Power conditioners and theremins

Having spent a week playing and recording theremin in a farmhouse in the Loire with a very variable mains quality which seriously affected the performance of my etherwave and gave our sound engineer plenty of extra work correcting the recordings, he recommended this product to me.

http://www.studiospares.com/mains-distribution/trojan-sps06-15a-power-sequencer/invt/348010?mc_cid=5eed6da3dc&mc_eid=46fe9b7bc4

He is very knowledgeable about audio electronics in general, but is not a theremin specialist, so I thought I would ask your advice before deciding whether or not to buy one...

I think you want a power conditioner - perhaps one that can regulate the output voltage and not just filter it.

Power sequencers are more for not popping the breaker when powering a bunch of PA amps up.  Sometimes conditioner and sequencer are combined in one box, which may indeed be the case for the SPS06, but I can't tell as there isn't enough info.

I think he gave me the wrong link.

This looks more like it. He had recommended the Furman brand when we spoke.

http://www.studiospares.com/mains-distribution/furman-m10x-e-merit-power-conditioner/invt/470870?VBMST=Power%20conditioner

That seems like a noise filter and spike protector, it wouldn't do any voltage regulation.  You can get these things in power strip form as well if rack mount is a problem - though once you have some rack stuff you might as well get one of these to act as a power distribution point (and give you some control lighting as well if you get the pull-out light version).

Were you mainly having trouble with noise at that venue?

How about something like this?

http://www.tripplite.com/products/series/sid/838

Hi Gordon.

I strongly suspect that problems with the EW are due to its (IMO) sad power supply design - The AC transformer does not (again, my opinion only) give a comfortable headroom, so if the mains supply drops significantly there is not sufficient DC voltage to drive the (on board) regulators correctly - 12V regulators need a minimum 14V DC into them..

I only suggest this because I recently bought an EW board, and wasted my money on a genuine EW PSU .. I get a little over 16V DC that is clean, before the regulators (with 240V AC in), so the AC only needs to drop to 210V and one gets into the zone where the ripple can get onto the theremin (past the regulators).

I dont suggest shoving a higher AC voltage in unless protection devices (thermal fuses and transorbs) are fitted and regulators changed to chunkier versions..

And I may be completely wrong regarding the problem.. I may just have got a crappy power supply..

But If its low AC voltage causing the problem, then you need a "conditioner" that raises the AC to 240V when its below that level, or a (say) 3V AC transformer in series with the EW AC output to boost the voltage a bit..

Fred.

ps - I have ordered some bits to put a Switch-mode replacement PSU together for my EW - I dont like being tied to AC anyway, and figured a supply that accepts DC 6V to 24V and outputs steady +/- 16V DC is a better way to go.. If it works ill let you know (probably a few months from now ;-).. But the EW will need slight modification.

Fred, from my experience I feel like contradicting you somewhat...

Seen that the EWS draws only a little current (+/- 30mA) you find rather the AC peak to peak values behind the rectifier diodes at the big loading capacitors. That makes that you have normally about +/- 19V (French or German 230V input) with a ripple of about 0.4V at the inputs of the 78L12 and 79L12 regulators which is IMHO more than enough headroom for these to give a clean output voltage. So, this should not be a source of problems in practice although I have to admit that, from an academic point of view, it is a poor design.

A theremin has in general much more and much more sensitive entry points for problems than the power supply, be it grounding issues, be it "over the air", and these would for me be the first subjects to examinate in case of an unclean audio signal.

What Gordon experienced in France is most probably due to another effect which can be found in many old houses in France: the power distribution inside the house is often still made with only two wires inside the walls and it's only in the wall plugs that the N and the GND pins are bridged. That leads to very "interesting" effects.

Hi Thierry,

Hmm.. Thanks for that info.. I think I should perhaps open my EW supply and see if there's a problem, cause I am getting 16V DC with about 1V P-P ripple having a good 240V AC in.. I haven't had any problems, but in my usual pedantic manner was a bit unhappy with these levels. (not a real bother cause I'm redesigning the PSU anyway..)

"A theremin has in general much more and much more sensitive entry points for problems than the power supply, be it grounding issues, be it "over the air", and these would for me be the first subjects to examinate in case of an unclean audio signal."

Yeah, I agree almost 100% with that - It was just the combination of having seen the EW voltages and the comments on this thread that lead me to cautiously suggest it may be this problem.. 19VDC @ 230VAC with the ripple you have seen should be ok for any reasonable mains input .

Unlike you, I haven't had my hands on many real EW's ;-) .. And this one is being seriously "abused" ;-)

I do have a spare "reference" board I wont be f*cking up though ;-)

Fred.

Hi Fred, it would perhaps be a good idea to check the current draw of your board. When doing so, I unsolder and lift rapidly one pin of each, the rectifier diodes and the big loading capacitors, connect them in the air and then back to the PCB through 10R 1% resistors to check the voltage drop across these. This method is quick and not very precise since it takes the quiescent current of the voltage regulators into account, but it allows a quick diagnose.

Hi Thierry,

I would have checked the board - except that I got exactly the same DC on both boards when driven from the same (Moog) transformer - So I assumed the voltages were normal. I now (after your data on expected DC voltage) suspect the transformer is below par -

I am not at all bothered by it.. One of my objectives in getting these EW boards was to make a battery / DC adaptor for the EW anyway (A SMPSU that takes any DC voltage between about 5V and 30V, provides +/- 18VDC from this, followed by linear regulators (non-standard - needs to drive big capacitance, so more a HF filter with crude regulation) -  giving clean +/- 15V that can connect to the EW board before the 12V regulators).. The only modification to the EW would be connecting the 2 unused pins on the power connector to the pre-regulator DC points on the board, this way it could be powered from AC or from the DC supply.

I dont even know why I bothered to buy the expensive Moog transformer - one of those impulse stupidities I sometimes do when on-line.  ;-)

Fred.