Power supply weirdness - Alesis Nano Compressor

[Mark Hammer]

I just picked up one of these over lunch for $20, so I'm pleased about that.  Didn't come with a PS but for $20 what do you want?  I can easily provide one.  The reference manual ( http://www.alesis.com/downloads/manuals/NanoCompressor_Manual.pdf ) indicates a 9vAC supply (830ma) and so does the chassis.  Since it's me who bought it, naturally I had to pop the lid off and take a look around (I keep a screwdriver in my desk, specifically for "curiosity-satisfaction" purposes).  The control elements are THAT2150 chips for those who wish to know.  A fistful of LM339 quad comparators in there, too, for the metering.

Here's where it gets weird.  The board has a pair of 3-pin regulators and what appears to be about as standard a form of onboard rectfiication as you can get (4 diodes, buncha caps before and after the regulators).  But get this, the regulators are 15-volt regulators.  How on earth does one provide the 17VDC that such regulators are supposed to require, from a 9VAC supply?

I'm mystified.  If the regulators are being used to provide something LESS than 15vdc, then why not use different regulators?  Something is perplexing here.  Anybody have some light they can shed?

[bancika]

Why not, you should just wire rectifier as voltage doubler, like this

[Mark Hammer]

True.  But that just gives you +/-9v (18vdc).  How do you get +/-15v (34vdc before regulation) from a 9VAC input voltage?

[slacker]

I don't know how you do it but my Microverb is the same that somehow gets +- 24 volts DC from 9 volts AC that then gets regulated by a 7812 and a 7912.
I don't understand it but a while ago I found this page about voltage doublers and triplers http://www.play-hookey.com/ac_theory/ps_v_multipliers.html hopefully it will make some sense to you
The power supply section on the Microverb has 6 electro caps in it and 5 1N4002 diodes so it could be that sort of thing.

[bancika]

True.  But that just gives you +/-9v (18vdc).  How do you get +/-15v (34vdc before regulation) from a 9VAC input voltage?

here it is:

[Mark Hammer]

So that provides 4 x 9v = 36v, minus a half-volt for each diode? (36 - 2 = 34)

[amz-fx]

Mark,

This is the schematic to the Alesis 3630 compressor. I would guess that the Nano is much the same.



regards, Jack

[Mark Hammer]

Much appreciated.  That doesn't look like it's going to deliver the +/-17v that the regulators will need, though, unless I'm missing something here.  Maybe what I'm missing is two different grounding points?

[Gus]

Clever supply!

[johngreene]

Much appreciated.  That doesn't look like it's going to deliver the +/-17v that the regulators will need, though, unless I'm missing something here.  Maybe what I'm missing is two different grounding points?

If it is 9V AC RMS then it is 2*(9/.707) = 25.5 Volts peak-to-peak. The supply schematic jack provided will give you close to +/- 25V unloaded.

--john

[johngreene]

Clever supply! what is that a wilson or something like that multiplier?

I have (had) an old Fischer stereo power amp that used the same kind of voltage doubler for the output tubes.

--john