SC/Jaycar kit-Maxie's mods advice wanted

Posted: 3/6/2005 6:22:22 PM

From: Noo Yawk Citay

Joined: 3/6/2005

OK, I'm obsessing on my upcoming theremin project and it'll still be a while before my Jaycar kit arrives from Oz.

I've checked Maxie's mods to this kit (on this site; his webpages are down I gather). First, is there a modified component layout diagram which shows where to add the mods onto the pc board? Or do we just 'wing it'?

Second, I was wondering if some of the mods might be ok to do with panel-mounted pots instead of trimpots. For example, the 'sound' section has the original mod to alter the waveform to a sine/saw mix. Could this trimpot be changed to a panel-mount knob to allow external sound quality adjustment?

Also, there's a mod to give more range to the volume control by replacing a 1k resistor between pins 2&3 of the IC with a 2.2k. Would it be ok to replace this with a 500-ohm resistor (approx.) and a 2k (or so) pot to allow external adjustment of volume sensitivity?

Does the Jaycar kit come with a hookup for an internal speaker as well as an audio-out jack?

I plan to mount my kit in a good-sized wooden box with 'real' loop and rod antennas. I want to make a nice, playable instrument for my gf. Was also thinking of mounting an internal 9V battery & holder, and having a 2-way switch to run off of internal or external power. Will this work?

Any other suggestions beyond these? Thx!!!
Posted: 3/7/2005 10:38:17 AM

From: Noo Yawk Citay

Joined: 3/6/2005

One other quick question: I found a nice wooden box (old) that I'd like to house my kit in. Dimensions are 13.5"x8.25"x2.75". Is this large enough, or is it too shallow? Thx!!!
Posted: 3/7/2005 11:55:18 AM
Jeff S

From: N.E. Ohio

Joined: 2/14/2005

I have one thing you might want to think about.
I don't know about the 2.75" dimension, but a box 13.5" long may be too short. If it is it will force you to compress the playing range of the pitch antenna, otherwise your volume hand will effect the pitch especially at the low end.
Posted: 3/7/2005 2:34:13 PM

From: Noo Yawk Citay

Joined: 3/6/2005

What would be a good minimum case length? 20"? Longer? I haven't seen any specific suggestions on this anywhere. Thx!
Posted: 3/7/2005 4:51:04 PM

From: Noo Yawk Citay

Joined: 3/6/2005

I do plan to make some mods and get a good tonal range and linearity. For those who have played a theremin (which I obviosuly haven't yet!) do you think this will be OK? The basic Jaycar kit is pretty small, but I want to beef it up as much as possible to make it a 'real' playable instrument, kind of like the Kees Enkelaar model (which I believe is based on the Jaycar kit or SC article/design).
Posted: 3/7/2005 6:25:26 PM
Jeff S

From: N.E. Ohio

Joined: 2/14/2005

The pitch to volume antenna distance on the RCA theremin is 17 1/8", and on the Etherwave Pro it is 19 1/2". I don't know what it is on the standard Etherwave, and I don't have my Kees available at the moment.
There is a very good reason for these distances as I have already stated.
The distance from your palm to your elbow would be a good reference, but I think anything in the 15" to 18" range would be fine.
Posted: 3/8/2005 2:55:12 AM

From: Undisclosed location without Dick Cheney

Joined: 2/21/2005

My standard Etherwave is 18" across the case. (The antenna mounts are on the ends of the case, so this may be regarded as the distance between antennas.)

Personally, I think I would have been happier if it was more like 27" across the case, but then, I'm a big guy.
Posted: 3/8/2005 8:56:54 AM

From: Italy

Joined: 2/19/2005

I am building the silicon chip modified too!

For how concerns modifications on pc, I soldered the new components on the top of the PCB following the original scematics and comparing them to the modified ones.

About your other question I don't really know what to answer, but I think trimport or panel pot ar not so different, so...

About the wooden case, mine is 40x17x8 (cm) I think it's big enough! :)
If you're interested I can send you some pic too.

I really hope, as you do, that willo come out a playable theremin. Silicon Chip's model is not that playable, it's good for sound effect, but it's hard do play because there is a really short range in pitch and all notes are near the antenna.

Volume controll sometimes affect pitch too, very fine tuning is needed to fix that.

We can keep in touch through this forum or by email to tell each other about our progress.. :)


Posted: 3/8/2005 12:03:28 PM

From: Noo Yawk Citay

Joined: 3/6/2005

Hey Mike, Good deal! Let me know what you plan to do. Which of the mods I posted above you will be doing, and any additional ones.

I think I'd like a switch for an internal speaker as well as a 1/4" kack out to my amp. Still thinking about a switch for battery power vs. adapter.

I will be looking for a longer case (around 18-20"), so thx for the replies above. I hope with a bigger antenna (I plan on 1/4" or 3/8" tube or rod, about 17-18" high unless someone else has a better suggestion!) and some tuning, I will be able to get a good playing range for tonal response.

Keep me posted on your kit, and I will do the same. (It may be 2-3 weeks for me, depends on shipping.)

Posted: 3/10/2005 9:56:55 PM
model citizen

From: Auckland, NZ

Joined: 3/8/2005

External pots instead of the internal trimmers are fine. You can also throw a resistor in series with the pot to reduce the range (usually done once you've found a near-perfect setting).

The box the Jaycar/Silicon Chip unit comes in is pretty small, maybe only 10" in length max. Comes with it's own driven internal speaker.

The 9v/adaptor idea is good, make sure you use a regulated supply though. You don't need a toggle switch for the battery/adapter, just wire in the correct DC socket (3 terminals) that allows the battery circuit to be disabled when the adapter is used. You can find these schematics on, or any of the DIY FX pedal sites.

I've heard of plates used in theremins being replaced with "proper" loops, but they have problems with sensitivity as the loops don't provide as much conductive area (possibly the volume circuit in pro theremins is a helluvalot more sensitive so that loop antennae can be used?).

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