcard / MP3 processing.

Please dont take any of the above as criticism, I am impressed! ;-)

My expierience with "Pro Thereminists" is that they are unlikely to directly "endorse" or "reject" a theremin - they are more likely (and more usefully) going to give advice about what they like and what they dont -

Be prepared to delay your retirement for a while ;-) You may yet need to do some tweeks - But if the thereminist is there for you to work with, your chances of really creating a classic improve hugely.


Posted: 3/26/2014 3:31:27 AM

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

"I would lay a wager that if you piped an RCA directly from its output through a cable to a sound card my Altermen sounds better. (-' " - Christopher from this thread.

Sorry for replying here, but I really dont want to get dragged into any conversation where the atmosphere "changes" or any argument is likely. I think its a bad idea for us to communicate here - it doesnt do either of us or TW any good when things go sour.. So I will give this quick reply and then vamoose.. Just say the word and I will delete this post.

I absolutely agree that your Altermen would sound better! (Apart from which, your soundcard would probably go up in a puff of smoke if you connected an RCA's output into it! - as you risk meeting the grim reaper if you touch the RCA output terminals  ;-)

But thats not the point - The RCA was an instrument designed to drive one of its speakers, the "instrument" is a "composite" and its speaker is an integral part.

When we design theremins today, we generally provide an audio output compatable with standard stuff (Line level audio).

There is, however, absolutely nothing to prevent someone making a theremin with built-in power amp and dedicated speaker, and not providing a line output.. But if one does provide a line output, the audio from this output should be good.

And I am not saying yours isnt good.. In MY OPINION its better than anything I have heard from you before (by a long way - with the exception of your vocal bucket) and better than quite a few theremins I have heard  but In MY OPINION I have heard quite a few theremins  which sound better.

Right now, given a choice between yours and an EW (all other aspects being equal) purely on sound, I would  choose yours - but this is because I would hope to fix the one thing I can hear that might otherwise cause me to pick the EW.. Also, its difficult for me to know how likable or otherwise your theremins sound would be without this sideband.. This is one problem I have that may be peculiar to me - When I hear some sonic aspect that bothers me, it tends to spoil it for me more than it seems to for other people..  

My opinions are completely irrelevant to anyone except me (or at least I thik they should be - I would like to replace IMO with MOO - My Opinion Only ;-) - And I actually wonder at times if they are REALLY of any importance even to me.. But if I stop expressing them, or "having" them, then I merge with the nothingness that in my opinion is everything.. Oh, I am happy to change my ideas and opinions - but I have a set of "requirements" that must be met before I can do this - its all tied up in a matrix of determinants that define what I "like" or "dislike" or am inclined to "believe" or to "doubt".. If, for example, my hearing was to change or my brain was to change, then I may like sounds or music that I dont at present... The "determinants" in my "matrix" would have changed.. ;-)

The above applies (for me) to both "subjective" and "objective" matters - Weight my matrix with stuff which contradicts "facts" in my matrix, and I get really excited - because I want to swap anything thats "incorrect" in my matrix "database" with stuff thats more "correct" even if this means ripping up my "certainties" - But my matrix also prevents me from being "nice" and pretending something is "true" when it biases me to think its "not true".

But I do know that just because I "like" something is no reason why anyone else in the universe "Should like" it - And likewise, if I "dislike" something and everyone else in the universe liked it, this would not "invalidate" my dislike for it!

And in "reality" there will be those who "agree" with some of my opinions, and those who will "disagree" - And there will be "valid" reasons for these which will almost certainly come down to the "determinants" in each of our individual matrices.


Posted: 3/26/2014 6:27:16 AM

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

I have just run your MP3 through my toolkit..

Paused playback when I heard the loudest "problems" and ran spectrum analysis for that point..

Its hard to be sure - the waveform and spectrum is generally clean - with a lovely distribution of odd and even harmonics..

But I believe that what I am hearing is a heterodyning of some kind between the difference frequency (and its harmonics) and 60Hz and its harmonics.

Unless I spent a lot of time on this, I cannot be precise or determine exactly whats going on - As I say, most of the time the waveform and harmonic spread are exceptionally good - I think you have something quite special here..

It seems that as the fundamental or harmonic with large level (2nd and 3rd are really strong  on your audio! even higher harmonics have healthy levels  ;-) approaches a harmonic of 60Hz, the "ghost" gets lower in frequency and dissapears - but as it moves away from this multiple, it becomes more audible - But I am only guessing this by the frequency that I stop the scan on when I hear the problem - I havent seen the ghosts in the FFT.. Or at least, none I can be sure of (certainly nothing as obvious as the FFT I did last time - that was easy, because the 60Hz and its harmonics were where they should be, not frequency shifted against your playing ;-) .

I suspect you have the VFO being modulated by mains - Its a real pain, a curse that inflicts us all! - I suspect that you are hearing hum coupled via the volume antenna and/or its oscillator / inductor/s, and that I am hearing "FM Hum" coupled to the pitch antenna and/or its oscillator / inductor/s.. I have heard similar "ghosts" on my prototypes and these were exhorcised by cleaning up the antenna signals (increasing voltage on the antennas and AC coupling them with a small C)

And all you can hope is that I am fussier than anyone else will be, because otherwise I think you will need to move your mains stuff well away from the antennas and HF components in order to fix these problems.

Please understand that this is definately a MOO ! - I have spent way more time on this than I should, and had this fact in my mind while doing it - which doesnt help the process of "due diligence" ;-) .. The cause of what I hear MIGHT only be in my ears / brain .. Or it might be some other mechanism like beating against some other signal / oscillator / ringing / who the f knows.


>> Final report:

# I found some "ghosts" and they do change frequency as pitch changes and reletive to the harmonics.. and they can reach normal "above acceptable" levels - I may still be wrong, but I am now more inclined to think that its not just my ears, and that other people might hear this! I also found a weird anomaly - Had been looking at one track, but superimposed FFTs from both tracks and they are different! - Your sound card has quite different frequency responces on each channel (assuming you have sent the same mono signal to both channels and havent messed with the recording..)!

Now I say goodbye to this thread! ;-) .. Hell, ive been paid hundreds of pounds in the past for doing less work! ;-)

Posted: 3/26/2014 12:55:47 PM

From: Colmar, France

Joined: 12/31/2007

There is IMHO nothing wrong with having 50 or 60Hz in the output signal, because

I analyzed some stuff recorded by coalport with his Hoffman Theremin. Its output signal has a little 60Hz AM and a lot of 60Hz FM. I could clearly see two pair groups of sidebands on each fundamental and harmonic of each tone in the Fourier view: One pair of sidebands at a 5 to 10 Hz distance (plus multiples) on each side is Peter's vibrato. The other pair of sidebands was at a 60Hz distance (plus multiples) which proves that there happens 60Hz FM in the RCA design.

Out of that, I found a constant 60Hz fundamental and its harmonics, that's the AM or hum.

I'm almost sure that the 60Hz FM contributes greatly to the warm and special sound of the RCA circuit!

May the shit storm begin... ;-)

Posted: 3/26/2014 1:20:23 PM

From: Canada

Joined: 8/1/2008


Posted: 3/26/2014 1:28:37 PM

From: In between the Pitch and Volume hand ~ New England

Joined: 12/17/2010

... Something smells and it is not the litterbox.

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