Simple Theremin suggestion?

[Paul Perry (Frostwave)]

Mathflan, do you think it is a power line kind of buzz?
(50 or 60 or 100 or 120 Hz).

[mathflan]

hi,

No it's not the power line buzz, it's from the LM358, I guess.
Th buzz appear when the sound is at a certain level.

[Paul Perry (Frostwave)]

Is the buzz a fixed frequency while it is there, no matter what happens?
Maybe an extra bypass cap from the power pin on that chip to earth.

[mathflan]

the better is that you hear that:

http://mathieu.vallet1.free.fr/index/Buzz.mp3

[Paul Perry (Frostwave)]

the better is that you hear that:
http://mathieu.vallet1.free.fr/index/Buzz.mp3

I'm not very 'golden eared', but I can't hear any problem there!
Maybe someone here can put it through one of those spectrum analyser programs & see if there is anything strange happening, but it just sounds perfectly normal to me!
I'd be interested to know what others think.

[mathflan]

hi perry!

But you doesn't heard this buzz, distortion which sound when I pump up the volume?

Perhaps it's typically the sound of a theremin?

[moosapotamus]

Your theremin sounds good, but I think I hear the "buzz" you're talking about. It's intermittent and seems to occur when the volume increases. There is a particularly accentuated volume swell near the end of your clip where it is very apparent. Sounds like lack of headroom / clipping, to me. Can the buzz be avioded by playing at lower volumes? Are you playing through a speaker?

~ Charlie

[Paul Perry (Frostwave)]

Looking at the sample on a scope (hardly an ideal situation!) there might be some higher freq oscilation ocurring on the peak of the waveform cycles, when the signal is loudest.
At low volumes the signal looks (to me) more triangular, but I can't imagine how it could be, if the signal is generated by heterodyning.
And I still can't hear a 'buzz', but that might be a function of my age :oops:  :shock:  :x

[mathflan]

thanks guys and thanks perry for looking in a scop , that's very cool!

I don't use the speaker , the mp3 that I recorded is from the line out and I pay attention on the gain on my mixer.
So the buzz comes from the circuit.
Perhaps it's a bad capacitor, I will check that.
I can remove the buzz by increase the resistor between pin 2 and 3 of the MC1496, It's work but when I increase the volume suddently, ( I approach my hand very fast) the buzz appear.

[Paul Perry (Frostwave)]

One thing to do, might be to check that the volume control voltage (pin 1 of the LM358 ) never goes above 4.3 volts.

A remote possibility is that the LM386 speaker driver has gone unstable because a speaker isn't connected (in my experience the 386 is problematic, but I know most people here love this chip!) and on peaks it is self oscilatign & spraying interference around. You could just remove it (if socketed) to test this. Or, easier, connect a speaker (it does have some kind of stabilising load connected already, though).

A further possibility is that the MC1496 is being overdriven on peaks. But I wouldn't have though that would make it sound 'buzzy', it should just flatten the output.. but maybe that is it. It certainly ties in wiht increasing the gain resistor between pins 2&3 of the MC1496.

Interestingly, the designer himself has given up on the 1496 & in his current theremin design uses two slope detectors instead of the heterodyne principle.

[mathflan]

Hi,
SO I did what You said but nothing remove the buzz.
In fact I think that I  did a bad setting on the volume. :oops:

I turned V2 trim at max and I turned T3 until the distortion dissapear.
I put my hand very closer to the volume antenna and I set up the volume before having a distortion.
after I turn Vr2 trim to adjust the volume so  to have a good response with  the hand.
Now I don't have Buzz anymore but the sound is lower.

I will post a MP3

Thanks you again for your help and your interesting

[mathflan]

One later.....

I found the solution to remove the buzz.

Just put a 1 K resistor between the Pin 6 of the LM358 and the ground.

Now, no more buzz, I 'm gonna able to play this theremin...

[Paul Perry (Frostwave)]

Thanks Mathflan! One day I'll get my Silicon Chip theremin back on the bench....... oh, who am I kidding!
Seriously, for anyone struggling with the Silicion Chip theremin, there are a few fixes on the Silicon Chip bbs.

[mathflan]

Hi paul.

I's been a long time!!!

I dig up this thread

I'am actually add three pots ,a low pas filter, a Pitch ajustement, a volume control and a Volume Adjust for the antenna.

I will the silico, theremin in a plastic box...

Ill post soon some pics

bye