PSU design assistance needed

[juniorhifikit]

I'm in need of some assistance designing a power supply for my little mixer project: +/-18V & +5V. The basic design that I'm working on is 16 channels each with four 5534's and a That 2180, so that's about 50mA times 16 channels is 800mA. The stereo bus (at this point) is six 5534's plus four SM2210's plus two BC184's, which I calculated to draw around 500mA; which totals 1.3 Amps. Trying to design an adequate supply, I have a few questions:

1. does the above seem correct for current draw?
2. If so, can I use LM317/337 for the +/- 18V legs (is 1.3 Amps too close to the "In excess of 1.5 Amp" rating of the LM317 TO-3 package?
3. Should my +5V logic leg be from a different transformer, or can I just use a separate coil on the same transformer?
4. How much voltage will drop across the rectifiers?
5. Little confused about how to select a transformer - let's see if I've got this straight:

If I'm drawing aprox 1.5A on each leg, the total current draw will be about 3A, multiplied by the peak-to-peak voltage I'm operating at (36V peak-to-peak plus some headroom for the drop across the regulators, etc should be around 40V?) so a transformer with a 120VA rating?

I found this transformer by Triad - VPT48-3300, which is rated at 160VA, dual primaries (so I can do 110/220V), secondary in series = 48V CT @ 3.33A, secondary in parallel = 24V @ 6.66A, regulation 8% (not sure what this means). Would this be adequate?

Here's my PSU design:

[mediatechnology]

This is a very good reference for you to start with. It should be on everyone's virtual bookshelf:

http://www.proaudiodesignforum.com/imag ... Design.pdf

National Semiconductor Audio Handbook Chapter 6 Power Supply Design

[JR.]

+1 that's a classic, was that the one with the green cover? I also vaguely remember a classic treatment in one of those old national books about heatsinks and thermal resistance relationships, junction to ambient, et al.

Of course to add a little esoterica to the mundane. In my TS-1, which I think has a schematic around here somewhere, I did a trick power supply, where I was running out of juice with my standard transformer putting out +/-15, and +5V from a single winding transformer. I was able to reduce a bunch of the power being wasted in the 5V pass regulator, but using a transistor switch, between the rectified transformer output and Unregulated supply reservoir cap for the 5V regulator.

Today, I would consider a DC-DC switcher for the 5V, but in a serious low noise design you might want to avoid a switcher.

JR

[mediatechnology]

John - The NSC Audio Handbook I have is the series with the dark blue bottom and - IIRC - a tan color bar at the top. Unfortunately the NSC books of that era were printed on newsprint and they haven't aged well.

I'm not sure where we discussed the TS-1 but I do have the sch here:

http://www.proaudiodesignforum.com/imag ... ec_TS1.jpg

[JR.]

Thanx.. the trick PS is in the upper left hand corner.. IIRC I gave the 5V 3-terminal regulator it's own roughly 10V unregulated supply so it wouldn't drop out of regulation with ripple. A significant power savings from the nominal 20V unregulated.

One perhaps unobvious part of that circuit, is that I add some hysteresis to the switch threshold comparison, to insure the switch turns off sharply (resistor from raw rectified AC). If you don't add this hysteresis, the switch doesn't turn off completely and overheats, not to mention it doesn't save you much power.

This is close to the efficiency of using a switcher, with far less complexity.

JR

[JR.]

Wayne- I didn't find my VR book in a quick search of several book shelfs... I have a pile of such stuff you might find interesting.. I'll talk to you off list about this, but I value what you are doing in archiving all this very good but obscure information, that served us so well when we were learning.

Since i never refer to this stiff any more, perhaps I'll ship some to you, if you don't already have it all...

Merry Christmas

JR

[mediatechnology]

Thanks John! I'm archiving some physical databooks for our member crazynightowl as well. I think I have the VR handbook. Let me see which edition I have...

BTW The National Audio Handbook edition I have is 1977. I think I have an earlier one as well.

[ronsimpson]

Normally a LM317/LM337 has a 120 ohm resistor from OUT to COM and a resistor (2k) from COM to Ground.
Your 120/2k are not consistent or correct.

[JR.]

Yup, the 120 or 130 ohm value doesn't matter much (its the ratios), but those spare diodes thrown in will muck up things, and it does matter which order they are in..

The fixed R generates a reference current, the current creates a voltage drop across the variable R.. get thee to an app note...

JR

[ronsimpson]

The diode from COM to OUT is fine. It is a good idea if the COM to Ground capacitor is large.
The problem is there is no COM to OUT resistor. Your output will be 1.25 volts.