[sdiy] Siel OR400 Transformer needed

Steve Lenham steve at bendentech.co.uk
Fri Jul 26 10:49:01 CEST 2013

Hi Maeghan,

Welcome to SDIY.

That schematic is quite useful, because they give us the DC voltages, 
making it easy to work out the transformer required.

The unregulated DC voltage from a full-wave bridge rectifier + smoothing 
capacitor arrangement is ABOUT 1.4 times the AC output voltage of the 

The schematic shows two lots of 20V DC, so you need a transformer 
delivering two lots of 20/1.4 = 14.3V. The nearest standard transformer 
voltage is 15V.

This can be described in catalogues in several ways, all of which mean 
the same thing:

2 x 15V
2 x 0-15
30V CT (centre-tapped, i.e. 30V in total with a tap point giving two 
halves each of 15V)

Some transformers will have only the three secondary pins shown on the 
schematic (the two ends of the winding plus the centre tapping) while 
others will have four. This means that they have two separate secondary 
windings (making them a bit more versatile) - in that case, you just 
need to strap the end of one secondary winding to the beginning of the 
other and use that common point as your centre tapping.

The only other thing that you need to know in order to go out and buy a 
replacement transformer is the power rating needed. For transformers 
this is specified in VA (volts x amps) and not Watts.

We can make an educated guess at the rating by looking at the mains fuse 
in the schematic. At 110V, the fuse is 200mA (0.2A). So the unit can't 
be drawing more than 110 x 0.2 = 22VA from the mains, otherwise the fuse 
would be wrongly specified (note for pedants: I didn't say that it would 
actually blow).

A common rating for transformers is 30VA, and that is what I would 
choose provided that it will physically fit into the unit.

So you need a 30VA transformer with two 15VAC secondaries. An extremely 
common part - any electronics supplier should have one.

Just take due care when working on anything using mains voltages. I'm 
aware of the "debate" over on AH and, while I consider some of the 
language overblown, there is a nub of truth in it. For your own sake, 
you should make sure that you are familiar with the basics of electrical 
safety - perhaps you already are, I wouldn't presume to know.

Hope this helps - good luck getting the unit resurrected!


Steve L.

Steve Lenham
Benden Sound Technology

On 26/07/2013 08:58, M Maeghan Skala wrote:
> Hello folks, this is my first post here. My name is Maeghan, I'm
> known on the web as pulsewidthmod.
> That being said, I'm looking to get a transformer for my Siel OR400.
> I blew mine in a real stupid accident.


> I"ve looked over the schematics for the Siel Mono, which shares the
> same power supply, and it doesn't specify the transformer needed.
> Some people on muffwigglers have said it could be 25v, 15v, and some
> have said 30v … all center tapped.
> Here's the schematic for the power supply:
> http://dc142.4shared.com/doc/KHy38vKD/preview008.png
> If anyone can help, it would be much appreciated :)
> cheers ~ maeghan/pulsewidthmod
> _______________________________________________ Synth-diy mailing
> list Synth-diy at dropmix.xs4all.nl
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