3875 regulated operating points and lm338 heatsinking

[pjanda1]

Its been awhile since I posted over here! I built an LM3875 with BrianGT boards a few years ago. All sorts of fancy parts too. Last spring I sold that amp and took a troubled little built-for-ebay LM3875 amp in partial trade. The thing is a mess with junky old surplus caps, a big ground loop and funky resistor values. But it works, and I'd like to make it a subwoofer amp.

The transformer the guy choose has way too much voltage for a 4 ohm load. It puts out about 36-37V rectified and under load. Rather than replace it, I decided to do something I haven't played with before. I'm going to build a regulated supply with an LM338. I know that is just over their rating of 35V, but I am going to take a chance.

My local store had a pair of versions of the TO3 package LM338s in stock, so I took them despite the price. They should dissipate some heat on their own, and I have some little AL angle scrapes (maybe 1/2 on each side by 3"?) that would fit nicely in the small chassis (which has an perforated top) to mount them. So, for the 4 ohm subwoofers (they'll be pretty efficient), I'd think of running the chips at 24-25V. But, given my lack of proper heatsinking, should I use higher rails? How high can I go and have it happily drive 4 ohm subs?

Paul
Wild Burro Audio Labs - DIY Full Range Speakers

[pjanda1]

2V over the rating, how about thermistors to knock of those extra volts? I'm trying to avoid ordering any parts though, and I know the local shop doesn't have them.

Secondly, I completely forgot that this thread might belong in the power supply forum. So mods, if you feel it does, make it so.

Paul

[AndrewT]

Hi,
buy the correct transformer.

[pjanda1]

Because this one has 5% more voltage than I need? 5% is pretty darn close in electronics. Many transformers with 35V secondaries would probably give 37V anyway, as so many are rated for 115V lines. The transformer isn't a problem. No matter what I use, I've got to decide what the best operating point is.

Paul
Wild Burro Audio Labs - DIY Full Range Speakers

[DigitalJunkie]

Is it a toroid transformer? If it is,you could probably just remove a few turns without too much hassle,maybe.

[multisync]

an old trick /technicque to drop a volt or 2 is to put a diode or2 in series with the supply line. ie put 2 bridges one afte the other your +/- 37 becomes +/- 35.

The Lm 338 has a maximum voltage differential, input to output of 40 volts.. So if your input is 37 and your out is 25 the difference is 12. Well within range. If you play your music loud the Lm338 will get hot and may shut down if the heat sink is too small. Remember 25 volts/4ohm is 6.25 amps peak( with no losses). You will need 2 regulators for stereo. I have run, just for fun mind you Lm3386 at near their max rating +/- 42 vdc into 2.5 ohms. There were 3 of them in parallel. Yes they did get very hot. But it did work. Music is not sine waves. It is possible to run chips/power devices near the max rating with small heat sinks provided you don't do it for very long. If you are not selling the amp and take it easy with the volume control the only thing that may happen is that the chip will shut down due to over heat, over voltage, over current. Then again it may blow up too.

A long while ago I took some ics that were rated for +/-18 volts max and ran them up to near +/- 28volts before they failed. I also tested a number of small signal transistors and found that collector to emitter breakdown would quite often be 100% or more ie double the published rating.

there are not too many semiconductors that fail when you exceed the max by 10%.

Not too long ago nearly every resistor in a circuit was 20%, a few 10% critical areas used 5% very critical, usually instruments were 1%. Electrolitic values were only a sugestion. ie -20+50%. Now everybody wants 1% or better

[pjanda1]

I know that the voltage differential is well within the spec of the 338, I just want to keep heat dissipation to a minimum while achieving the best op point for the chip. I'm not to worried about blowing the 3875's either, I have more. The thing is, I want to maximize the fun. This amp will mainly function from 20-40hz, and while I won't need a ton of power (to drive little tapped horns), more wouldn't hurt. What I want to avoid is losing the peaks to the chip's protection function. It appears to me, based on my understanding of the data sheet, that you get next-to-no power into 4 ohms when you cross 27V or so. Yet plenty of folks seem to advocate running them a little (to a lot) hotter, which will also keep my regulators cooler. I'm tempted to stay down around 25-26V. When the movie goes "boom" I don't want them to run into protection mode and deny me the fun.

I know music is transients, but thermal concerns about the regulator are different, right? Won't it be dissipating more heat when the amp isn't working hard? (which will be most of the time).

So, rather than getting mired in what I need to do to the transformer, what experience or opinions do folks have about rail voltages for the 3875? Am I shooting for 25V, or closer to 30V? That question applies whether or not I am regulating and whether or not I buy a new transformer.

Paul
Wild Burro Audio Labs - DIY Full Range Speakers

[pacificblue]

Originally Posted by pjanda1

Won't it be dissipating more heat when the amp isn't working hard?

No. When the amp isn't working hard there will be a slightly higher voltage drop across the regulator, but much less current flowing.

[multisync]

if you have lots of amp chips run them in parallel and then you won't have to use regulators and you will get close to 100watts @4 ohms. each chip will dissapate 50 watts. Like I said I tested 3 chips in parallel with +/- 42 volt rails. no problem and the chips did not go into protection. Ran hot yes. Needed a bigger heat sink. Like you said the peaks are only momentary, most of the time.

[AndrewT]

Originally Posted by pjanda1

Many transformers with 35V secondaries would probably give 37V anyway, as so many are rated for 115V lines.

rubbish!
A transformer rated at 115/230:35Vac will give out 35Vac when fed with 115Vac and fully loaded, subject to the manufacturer's tolerance.
I would expect the tolerance to be less than +-1%.