Need to make a powersupply for small soundcraft mixer

[ikiru]

Hello, Some time ago, the power supply for my spirt folio 10/2 was lost in a break and entry, rendering the board useless. Contacting soundcraft as you can imagine was useless. They could not a) sell me a new supply of b) tell me what I needed to build...... So i'm assuming a +-15V supply? Does anyone have the schematics for this mixer? What sort of current would you think this thing would draw? It would be nice to get it up and running again, for something to use until I build up one of those nice mixer designs NYD has provided! Here is a photo of the board.

 

[Bjorn Zetterlund]

In the UK, power supplies for the Folio range are usually available as spares...they are known to be *****, to make a long story short. We have some folios at work, and always make sure to bring spare power supplies whenever they we take them out. They don't do phantom power so well either. In fact their cheapest 'notepad' 4-input mixer blatantly hums whenever phantom is switched on.

If I remember correctly, the 'wallwart' is simply a transformer, with rectifier and smoothing/regulating being done in the mixer itself. Unfortunately, this means that it's less easy to replace the 'wallwart'; if you get one with too low a voltage, the regulators in the mixer will not have enough of a voltage drop to work with to make smooth DC and the mixer will hum. If you get one with too high a voltage, the regulators will end up having to dissipate too much power and may end up blowing themselves up...or they may just get very hot. Given that the Spirit series mixers are done to a budget, they are not that forgiving if you get it wrong. Easiest solution is to try to get a new supply unit...and get two!

Bjorn

 

[Mark Burnley]

...or open up the mixer, trace the cables/tracks from the power socket and see what voltage regulators are used. Armed with this knowledge (guessing +/-15V from 7805/7905 or LM317/LM337 etc) you can then just build a box with a transformer in of suitable voltage...

Find out how many connections there were from the PSU too- it may do the trick of getting a +/- supply from a single secondary of the transformer, or it may need three connections from a centre-tapped secondary transformer. It may also derive its phantom from the AC supply.

(or get Bjorn to measure the loaded and unloaded outputs of those genuine transformers :wink: )

Mark

 

[ikiru]

Thanks for the responses, very helpful so far! So I cracked it open to peek inside (that took a while!) First off it was a line lump, that has a three pin connector (which tells me I need a center tapped transformer correct?). there is a lm7915 and a lm7815, and there is also a lm317 which is hanging out by the phantom power switch. So I guess it is -+15V, does that mean I need a transformer a little larger than 15V? and what amp should I be looking for?

The interesting part about opening it up was to discover that it there aren't any surface mount chips. the Mic preamp looks similar to this one
http://sound.westhost.com/project66.htm
except the two pairs of transistors are 2sb747 and 2sc2240. there are two opamps on each channel both njm072bl.
maybe while I have it open i should do a little modding...............
:grin:

 

[ikiru]

The one problem with replacing the opamps, is that they are sip-8 packages.....any ideas? the only thing I could find in sip8 were njr5532's, while I have no problem with 5532's, aren't the njr's supposed to be the bottom of the barrel?

Thanks again,
Jay

 

[maxkriza]

hi there
the transformer you need is 19-0-19 volts AC @ 400mA
Cheers

 

[ikiru]

Thanks max!
I love this place :thumb:

Now would it be ok to up the amps a bit? (since I am thinking of trying some 5532's in place of some of the 072's?)

awsome!

jay