Opamp in theremin

I just wonder if it will improve the sound of my theremax if put superior opamp (AD8065,OPA627,AD845 etc) in place of lm748.

Seen the low and asymmetric supply voltage of the Theremax, this would only be a waste of money. The best and powerful gearbox won't help if the motor driving it is weak...

Interesting that you bring this up, Thierry.

I have thought about replacing the Paia power supply with a regulated, well-filtered, bi-polar power supply. I've also wondered if it would work better with a little higher voltage. Also, I'm not sure why they chose such high frequencies -- I'd much prefer to run under 200khz rather than around 500Khz as it does now.

Any thoughts on this?

Q: Why does Paia work with such high frequencies?
A: With simple pitch oscillators without additional coil(s) for linearization/range optimization, the frequency deviation (max pitch range) is proportional to the free-run frequency of the oscillator and the thickness of the pitch antenna. It is inversely proportional to the capacitance of the tank circuit's capacitor. So in order to get the actual pitch range with this very thin pitch rod, they needed a small capacitor in the tank circuit (100pF on the pitch side, 68pF on the volume side) and a relatively high free-run frequency.

Remark: I have already thought of using thicker antennae on the theremax and modifying the oscillators for a more stable and linear operation at lower frequencies but finally I found that the remaining circuitry was not good enough to justify these modifications - it would turn out in a complete redesign from A to Z and that isn't worth it. The theremax is and remains a low-cost theremin with a single advantage - the beautiful case.

Q: Why not use a bipolar power supply with the theremax?
A: The circuit is basically designed for a monopolar power supply (ground at the most negative point). There is an auxiliary voltage of Vref = 1/2 Vcc generated which acts as a floating virtual ground or reference voltage for the comparators and the op-amps. Internal voltage stabilization on Vcc is done with a Zener diode, Vref is not stabilized at all. What one could try is:
a) better stabilization of Vcc and increasing it slightly i.e. with a 78L10 voltage regulator.
b) dividing the Vref circuitry in order to have separately stabilized Vrefs for the comparators (which pollute it with rectangle signals) on one side and the op-amps on the other side, using i.e. 2 different 78L05 voltage regulators. Total cost around 8$ plus a new unstabilized 14-16V external power supply.

I'll think about the practical realization of this solution and (hopefully) be able to post details here this weekend.

[i]"the frequency deviation (max pitch range) is proportional to the free-run frequency of the oscillator and the thickness of the pitch antenna."[/i]

The Theremax has a huge range -- over seven octaves -- though it is nearly impossible to play at the extremes. I swapped the thin rod for a thicker one with the result that I now I have about 3 1/2 playable octaves.

[i]"So in order to get the actual pitch range with this very thin pitch rod, they needed a small capacitor in the tank circuit (100pF on the pitch side, 68pF on the volume side) and a relatively high free-run frequency."[/i]

Might be worth a try to lower the pitch free-run frequency a little -- probably not clear down to <200Khz but perhaps enough to get it out of the AM radio band.

The theremax has plenty of range (frequency deviation) so I could sacrifice some range and still have a useable instrument.

[i]"I have already thought of using thicker antennae on the theremax and modifying the oscillators for a more stable and linear operation at lower frequencies..."[/i]

The range below Middle-C is kind of rough sounding and I think this is due to the unregulated power supply and the faux-ground.

[i]"The theremax is and remains a low-cost theremin"[/i]

True, none-the-less I do like its tone color. The only way I'm able to get rid of the roughness is to: connect the theremin's audio output through an Ebtech hum eliminator AND to wear a wrist strap and connect that to a gound point on the Theremax.

[i]"There is an auxiliary voltage of Vref = 1/2 Vcc generated which acts as a floating virtual ground or reference voltage for the comparators and the op-amps."[/i]

Exactly -- and I think this doesn't work so well. The tone is often rough and I think the cheap power supply scheme has something to do with it.

Thank you for the reply -- I await your suggestions for mods!

I have a general purpose split supply circuit which is probably suitable for the Theremax:

http://www.element-14.com/community/docs/DOC-24787

Fred.

I've found some virtual ground circuits, but it seems that yours is much better that ones on this site: http://tangentsoft.net/elec/vgrounds.html (http://tangentsoft.net/elec/vgrounds.html)

[i]"I've found some virtual ground circuits, but it seems that yours is much better that ones on this site: http://tangentsoft.net/elec/vgrounds.html"[/i]

There are some good splitter circuits on this site - What my circuit does is provides both the mid voltage and the regulated supply voltage, which makes life easy if one wants to change the supply voltage, as only one resistor change is required.

The circuit was originally designed for a variable supply with Mid-V tracking the output supply V - a single variable resistor allowed this function to be achieved.

Design of power circuits is fraught with unexpected problems - The PSU is THE most critical aspect of any and every electronics design IMHO.. If the supply has 'dirt' on it (particularly, but not exclusively, high frequency 'dirt') them much of the care taken in other aspects of the design is a waste of effort. Many (most?) designs are undertaken before the power supply is even thought about - it is left until last, and cobbled together quickly without enough care... The supply, decoupling, and related issues need to be in full focus at every stage of the design if optimum performance is to be achieved.

There is no single PSU design which is suitable for all applications - the circuit I have given is likely to work if connected to the Theremax directly from the Op-Amp outputs.. but some trimming of the (particularly load) components will optimise its performance for this application. I do not tend to use this circuit much anymore - but it proves useful when retro-fitting an existing design with a better supply.

The circuit looks complex, but for low current applications (where one does not need additional bypass transistors or need to deal with large dissipation) it can be built on a small board quite cheaply.

Fred.

Fred, thanks for the link. Saved to "Favorites".

The better power supply may be a basic requirement for improving the stability and sound quality of the Theremax. But it will not solve all problems. I'm still making tests how I divide best the circuitry which uses this mid voltage by two and decouple the parts.