Theremin Power Supply

My test bench power supply works beautiful when I light up my theremin board. It was only when I started looking for a wall-wart cost effective solution that I ran into trouble.

Back in the 70’s & 80’s wall-warts seemed to be mostly a transformer and filter to give a reasonable low wattage DC alternative power source. Today they use a Switch-Mode Power Supply which operates at a high frequency to keep their physical size down while filtering, not as durable but cheap!

Fred brought to my attention all these strange frequencies in my theremin sound and listed some possible sources. This focused me back to my earlier days of troubleshooting when I used power from batteries and a good earth ground.

Using batteries my audio signal became a site to behold, the epitome of smoothness, like a babies… no the morning glass on the lake before you water ski!

I believe heterodyning is the living soul of the theremin but her heart must be the power source.

The nice thing about building your own theremin is you should be able to keep it properly tuned for the rest of your life. The best thing about designing a theremin is you can regroup, adapt and modify.

I can now effectively use a SMPS wall-wart on my theremin design, thank you Fred, but if others needed a new wall-wart it would be better if you ordered the correct one from the original theremin manufacturer or you just might create a nightmare! LOL

Whilst on the subject of using a wall-wart, you should use the lowest voltage if switchable you can get away with that does not change the heterodyne tone or add distortion. You want to keep any excess voltage (heat) outside of the theremin enclosure. AC or DC wall-warts may say 12v but theremin loading can be so low on some designs it may be more like 14 volts. The excess voltage turns into heat and must go somewhere.  

This is one of the methods I use to avoid thermal drift and I don’t need a theremin warm-up period. Room temperature works perfect for me.

I kept the timbre plain for study, held some notes steady, no reverb, yet there is a natural brightness, theremin to sound card direct. Now a better signal to SMPS "power supply noise" ratio: Phoenix.mp3  260k

Christopher

Edit: I could use a non-switching fixed 12vdc power supply sitting on the floor if there was something reasonably priced out there (do you know of any?) but what fun is that?  How can you tell if a power supply is switching or linear if they don't state it? Are all wall-warts today switching?

Most wall-warts are switching.

SMPS are far more efficient and lower cost - but this often comes with the hidden price that, although the voltage is regulated, and there is plenty of power, the HF ripple on the DC can be high.

I use SMPS, but do quite extensive filtering on the DC from these - "Overkill" one might say - Best (IMO) if you want 12V, is to get a 18V SMPS, use a linear regulator to drop this to 15V, then another linear regulator (7812 or equivalent) to drop this 15V to 12V.. Each of these linear regulators having its own input and output capacitors - and I also put an inductor between the output of the 18V SMPS and the input to the 15V regulator, and an inductor between the output of the 15V regulator and the input to the 12V regulator.. Oh, put a resistor of about 1k to load the 15V regulator a bit (depends on which regulator you use)..

The above components cost about £5, and can be built into a small external box between the wall-wart and the theremin if you are not designing / building your own theremin.

If one is actually designing and building a theremin (or any electronics) the power supply should be in ones thoughts right at the start - its not just about voltage and current.. the whole board and the power distribution on the board needs to be optimised at the design stage - I would say that with any musical project (and most other electronic projects) perhaps 50% of what differentiates between a clean well designed product and one which works but which perfectionists will not like, is down to the power supply and related matters.

At a more pragmatic level, if one has a design and this care has not been taken, improvements can often be made by replacing decoupling capacitors with higher quality parts having bigger capacitance - 100n decoupling is a standard - if one gets good 470n or 1u equivalents (good being ones with lower series inductance and resistance) one can often greatly improve performance.. Also, ignorant designers often completely mess up the regulator stage, put big electrolytics on the regulator output for example - If your device has such idiocy, you might get improvement by correcting these errors - but if such basic errors are made, it is likely that other rubbish design makes the product a lemon.. The PAiA theremin is, IMO, one such lemon.

If one has a dual-supply circuit, and need to run this from batteries (or from a single output wall-wart), one can often achieve this by using a DC-DC converter.. These are small SMPS circuits which take a DC input voltage and output a +V and -V.. Standard parts take 12V or 5V input, and output +/- 12V or +/- 15V at 1W or 2W per supply (various power outputs are available, the bigger, the more you pay - 1W / output can be bought for about £5).. The output from these IMO are not suitable for directly driving audio / theremin circuitry.. But if you want +/-12V, get a DC-DC with an output of +/-15V, and use filters and linear regulators to drop this to +/- 12V.

One final thing.. Keep SMPS of all kinds away from any coils etc.. Some of them radiate magnetic fields which can mess things up.. Most dont - Most use torroidial coils to contain the fields (and improve their efficiency) - But one can still get capacitive coupling causing problems.

As for heating.. The heat dissipated by a linear regulator is easy to calculate and usually low for the currents required by theremins..  First, determine the difference voltage across the regulator (Vin - Vout) then the current drawn by the circuit - The power dissipated will be the voltage across the regulator times the current..

So if one has 3V across the regulator (say Vin = 15V, Vout = 12V), and are drawing 100mA, dissipation will be 3*0.1 = 300mW ...

Fred.

*on my latest designs I have been playing with my own designed SMPS - this starts up asynchronously, but when the theremin oscillators are running, synchronises to the reference oscillator and therebye overcomes all and any possibility of  problems. I am looking at making this as a module to fit into theremins like the EW, so one could power them from a 6V to 12V battery or DC wall wart.