Anyone interested in opto-couplers for VCA use? Electronics Goldmine is running a sale (until Sept 3 @ noon) quantity 10 for $5:
http://www.goldmine-elec-products.com/prodinfo.asp?number=G15396C
Outside of the Anderton guitar compressor done long ago with a homebrew opto-coupler (LED black electrical taped to CDS cell) I've never used them myself. The digital revolution probably killed these things (along with RoHS).
NPN Colpitts w/ ILYA mixer
Inspired by ILYA's mixer circuit here, I thought I'd give it a go with one of the oscillators in this thread, the NPN Colpitts:
ILYA's mixer is Q5, after which I've added buffer Q6 & Q7, with another pole of LPF via R12 | C10.
Here it is in simulation doing 1kHz audio. I have no clue if I sized the tank LC correctly for proper use, this is more of an initial feasibility study. Haven't breadboarded it either. Waveform looks like it might sound kinda buzzy. Sim shows ~50Vp-p swing at the antenna, and very low current draw.
LTSpice file is here.
Good question! FredM used to do a bunch of overnight sims in Spice to get just a few seconds of audio.
Googled it and found the *.wav out option for LTSpice:
.WAVE "c:/out.wav" 16 44100 out
Added the above to the spice file which generated a wave file, though it seems limited to +1V max, so I had to make the output resistor a voltage divider. Lowered the frequency to 500Hz to get more harmonics in the human hearing range, manipulated the result in Adobe Audition (waveform replication to 2 seconds, DC offset removal, envelope) and saved as MP3 here.
OK so lets take this Spice simulation to its musical conclusion. When I see a waveform like this, my immediate thought is what will it sound like as a musical instrument, not just a waveform. Dewster created a sound file which gives you a most rudimentary idea of the timbre. In this case a bit "buzzy" maybe but I could do with even more harmonic complexity. But from just that 2 seconds it's hard to tell much.
I now have to ask what would this waveform really sound like in a more theremin-like environment? So I downloaded the .MP3 - converted it back to .WAV (better to have started with the .WAV but that won't really make much difference here). I then edited the file to get a single note from it as the original cycles over the two seconds so I wanted to clip out a piece that might be used as a single tone.
Then I imported that into the Kontakt Sampler and created a .nki sound sample file from it that I could play on a keyboard - the sampler takes care of this for me - reassigning to correct pitches as I play. I changed the ADSR envelope of the sample to have a much slower attack that is more appropriate for a theremin. Changed the decay and release a bit too to be more in line with what I would expect a theremin to use.
Then I loaded up the sound in Cubase as a Kontakt plugin on a MIDI file I played in on the keyboard. In this case the melody to Orlando Gibbons "The Silver Swan". I had the lute MIDI accompaniment already input from my accompaniments project. Then I took the sample and added a bit of digital delay to it with a Waves plugin. Then added little vibrato on it from another digital plug in I have for that and finally added some reverb of various sorts. So I tried to process it as I might a theremin (other than the slight simulated vibrato which I could have done better in other ways).
Finally I assigned a lute sample I have to the accompaniment MIDI part and recorded the result.
Now of course this does not sound exactly like a theremin. I could have assigned portamento to it and created a more natural perfomance with that, but this was just a quick test. I think it gives a reasonable idea of what this waveform might sound like in a musical setting.
https://soundcloud.com/rk53-1/theremin-waveform-test-1-silver-swan
So Splice simulation is fine for seeing what waveforms might result, but the "proof of the pudding" must be in the musical output and how that might conceptually sound as played on a theremin. Interesting little experiment.
The buffers Q2 and Q4 sharing RC decoupling with Q1 and Q3 is a bad idea (my bad). Adding a 10k resistor to the base of the mixing transistor seems to help amplitude.
The more I mess with bipolar transistors in analog Theremin circuits the more I see how ideal FETs are instead. I may order a bag of J113 from Digkey and try them (25 cents each @ quantity 25).
Rich: Amazing! You're clearly more obsessed than I am! ;-)
ILYA: Eliminating R10 kills heterodyning in the simulation. The small capacitor didn't seem to do anything, but I may give it another go.
Waiting my LM3700, I thinking about the idea of application which processes, say, a wav file of prerecorded (or pre-calculated) portamento full range glissando, on the one side and, say, a midi file on the other, and finally calculates the more or less realistic musical voice.
Actually the full range glissando is a bank of all possible waveforms.
Obsessed with sound. You bet.
It's interesting in all the technical talk that goes on here that the discussion of sound is almost always given the least time, yet its the most important part of any instrument.
The problem with MIDI is that it doesn't have enough resolution to create a true portamento so I agree that having preset microtonal sets that can be applied through MIDI to create better portamento may well be an interesting way to go.
Obsessed with sound. You bet.
It's interesting in all the technical talk that goes on here that the discussion of sound is almost always given the least time, yet its the most important part of any instrument.
The problem with MIDI is that it doesn't have enough resolution to create a true portamento so I agree that having preset microtonal sets that can be applied through MIDI to create better portamento may well be an interesting way to go.
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