Hi,
I need some help coosing a toroidal transformer. It's for a class AB SKA GB150D amp with about 200W per channel in 4 ohms.
For the same price I can go with 2 independent transformers (1 for each channel), 400VA each, dual secondary 0-30V, 0-30V from Toroidy. Audio grade.
OR
One single transformer with 4 secondaries, 0-30V, 0-30V, 0-30V, 0-30V, with 600VA from Canterbury Windings. Audio grade.
What should I choose?
Thanks,
I need some help coosing a toroidal transformer. It's for a class AB SKA GB150D amp with about 200W per channel in 4 ohms.
For the same price I can go with 2 independent transformers (1 for each channel), 400VA each, dual secondary 0-30V, 0-30V from Toroidy. Audio grade.
OR
One single transformer with 4 secondaries, 0-30V, 0-30V, 0-30V, 0-30V, with 600VA from Canterbury Windings. Audio grade.
What should I choose?
Thanks,
2x400va = 800va total 1x600va = 600va . If you have the space two is better than one for the same price from my view point. What do they define as audio grade transformers ? 😉
It's easy.
Add up the total maximum power output.
Your total comes to 400W.
A suitable transformer is 400VA to 800VA for all domestic music duty.
BTW,
you won't get to your target 200W into 4ohms with a 30-0-30Vac transformer.
35-0-35Vac should achieve your 200W target if you build it with sufficient current capability.
2nd BTW.
Amp Guru has his own Forum. They know how their amplifiers work.
Add up the total maximum power output.
Your total comes to 400W.
A suitable transformer is 400VA to 800VA for all domestic music duty.
BTW,
you won't get to your target 200W into 4ohms with a 30-0-30Vac transformer.
35-0-35Vac should achieve your 200W target if you build it with sufficient current capability.
2nd BTW.
Amp Guru has his own Forum. They know how their amplifiers work.
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Audio grade means electrostatic shielding between primary & secondary and GOSS band.
Andrew, I know 400VA would probably be enough but we all like to oversize a bit. And 4 ohms speakers can go lower in some frequencies and the amp must have a PSU capable of dealing with it.
I guess it's more power vs better regulation and dual mono vs single transformer(although with 4 independant windings). In the end what would be the better deal?
Andrew, I know 400VA would probably be enough but we all like to oversize a bit. And 4 ohms speakers can go lower in some frequencies and the amp must have a PSU capable of dealing with it.
I guess it's more power vs better regulation and dual mono vs single transformer(although with 4 independant windings). In the end what would be the better deal?
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To be honest I don't need 200W. The GB150D is rated 150W in 8/4 ohms but Greg says it's more like 200W at 4 ohms. With 35Va It's 150W/8 ohms and NOT recommended for 4 ohms duty. Greg recommends 30Vac for 4 ohms speakers.
I calculated:
30Vac x sqrt(2) = 42.43Vdc
42.43 - diode losses - amp losses = 42.43 - 1.4 - 2 = 39Vdc
39/sqrt(2) = 27.58V
27.58^2 / 4 ohms = 190W ~ 200W
If speaker impedance decreasses to 3 ohms in some frequencies, I get 253W
I gave you the guide.
VA = total maximum output power, times one to two.
If you are now targeting 150W that comes to 300VA to 600VA.
VA = total maximum output power, times one to two.
If you are now targeting 150W that comes to 300VA to 600VA.
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I'm targeting 200W per channel because it's what Greg says the amp gives with 30Vac. So 200-400VA, I understand that. Both my options cover that. But... what is the best solution, performance wise, sound wise... 2 seperate transformers or 1 big one with 4 secondaries? Is it indiferent?
Yes, I understood that. 200-400VA per channel = 400-800VA total power for two channels. Both my options cover that - a big one with 600VA or two small ones each 400VA = 800VA total. The big one will have better regulation but the small ones give more total power.
With due respect, losses will be WAY higher than 2V:
1) you can count on 42.43 V available at full power only if it´s a regulated supply, not the case here.
Conventional C input supplies typically lose between 10% (if oversized low DC resistance transformers are used) to 20% if rated "just enough" as in Home Audio use.
Count with some +/- 35V rails, under load, best case, standard (not way overkill) design.
2) voltage drop in a typical power stage, adding Vbe drops (about 1 V) plus Vce(sat) : some 4V in most cases (just check transistor datasheets) means that in general you lose at least 4V there ... not counting drop across emitter ballast resistors.
As in: 5A (peak) into .33 ohms adds up to more then 1.5V just there.
All losses add up, of course.
3) you will also have ripple (a couple Volts) which of course must also be discounted, unless you want a 100/120Hz sinewave modulating your audio.
In practice, amps with +/- 42 V rails (I have built some 10 thousand of them) give you about 70W RMS into 8 ohms and 100/120W RMS into 4.
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With a conventional design yes, but with the SKA output mosfets losses are only 2V.
http://www.ska-audio.com/Forum/YaBB.pl?custompage=products
Again, with a conventional design yes but the SKA is different. The SKA is rated 150W into 4 ohms with +/- 45 V rails but Greg Ball says it can produce in excess of 200W into 4 ohms with these rail voltage.
http://www.ska-audio.com/Forum/YaBB.pl?custompage=products
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Andrew, thank you for your pacience. I learn a lot from your posts.
I should have simplified my question. In the end, what I want to know is if a 4 secundary bigger transformer has advantages over two smaller ones.
The bigger transformer whould have better regulation but perhaps more cross-talk? The core and the primary are shared between channels. Is there anything else I missing?
I found a post in another thread were you, Andrew, state that one trafo with 4 secs is better.
The post is: http://www.diyaudio.com/forums/parts/235774-multi-secondary-toroids.html#post3489221
Do you still subscribe to what you said?
I should have simplified my question. In the end, what I want to know is if a 4 secundary bigger transformer has advantages over two smaller ones.
The bigger transformer whould have better regulation but perhaps more cross-talk? The core and the primary are shared between channels. Is there anything else I missing?
I found a post in another thread were you, Andrew, state that one trafo with 4 secs is better.
The post is: http://www.diyaudio.com/forums/parts/235774-multi-secondary-toroids.html#post3489221
Do you still subscribe to what you said?
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You'll find that I discussed creating a 4secondary transformer some years back.
I liked that arrangement. I have actually built two 4secondary toroids.
One from a trifillar 25Vac transformer by adding a 4th winding after separting the trifillar into three separate windings.
The other started as a dual 50Vac that I tapped into the middle of the two windings to create a 25-0-25Vac, 25-0-25Vac dual winding centre tapped. I think I have discussed the modifications of both in old Threads.
And yes I still hold that viewpoint, despite a contra reply posted yesterday, where we are on opposite sides of the opinion fence.
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I think you are misreading the SKA's supply rail voltages.
Yes, the CFP style topology does allow the SKA to nearly become a rail to rail output, but the rails sag significantly while delivering maximum power.
You have to allow for the difference between quiescent voltage at the amplifier and full power voltage at the amplifier. A good toroid+PSU may sag by only 5Vdc when delivering a total of 400W into 4+4ohms. A high transient power type build might sag by 20Vdc in achieving that same total output.
Dear Paulo, SKA states different power/voltage/impedance ratings in the same page, which is fine, but you can´t assume all are met on all conditions at the same time.
Yes, he rates the amp as 150 and even 200W in certain cases ...but IF:
He does not say that with such low rails you will *still* have your 150W, even into 4 ohms.
In fact: 150/8 means 55V rails drop to 50V or 10% sag ... and that with very good transformers.
So your 42V rails will drop (best case) to 38V ; discounting 2V you´ll have 36V peak or 25.4 V RMS so 80W into 8 ohms, and *maybe* 150 into 4 ohms.
I say maybe because if supply sags 10% into 8 ohms load (official SAK rating), into 4 they will sag at least a little more , easily 15% .
But in any case, don´t take my word for it, you´ll *measure* that when you actually build the amp.
Take care 🙂
Yes, he rates the amp as 150 and even 200W in certain cases ...but IF:
You are starting with +/-42V ones, WAY lower.
He does not say that with such low rails you will *still* have your 150W, even into 4 ohms.
In fact: 150/8 means 55V rails drop to 50V or 10% sag ... and that with very good transformers.
So your 42V rails will drop (best case) to 38V ; discounting 2V you´ll have 36V peak or 25.4 V RMS so 80W into 8 ohms, and *maybe* 150 into 4 ohms.
I say maybe because if supply sags 10% into 8 ohms load (official SAK rating), into 4 they will sag at least a little more , easily 15% .
But in any case, don´t take my word for it, you´ll *measure* that when you actually build the amp.
Take care 🙂
JMFahey, what you say makes sense to me but...
I have access to the sealed part of the forum. Greg especifies 42 - 45 Vdc for a 4 ohms build. With +/- 45 V Greg said the amp gives 200W.
Greg it talking about unloaded rails.
I hope is OK to quote Greg. AFAIK he is away from his forum at the moment.
The transformer voltage is not the issue. I want to build a 4 ohms capable amp and I'll go with the voltage especified for that (a 30Vac transformer for about 45Vdc per Greg). The issue is if I'd be better with a single big lower regulated transformer or with two independant smaller ones instead. Same number of secundaries but regulation and cross talk might be issues, I don't know.
I have access to the sealed part of the forum. Greg especifies 42 - 45 Vdc for a 4 ohms build. With +/- 45 V Greg said the amp gives 200W.
Greg it talking about unloaded rails.
I hope is OK to quote Greg. AFAIK he is away from his forum at the moment.
The transformer voltage is not the issue. I want to build a 4 ohms capable amp and I'll go with the voltage especified for that (a 30Vac transformer for about 45Vdc per Greg). The issue is if I'd be better with a single big lower regulated transformer or with two independant smaller ones instead. Same number of secundaries but regulation and cross talk might be issues, I don't know.
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One thing that might help me deciding is the layout. The first pic is a dual mono, the second is with a single 4 secondaries transformer. Is one better than the other?
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