I've been doing some searching and haven't found any thorough explanations of how to properly match a transformer to a Class D amp. I'll admit up front... I'm quite confused on this one.
I've studied Slone's writings on sizing a transformer, and his calculations all seem to be topology invariant... unless he is making the unstated assumption that the amp is always Class B. It seems natural to think that the efficiency of the amplifier itself will have an impact on the required size of the transformer, but no such efficiency coefficient is to be found in his calculation.
So I did some less "scientific" research, and took a look at hundreds of posts here, elsewhere, and looked at commercial Class D offerings to see what transformer sizes were being used for various amplifier output ratings. It looks like it's all over the board. I see some here using 1kVA or 1.5kVA trafos for 400W outputs. I see in some commercial products what looks like 500VA~800VA trafos for 1000W+ output. Using Slone's method, 1000W looks like it would need some 1.5kVA, but I'm not sure if that applies only to Class B or to all topologies.
Could someone point me in the right direction? I'm sure the information is already on the Web somewhere, probably in multiple places, but my Googling skills seem to be lacking lately.
I've studied Slone's writings on sizing a transformer, and his calculations all seem to be topology invariant... unless he is making the unstated assumption that the amp is always Class B. It seems natural to think that the efficiency of the amplifier itself will have an impact on the required size of the transformer, but no such efficiency coefficient is to be found in his calculation.
So I did some less "scientific" research, and took a look at hundreds of posts here, elsewhere, and looked at commercial Class D offerings to see what transformer sizes were being used for various amplifier output ratings. It looks like it's all over the board. I see some here using 1kVA or 1.5kVA trafos for 400W outputs. I see in some commercial products what looks like 500VA~800VA trafos for 1000W+ output. Using Slone's method, 1000W looks like it would need some 1.5kVA, but I'm not sure if that applies only to Class B or to all topologies.
Could someone point me in the right direction? I'm sure the information is already on the Web somewhere, probably in multiple places, but my Googling skills seem to be lacking lately.
Hi,
Slone's methods are for a no compromise class B topology, hence all the added losses, if you used that method for class d you'd have extreme overkill.
You can still use it as a reference though but keep in mind the differences in topology.
Assuming class d amps are ~90% efficient (close enough), if you want 100W at the load, then 100W+10%, the only voltage drop of worth is that of the rectifier diodes, so you can take that into account when working backwards to get your secondary requirement. Double that VA rating for a 2 channel amp off one transformer.
The OEM penny pinch method is to use ~70% of the calculated requirement based on the fact that music signals are not pure sine waves and therefore dont' demand the same kind of power.
Then you have everybody else who swears bigger is best.
The determining factor might be that using a higher VA rating might get you a transformer that has a tighter load regulation factor.
Hope that helps ya.
Slone's methods are for a no compromise class B topology, hence all the added losses, if you used that method for class d you'd have extreme overkill.
You can still use it as a reference though but keep in mind the differences in topology.
Assuming class d amps are ~90% efficient (close enough), if you want 100W at the load, then 100W+10%, the only voltage drop of worth is that of the rectifier diodes, so you can take that into account when working backwards to get your secondary requirement. Double that VA rating for a 2 channel amp off one transformer.
The OEM penny pinch method is to use ~70% of the calculated requirement based on the fact that music signals are not pure sine waves and therefore dont' demand the same kind of power.
Then you have everybody else who swears bigger is best.
The determining factor might be that using a higher VA rating might get you a transformer that has a tighter load regulation factor.
Hope that helps ya.
I just spent the evening at the B&O ICE Power site.
They claim 79% efficent into 4 ohms and 82% at 8 ohms.
Those are believable numbers, others I have seen are not.
First timers' always forget the power factor for cap input supplies. A 1KVA transformer will only drive about 700W into caps around 10,000µF or so, bigger caps only make the power factor worse (no, you can't 'make up' for a weenie transformer with big caps).
Duty cycle on rap bass and some lead guitar may be as bad as 50%, but driving a 45* load will make up for this. A wash.
100W + 100W at 8R would give X2 at 4R. Divide by 0.8 for efficency and you need 500W in. Divide by a 0.7 power factor and you need a 700VA transformer for your 100W stereo amp that will be 4 ohm stable into a real loudspeaker (a 45* load).
Less is less.
You can figure your design around a 25% duty cycle, I have done this before. But the bigger transformer sounds better.
I also burned up a 25% duty cycle transformer when bench testing an amplifier into clipping using FM radio as the source material (of course no one you ever met has driven an amplifier into clipping, right?).
Don't try and save too much money on the transformer, in the long run it doesn't pay.
They claim 79% efficent into 4 ohms and 82% at 8 ohms.
Those are believable numbers, others I have seen are not.
First timers' always forget the power factor for cap input supplies. A 1KVA transformer will only drive about 700W into caps around 10,000µF or so, bigger caps only make the power factor worse (no, you can't 'make up' for a weenie transformer with big caps).
Duty cycle on rap bass and some lead guitar may be as bad as 50%, but driving a 45* load will make up for this. A wash.
100W + 100W at 8R would give X2 at 4R. Divide by 0.8 for efficency and you need 500W in. Divide by a 0.7 power factor and you need a 700VA transformer for your 100W stereo amp that will be 4 ohm stable into a real loudspeaker (a 45* load).
Less is less.
You can figure your design around a 25% duty cycle, I have done this before. But the bigger transformer sounds better.
I also burned up a 25% duty cycle transformer when bench testing an amplifier into clipping using FM radio as the source material (of course no one you ever met has driven an amplifier into clipping, right?).
Don't try and save too much money on the transformer, in the long run it doesn't pay.
DJK,
Your post is very contradictory to the discussion in the UcD power supply thread.
According to calculations there, 1000VA/2*40V with 33.000uF per rail should be enough to drive UcD400's up to 3 ohms (>550W/channel?)
Do you believe these calculations wrong? What would you recommend?
There's also the "hard limit" - UcD400 V2 pumps 20A max.
Would love to hear your comments.
Your post is very contradictory to the discussion in the UcD power supply thread.
According to calculations there, 1000VA/2*40V with 33.000uF per rail should be enough to drive UcD400's up to 3 ohms (>550W/channel?)
Do you believe these calculations wrong? What would you recommend?
There's also the "hard limit" - UcD400 V2 pumps 20A max.
Would love to hear your comments.
Want to have and need to have are two different things.
Don't forget that duty cycle as we are not talking about powers running full power 100% of the time which will "never" happen. Also a transformer rated at 500 VA is rated 500 VA continuous with a certain temperature rise. It will happily provide 750 VA during shorter intervals and since it is not continous it will do this without getting warm.
The general rule IMHO, is thus to take as much as you want to pay for. Lars at LC Audio had very good results using 80 VA (or possibly 50 VA) with his 200 W ZAPpulse. I myself would be very happy with 500 VA for a stereo UcD400 driving 8 Ohm speaker.
Don't forget that duty cycle as we are not talking about powers running full power 100% of the time which will "never" happen. Also a transformer rated at 500 VA is rated 500 VA continuous with a certain temperature rise. It will happily provide 750 VA during shorter intervals and since it is not continous it will do this without getting warm.
The general rule IMHO, is thus to take as much as you want to pay for. Lars at LC Audio had very good results using 80 VA (or possibly 50 VA) with his 200 W ZAPpulse. I myself would be very happy with 500 VA for a stereo UcD400 driving 8 Ohm speaker.
You obvously don't understand things like 'duty cycle', and 'power factor'.
"Don't forget that duty cycle as we are not talking about powers running full power 100% of the time which will "never" happen."
As the service manager in a hi-fi shop I found that many customers drove their amplifiers into clipping on a regular basis, if not all the time. What is the duty cycle of a square wave?
"Lars at LC Audio had very good results using 80 VA (or possibly 50 VA) with his 200 W ZAPpulse. "
Sure, driving 2W out at a duty cycle of 4%, so what?
If we are going to brag about doing stupid things...I know someone that mounted a 10HP lawnmower engine in his pickup truck. It got in excess of 100MPG and would go maybe 60MPH on flat, level ground with no wind.
I have about a ton of Yamaha, Sony, and Denon transformers that have the thermal fuse out in the primary, why I'm saving them I'm no longer sure.
Feel free to ignore my comments based on 30 years of experience in the hi-fi business.
To finish the story about the 25% duty cycle transformer that burned up during testing, I also provided a 50% duty cycle transformer I recommended they use in production, and a 100% duty cycle transformer just to show what a difference in sound it made.
While they cheated and used the 100% rated model at the trade show, to their credit they rejected the 25% model and sold the amplifier with the 50% model.
You guys are welcome to do what you want.
(are you all bean counters, or are some of you DIY hobby types?)
"Don't forget that duty cycle as we are not talking about powers running full power 100% of the time which will "never" happen."
As the service manager in a hi-fi shop I found that many customers drove their amplifiers into clipping on a regular basis, if not all the time. What is the duty cycle of a square wave?
"Lars at LC Audio had very good results using 80 VA (or possibly 50 VA) with his 200 W ZAPpulse. "
Sure, driving 2W out at a duty cycle of 4%, so what?
If we are going to brag about doing stupid things...I know someone that mounted a 10HP lawnmower engine in his pickup truck. It got in excess of 100MPG and would go maybe 60MPH on flat, level ground with no wind.
I have about a ton of Yamaha, Sony, and Denon transformers that have the thermal fuse out in the primary, why I'm saving them I'm no longer sure.
Feel free to ignore my comments based on 30 years of experience in the hi-fi business.
To finish the story about the 25% duty cycle transformer that burned up during testing, I also provided a 50% duty cycle transformer I recommended they use in production, and a 100% duty cycle transformer just to show what a difference in sound it made.
While they cheated and used the 100% rated model at the trade show, to their credit they rejected the 25% model and sold the amplifier with the 50% model.
You guys are welcome to do what you want.
(are you all bean counters, or are some of you DIY hobby types?)
DJK,
We are only asking you your opinion, and when confronted by other people's opinion you call them bean counters... I believe that is unasked for. It's a discussion and I want to see opinions from different people, also yours. Just enlight us with your 30 years of knowledge and present us with your recommended setup.
So again, I'd love to have your opinion for PS and transformer for this setup:
I want to be able to run 3 ohms on 2 UcD400's, fully loaded. Feel free to enter a duty cycle that you feel is "difficult".
Thanks.
We are only asking you your opinion, and when confronted by other people's opinion you call them bean counters... I believe that is unasked for. It's a discussion and I want to see opinions from different people, also yours. Just enlight us with your 30 years of knowledge and present us with your recommended setup.
So again, I'd love to have your opinion for PS and transformer for this setup:
I want to be able to run 3 ohms on 2 UcD400's, fully loaded. Feel free to enter a duty cycle that you feel is "difficult".
Thanks.
Hi,
Take a deep breath. Just because you were disagreed with is no reason to take it as a personal assault which you seem to have done.
As to the question of who understands things like duty cycle...
You do realize that a 50% duty cycle =0W output? What the hell is a 45* load anyway, and what the hell is a 25, 50 or 100% duty cycle transformer?
You do realize a 4% duty cycle = near clipping? While a 25% duty cycle = about half power?
This isn't an attack really, I'm just a bit confused by your lingo, maybe you can help me out.
I had previously decided on an 800VA transformer for a stereo UCD400 driving 4 ohm loads, 1000VA would of course be nicer, hardly a necessity though. I think even an 800VA would handle a pure sine wave test with reasonable heating, for maybe an hour or so, not 24.
Thanks
Take a deep breath. Just because you were disagreed with is no reason to take it as a personal assault which you seem to have done.
As to the question of who understands things like duty cycle...
You do realize that a 50% duty cycle =0W output? What the hell is a 45* load anyway, and what the hell is a 25, 50 or 100% duty cycle transformer?
You do realize a 4% duty cycle = near clipping? While a 25% duty cycle = about half power?
This isn't an attack really, I'm just a bit confused by your lingo, maybe you can help me out.
I had previously decided on an 800VA transformer for a stereo UCD400 driving 4 ohm loads, 1000VA would of course be nicer, hardly a necessity though. I think even an 800VA would handle a pure sine wave test with reasonable heating, for maybe an hour or so, not 24.
Thanks
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
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
Yves Smolders said:
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.
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.
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.
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.
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.
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.
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.
richie00boy said:
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.
richie00boy said:
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.
classd4sure said:
--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.
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.
Thanks
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.
Thanks
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