Salvage transformer

[DJPhil]

I've been reading and reading, so much to know!

I had a quick question about this transformer I picked up on the cheap. I think it may be a candidate for a medium sized amp but I'm getting dizzy swimming through all the datasheets. I'm on the brink of being sure of this, but I figured I'd better check my math.

This amp will be for a friend who's likely never to gig with it, but will occasionally be practicing with a band. He's been getting drowned out with his current 15W practice amp so we thought we'd build something a bit better.

The transformer is an EI with bell shielding and was listed as 36VCT 3A with a shipping weight of about 3.5Kg. Once it arrived I tested it at 21.7VAC RMS on each secondary with no load. I'm guessing that it'll settle down nicely to about +/- 25VDC or so once I get the power supply built. As a bonus, it's got a 5.73VAC RMS winding on it tempting me to add some freaky digital circuitry for no good reason.

The voltage seems just about right for a LM1875, and I think there may be (more than) enough current to do a two channel build. I like the idea of using the 1875 as opposed to most of the other chips in this range I've found on Mouser. They seem to try to do everything for you to cut component count and the 1875 seems to keep things fairly simple.

Does this seem like a good way to go?

[J M Fahey]

Yes.
I think you will have almost +/-30V (probably +/-28V) available; with a 3 A current capability, it's interesting.
It will power 2x 1875 or 1x 3886, which is quitre simple too.
You'll get 50W (even almost 60) into 4 ohms (2x10" would be great).
That amp will hold its own both in the studio and live.
It will *not* be drowmed.
I suggest making a powered enclosure, with the amp on the floor (easier and stronger) and just a power switch, fuse, input jack and volume pot on a small plate in the back.
His actual 15W amp can drive it.
It's the best of both worlds: he can practice at home with the little one, add muscle when needed.
Good luck.

[DJPhil]

Many thanks for the feedback, sorry it took a while to get back.

Work on this has been postponed a while by my friend, but it just gives me more time to plan.

I like the powered enclosure idea. I think I'll be remaking the guts of his little practice amp, as it's got some terminal issues. Who ever heard of powering a 4558 with +/- 25VDC!!!  :loco It's actually designed that way in the schematic! Still, the modular approach would work perfectly for him, as the powered enclosure wouldn't have to ever leave their practice area, and we can focus on rebuilding the little amp that couldn't. I'm really looking forward to tone tasting with him and tweaking things to taste. It'd be a whole lot easier if I spoke musician, but I can't even play a kazoo!  xP

In case anyone else is interested, I picked up the transformer I mentioned from Fair Radio Sales in the US. It's in their Transformer/Choke section under Low Voltage, part number 701-121 (use browser find to get there quick, usually ctrl-F). I picked up two sets of banana jack to alligator clip test leads to make the $10US minimum order (~$12US total). They use standard UPS rates for shipping, so it was ~$22US shipped. It was in new stock condition with factory leads and well packaged. Pretty sweet deal if you're not set on toroidal! They've got a bunch of other transformers there as well.

Hope that helps, and thanks for the idea! I'll keep this thread posted as things progress.

Edited: Hit post instead of preview when I was half done.