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Old 18th December 2004, 09:53 AM   #1
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Question Question- Toroidal 25vac (ebay)

I am currently new to chipamp DIY audio stuff but I really want to get into this sort of thing.

Just a couple of questions.

I am planning to purchase and build a Brian GT LM3875 Amplifier Kit (stereo) but having a hard time tracking down a toroidal Transformer. Would the following transformer be compatible w/ this kit?

http://cgi.ebay.com/ws/eBayISAPI.dll...859938899&rd=1

That 25VAC dual secondaries, 600VA runs about 40USD when auction completes.

Is this toroidal worth it?

Is the quality any good?

Are there any places to source toroidals in Canada (spec. Toronto)? ( already checked out plitron, but seems quite expensive)

Thanks for any help.
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Old 18th December 2004, 01:13 PM   #2
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Default yes, IF.....

its definately a good price and a good size, you can run 4 amps off that, probly.

now the but!!! at 25vac, unless you run a regulated supply, too much hassle in my oppinion, you'll end up with about 35 to 37 volts after rectifying and smoothing. that puts you in the 8ohm speaker bracket, i wouldnt run that high for 4ohm. i personaly wouldnt run that high for bridge or parallel either but thats me.

if your happy running 8ohm or want to try and build a regulated supply, then yes its a suitable transfo, at a good price. c ya later, steve.. ..
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Old 18th December 2004, 01:19 PM   #3
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Defnately agree about bridge. But for parallel, it is fine as long as the load doesn't go below 4ohm.

With a bridged amp using 2 OPA541 chips running at +/-35VDC, The chips started on fire driving an 8ohm load (4 per chip) twice before I ran out of chips and gave up.
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Old 18th December 2004, 01:52 PM   #4
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Hi,

overhere $40 for a 600VA toroidal on the net isnt that impressive.

Toroidal transformers have the secondary windings laid over the primary ones.
It is relative easy to unwind the secondaries to the desired voltage value.
Just remove the tape, unwind till you reach 22Vac max, and retape the thing.
The toroidal transformers i see in general are fitted with low quality tape.
Taping is easy, and you have the option to go for solid quality tape in any color.
Personally i favor traditional black tape, Holden&Fisher style.

While you are remodelling a toroid, you might just as easy fit it with a resin core.
This adds ; better cooling, solid attachment to the chassis, and vibration damping.

btw:
600VA is kind of hefty, anything over 200 VA per channel isnt worth the bother with these chip amplifiers.
The alternative is to put a high power resistor in the powersupply,
between 2 electrolytic capacitors it makes a CRC filter that lowers ripple, and lowers the voltage to a level that fits the chip amp.
I am not sure if it is worth it with LM3875's, this way you loose a lot of the power of the toroid in the resistors as transferred heat.
But it is easy, and Dale resistors can be had cheap up till 100watt types.
Most GC builders seem to favor very small chassis, in that case CRC filter/voltage regulation is out of the question.
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