S/C Jaycar - Continued

Continued from here (http://www.thereminworld.com/forum.asp?cmd=p&T=955&F=3&p=3) :

[i]"I have an old jaycar theremin, well from the jaycar design but built from scratch, with some of the mods mentioned in this forum. It is not a great instrument but playable. I was exited when my neighbour siad he had the new MKII Jaycar (2009 project) I have put it together and it seems OK so far. However when I put the lid on the volume gradually drops away. It looks like a bad design having the speaker magnet so near the coils. Has any one seen this problem? Does anyone have any comments on the MKII design?"[/i]

Magnetic fields CAN severely affect the tuning of the coils (IF Transformers) in the S/C / Jaycar theremins. What happens can be easily demonstrated by trying to tune an IFT with a magnetized screw driver.. the screw driver does not even need to touch the slug - one can hear tuning change as it gets within 10mm of the can!

Try connecting a flying lead to the speaker, and removong it from the vicinity while you put the lid on.. this will prove / disprove whether the speaker is the problem.

If it behaves without the speaker, this is NOT proof that magnetic fields are the problem - The speaker also acts as a large grounded (capacitively coupled) object which can upset tuning..

To eliminate this as the cause, you need to construct a dummy speaker (plastic bowl or packing foam shaped aprox like the speaker, wrap this in aluminium foil, connect this to ground.. fit lid with this dummy speaker - If the theremin stays in tune (or the volume circuit works) with this lid fitted, then you can be almost certain that the magnetic field from the speaker is the cause of the problem..

How to fix it? (A) Try to move the board or speaker so that IFT's are away from the speaker..
(B) Put speaker socket on case and have speaker external (C) Forget the speaker, just use line out (4) Build a bigger box, place speaker on the base (facing down) next to the PCB, so that there is no possibility of field interference.

It is also a good idea to mount the board upside down, and have holes in the bottom of the box for adjusting the IFT's.

And yeah - not too clever to design a theremin with large permanent magnet close to ferrite tuning slugs.. ;-) .. It is possible sometimes, with much trial and error, to compensate the 'bias' added by the magnet (sometimes it is not possible, as the field pushes the ferrite close to saturation, and not enough adjustment remains).. But such a needless pain in the RS!

It is my view that theremins should be designed in a way such that one can adjust and tune it, and any processes required (such as re-assembly) after such adjustment has been performed, do not affect the adjustments / tuning. This seems like such a no-brainer, I simply cannot understand why so many theremins do not follow this obvious 'rule'..

Fred.

I just did never install this small crappy speaker in my Jaycar theremin. It sounds hundred times better when connected to a good amplifier. So why should Iook for additional difficulties?

[i]"I just did never install this small crappy speaker in my Jaycar theremin. It sounds hundred times better when connected to a good amplifier. So why should Iook for additional difficulties?" - Thierry [/i]

I absolutely agree with you, Thierry!.. Alas, some people like the idea of having a in-built speaker.. and there is nothing 'wrong' with having this - it does make life easier if one wants a self-contained unit.

In my view, the best solution is to construct a larger box (this is needed anyway to get reasonable distance between the antennas) and to mount the board and speaker on the base of this box.. With all capacitive and magnetic components firmly fixed into place and positioned for minimum interaction, it is possible to adjust / tune the theremin and not have the problem of this tuning being upset when the box is closed.

The mechanical construction of the SC/Jaycar theremins is, IMHO, absolutely horrible and crazy! - And if one MUST have an internal speaker, then at least put a reasonable speaker (and amplifier) into the box. The box is too small anyway (even if one does not fit the speaker) .. Building the theremin into a reasonable sized box gives plenty of space to fit a speaker ON THE BASE (as in, sound comes out of the bottom of the theremin).

Without a bigger box, IMHO, these theremins will always be no more than toys anyway.

Fred.

If that theremin is small and has built in speaker - it is a toy. Why don't make separate speaker with amplifier that isn't big and can be taken everywhere? I've made one little cheaply (~40$) JVC and Tonsil speakers, built in amplifier based on TDA7375 with copper pipe radiator, universal laptop PSU made in china (needed additional components and shielding as it was interfering with theremin). It's small and cheap, I use it with my theremin. Sometimes we watch movies at university after classes, speakers there aren't good so I take mine and people are astonished how powerful this little thing is. It can be used with mp3 player but needs preamplifier(3$). photo (http://s1123.photobucket.com/albums/l557/Arsimantur/?action=view¤t=100_2785.jpg)

To make magnetic shielding use Electrical steel (http://en.wikipedia.org/wiki/Electrical_steel). If you can't find it you can get it from old transformers. Simply boil them in water and then you can split core from rest. Then use screwdriver to split the core. Bend strips and glue them together with strong epoxy glue.


Continued from here (http://www.thereminworld.com/forum.asp?cmd=p&T=4667&F=0&p=3)
Why is there a limit of 26 posts in a topic?

Measures done on C36.R39.R40: http://s1123.photobucket.com/albums/l557/Arsimantur/Theremax/C36_R39_R40/ (http://s1123.photobucket.com/albums/l557/Arsimantur/Theremax/C36_R39_R40/)

Whatever I do, I see really close to sine wave (some harmonics appear). So problems are somewhere in volume part or VCA. I wasted more than a week, day after day looking at pitch part! Arghhhhh... Ok, time to do some measures there...

[i]"If that theremin is small and has built in speaker - it is a toy."[/i]

Yes, I would agree that the way the S-C/Jaycar/EPE theremins are constructed does turn them into "toys".. But at the "heart" of these units, there is a [i]reasonable[/i] instrument.

Enkelaar took the most basic S/C board, used the power amplifier as a wave-shaper giving line-level output, put the board in a reasonable sized box with decent antennas, and did a few other minor modifications, and his theremins are reasonably playable (but highly non-linear.. The S/C theremins have no EQ of any kind) and popular.. Certainly not a "toy".

If one modifies (change of component values on the oscillator circuits, drop the frequency a bit, add a EQ coil) one can improve linearity.. But (IMHO) it is not worth doing much more than this.

One does not need to go to the extreme of mu-metal screening.. External magnetic fields rarely bother oscillators - it is really only when there is gross "malpractice" (having a speaker voice-coil only a inch or so from critical ferrite inductors) that one gets problems. On my H1 theremins I had a large speaker only about 2" from the board, but could tune the theremin without having to open it or move anything - The speakers were good quality with low level of field radiation, and all worked well.

Why the limit on thread length? Something to do with limitations on this site, we have been told.. Its ok when one gets used to it - not too difficult for the last person posting to link to a continuation - or start a continuation and edit your last posting to link to this.

Fred.

The theremax has a simple germanium diode mixer. Don't waste your time with looking for a clean sine wave - you won't find one.

Out of that there is lot of signal straying between the wires which go to the front panel. Spacing and placing them carefully (I placed mine on three different levels in order to get them far from the circuit board and far from one another) may reduce these effects but the output signal will never be clean.

I think that John Paia was good when developing analogue synths, but he has obviously not been a RF Guru.

[i]"I think that John Paia was good when developing analogue synths, but he has obviously not been a RF Guru." - Thierry [/i]

As I understand the history, Paia was started as a tax "fiddle" - a way by which their hobby electronics could be made tax deductable.. it was not originally intended to be a "real" enterprise - they advertised product just to comply with legal requirements - and, as I understand it, were surprised when people actually started buying their products.

The products have, IMHO, always been at a "hobbyist" level - And this includes their analogue synths, Theremin, and everything I have seen from them. I am not saying they are all 'crap' - Some of their stuff has been interesting and innovative - I learned a lot from debugging and modifying some of their stuff
(the "Gnome" for example) constructors were having problems with - and their comparatively low price made electronic music accessible to many who would otherwise never have bought into it.

But if one is looking for well designed Pro instruments, IMO there is no point in looking at Paia products - none of which is remotely comparable to the design competence one expects from a reasonable / good / brilliant electronic engineer.. Absolutely no comparison to anything designed by Bob Moog, for example.

A lot of people will probably disagree with, and be angry about much of what I say above - My comments are purely from a design perspective.. What really matters is the end result. If a design works, and gives results people like, then it really doesnt matter what is on the circuit board or how "ugly" waveforms etc on that board are. A LOT of people use and like Paia products - and this is all that really matters.. It is really only when one wants something the product cannot provide, or when there is a problem, and one goes digging about with a 'scope, that one comes across the limitations.

Fred

This was moved to the proper subject heading.

Here you can find some samples: here (http://www.thereminvox.com/filemanager/list/1/) and here (http://www.thereminworld.com/sounds.asp). Open it with audio editor to see the waveforms.

I found out what was causing that strange spikes and distiortion on audio input - audio wire was running too close to wires comming from volume oscillators to pot! I'll move that wires and probably use shielded cable.

I analyzed measures I got on C36.R39.R40. Upper harmonics appear only when pin2 and C30 are in place while without R42 and R40. There is no change when only with C30 or pin2. R42 does no change, R40 just slight change in the highest register. I guess that distortion appears when square is being generated and it escapes through C30 to C29. Anyway signal doesn't look as bad as on pins of IC2, where it's clipped, it's happening somewhere in VCA. Transition from clipped sine to square is ok.

Now I understand why theremax isn't well designed. When I was looking for a theremin, I couldn't find anyone saying 'Don't take theremax as it's not well designed, because...' but something like 'theremax is good for start, has some flaws, but there are plenty of modifications, sound is better than EW and when equpped with pipes in place of wires for antennas - it has a good linearity'. I was going to build one from scratch(copy), I couldn't find PCB for EW and I knew that designing PCB is not a great idea, so I made theremax which PCB I found on google. That's my story :) Maybe some day after I master playing I'll buy Moog's or Fred's (when you develop one).

It's not easy to write that something is crap since things which are crap in my eyes may be seen and valued in a different manner by others.

Out of that it should not be necessary to point out things which can be easily seen by someone who is experienced in analogue RF designs. And since the latter is in my eyes a basic requirement for building successfully theremins...

There are enough kits on the market (the Moog Etherwave comes with a ready soldered and pre-adjusted circuit board for example) for those who are not enough experienced in this domain. But even when you buy the Theremax as a kit, you should be at least a HAM radio amateur in order to make it work correctly. If not, you will loose too much time and quality of work because of your lack of experience and equipment.