Quote Hugh Robjohns:
Spouting personal opinion and half-knowledge dressed up as facts is not particularly helpful or constructive!
Now, now, Hugh! Calm down!
There is nothing wrong with keeping it simple. Simple = reliable and in a small PSU that is all that counts.
A PSU has transformers, a rectifier or two, some capacitors and Z diodes. It also needs some cooling for the rectifiers and a fuse or two scattered here and there in case something goes wrong. If you want to get sophisticated, you could add metering to tell the user how much power and-or current is being drawn. But that's it.
We are talking about supplying a hand-full of voltages to a device that only ever draws its current within a very small range. This is not a DC supply to some critical motors that pull rods up and down in a nuclear power plant, but a thing that just sits there and uses more or less the same (relatively small) amount of current day in and day out.
I have spoken to several 'desk people' on this subject and 'as a man' they tell me that they were instructed to out-source the manufacture of as many parts of the desks (and anything else they made) as possible. Whereas before they would have a metal shop for frames and stands, a paint and print shop and a geezer building PSUs, these items were built elsewhere and were usually off-the-shelf. This meant that the manufacturer freed up capital, but it also meant that the individual item became laughably expensive and getting something made to specific requirements became almost impossible. That's where the £10,000 PSUs and £6,000 patchbays come from.
I spent two years of my life building desks and we did everything in-house. And I do mean everything. We had a screen printer, a UV cabinet, a CNC machine to cut the strip plates and drill the holes, a wood shop with format-table-saw for hand rails, tops and sides and things and of course a welding shop for frames and stands. The only thing we did not do was wind the power transformers because Guenther up the road had a trafo-winding shop and we bought in Siemens audio transformers for the inputs.
The largest desk we built was, if my memory serves me rightly a 56-frame with 2" strips and the usual patch-bay, monitor section, bla, bla. The customer could have any configuration. Literally any. 12dB eq, 18dB eq, one, two, three parametrics, VU metering, peak meters, odd number of channels. XLR on the back, Hartings and XLR on the back, Hartings and patch-bay only. In buckets, in one big lump, whatever. You know, the way it used to be before the marketing boys took over.
Because we were just a handful of people and only three of us worked full time (myself, crazy Ralf who came to work in underpants and V from the infamous mobile recording featuring Petra, a bucket, a piano, an A60, two 441s, a trestle table and a pair of binoculars) it was inevitable that one got a good idea of the problems involved with every step of manufacture. And yes, I did build power supply units, lots of them!
Of course they did not spark alpha-beam charge-centres and they did not telephone their Aunt in Maryland to report that a voltage had drifted from 17.8 to 17.3V and they were not made out of off-the-shelf units bolted together. They were bog-simple and bloody big.
I went on to build amps, crossovers and speakers and these too were bog-simple and bloody big. To be honest, one of the reasons I made them simple was because I did not have the know-how to make them complicated. But those that I know of are still working today.
Having done God-knows how many thousands of PAs, mobiles, studio sessions and all bits in between, I can assure you that audio equipment has to be one thing above all other things, no matter what - and that is reliable. A features list as long as your arm is of no use if it doesn't work. A THD spec with more noughts than you can point a stick at is pointless if you can't hear anything. A dream signal to noise ratio is bloody silly if it is not making any noise at all.
A mixing desk makes very simple demands on the power supply. It just says "Could you please provide me with these few DC voltages at these currents and don't drift or do anything silly and no Gremlins please."
It doesn't vary by much, or flicker on and off or suddenly become a generator the way so may industrial applications do. It just sits there and asks for three square meals a day and the colour supplement on Sundays. All it wants is to be safe in the knowledge that each meal is available in double portions. Just in case. And that means that the PSU has to be bog-simple and bloody big.
A final thought on fancy power supplies:
Q: How many audio engineers does it take to change a light bulb?
A: None. - I just don't understand it. It ought to be working!
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