Help with choosing Transformer and calculating output power

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I'm looking for a large transformer to power my amplifier.
It's a amp driving a single 4 ohm subwoofer.

I found this 350VA transformer browsing the web. 55-0-55V
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=129-095

I'm wanting to get between 300-400W RMS into a 4ohm speaker, estimated I'd need at least 50V rails. Would this transformer work, or would I need to go higher VA?
 
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the ratings depend on a lot of factors, like the minimum load you want the amp to drive, rail voltages, output losses, transformer regulation, etc.

my rule of thumb is to over-size transformers and use transformers rated at 4x the output of the amp. So in your case, look for a 1-1.5kw transformer.

This is ridiculously over-engineered, and for audio signals it is not unreasonable to use transformers that are 50% of the rated power of the amp. so you can essentially use a 200w transformer for your project.

In terms of the math, here is what I do:

assume that you want to drive down to 4ohm. a 55vrms transformer will output about 80v DC, without any load. it is not uncommon for those guys to have a 10% regulation under full load. so you are talking about 70v rails.

To output 400w on 4ohm, you need to output 800w peak, or 14amp peak current (sqrt(800w/4)). or 980w out of the transformer (1kw is close enough).

so your transformer should be rated at 55vrm and 14amp. That is one heck of a transformer.

If you want your transformers to handle 2ohm load, your figures will go up substantially.
 
Hey, thanks, that link was very helpful.

After reading the article, I'd need a minimum of 500VA for ~350W or 600VA for 400W Maybe even more :eek:

I guess I'll be getting two of those 350VA transformers, or I'll see if I can get a larger transformer that costs about the same as 2 smaller transformers.

Now instead of getting a 55V trans, that's 14A, which I'll look for BTW, I'm thinking of going dual transformers using each one to power a rail, with the secondaries on each trans in parallel.

I'm hoping that would give me more than 14A per rail depending on the transformers.

*I have overbuilt the amp intended for the project, so I'm not worried about the amplifier itself not handling the transformer. I used 80V caps per rail, and 5 pairs of MJL4281/4302 (350V/15A/230W) on large fan-cooled heatsinks.*

Being that it's seeming to be costly to get a transformer to produce such high volume, and I won't blast my sub all the time, should I maybe go for 200-250W instead @ 4 ohm? I already have a transformer that goes to another amp I'm retiring (EI trans though) that can do that. I'm wondering if the extra power will really make a difference or not, I haven't tried the 250W trans yet on this amp.

I'm wanting the amp to sound BIG, even at lower volumes. I used lots of caps and output transistors to help with that, so the load is driven with authority, but I've read here that the toroids are the best, and some say the bass is tighter, which is the reason I'm looking for those instead of the EI trans.
 
With music signals, you can get away with even a 200VA transformer for a two channel 350W amplifier. It will hardly overheat, even at full volume near clipping, however, supply rail sagging will be substantial, thus requiring up to 30% higher supply voltages when idle.

The point of using higher VA ratings is just to prevent rail sagging, so a 350VA unit will probably suit your needs if you ask for somewhat higher than required secondary voltages and your amplifier can cope with them when idle.
 
Eva said:
With music signals, you can get away with even a 200VA transformer for a two channel 350W amplifier. It will hardly overheat, even at full volume near clipping, however, supply rail sagging will be substantial, thus requiring up to 30% higher supply voltages when idle.

The point of using higher VA ratings is just to prevent rail sagging, so a 350VA unit will probably suit your needs if you ask for somewhat higher than required secondary voltages and your amplifier can cope with them when idle.

Thanks Eva, you answered my next question. My Filter caps are 80V, and I don't really need 80V rails (77.7 with 55V trans no load) so if I get a trans that provides that much no load, and maybe sags down to 55-60V full load, it would still suit my needs. If a single 350VA trans can do that, then that's what I'm looking for.

**EDIT** Looked at physical size of trans specs, it's perfect
 
tlf9999 said:
To output 400w on 4ohm, you need to output 800w peak, or 14amp peak current (sqrt(800w/4)). or 980w out of the transformer (1kw is close enough).

so your transformer should be rated at 55vrm and 14amp. That is one heck of a transformer.

There is absolutely no need to consider peak current or power! You are outputting AC from your power amp not DC.

As a rough rule of thumb the required VA is double your wanted output power. To find the rails, work out the peak voltage required for your load then add about 10V on top.

If you go to my website www.readresearch.co.uk and look at the Utilities section, you will see a spreadsheet called Amplifier Designer which will let you calculate precisely what rails you need and the VA to meet your chosen power output all day long.
 
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EWorkshop1708 said:


Thanks Eva, you answered my next question. My Filter caps are 80V, and I don't really need 80V rails (77.7 with 55V trans no load) so if I get a trans that provides that much no load, and maybe sags down to 55-60V full load, it would still suit my needs. If a single 350VA trans can do that, then that's what I'm looking for.

**EDIT** Looked at physical size of trans specs, it's perfect

I think that you are pushing it a bit to have pretty much 78V rails with 80V caps!!! I'd be drping it down to 70V at most. for 78V rails you should really be using 100V caps! with 80V caps 64V rails are roughly ideal (based on the guidline of running them at 70-80% of the VW rating).

Tony.
 
Ok, so you are saying I should go with a 50V-0-50V trans instead due to my 80V caps? (50*1.414=70.7V)

Either way, it's looking like I'll need more than 350VA because I need to provide 9-10A of current to drive 4 ohm, while that trans falls just short of that.

I'll probably get two transformers to get 700VA. Still not a bad price for that much power.
 
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that's my oppinion :) running them so close to their VW rating is I would say flirting with danger... I can't remember off the top of my head the % headroom for safe working...... you might be able to get away with slightly higher rails, but I'm not 100% sure.... anyone else care to comment :) at the very least the caps life will be shortened running them close to rated voltage, at the worst your electricity supply could vary by +- 20% in which case (if on the + side) your caps would probably be toast!!
deviations of +- 10% in supply are probably more likely....

Tony.
 
after u will connect the amp the voltage wont be 78 ,it will drop down a little .

there should be no problems running the caps at that voltage if they are rated up to 80V ,its not reccomended though ,it also depends on the quality of the caps.
i was running 35V rated caps with 34V rails for more then 2 years with no problem .
 
Hi Eworkshop,
your 50Vac will give 70.7Vdc as you said.
Add on the transformer regulation of 4% to 6% giving 73.5Vdc to 75Vdc. subtract diode drop at idle current say about 1V. final voltage at rated mains input voltage 72.5Vdc to 74Vdc.
If you now add in the mains variation from +6% to + 10% you will be way over 80Vdc. You decide if you want to take that risk. I would recommend 100V for your 50Vac transformer. for a 55Vac transformer you definately need 100V caps.

Regarding VA rating I would recommend 1.5 times the maximum output. So you need 600VA for 400W. This will have good bass if the caps are about 2mF to 3mF per amp of peak output current.
Your 4ohm speaker will have a minimum Z of about 3ohms. the Vpk will be about 60Vpk leading to Ipk=20A. you then need about 40mF to 60mF on each rail. You could go higher on VA rating and this would reduce your cap size slightly (30mF to 40mF). Alternatively, as suggested by others, go down on VA rating and increase the cap size to compensate. Half the VA would probably benefit from doubling the caps to 80mF to 120mF on 300VA transformer. This will still give good bass. The continuous output power will be slighly down maybe 300W but it will play music like a 400W amp.

Finally 5 pairs MJL4302/4281 will take a 4 ohm load to
 
I think my caps will be ok for a 50V trans, but if the volts are real high, I could go 45V.....I did get some modern, high quality caps that are small in size, but very heavy. They weren't cheap either. :eek:

I have an amp (that I'm replacing with the one I'm building) that has a 45-0-45 trans with 5600uf 75V caps. About 240W RMS. So my 80V caps aren't much higher, but I'm wanting to get more power than this old amp. This amp has a low AC hum, and I think it's probably the small value :rolleyes: 5600 uf caps.

If I used the transformer from this older amp for the new amp I'm building, would I get more power than 240W, and better sounding bass with this transformer than the old amp, simply because I have much more uf than 5600 per rail??? (Mine has 23,500 per rail 5x4700) Also my amp does NOT have output protection, so I believe this, along with 5 pairs of outputs will give me more peak current and watts, but I'd like to know if this would affect RMS?
 
Hi E Workshop,

The supply voltages you need will also depend on the topology of the amplifier output stage. For example a MOSFET amp can easily waste 6-8V in a Vas/SF output stage whereas a CS configured output (e.g. my Killer amp topology) wastes typically <2V.


Your existing 45V-0-45V should give about +/-66V under no load and depending on the VA rating sag to just below 60V. Add ripple at say 1V (with extra C) and an efficient output stage topology and you should go close to 250W from one channel at 8 ohms! Depending on transformer regulation you could see 300W into 4 ohm on one channel (see my website).

Given it's music you're listening to, a VA rating of 300W would suffice as long as you add enough C to keep ripple low. I might just add that an amp design with poor PSRR may need extra VA and C to compensate.

Cheers,
Greg
 
Rail loss is caused by gate drive voltage threshold as well as VAS saturation threshold...

In typical complementary Mosfet amps rails loss is as follows...

1 Volt for VAS + 3 Volts for gate drive threshold + 1 volt for RDS+RS degeneration = 5 to 6 Volt of total rail loss....
Solution: Run VAS at least +-8 Volts higher than output stage voltage to over come rail loss and boost efficiency...or use reversed output configuration with floating supplies and grounded output...

Anthony's AV 800 Rail loss = 10Volts max

Zeta amp [very good design by Lars Clausen]Rail loss = 5 Volts max..

Killer [the basic instint] [Greg Ball's excellent design] Rail Loss = 2 V max

Quasi's N-channel[based on Siemens design]= 0.75V max

regards,
K a n w a r;)
 
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