Hi, I am trying to determine the VA I need for my two channel amp. I am going to go through the calculation to verify I am right in my assumption. So bear with me.
I am thinking of using a 30V 30V CT toroidal transformer to get +/-40VDC. I am going to limit the swing to +/-30V to give 10V headroom to keep the VCB lower. So I assume 4ohm speaker load. So the peak current is 30V/4ohm=7.5A. But I am going to call it 8Apeak. But this is drawn from say +42V supply(diode drop to give 40V). So the power drawn is 0.707 X 8A X 42V = 226W. But since this is half the cycle, so it should be divided by half. BUT we still have the negative half. So the final result power dissipation is still 226W per channel and 452W for the complete amp.
The IPS and VAS is going to draw say 50mA. that is 50mA X 84V= 4.2W ( this is DC) so two channel is about 10W. So the whole thing is 462W. But let's call in 500W total.
From this calculation. a 625VA toroidal transformer should be good enough. Am I rignt?
I am thinking of using a 30V 30V CT toroidal transformer to get +/-40VDC. I am going to limit the swing to +/-30V to give 10V headroom to keep the VCB lower. So I assume 4ohm speaker load. So the peak current is 30V/4ohm=7.5A. But I am going to call it 8Apeak. But this is drawn from say +42V supply(diode drop to give 40V). So the power drawn is 0.707 X 8A X 42V = 226W. But since this is half the cycle, so it should be divided by half. BUT we still have the negative half. So the final result power dissipation is still 226W per channel and 452W for the complete amp.
The IPS and VAS is going to draw say 50mA. that is 50mA X 84V= 4.2W ( this is DC) so two channel is about 10W. So the whole thing is 462W. But let's call in 500W total.
From this calculation. a 625VA toroidal transformer should be good enough. Am I rignt?
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Not sure why you would want to artificially limit the swing. Assuming that you are not boosting the rails to the front-end, and a conventional topology, your output swing will be inherently limited anyhow to within a few volts of the rails or so.
A stiff 60VCT transformer will give you around 150W into 4R and perhaps 80W into 8R. If you want to build a stereo amplifier with a single transformer, designed for 150W into 4R then you might choose a unit rated between 300 - 600VA. Of course an even bigger one will not hurt either.
For any transformer >300VA I would suggest using a soft starter in series with the primary windings.
A stiff 60VCT transformer will give you around 150W into 4R and perhaps 80W into 8R. If you want to build a stereo amplifier with a single transformer, designed for 150W into 4R then you might choose a unit rated between 300 - 600VA. Of course an even bigger one will not hurt either.
For any transformer >300VA I would suggest using a soft starter in series with the primary windings.
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Thanks for the reply. The reason I use low voltage because I want to use high quiescent current to run the first 5W as Class A and lower dissipation.
I artificially limit to swing to 30Vpeak to avoid the increase the CCB loading the previous stage. Of cause, I can choose not to limit the output also. It all depends on the input of the music.
As is, the power calculate from 30V peak is 112W already. It's plenty. I was even debating lowering the rail even more and raise the quiescent current even more to get more Class A power.
I just want to confirm a 625VA transformer is a good choice.
yes indeed that is close to what I call lab amplifiers, you know running sine waves into calibrated loads and waiting a couple of hours for the transformer temps to rise 40 degrees. Most home amplifiers are rated into 8 ohms, designing stable amps for 4 ohm ratings usually have 4 x the output devices and massive heat sinks to match. Isn't this usually overkill listening to music in homes on a single set of speakers? (wasteful of precious resources aluminum, , iron and copper). if some high end speaker dips into 4 ohm territory for a hundred Hz, I don't think it needs a full output section to match. I reckon it simplifies spec'ing for crest factor and load magnitude, phase. Most speaker designers know voltage drive not current is the limiting factor for high SPL's.
Don't forget to account for transformer regulation and efficiency. toroid's are usually much better than EI cores pound for pound. EI types can have better isolation if that matters. What about audible buzzing of laminations and / or windings? if you don't specify vacuum varnish most times they wont/can't guarantee audible noise.
better to specify dual windings for grounding and interference. Ideally common ground is made after the PS filtered outputs. Also twisting secondary wires to cancel EMI from rectifier current pulses is easier with 2 pairs.
Tortran - High Efficiency Toroidal Electrical Transformer Design Guide - Bridgeport Magnetics Group
Don't forget to account for transformer regulation and efficiency. toroid's are usually much better than EI cores pound for pound. EI types can have better isolation if that matters. What about audible buzzing of laminations and / or windings? if you don't specify vacuum varnish most times they wont/can't guarantee audible noise.
better to specify dual windings for grounding and interference. Ideally common ground is made after the PS filtered outputs. Also twisting secondary wires to cancel EMI from rectifier current pulses is easier with 2 pairs.
Tortran - High Efficiency Toroidal Electrical Transformer Design Guide - Bridgeport Magnetics Group
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Hi,
Class A and 4 ohm loads never go together that well.
First 5W into 4 ohms class A is first 10W into 8 ohms class A,
and FWIW first 20W into 16 ohms, if it does 40W into 8 ohms.
I don't remotely understand the 10V limiting of the supply,
it simply makes no sense, simply use a lower supply.
Your amplifier does not continually run at full power
with music, only sine waves in a lab, and the real
average load on music at clipping on the transformer
is lower than RMS implies, about half, moreorless.
rgds, sreten.
Class A and 4 ohm loads never go together that well.
First 5W into 4 ohms class A is first 10W into 8 ohms class A,
and FWIW first 20W into 16 ohms, if it does 40W into 8 ohms.
I don't remotely understand the 10V limiting of the supply,
it simply makes no sense, simply use a lower supply.
Your amplifier does not continually run at full power
with music, only sine waves in a lab, and the real
average load on music at clipping on the transformer
is lower than RMS implies, about half, moreorless.
rgds, sreten.
I don't understand the "Lab amp". What's the difference?
So bottom line, you think I am over killed?
Thanks
My speaker is spec 4 ohm. That's the reason.
I just design in 10V min between collector and base of the power transistor to limit the collector base capacitance that might create a pole. I am not definite about this yet. Just a thought.
just build it, test it and learn from it...
a 300VA traffo will not stop delivering power at 301VA,
a 300VA traffo will supply 600VA in a minute...
sine wave power has a lot more energy content than real music...
a 300VA traffo will not stop delivering power at 301VA,
a 300VA traffo will supply 600VA in a minute...
sine wave power has a lot more energy content than real music...
I am not going to buy an under power transformer and have to buy another one. My only question is whether the 625VA is good enough. I found one on sale for good price, I just want to verify and quickly buy it.
I think yer buying two times the transformer you need and wont have enough voltage on the rails.
I think you should design for 8-6 ohms and live with the voltage sag due to poor regulation for 4 ohm loads, this will actually help SOA of the output devices.
Class AB with high bias with 40V rails will be at ~ 75W at 8 ohms call it 65% eff. due to higher bias.
2*75/0.65 W +8W ~ 239 W need 250VA-300 VA transformer > shop for a standard toroid to give 42 rails at half load current.
I speak from experience!
I think you should design for 8-6 ohms and live with the voltage sag due to poor regulation for 4 ohm loads, this will actually help SOA of the output devices.
Class AB with high bias with 40V rails will be at ~ 75W at 8 ohms call it 65% eff. due to higher bias.
2*75/0.65 W +8W ~ 239 W need 250VA-300 VA transformer > shop for a standard toroid to give 42 rails at half load current.
I speak from experience!
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Thanks so much, this is the answer I am looking for.
Ha ha, the one I am looking at is not much more than a 300VA because it's on clearance!!! I rather keep the rail at +/-40V and jack up the quiescent current. This is for home use, I want quality rather than quantity in power.
I thought the higher the VA, the stiffer the rail and the less distortion I'll get.
You think 40V rail is too low? There is a 500VA 35V 35V CT, but it's more expensive as it is not on clearance.
Thanks.
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nah you'll never hear it unless yer bridging quad amps, put yer money in caps to lower the ripple voltage.
better yet raise the rails.
On a related question. My Acurus has a +/-80V rail. I see the primary has 4 wires, it must be two 110V primary in parallel. Can I put them in series to cut the rail be half? To +/-40V?
yeah but you cut the VA in half too.
VA is what you pay big bucks for , don't throw it out the window.
VA is what you pay big bucks for , don't throw it out the window.
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Thanks, I guess we cross posted, I was asking you whether you think 40V is not enough. That there is a 35V 35V 500VA, but it cost more because it's not on clearance!!!
Thanks
This is what I don't understand why the VA is cut in half. BUT I thought that would be too easy!!! Can you explain?
flux density = volts*sec per turn
you doubled the turns for the same volt seconds so the flux density is half
you doubled the turns for the same volt seconds so the flux density is half
Just get a 1KVA ... get it over with 😀 .
You might build a bigger amp in the future. Go with a higher voltage , too.
I can just set my capacitance multiplier divider to limit my VAS's swing to a
much lower voltage.
Leave your project "open ended" - for future embellishments.
OS
You might build a bigger amp in the future. Go with a higher voltage , too.
I can just set my capacitance multiplier divider to limit my VAS's swing to a
much lower voltage.
Leave your project "open ended" - for future embellishments.
OS
yup go for 63V volt rails, why 63 because that's the sweet spot when shopping for big caps.
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