Dual 12V preamp power supply

[tubeing]

Hi, I'm quite new to power supply design so I'm asking to you PSU gurus what could be the "best" design to realize if I need a dual 12V psu to feed an analog preamp board. I need about 300mA per rail, Vin is +/-15V rectified. I'm looking for a good design in terms of noise, line and load regulation and stability. Nothing extreme, but a reliable solution, better if easy to realize: a good trade-off between performances and complexity (number of components, layout issues, stability etc etc). I found that the +12/-12V psu of my analog board has been done with "raw" 7812 and 7912, unmatched, and I need to replace this ultra simple circuit with something with better performances. I've read a lot of posts but I need a good starting point.
Thanks in advance.
Stefano

[wintermute]

Hi Stefano, I'm certainly no guru, and rather new to ps design myself. You could consider going for an LM317/LM337 type setup. For best performance however I'd suggest having 17V post rectification (on input to the regs) to get your target of 12V. I don't think it is any accident that in the datasheet the Vin to Vout differential is 5V in all performance specs

something like this -> Power Supply for Preamps

Certainly in the sims anything less than 5V differential results in higher ripple on the output, I haven't tried lowering the voltage on my prototype circuit yet but will be one of the things I investigate to see whether reality matches the sims.

Also do consider at the least doing a simple CRC before the reg (I'm going for overkill with CRCRC) lower ripple before the reg should translate to lower ripple after However this will increase the post rectification voltage yet again if you want to maintain that 5V differential.

My as yet unproven design is a bit unconventional in that it uses LM317's for both positive and negative rails, hopefully it won't be too much longer (its been over a year) before I actually build it. However I don't think it falls into the "good tradeoff between performance and complexity" category

Tony.

[tubeing]

Thanks Tony,
I'm used to work with LM317/337 since I started to build my electronic stuff years ago!
Now I'm looking to something better in terms of load/line regulation and noise.
But...I can give it a try anyway, must be surely better than the original combo 7812/7912
Have a nice day!
Stefano

[semi932]

Believe me I'm no expert just an experimenter but I've had success and enjoyment using no regulation.

It's hard to hit exactly + - 15v but most pre's allow a little flexibility in that area.

If it's using op-amps usually good till 18v or so.

If you can find the right ac source you can get right about 15.

Seems to be a lot cleaner especially in the high end where noise is most notable.

I usually use filter cap then bypass film and then lytic after rectification.

Not technically correct I know but I like the result.

Off the beaten path.

[wintermute]

Hi Stefano, everything I've read points to LM3x7 as being a bit step up over 7x12. However if you want to go to the next level you probably need to look at shunt regs, I guess this is where your comment about stability came in. I looked at Salas' shunt reg but decided to go down the LM317 path as it seemed simpler

the following might be of interest Simple Voltage Regulators Part 1: Noise - [English] and Simple Voltage Regulators Chapter 2: Output Impedance

Tony.

[agdr]

tubeing: you might also want to take a look at AMB's sigma22 dual rail board:

The sigma22 Regulated Power Supply

Just leave off the 4 rectifier diodes D1-D4 and caps C1-C4, jumper the D1 and D4 diode holes instead, then you can use the "transformer" connector to feed in your +/- 15V DC (pin 1 = V+, pin 2 and 3 GND, pin 4 = V-). In the "parts" section (scroll way down) he specifies a 30V CT transformer (15V each secondary half) for +/-12VDC out, so you should be OK with +/-15VDC in. The board uses power mosfets on the output.

[tubeing]

Tank you very much to all of you!
Sigma 22 seems to be very intresting...but it will not fit into the case of my preamp, not enough room...I think I'll try the Belleson regulators while building up some different circuits based on what you told me!
I'm lucky, the Euro is quite strong now...

Thanks again.
Stefano

[E.T.]

I don't know about esoteric psu's but you could snatch an unsuspecting copy of Walt Jung's book Audio IC Op Amp Applications. He includes an inexpensive and good supply using regulators.





You got to know when to code 'em, know when to modem.....

[hesener]

You may want to use a LM317/337 regulator (at 13V) followed by cap multipliers, that should fit with your space constraints and give very low noise. If you have more space, a shunt regulator will certainly be better sonically.

just my two cents.....