Hows this (toroid transformer) for a GC

[cid499]

Hey all,

A non-electronics guy here. Been ogling over some gainclone plans and have decided to start parts hunting.

http://surplusgizmos.zoovy.com/product/TORO1

How's this for a transformer? I don't really understand transformers much, but from what I've read, it looks like this one has "dual primaries" and only one "secondary."

Is this correct? How's the voltage and VA rating?

Even if its not good for a GC, it may be good for somebody else's project out there, and it is relatively cheap.

Phil

[BWRX]

Hi Phil.

That transformer has an 81VAC center tapped secondary. As they allude to on that web page, after rectification you will have approximately 114VDC, so 57VDC per rail. That rail voltage is too high for a chip amp.

[cid499]

Thanks for the reply. Perhaps you could clarify a few things about the chip voltage ratings.

This is taken from the data sheet for the LM3875:

Supply Min 20 Volt
Supply Max 84 Volt

Does this "Supply Max" rating of 84VDC refer to the 114VDC estimated for the transformer in the link?

I do have another transformer sitting around my house with single primaries, and single secondaries is (+-19.8V). It seems that the approximate DC voltage of this transformer (19.8/0.707 = 28VDC). Would this make a more suitable transformer?

Thanks,
Phil

[BWRX]

The min and max supply voltages you quoted from the data sheet are for the total voltage between the V+ and V- pins of the chip. If you use a single rail supply you would ground the V- pin and can apply up to 84V to the V+ pin. If you use a split rail (+/-) supply where the + and - voltages have the same magnitude, you can apply up to +/-42V (for a total of 84V between the V+ and V- pins) to the V+/V- pins.

What is the VA rating of the other transformer sitting around your house? If it has a 120VAC primary, two 20VAC secondaries, and a decent VA rating, it sounds like it would be more appropriate to use.

[cid499]

I don't have the VA rating in front of me, but I think it is reasonable. Excellent! It's even better when I can use parts that I have laying around.

Are there disadvantages to using a single transformer with only single secondaries? If I read correctly, there are often 'humming' issues with using this kind of setup.

[taj]

There's nothing wrong with single primaries, as long as they are 'expecting' the voltage you plug them into. Usually, if they have dual primaries, it's for combining them in different arrangements to accept different line voltages: 115 V with both wired in series or 230V when wired in parallel, to generate the same rated secondary voltage.

The single secondary will cause you more grief though. Amps nearly always require dual polarity DC power. (Plus and minus output voltage.) A single secondary will (by default) only give you one polarity. So you need two secondaries or one with a center-tap, one to create the positive and one to create the negative polarities for your amp. The center-tap wire becomes the 0-volt halfway.

You can use two identical single-secondary transformers and connect them in series, using the connected wires as a center tap. One is used to generate positive DC, the other negative (assuming you have a bridge rectifier connected appropriately).

Check out a bunch of amplifier [unregulated] power supply schematics. You'll get the picture pretty quickly.

For a chip-amp you'll probably want 125 to 300 VA per channel.

I used a pair of 22v+22v@300VA transformers for my chip-amp; one for each channel. Many would consider that overkill and would only use one of these for both channels. But some would consider it necessary. Depends on how hard you push the amp.

..Todd

[cid499]

I believe the toroid that I have is center-tapped. There are three wires that comprise the "secondary." The sticker on the transformer labels it as +19.8, 0V, -19.8.

[Nordic]

I would not consider 300va as overkill... even at this rateing they run lukewarm after a few hours...

[AndrewT]

Originally posted by cid499
I believe the toroid that I have is center-tapped. There are three wires that comprise the "secondary." The sticker on the transformer labels it as +19.8, 0V, -19.8.

Hi CD,
the 19.8-0-19.8 is the AC voltage from your centre tapped transformer.

Check the open circuit voltage coming from your transformer. It can be very different from what is printed on the label. This will then determine the open circuit DC voltage after you have rectified the output.
You should then add on a factor for high mains supply voltage and use this new maximum to select PSU capacitors that match or exceed the PSU voltage.

Some of the circuits use a PCB that is designed for dual secondaries and need a little modification to operate correctly. This causes more than a little grief.
There are experts here to guide you through this if necessary.

[DJ Exprice]

Just don't forget the nut, bolt, and washer!