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LC oscillator with very low capacitance?

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Sulaiman

Thu Jul 14 2011, 12:15PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I agree with the approach of two rf oscillator outputs mixed down to audio with one of the oscillators frequency determined by variable capacitance (could even be one plate or rod with your hand ... e.g. like a Theremin.
One transistor for each oscillator, one for the mixer should be enough.
If you want to an SA602A (still easily available and cheap cGBP2.5) has an oscillator and a mixer function...very easy to use.

Proud Mary

Thu Jul 14 2011, 01:16PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Here is a Theremin circuit as suggested by Radiotech (i.e. Ð¢ÐµÑ€Ð¼ÐµÌÐ½) but it might prove challenging for someone just starting out: all those tuned circuits to be aligned...

Ash Small

Thu Jul 14 2011, 01:32PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

On the subject of theramins, here is a link to a short, psychedelic video of Miss Alice Malloy playing her theramin bra:

'Jason from Theremin World writes: "She's turned a couple of metal dome theremins into a theremin-bra......"'

Proud Mary

Thu Jul 14 2011, 06:35PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Ash Small wrote ...

On the subject of theramins, here is a link to a short, psychedelic video of Miss Alice Malloy playing her theramin bra:

'Jason from Theremin World writes: "She's turned a couple of metal dome theremins into a theremin-bra......"'

What's this, Ash? A prurient interest in women's underwear, is it ? Whatever next!

Ash Small

Thu Jul 14 2011, 07:14PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Proud Mary wrote ...

.
What's this, Ash? A prurient interest in women's underwear, is it ? Whatever next!

I felt it was relevant to this thread, after Radiotech's comments.

It's the most bizzare theremin I've ever seen. I know the video isn't up to much, but apparently she made it herself, in the true spirit of 4HV.

I certainly didn't post it because of sexism or whatever,

This is a quote from the Audio Artists website "The incredibly outrageous, beautiful & talented Miss Alice Malloy plays her amazing homemade theremin bra at Don Hill's - Stunned and amazed, the crowd has yet to recover."

Also featured on the website are sereral other 'unusual' synthesizers, etc.

I must say, though, that I prefer Jon Spencer's theramin playing.

Proud Mary

Thu Jul 14 2011, 07:17PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Ash Small wrote ...

Proud Mary wrote ...

.
What's this, Ash? A prurient interest in women's underwear, is it ? Whatever next!

Ha ha, got you there, Ash!

radiotech

Fri Jul 15 2011, 05:58PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Bringing this back to earth..

An experimental Theremin can be made from 3 old style transistor
radios, preferably the big kind with large tuning capacitors.

If you have a communications radio receiver with a BFO you just
need one transistor radio.

Remove the back cover from the transistor radio and attach a stiff
wire to the 'oscillator' gang terminal of the radio. Set the radio to
a station about 1000 kHz and verify that bringing your hand near
the wire can detune the radio, perhaps even cause it to change stations.

The oscillator frequency is 455 kHz higher than then the frequency on
the dial.

You need to bring a wire from the antenna of the communication receiver
close to the radio and tune in the oscillator frequency, which will be an
un modulated carrier. Now swtich the BFO on and 'zero-beat' the carrier.

Now when your hand approaches the wire you have rigged up, a variable
frequency will be heard in the speaker, controlled by your hand.

Without the BFO, using the three transistor radios, you have to get one of
the radios to radiate its oscillator into another radio to produce a modulated carrier at some station free point of the dial. Placing them close together
can do this but it is tricky. Then the third, prepared radio will work as
it did with the communications radio.

You can the circuit for the old RCA Theremin on the Nostalgia Air
website. Look at it and decide what the UX120 tube does.

Proud Mary

Fri Jul 15 2011, 06:27PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

radiotech wrote ...

Look at it and decide what the UX120 tube does.

It looks to be configured as an AGC diode to control AF gain.

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