Console psu project

[bluesbaz]

I have had this console in my garage for some time and finally had a chance to test it on a similar psu, it worked... Mostly.
In the interest of getting it up and running I need to build/buy a psu. The board requires -+18v I've looked for a linear off the shelf job but, I've hit a snag everyone wants 1.5k plus for such a thing. I've put a schematic together from the Amek psu, but if any one has any suggestions, I would appreciate it.

 

[bitman]

I have a big power transformer that came out of a power supply for a Pacific Recorder Broadcast Console.
It weighs quite a bit but it's yours for the cost of shipping.

I also have 3 big can style caps that came out it too.

 

[jackies]

How large is your console?
I recall my amek psu has more than one transistor or LM338K on positive channel.
Just one for the negative though.
It's specced 8A +18v and 5A -18v...

 

[volker]

You should specify a rough current consumption first.
Other than that, a rectifier or two after the transformer probably wouldn't hurt.
Take a look at your schematic again. Or any other, there are loads around.

 

[sodderboy]

What is 1.5k?  If you mean $1,500 then you are looking at beasts much bigger than the circuit you posted.
Re the circuit, you really need a rectifier between transformer and filter caps. 
Check surplus places like mpja.com or surplussales.com.  Once you figure the current capacity needed, overrate it by 30%.
Mike

 

[bluesbaz]

Sorry I missed that one its 5a +-18v

 

[Rob Flinn]

I built pretty much exactly the same AMEK PSU (but I didn't forget the rectifiers   ) for my 39 i/p Trident.  It works very well.    One thing I would say is that if you are drawing 5A then you need to be aware the the LM 338k will only handle 5A if it's not dropping much volatge across it.  Look at the data sheet.  The more voltage it drops the less current it will deal with. 

Also I used 2 big torroidals, 1 for the + V side & the other for the - V.

 

[hitchhiker]

I have a 3 space rack mount +18v -18v 20 amp cadac power supply that came with my console. It has a fan and I've since replaced it with fanless supplies in my simple one room studio. It has a about 25' cable, so you may have a spot that would work for placement. It's mega heavy. It runs on adjustment up to 21v. Give me a PM and maybe we can save you a big DIY job for not much more than the parts cost.
Lance 

 

[boji]

Not sure the Amek is a good benchmark...

 

[bluesbaz]

Do you have any suggestions Boji?

 

[sodderboy]

That CADAC sounds like a good deal. 
If you start to look for 18VDC 8A power supplies you find a lot of nothing.  You can fudge a 9-15 volt supply to go to 18 if you use the 100VAC input and change the voltage divider at the regulator, but two of those will still run over $300.
What happened to the original PSU?  Can that be rebuilt?  Or is it a DIY console?
Mike
PS: it looks like linear is dying a slow death- plenty of switchers out there. . . .

 

[bluesbaz]

Never received the psu to begin with.
The full spec is +-18v 5a, +5.75v 2a, 48v 300ma
-s

 

[AudioJohn]

You will be able to make your PSU quite cheaply - but as already mentioned - you need to have 2 bridge rectifiers after the transformer!
A 300VA transformer with 0 - 18 and 0 - 18 volt AC secondary windings should be fine, toroidal types are not too expensive. You can reduce the voltage rating of the main smooting capacitors (C1 + C9)  to 35 volts, this will also dramatically lower the cost of these components. I don't like the 'linked centre tapped' capacitors C13/C12 and C5/C4 - not sure that I have ever seen anything quite like it! Just have a single small capacitor across C1 and C9 and omit the linking.

I am also not a fan of adjustable presets, having seen too many faults in this area - so go for fixed resistors, such as R6/R1 = 120R and R3/R7 = 2K7. This will give 17 volts, which will be fine.

Also you will need a +48 volt Phantom supply - this will need a separate transformer, rectifier and regulator - or pop in a cheap SMPS module perhaps? This could be done at a later date when you have got the rest of the mixer working correctly of course.

It is possible to use voltage doublers to derive Phantom from the same transformer as the audio supplies - but this is easier on lower current PSU's where it is possible to use negative and positive regulators and a single bridge rectifier.

Pity that negative versions of the LM338 are not available......

 

[bluesbaz]

Good call on the caps. I also added the bridge rectifiers. I guess I left out the rectifiers because I dont understand the circuit.
How is the regulator regulating on its adjustment terminal. I figured if it can reg on an adj terminal maybe it doesn't need rectifiers. Really though, can someone explain the general idea of whats happening on the neg leg of this thing.

 

[gemini86]

Regulators do not rectify AC voltages...only regulate DC voltages as your current draw goes up and down. So you first need a basic linear supply BEFORE your regulator (transformer/rectifier/filter cap)

 

[sahib]

How is the regulator regulating on its adjustment terminal. I figured if it can reg on an adj terminal maybe it doesn't need rectifiers. Really though, can someone explain the general idea of whats happening on the neg leg of this thing.

Read page 4 starting with Application Hints.

http://www.national.com/ds/LM/LM138.pdf

Yes you can actually regulate AC voltges too but you need DC for your circuit to function. A diode or group of diodes (bridge) is used to rectify AC into pulsating DC and a reservoir capacitor (C1 and C9) is used to minimise this pulsation. The regulator (LM338) regulates it and provides a constant voltage output at constant current.

Not much interesting thing is happening on the negative side. It is just another positive regulator except its positive side is tied to common ground which is the reference point. So, normally what would be the ground is now at a negative potential in relation to the reference point.

 

[Harpo]

As already mentioned by AudioJohn, you'd better change the voltage setting network of (R5||R7)/R6 and (R2||R3)/R1. If the wiper of the trimmers loose contact (R5 or R2), the paralleled 10Ks will set the regulator for 29.6V output if there is enough voltage in front of the regulators. This might get an expensive fault. If for whatever reason you want the output voltage adjustable, place R7 in series to R5 and R3 in series to R2. Keeping R1 and R6 220R, change R7 and R3 to 2K7 and make the trimmers R5 and R2 500R. This will give you a range between 16.6V and 19.4V with your wanted 18V at trimmer in its center position. I'd also up the current rating of D1 and D2 if caps C1 and C9 fail/dry out one day.
Just another 2ct.

 

[bluesbaz]

Thanks for the input I like the idea of the more limited trimmer, and I'll read the hints on the data sheet a few more times until i get it.

 

[boji]

Not sure the Amek is a good benchmark...
Do you have any suggestions Boji?

In my limited expirence with psu's the 2u amek psu I first owned blew on me. Its fans never had the cfm to keep it cool, even after a good cleaning. My only suggestion is what ever design you choose to model yours by, over engineer the thermal transfer. Good luck, post picts!

 

[Harpo]

... and I'll read the hints on the data sheet a few more times until i get it.

good idea.
As drawn you have output voltage varying between 1.4V and 4.6V (current setting resistor 220R connects between reg.out and adj., not in between the series resistors) and with the assumed load you'd need a river to cool the regulator down with a 25VAC secondary in front. I doubt, you could get away with a single regulator per rail. Allowing +/-10% line voltage variation and 10% ripple, you'd need a 20VAC secondary, 152VA per rail to deliver 5A at 18VDC. That is 58.6W power dissipation for the regulator in a high mains scenario. Maybe my ugly psu calculator can help you playing with numbers.
Changing D1 and D2 to a higher voltage type won't change their current rating. For usual in a console there is lots of capacitance connected and kicking back to the regulator output, not only your C11,C15,C3,C7 (just add up the decoupling caps on each channel).