ferguson@tc.fluke.COM (Ed Ferguson) (05/16/91)
I'm adding voltage regulation to my audio amp to improve performance. Vout is +/- 28 V and the amp draws 3A on each peak. National makes an adjustable 3A positive regulator (LM150) but I don't see a 3A negitive adjustable regulator offered. Could two LM317Ts (1.5A each) and two LM337Ts (1.5A each) operate in parallel to supply the +/- rails at 3A? Or is a regulator with a series pass transistor the better way to get more current? Ed Ferguson
stevem@specialix.co.uk (Steven Murray) (05/17/91)
ferguson@tc.fluke.COM (Ed Ferguson) writes: >I'm adding voltage regulation to my audio amp to improve performance. >Vout is +/- 28 V and the amp draws 3A on each peak. National makes an >adjustable 3A positive regulator (LM150) but I don't see a 3A negitive >adjustable regulator offered. >Could two LM317Ts (1.5A each) and two LM337Ts (1.5A each) operate in >parallel to supply the +/- rails at 3A? Or is a regulator with >a series pass transistor the better way to get more current? >Ed Ferguson Sure! But you need to share the current between them with resistors. Typically you would put a 0.1-0.2 ohm in each output lead, then tie. Have a look at the specs and apps in the data book for the devices. One circuit you probably won't see in the data book that does work is a couple of LM317's set up with output adjustment resistors, followed by load sharing resistors. You DO get output variation - but not a lot. 0.2 ohm resistors will give you 0.3v load variation from 0 to 3A. There does seem to be a 3 amp adjustable reg: LM333T. Anyway, you might want to watch out for heat dissapation - something that's easily overlooked. You might end up deciding to use 3 regs on each side, just to be really sure the heat doesn't exceed ratings. I prefer using regs to Transistors (except on large quantity work) - a couple of extra pennies for smarter regulation. Do put in all the decouple caps and protection diodes recommended in the data sheets too - its so nice to do projects once instead of twice! -- Steven Murray uunet!slxsys!stevem stevem@specialix.co.uk I am speaking, but | If these are your opinions, then we are in agreement!! not for my employer.| Flames, spelling errors, complaints > /dev/null
jfw@ksr.com (John F. Woods) (05/18/91)
ferguson@tc.fluke.COM (Ed Ferguson) writes: >I'm adding voltage regulation to my audio amp to improve performance. >Vout is +/- 28 V and the amp draws 3A on each peak. National makes an >adjustable 3A positive regulator (LM150) but I don't see a 3A negitive >adjustable regulator offered. >Could two LM317Ts (1.5A each) and two LM337Ts (1.5A each) operate in >parallel to supply the +/- rails at 3A? Or is a regulator with >a series pass transistor the better way to get more current? First, the LM145 is a 3A negative adjustable regulator. You can't just wire regulators in parallel, they don't share well; it is fairly simple to devise a circuit which makes them share better, but the regulation probably suffers (the data book isn't clear on the behavior of the circuit, but it looks fishy to me). A pass transistor is probably the way to go; use the right circuit and it will even benefit from the current limit built into the regulator.
mhk@HQ.Ileaf.COM (Mark Kaepplein x1517) (05/23/91)
I second using pass transistors. I would hesitate using any standard regulator, and by using resistors to balance the load between regulators, you negate any benefit regulation might give you. Why don't I like regulators? Current limiting. Nearly all of them do it. A simple zener referenced pass transistor sounds better and an IC voltage ref. ought to be better still. If you want to try hearing the degradation resistors cause when they are on the rails, bypass your rail fuses with wire. Yes, if you have a problem in the amp, you could lose the output devices, but I run my amp that way and it sounds much better.