[sdiy] Roland 191J PSU

[Elby Designs]

The general rule of thumb is that you need a good 3V overhead into a
regulator over the desired output. So for a 15V system you need 18VDC in.
This allows the regulator to work properly (LDO - low-drop-out - devices can
work with less). On top of this you usually need to allow for voltage drops
across rectifiers etc and for variations in mains voltages. With this
factors in mind you should find that you need something like 20VAC so 22VAC
sounds like a sensible values for a company to ensure good operation around
the world. For the single user, 18VAC should work fine as well - worst case
scenario, you live in an area where the mains voltage is always at the
lowest level the supply company maintains, it is freezing cold and you are
having an off day i.e. everything is stacked against you, then the regulator
might show non optimum performance - your 15V rails have a bit of ripple on
them!.

So yes, 18VAC should be fine. You need to make sure you have the same or
similar VA rating otherwise you may not have enough current.....

Best Regards
Laurie Biddulph
Mobile: 0400 257 645
Web: www.elby-designs.com


-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Louis van
Dompselaar
Sent: Friday, 17 December 2010 11:08 PM
To: sdiy diy
Subject: [sdiy] Roland 191J PSU

Hi List,

The Roland 191-J PSU is +/-15V but uses a 2x22V transformer.  Would there be
any reason why this wouldn't work with a 2x18V transformer?
The design is pretty straight forward, I can't really see why it would need
such a relatively high voltage transformer.

I need to replace one but can only get 2x18V or 2x24V.  The latter would
probably be too much.

Louis

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