Hi all,
I need to calculate the power for a trafo substitution. Here I put the schematics from the psu:
I don't remember how to calculate what's the watts power and how much AC volts have to be the trafo. The amplifier has an output of 150W 4 ohms, and my intention is to use a toroidal trafo. Please can you help me?
Grateful!!!
I need to calculate the power for a trafo substitution. Here I put the schematics from the psu:
I don't remember how to calculate what's the watts power and how much AC volts have to be the trafo. The amplifier has an output of 150W 4 ohms, and my intention is to use a toroidal trafo. Please can you help me?
Grateful!!!
Attachments
Hi,
for +-50Vc you need a dual secondary 35+35Vac, or a centre tapped 70Vac.
For a 150W amplifier you need 150VA to 300VA.
If your design can only produce 150W from +-50V there is something wrong with it.
A BJT output stage should get around 200W into 4r0 and a FET output stage a bit lower, probably around 170W into 4r0.
Maybe your design is really a 110W into 8r0 amplifier that can drive a resistor load of 4r0 but cannot drive a reactive 4ohm speaker adequately.
for +-50Vc you need a dual secondary 35+35Vac, or a centre tapped 70Vac.
For a 150W amplifier you need 150VA to 300VA.
If your design can only produce 150W from +-50V there is something wrong with it.
A BJT output stage should get around 200W into 4r0 and a FET output stage a bit lower, probably around 170W into 4r0.
Maybe your design is really a 110W into 8r0 amplifier that can drive a resistor load of 4r0 but cannot drive a reactive 4ohm speaker adequately.
Thank you so much Andrew,
The schematics is from a Gallien Krueger bass amp. The power supply that originaly was designed by the manufacturer probably has to be work a little at the limit because this psu was changed in the last models and the manufacturer has to do changes and modifications to support the old model I have.
The old psu was burned and the trafo was discontinued and the power supply was replaced for the schema that I added.
Here I let you the complete schematics.
[URL=http://www.4shared.com/document/_QPgXjvy/MBE_series.html]
Thank you so much.
The schematics is from a Gallien Krueger bass amp. The power supply that originaly was designed by the manufacturer probably has to be work a little at the limit because this psu was changed in the last models and the manufacturer has to do changes and modifications to support the old model I have.
The old psu was burned and the trafo was discontinued and the power supply was replaced for the schema that I added.
Here I let you the complete schematics.
[URL=http://www.4shared.com/document/_QPgXjvy/MBE_series.html]
An externally hosted image should be here but it was not working when we last tested it.
[/URL]Thank you so much.
For some reason the link don't work. Here is the good: 4shared.com - document sharing - download MBE series.pdf
It will stick 150w into 4 ohms, with 45 volt rails (after rectifying etc).
Assume 50% efficiency, you need at least 300VA, but...
I'd say than you'll need more than 300VA. When in use, it only supplies power in very short bursts. This means that, instead of something that will do 300VA continuous, you'll need more like 500VA, because that's what it will supply in those short bursts.
For example, I used a 150VA transformer for my little 30W guitar combo. Doing so means it will push 60W into 4 ohm, but it's happy supplying those high peak power requirements.
Chris
Edit - it also means that your PSU won't be the limiting factor...
Assume 50% efficiency, you need at least 300VA, but...
I'd say than you'll need more than 300VA. When in use, it only supplies power in very short bursts. This means that, instead of something that will do 300VA continuous, you'll need more like 500VA, because that's what it will supply in those short bursts.
For example, I used a 150VA transformer for my little 30W guitar combo. Doing so means it will push 60W into 4 ohm, but it's happy supplying those high peak power requirements.
Chris
Edit - it also means that your PSU won't be the limiting factor...
That's a LOT of pointless overkill (and a big waste of money and weight). Short bursts of power are supplied by the capacitors, and transistor amps are far more efficient than 50% - 300W would be fine, and 225W would as well.
Try looking what's used commercially.
Nigel,
well said.
A 150W amplifier gives good value for money when paired with a transformer between 150VA and 300VA. Anything more is extra money for a small benefit.
However, it's going to be expensive getting a custom wound 33+33Vac transformer.
30+30Vac and 35+35Vac are easy to source.
If 35Vac is adopted them max power should be around 200W into 4ohms and the matching transformer becomes 200VA to 400VA. But only if the 2pair output stage can survive the extra voltage and current that 200W imposes.
well said.
A 150W amplifier gives good value for money when paired with a transformer between 150VA and 300VA. Anything more is extra money for a small benefit.
However, it's going to be expensive getting a custom wound 33+33Vac transformer.
30+30Vac and 35+35Vac are easy to source.
If 35Vac is adopted them max power should be around 200W into 4ohms and the matching transformer becomes 200VA to 400VA. But only if the 2pair output stage can survive the extra voltage and current that 200W imposes.
Thanks to all.
This is what I found following your advice. Toroid encapsulated, 230Vac, 2x35Vac, 300VA, 46,98€. Have a very good aspect and a very good price too, I think.
http://docs-europe.origin.electrocomponents.com/webdocs/0028/0900766b800284bc.pdf
This is what I found following your advice. Toroid encapsulated, 230Vac, 2x35Vac, 300VA, 46,98€. Have a very good aspect and a very good price too, I think.
http://docs-europe.origin.electrocomponents.com/webdocs/0028/0900766b800284bc.pdf
that should get you ~+-51Vdc and if the amp is assembled to pass current then expect 200W into 4r0 when fed with 235Vac.
What, may I ask, recharges the capacitors between their discharges? Because it isn't a constant, gentle current draw.
I've also looked inside commercial amps. The power supply is usually seriously under-done. For example, how are two 3,300uF caps going to supply 50w per channel? The transformer was also designed to manage around 100w. This isn't enough when, in fact, most class AB designs have efficiency near 50%. If there's a heatsink, there's power losses...
Amplifier Efficiency
Chris
It is true that many commercial designs are lacking in the power supply department, I've even seen an amplifier where the power LED started blinking in rythm with the music.
The reason for this is as I understand it: cost, it is costly to put a bigger transformer in there and caps are expensive as well. Saving 10 cents a part might not seem as much, but if you are producing 50.000 amplifiers the 10 cents saving is suddenly 5000 dollars saved. If you start using lower quality parts everywhere you can drastically reduce costs a lot, at the expense of running parts close(or over) spec.
The awesome thing with DIY is that you can make it as good as you want, if you want those 100.000uF per channel you get those. In most cases you are only making one item, why not make it as good as it can be, within reasonable limits of course. Where the "reasonable" line is drawn is up to the user to decide upon, what is common practice for some is heresy to others.
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Kolbjørn
The reason for this is as I understand it: cost, it is costly to put a bigger transformer in there and caps are expensive as well. Saving 10 cents a part might not seem as much, but if you are producing 50.000 amplifiers the 10 cents saving is suddenly 5000 dollars saved. If you start using lower quality parts everywhere you can drastically reduce costs a lot, at the expense of running parts close(or over) spec.
The awesome thing with DIY is that you can make it as good as you want, if you want those 100.000uF per channel you get those. In most cases you are only making one item, why not make it as good as it can be, within reasonable limits of course. Where the "reasonable" line is drawn is up to the user to decide upon, what is common practice for some is heresy to others.
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Kolbjørn
You were refering to 'short bursts of high power', the capacitors recharge between those short bursts.
Are you perhaps in the wrong forum?, this is 'musical instruments', we don't use HiFi amps, we use PA amps - and they are rated for continuous full power useage, yet don't use the massive over-kill transformers you're suggesting.
In that case, why won't most (if not all) PA gear stick x watts into 8 ohm, and exactly 2x watts into 4 ohm? Even the expensive stuff (maybe £500 for a basic 2ch thing) just won't have a big enough power supply.
Here's an interesting example, made by crown. a reputable company, they make some good stuff.
http://cpc.farnell.com/jsp/level5/module.jsp?moduleId=cpc/439892.xml
The short bursts of high power are dispensed by the capacitors, and the transformer, which has to constantly recharge them, in very very narrow time gaps. That's what the steep uptake is when you look at a loaded PSU through a scope. The transformer has to supply maybe 150VA continuous, but it's doing that in peaks of maybe 500VA, 100 times per second. It has to be able to cope with that current draw.
Here's an interesting example, made by crown. a reputable company, they make some good stuff.
http://cpc.farnell.com/jsp/level5/module.jsp?moduleId=cpc/439892.xml
The short bursts of high power are dispensed by the capacitors, and the transformer, which has to constantly recharge them, in very very narrow time gaps. That's what the steep uptake is when you look at a loaded PSU through a scope. The transformer has to supply maybe 150VA continuous, but it's doing that in peaks of maybe 500VA, 100 times per second. It has to be able to cope with that current draw.
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I think chris661 is alluding to form factor and its effects on transformer losses and load regulation. Try Power Transformer Facts Bulletin No.1 - Application Notes on Rectifier Transformers Basically, the transformer will only be passing current for part of the cycle to charge the capacitors. This increases the copper losses in the transformer and makes load regulation worse (4 times the current for a quarter of the time).
Since the maximum theoretical efficiency of a class AB amplifier is 78%, and, with an inductive load, 50% or less is common (Amplifier - Wikipedia, the free encyclopedia), 300VA is the MINIMUM to get 150W at the output. However, this is for a bass amp, so the output waveform will be peaky and 300VA should be fine. For any kind of continuous output (e.g. for a keyboard), bigger could usefully be chosen.
Since the maximum theoretical efficiency of a class AB amplifier is 78%, and, with an inductive load, 50% or less is common (Amplifier - Wikipedia, the free encyclopedia), 300VA is the MINIMUM to get 150W at the output. However, this is for a bass amp, so the output waveform will be peaky and 300VA should be fine. For any kind of continuous output (e.g. for a keyboard), bigger could usefully be chosen.
Much PA stuff does supply double in 4 ohms, as does much HiFi stuff - it really depends how you rate your amplifier - and no decent manufacturer would rate it as it's absolute maximum possible.
For an example, an old Grundig Receiver, the R45, was rated at 45W per channel RMS in 4 ohms (Germany uses 4 ohms instead of 8 - I've no idea why?). I measured one (because I'm nosey like that), and it actually provided 76W RMS per channel, continuous sinewave, both channels driven.
My Behringer PA mixer/amp is rated at 400W in 4 ohms, and 200W in 8 ohms, exactly double - as most are. I've never measured it, as I've no reason to, but I would expect it to better both those values, and better the 8 ohm one by more than the 4 ohm one.
BTW, the reason you get less than double power in 4 ohms isn't the PSU as much as the amplifier, there's a certain amount of resistance involved in the amp, so doubling the current doubles the voltage drop in the amp, plus the PSU will sag further as well.
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