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Old 25th December 2006, 03:31 PM   #1
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Default too high voltage?

hi, i bought a 25v toroid, which when i put a meter across read 28v, which then turned into 39v after rectification. i was wondering if this will be ok for the lm3875 with use with an 8 ohm speaker?

will the toroid 'settle' so to speak, i mean, will the voltage drop to the quoted figure?

thanks in advance
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Old 25th December 2006, 03:39 PM   #2
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Default Re: too high voltage?

Quote:
Originally posted by seventynine
hi, i bought a 25v toroid, which when i put a meter across read 28v, which then turned into 39v after rectification. i was wondering if this will be ok for the lm3875 with use with an 8 ohm speaker?

will the toroid 'settle' so to speak, i mean, will the voltage drop to the quoted figure?

thanks in advance
This is normal. It's not gonna settle or burn in AFAIK. It will vary depending on the mains voltage (which I believe has a 'tolerance' of +-10%, but I could be wrong there). It'll also drop somewhat under load I believe.


I have no idea if it's going to be ok with an 8 ohm speaker, so I will let someone else answer that



EDIT: the tolerance is apparently +10% -6%, more http://www.russandrews.com/article-4...gfrequency.htm
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Old 25th December 2006, 08:12 PM   #3
Minion is offline Minion  Canada
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Most Transformers will be a couple Volts above there rated Voltage when there is no Load on the Transformer but once it is conected to an active Circuit that is drawing Power the Voltage will settle down to closer to it"s Rated Voltage....



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Old 25th December 2006, 08:41 PM   #4
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Default Re: too high voltage?

Quote:
Originally posted by seventynine
hi, i bought a 25v toroid, which when i put a meter across read 28v, which then turned into 39v after rectification. i was wondering if this will be ok for the lm3875 with use with an 8 ohm speaker?

will the toroid 'settle' so to speak, i mean, will the voltage drop to the quoted figure?
The voltage drop will depend of how big the tranformer is, bigger less voltage drop but expect 5% at least. The toriod will give 25 volts at rated _resistive_ load (which an amp not is) and also rated input voltage.

Up to 28 VAC is the very limit => 25 VAC
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Old 26th December 2006, 10:34 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
28Vac from a 25Vac transformer is a bit higher than usual. It is +12%.

BUT, what voltage is it expecting at the mains input?
Did you measure it with the correct mains input voltage?
Will it run at rated voltage or is your house normally running at a different voltage?

Once you know what to expect at the input and output of the transformer, then we need to take account of voltage when actually operating.
As already stated the transformer voltage will drop when under load.
Some chipamps draw a high quiescent current. The voltage from the PSU will be lower when operating into a high Iq chip than into a low Iq chip.

Build it and run it and then tell us the measured voltage at your normal listening time.

25Vac is OK for your planned 8ohm speakers. Do not be tempted to use 4ohm or parallel 8ohm for some special occasion.
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Old 26th December 2006, 06:39 PM   #6
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toroid expects 230ac - ive not checked what my mains are but i suspect it to be about 250 as we are fairly near the power substation (im sure someone told me this made a difference)

will finish it off tomorow now as im too intoxicated to make a good job of it
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Old 26th December 2006, 08:04 PM   #7
CarlosT is offline CarlosT  United States
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Interesting experience I had was related to a pilot light...

I was hanging off the pilot light off the DC power V+ and common. My little 90VA 20V + 20V was putting out about 31V + 31V under no load after rectification. I had a Sylvania old school pilot light with a red jewel lens with a 28V 28MB bulb. Well...when I hooked up the pilot light, don't you know that the votage on that leg of the DC power dropped to like 25V. I got worried about it and decided to power the pilot light right off the AC 110V (of course with an appropriate 120V 120MB bulb). Did I need to worry about this? I didn't wanna create some kinda imbalance between the V+ and the V- voltages but maybe this was just the PSU settling in on the terminal power values and I had nothing to worry about.

Thoughts? Comments?
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Old 28th December 2006, 05:44 PM   #8
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its dropped down to about 37-38 under load, the heatsinks are fairly hefty and the ic's arent even getting warm so i think ill be alright.

whatdo i watch out for?

(will get some pics up soon also)
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