Jaycar MKII - 12 V AC power supply - please help!

Posted: 8/28/2013 11:33:24 AM

From: Melbourne, Victoria

Joined: 8/28/2013

HI - my 12 year old son has soldered the kit - we now find we need a 12V AC power supply 250mA.  I went back to Jaycar and they only sell an 12V AC 500mA.  The techo told me I would fry the theremin as the power supply is unregulated and would deliver 24V AC instead.

Note that the MKI used a 9V DC supply - this one is different.

1. Is it right that a 500mA 12V AC source will not work?

2. Can anyone who has built one of these tell me what AC supply they have used?

3.  The Jaycar guy also told me I could use a 12V DC but it could not be a switchmode type (which they did not have either).  If I use a 12V DC battery - where do I connect it (somewhere in the diode circuit?)

Thanks if you can help out an electronics ludite...- MJL.


Posted: 8/28/2013 11:55:43 AM

From: Colmar, France

Joined: 12/31/2007

The milliamps (250 or 500) are in general not important. But on very cheap transformers it is common that the free-run voltage may largely exceed the nominal voltage, especially if only a very little current (compared to the maximal transformer's current) is drawn. And since the Jaycar consumes about three times nothing, the guy is right that a too powerful (and too cheap) transformer may make it go up in smoke.

But he is right about using a 12V to 15V (regulated but not switched mode) DC power supply. This one can directly be connected on the power supply plug as the AC power supply would. The internal diode quartet will take care about polarity and so on, so there is no need for internal modifications.

Unfortunately it has become a common behavior among new TW members to not to fill out their profile completely (especially their location) which makes it difficult to point them towards a local or online store in their country which carries the needed products...


Posted: 8/28/2013 2:19:53 PM

Joined: 3/30/2012

"we now find we need a 12V AC power supply 250mA.  I went back to Jaycar and they only sell an 12V AC 500mA.  The techo told me I would fry the theremin as the power supply is unregulated and would deliver 24V AC instead."

I feel that 24 volts (100% regulation factor) is too high an estimate for a 6VA transformer. With low VA transformers such as these I would expect a regulation factor of around 30% to 50%. That is, at a nominal 12 volts the unloaded voltage would be in the order of 16 to 18 volts.

(I guess the way to find out is to ask Jaycar to measure it at the desk for you. Depending on the store, and how busy they are at the time, and how you go about asking, I find that most of their sales assistants are happy to help like that.)

I'm not familiar with the project's circuit, but if SC originally specified 12 volts AC @ 250mA (a very low 3VA), then, knowing it's a low current application, I guess they took that regulation factor range into consideration?

This type of AC plug pack is becoming increasingly harder to find these days. It's really only legacy equipment that requires AC. Nowadays, it's switch-mode DC input.

I don't know where to find a plain 12 volt DC regulated PS in a box. That's the sort of thing I'd expect to make myself.

*** As a further thought: if it turns out that the Jaycar 12 volt plug pack has an unacceptably high unloaded voltage, then possibly try their 9 volt AC version -- which may come in at a more acceptable unloaded voltage. Again, without knowing the tolerance allowed for in the design, it's a hard call.

Posted: 8/28/2013 10:10:41 PM
RS Theremin

From: 60 mi. N of San Diego CA

Joined: 2/15/2005

MJL said: "If I use a 12V DC battery - where do I connect it (somewhere in the diode circuit?)

Use two 4 cell holders to create a 12v battery pack. I would imagine this theremin using AA batteries would last months on alkaline and years if you turned it off every time. Otherwise you will spend more money on the power supply than the theremin.  Batteries will give you the cleanest sound. You should get a good earth ground by plugging into a amplifier that uses a three prong plug. Otherwise the theremin will misbehave. Plugged into a laptop, using headphones or just computer speakers by themselves probably won't give you a good earth ground.

Edit 1: You really need an extra cell, 9 total to get the voltage up to 13.5, I forgot to consider the 1.2v loss across the diodes and battery decay.

Schematic 2009

If you can't find the proper plug, solder on any two lead connector you find available at the bottom side of the power plug jack. The +/- direction can be reversed without issue as the full wave diode setup will direct power in the proper direction.

Switching power supply wall-warts work when a theremin is designed to give the much needed high frequency filtering, otherwise you might hear birdies and things in your finale sound. But you still need a good earth ground from somewhere. If you have a 12 dc wall-wart from something else try it, it won't hurt anything and can be connected in either direction.

This is an excellent theremin to develop soldering skills and get a feel how an authentic theremin actually works.

This box will teach your son about tuning for the Null point which is important for most other theremins he encounters in the future. He will experience the magic and the frustration of theremin building.


Edit 2: You want to use the minimum voltage that works at the power jack terminal (13.5v). The regulator will dissipate any extra voltage/power as heat. A theremin should never have an internal heat source as this will cause thermal pitch drift, the audio tone drifting higher or lower.

Posted: 9/5/2013 12:39:56 PM

From: Melbourne, Victoria

Joined: 8/28/2013

Thank you so much for your suggestions.  I went with 9AA's and we certainly have some theremin like behaviour out of the kit after the tuning (though the pitch antenna range is not that great - say 20cm).

Next question is that the "voice" VR2 does not seem to be doing anything, at least to my ear. Should I be noticing a distinct difference going from "soprano" to "cello"as this is turned one way or the other?


Cheers -M

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