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Old 7th February 2005, 11:05 AM   #11
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Quote:
Originally posted by Yves Smolders
ClassDforsure,

That would already exceed any real-life test I believe... except maybe continuous max-power near-clipping bass for multiple hours, like in discotheques.

If a sine wave at full output can be handled at 3 ohms for an hour, there is *no way* I'd ever run into problems with my speakers.

Yves
Yep, that was my train of thought when I decided on 800VA.

Considering OEM's under-rate their transformers by 30% based on the statistics of audio verses a full sine wave.

I did ignore the non ideal effiency though, but if Jan-Peter say's his module is >90% I take his word for it. The other factor I took into account was that while I do like to crank it up at times it is never for very long, my speakers are just too efficient to require it.

I should state this was an example of me trying to pinch what pennies I could without making too huge a compromise. Ideally I'd have gone for a 1000VA.
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Old 7th February 2005, 11:09 AM   #12
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The sizing of a transformer is noting at all to do with class of amplifier (neglecting class-a with large quiescent current requirement). The fact that a class-d amp may be 90% efficient is simply talking about the heat losses in the amplfier.

The sole factors are voltage required to drive your desired power into your desired load, and duty cycle. If you assume worst case of 100% duty cycle (50% on, 50% off, i.e. sinewave) then a good approximation for an amp with little or no quiescent curent is transformer VA is 2x speaker power. At the end of the day, it's all about the acceptable temperature rise of the transformer.
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Old 7th February 2005, 11:13 AM   #13
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djk,

I do understand these things quite well and I have also read the information given by people I trust quite a lot. These people are people like Bruno who designed the UcD module, Nelson Pass and Lars from LC Audio as well as many others.

I'm not bragging about doing stupid things at all. I'm being realistic which is a totally different thing.

Sure some people drive their amps into clipping on a regular basis. Even I do sometimes.

I read classd4sure (but let him speak for himself) as he doesn't quite understand what you are talking about and quite frankly, at least to me, most of your post does not really make sense at all.

Thanks for the offer of letting me ignore your experience of 30 years in the business. I certainly will and I will also continue posting my views on things. Not to be less grand than you I also offer you the same and you can happily ignore my 30 years of experience without upsetting me the least.
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Old 7th February 2005, 11:14 AM   #14
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I'm settling for 1000VA for stereo UcD400 as well... should do just nicely without ever breaking a sweat.

I even think a 3rd channel wouldn't be a problem on 1000VA. (my center is an easier load though, 120W/8ohms max, dips to 5 ohms)

By the way, is the input gain identical on the 180 and 400 model UcD's?

Now i'm looking for a nice case to put it all in.
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Old 7th February 2005, 11:33 AM   #15
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Quote:
Originally posted by richie00boy
The sizing of a transformer is noting at all to do with class of amplifier. The fact that a class-d amp may be 90% efficient is simply talking about the heat losses in the amplfier.

The sole factors are voltage required to drive your desired power into your desired load, and duty cycle. If you assume worst case of 100% duty cycle (50% on, 50% off, i.e. sinewave) then a good approximation is transformer VA is 2x speaker power. At the end of the day, it's all about the acceptable temperature rise of the transformer.
Not quite true. Class A is a totally different thing since high currents are drawn from the power supply even at idle. In the case of SE amps the efficiency os very low and the favourite example here is SoZ which draws 1200 W continuously for a power output of 50 W. No "twice the rated power" is going to be enough there.
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Old 7th February 2005, 11:36 AM   #16
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Quote:
Originally posted by richie00boy
The sizing of a transformer is noting at all to do with class of amplifier. The fact that a class-d amp may be 90% efficient is simply talking about the heat losses in the amplfier.

The sole factors are voltage required to drive your desired power into your desired load, and duty cycle. If you assume worst case of 100% duty cycle (50% on, 50% off, i.e. sinewave) then a good approximation is transformer VA is 2x speaker power. At the end of the day, it's all about the acceptable temperature rise of the transformer.
Power lost as heat in the amp is power that doesn't get to the speaker though. With class d I personally feel it negligible, this isn't the case with other classes though is it?

Why 2X speaker power? When I came up with 800VA for my case, I have two 400W 4Ohm speakers and was going to use one transformer to power them both. You just mean for stereo right? I think you're saying an 800VA would be a approximation in this case and hoping you're not saying i'd need a 1600VA, but it would be nice anyway, if for nothing else, perhaps as a means of theft prevention

UrSv, I personally will be looking forward to and reading your posts.

Damn it got ugly fast in here.
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Old 7th February 2005, 11:36 AM   #17
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Quote:
Originally posted by Yves Smolders
I'm settling for 1000VA for stereo UcD400 as well... should do just nicely without ever breaking a sweat.

I even think a 3rd channel wouldn't be a problem on 1000VA. (my center is an easier load though, 120W/8ohms max, dips to 5 ohms)

By the way, is the input gain identical on the 180 and 400 model UcD's?

Now i'm looking for a nice case to put it all in.
1000 VA for 2-3 modules is perfectly fine.
Yes, the gain is 26 dB for both modules.
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Old 7th February 2005, 11:47 AM   #18
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Quote:
Originally posted by classd4sure


--snip--
Why 2X speaker power? When I came up with 800VA for my case, I have two 400W 4Ohm speakers and was going to use one transformer to power them both. You just mean for stereo right? I think you're saying an 800VA would be a approximation in this case and hoping you're not saying i'd need a 1600VA, but it would be nice anyway, if for nothing else, perhaps as a means of theft prevention

UrSv, I personally will be looking forward to and reading your posts.

Damn it got ugly fast in here.
"Twice the rated power" is rule of thumb used by some DIY:ers to estimate the transformer rating wanted (not needed). Please remember that there are twice (or very close at least) as many thumbs out there as there are DIY:ers so there are many rules and this is just one. Please also remember that an amplifier rated at 100 W into 8 Ohms would then call for some 200 VA of transformer (assuming we are talking Class B, AB or D/T/G/H et al). Fine, but what happens if we start using the amp at 4 Ohms? Does our amplifier then stop playing? Not at all, but more power is drawn from the power supply. To put some perspective into this I would recommend looking at consumer amplifiers, excluding the most expensive high-end and above segment, and to count the number of amplifiers that have a transformer rated at twice the total rated power in 4 Ohms. There are most likely very few...

And thanks for wanting to read my posts.
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Old 7th February 2005, 12:01 PM   #19
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Quote:
Originally posted by UrSv


Not quite true. Class A is a totally different thing since high currents are drawn from the power supply even at idle. In the case of SE amps the efficiency os very low and the favourite example here is SoZ which draws 1200 W continuously for a power output of 50 W. No "twice the rated power" is going to be enough there.
I don't wish things to start getting ugly either, but I did add the caveat "neglecting class-a with large quiescent current requirement" in my first para, which you seem to have neglected
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Old 7th February 2005, 12:02 PM   #20
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Yeah, that'd be a great rule of thumb based on 8ohm speakers I think, would still be able to drive 4ohms easily. I calculate based on 4 ohms though since that is what I'm concerned with.

I believe transformer underrating is one of at least a few reasons why most (of the cheaper) consumer amps don't have a power rating that doubles when halving the load. They've already underrated it as much as they can based on 8 ohms and maybe are only willing to sacrifice an extra few watts before transformer heating does become a concern.

One interesting question does come to light by all this mess, and that is increasing transformer size to counter act a poor power factor. Would that be as aimless as I think it is? It might lead one to believe we can get away with using 200 000uF in a supply providing we had a 20 000VA transformer. PF correction would be the obvious option.

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