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Old 27th July 2007, 07:29 PM   #1
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Join Date: Jul 2007
Default transformer specs

Hey guys,
im building a gain clone (i believe) using only half of an lm1876 before building another bridged one to the full 40W potential(specifically for a guitar amp). Got the circuit working using some dedicated power supplies from a lab, but i'm wanting to build my own power supply. Now im wanting to just copy the below schematic but i have a few questions.
When checking out transformers at dikikey, what kind of VA should i aim for with the lm1876 operating at max 20W? Also im aiming for secondary voltage going about 15 - 0 - 15, but i don't get how they mean with parallel voltage or series voltage. and lastly on the transformer what difference is dual vs. single winding?
On another note, i know how the bridge rectifier works, but what kind of rating on that should i be looking for if my ideal output is +15 and -15, with current of im guessing around an Amp?

looking at digikey here --> under power transformers.

this is my first post so i hope im not laying down to many questions right off the bat. any help would be loved
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Old 1st August 2007, 11:30 AM   #2
gootee is offline gootee  United States
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Hi, magelord04.

There is a good Application Note about power requirements for various chipamp configurations. Go to and enter AN-1192 into the search box. That should give you the methods needed to calculate what you need for your (and any) particular chipamp(s) setup.

And I guess if you have it working with lab power supplies, then you can measure what you're pulling from them, to help determine what VA rating your transformers might need to be. I'd shoot for at least 160% of what you think you'll need.

The transformers you're looking at (I didn't look at your link) probably have dual secondary windings/outputs (i.e. four wires out, in two pairs). Those are usually rated for both series and parallel connections of the secondaries. If you connect the secondaries in parallel, you get half the voltage compared to series, but twice the current. If a dual-secondary transformer were used as in your schematic, you'd get half of the series-rated voltage from each side (i.e. 1X the parallel-rated voltage), and half of the parallel-rated current from each side.

I try to never buy a transformer that has a single center-tapped secondary, as shown in your schematic. Dual secondaries are much more versatile, but can still be connected as in your schematic. But they can also be used to create a dual + and - regulated supply when using two positive regulators (which is handy when neg regulators aren't available for the current rating needed), which can't be done with a single center tap.

For the bridge rectifier, if you're not using discrete diodes, I'd just standardize on something like a 100V 8A model (or whatever), that could be used for many different projects, and buy 10 or so (if that gives a price break) if you think you might ever want to build more power supplies.

- Tom Gootee
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Old 1st August 2007, 01:58 PM   #3
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Join Date: Jul 2007
thanks a bunch,
looks like i might go for one of apex's toroids. i appreciate your advice.
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Old 1st August 2007, 04:19 PM   #4
gootee is offline gootee  United States
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Join Date: Nov 2006
Location: Indiana
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You're welcome. (I was actually surprised that no one else had answered you, yet. But, it seems like the message-traffic has been slower, here, in the last few days. Maybe a lot of people are on vacation/holiday, right now, or something.)

Note, too, that there are also lots of transformers at , and also at .

By the way: I _highly_ recommend that you also order both of their free paper catalogs, for future reference. They make it SO much easier to find (and to find out about!) things than when using a website. They have thousands of pages, with almost everything electronic you can imagine. Mouser's service is truly excellent (as is Digikey's, of course), and they have no minimum order nor any "small-order fee", and have most things available in qty 1, and same-day-ship until 8PM CST. They also usually have direct links to datasheets, and to manufacturers' websites, in each line of search results. And Allied's new catalog has color photos of almost everything. They're extremely useful references to have on hand.

(OT P.S. really surprised me, when I was looking for a whole bunch of different Molex gold-plated 0.1-inch KK type PCB connectors and housings and pins and headers, etc, for my Curve Tracer product's production inventory: Allied's prices for most Molex stuff were something like 60% (or less) of both Mouser's and Digikey's prices!

- Tom Gootee
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