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Old 10th January 2003, 07:09 AM   #1
sangram is offline sangram  India
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Default Transformer: How large?

Well, how large?

Are there any thumbrules, apart from 'The biggest that fits into the case'?

I can only get EI core transformers. No toroids. However, the ones I do get are reasonably decent, with little to no rattle and hum

If the amp is say, 200 watts, what kind of transformer am I looking at? 300 watts? 450 watts? How much is too much?

Thanks

In specifics: I want to build a pair of dual drive subs (stereo 'psuedo-subs' using regular 12" hifi woofers) using a 6 db/oct passive rolloff at about 100 Hz, and a pair of TDA 7294s running in bridge mode. The manufacturer claim is 170 W into 16 ohms in such a config, which for me is bang on target. I have a pair of Jamo E 470 floorstanders which have good bass but my room is just killing the bass. Suffice it to say that there is little I can do short of adding more low bass in the spectrum. I do NOT like using tone controls, or else that would've been the best thing to do...
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Old 10th January 2003, 07:24 AM   #2
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here the formula taught to me by an electrical engineer
first identify the amount of voltage and the current drawn by the amp and then factor in a ratio. i would show an example for better understanding

for instance i need 12 vdc unregulated and the circuit draws 1 amp

i take a 9 vac transformer that should give me about 12vdc if i used a cap input. then use 12 vdc * 1 amp to give me the va rating of the power requirements of the circuit. i should get now 12va. Of course we can't use this value the transformer would heat up and voltage would sag. So we normally factor in 1.8(really mininum). So now times 12va with 1.8. We should get a 21.6va transfromer for this circuit. 1.8 factor is a rule of thumb. Normally with experience, i tend to use a ratio of 4 instead of 1.8 as EI are not known for superb regulation.

Some transformer makers also tell us the regulation rating of the EI they make. Normally EI regulation is around 10% or more. So when full load the voltage would sag to 0.9 or 90%. Therefore with ratio of 4 i don't think u can go wrong.
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Old 10th January 2003, 07:24 AM   #3
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for Class AB is genrally for a 200watt amp your need a 400va transformer, so double.
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Old 10th January 2003, 08:15 AM   #4
sangram is offline sangram  India
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Thanks guys

Maybe somewhere above 2.5 or so would be a comfortable compromise between money and power, I guess. I cannot afford 800 watt transformers to run 7294s, seems that about 400/450 watts is a good bet for 180 watts RMS, say what?

NickC... is factor of 4 really necessary? I guess for a sub it may be, but I am daunted by the 2 x 400 VA transformers required for each sub... However the regulation is also an issue to about 15% maybe (Indian construction ) so I still am thinking...

Is there anything as too large a transformer??
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Old 10th January 2003, 08:26 AM   #5
UrSv is offline UrSv  Sweden
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As many opinions as there are people will be the result I think.

For a Class B or AB amp at 180 W I would be more than happy with 300 VA for this application. Use reasonable amounts of capacitance say 22000 uF maximum per rail in order to not suck too much juice out of the transformer.
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Old 10th January 2003, 11:25 AM   #6
sangram is offline sangram  India
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Is 250 VA too little? Reason I ask: I have a spare 25V 250 VA lying around and it was initially planned for a nice 140 watt amp using two TDA 7294s in stereo mode, but the price of the 7294 is making me lust for the bridge amps (though the IC is one of the smallest cost items in the sub, thankfully I have the drivers). Also this particular transformer is as common as a common cold, so it is easy to get ones hands on...

Now I'm bargaining

I am succesfully running a 70 watt bass and guitar rig based on the same IC off a 120 VA 20VAC transformer.... That makes the ratio about 1.7, slightly underpowered by some standards.

But I like 300 VA. It's a nice convenient size, I just wouldn't want to waste a perfectly capable transformer.
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Old 10th January 2003, 12:45 PM   #7
UrSv is offline UrSv  Sweden
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That would be just fine IMHO. Use it and feel good about the use of a part at hand is my recommendation.
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Old 10th January 2003, 01:11 PM   #8
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I posted on this a while back and got rather disbelieving responses, but I'm running two 3886s and all associated crossover op-amps + regulators of a +/-25V 30VA transformer quite happily.

The rating was calculated for me by a bloke who designs well respected pro-audio amps for a living, so I trust him. He says it's not necessary to match the transformer to the power rating of the amp for audio - you can de-rate the transformer because of duty cycles in the signal you're running through the amps and things like that. He's turned out to be right - two 50W amps run fine off a 30VA transformer. No problems at all.
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Old 12th January 2003, 05:46 AM   #9
sangram is offline sangram  India
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Well, cool 225 VA sounds good, 30VA is something I'm not gonna try. How much power are you getting out of that??
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Old 12th January 2003, 06:46 AM   #10
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30VA is certainly something that I wouldnt recommend. For 2x50W, assuming a capacitor input power supply, if both channels peaked at the same moment, that tiny transformer would be expected to do 330VA (plus copper losses / core losses) now I think that we can safely say that it will not succeed at that. Of course, if the supply caps are nice and big (and sufficiently low ESR), this will take some of the stress off the transformer but it's still far from being a good solution. And all of this is ignoring the active x/overs & regulators etc. that are fed from that same transformer.
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