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Transformer power - numbers game
Transformer power - numbers game
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Old 1st March 2006, 02:42 AM   #1
taj is offline taj  Canada
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Default Transformer power - numbers game

I'm trying to square this circle...

An eBay vendor selling toroidal power transformers, offers one he calls 39+39v output. (Which, on the face of it, sounds fine for my Leach amp project.) When it arrived, the label on the transformer says 35+35v secondaries (115v primaries). The eBay vendor explains that 120 volt input is about 8% higher than 115v label, therefore the output will be 39v.

My math sucks, but regardless, my calculations make this primary voltage difference less than 5%, which only boosts 35v to like 36.75v.

Secondly, I thought the North American power grid ran at 117v nominal. Mine measures exactly that. So the difference isn't going to be as noticable as eBay vendor suggests.

With regulation effects considered, I expect the voltage to probably be a bit higher with light loads (it's a 700 VA unit), but still not as high as 39 volts.

Am I off-base on this?

- TAJ
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Old 1st March 2006, 04:49 AM   #2
acenovelty is offline acenovelty  United States
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Well sir, your math is Ok, but have you wired it up and put a meter on it? Personally had lots of torroids measure higher or lower than the label. Then again, you did get it from e-bay. Caveat emptor as they say.

Prof. Leach says,"T1 - The power supply voltages should be in the range (+ and -) 56 V to 59 V with no load on the amplifier."
And further recommends (80VCT@6.25A)

Even if it were 39-0-39, you would still be below the specs.
39 x 1.414 = 55.146 no load. Which will give slightly less than the 120 watts advertised even if it really is 700 VA.

From http://www.partsexpress.com
AVEL Y236853 625VA 40V+40V TOROIDAL TRANSFORMER
Price: $65.88 EA
625W 40+40V 7.81A

AVEL Y236903 800VA 40V+40V TOROIDAL TRANSFORMER
Price: $69.93 EA
800W 40+40V 10.00A

If you paid significantly less, then add or remove some winding and be happy. Otherwise, be prepared for a likely ugly scene with the seller over mis-representation. If you used paypal, probably just kiss the money goodbye. Credit card, you may have a case for returning it and the credit card company will charge it back to him. Lots of hassle over a readily available device.

I personally never argue with a man over his price, he knows what his stuff is worth. Buy the best, you'll never be disappointed.

Regards
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Old 1st March 2006, 05:26 AM   #3
taj is offline taj  Canada
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I appreciate the reply acenovelty.

Nope, haven't gotten around to wiring it up yet.

I was much more concerned/confused about the theory here than whether I got ripped off on eBay. I didn't at any rate ($50). The toroid is fine, and if it turns out to be inappropriate for the Leach amp, then I'll chuck it in my parts box and keep my eyes peeled for a suitable project. (The eBay vendor doesn't seem unreasonable - despite his incomprehensible math.)

And say it's a tad below spec (say 78vCT as advertised)... What's the side effect? Lower power? (Who cares.) Anything else? Starving anything?

Also, I've been warned that when considering regulation effects, the formula seems to work out closer to RMS*1.5 if we're talking about light loads and typical regulation. Now, 1.5* puts this coil's output near the upper end of the good professor's recommendations.

Thoughts? (yeah, I know... "plug it in and measure it".)

-Todd
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Old 1st March 2006, 05:48 AM   #4
acenovelty is offline acenovelty  United States
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The theory postulated by the seller is bogus. Allows for over rateing the product, but false.
Below spec only means less power, and not much in this case.
A new one at spec from a reputable dealer is only $20.00 more.
The formula is voltage x 1.414 = ? unloaded. Put a load on it (like the transistors) and the value goes down. The bigger the load the lower it goes. Less and less power just when it is needed. Sagging rails usually result in distortion along with heat. Compensate with more and bigger caps, 63V caps are still not cheap enough to make up for the $20.00.

Fix it yurself by adding or removing windings to get to 40-0-40 and be happy.

Be well
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Old 1st March 2006, 10:05 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
test your transformer on open circuit voltage.

Your caps with a low bias on the amp will run at a DC voltage closely based on this.

Vdc~=[Vac*1.414*.99]-0.7

If Vdc is lower than the range Leach recommended then you will have less power output, so what. 100W is only 1db below 125W
Use the money saved to buy the extra 63V caps instead of 75V/80V caps and more of them. After the secondaries build everything as a monoblock.

If you run the amp at lower voltage it will still work well and can drive a slightly lower impedance or more reactive speaker due to less voltage stress on the output and driver devices.

If you really want to impress with the power numbers then add extra secondary turns to achieve the supply voltage you want. But you will have to work with quite long lengths of wire to add 5Vac to each secondary.
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Old 1st March 2006, 10:45 AM   #6
jacco vermeulen is offline jacco vermeulen  Netherlands
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80 volts capacitors ? A 40Vac toroid only needs 63V caps.

Using a 35Vac transformer on the Leach still requires 63V capacitors, 50V models will not do.
I'd do what Ace says,adding some windings of bell wire on the secondaries is cheap and easy. You can tailor the secondary voltage just like you want it.
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Old 1st March 2006, 06:24 PM   #7
AndrewT is offline AndrewT  Scotland
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Hi Jacco,
it's Leach that says 75V caps for the power supply.

The reason being the recommended 40Vac transformer which at nominal voltage will give 56Vdc. But after adding 5% transformer regulation and 6% mains supply tolerance gives 63Vdc unloaded capacitor voltage.
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Old 2nd March 2006, 08:31 AM   #8
taj is offline taj  Canada
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Thanks folks.

I'm going to plow ahead with the 35+35v transformer and live with with reduced output power, which isn't a big issue with me. Meanwhile, if I find a good deal on a suitable 40+40v unit, great. I can always find another amp project to suit the 35+35v one. Also, it just happens that I have at least a dozen new 63v 10kuF caps sitting in my parts box, so I'll probably use 2 per rail for this.

Andrew,
What's the best way to test open circuit voltage? Should I just measure the open secondary leads with my DMM, or build a rectifier/filter circuit and measure its DC voltage? or??

By "after the secondaries, build everything as monoblock" you mean separate rectifier/filter circuits for each stereo channel? That's kind of what I planned.

..Todd
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Old 2nd March 2006, 09:55 AM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
a safe way to test open circuit (not necessarily best).

Insert all the secondary wires into separate terminals of a terminal block so that no accidental shorting can occur.

Wire up the primaries similarly to avoid accidents.

Stand back and switch on.
Check your output voltages with your DVM set to 750Vac. Once you have ballpark figures you can reduce the DVM scale to an appropriate level.

Yes, separate +-Vrails to each channel.
Are you using 4ohm or 8ohm speakers? +-20mF per channel might be a little light for 4ohm. Leave room if you can to experiment with +-30mF (3mF/Apk) smoothing. You may find no difference or like/dislike the change. At 8ohm, 15mF to 20mF should be adequate.
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Old 3rd March 2006, 02:45 AM   #10
taj is offline taj  Canada
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Cool. Thanks Andrew.

Funny, I wired it up pretty much exactly as you described - before reading your message. It measures 40.2 volts on each secondary with no load. The primary measures 117v as I expected.

Now I'm back to being confused about the 35+35 @ 115v. label and the eBay vendor's math, but excited about the amp project nevertheless.

..Todd
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