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Old 8th October 2005, 07:48 AM   #1
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Default Help with choosing Transformer and calculating output power

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/showd...number=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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Old 8th October 2005, 04:18 PM   #2
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350VA is not going to be anywhere near enough for 300-400W.... to get an idea of what you will need have a look here ---> http://www.tnt-audio.com/clinica/ssps2_e.html

Can't help out on the rail voltage side......

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Old 8th October 2005, 05:07 PM   #3
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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.
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Old 8th October 2005, 05:26 PM   #4
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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

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.
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Old 8th October 2005, 05:47 PM   #5
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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.
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Old 8th October 2005, 05:57 PM   #6
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Quote:
Originally posted by Eva
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
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Old 8th October 2005, 06:13 PM   #7
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Remember to use current sources and zener biasing in your circuit in order to prevent the working point from shifting when rails sag.
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Old 8th October 2005, 06:41 PM   #8
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Quote:
Originally posted by tlf9999
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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Old 9th October 2005, 01:56 AM   #9
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Quote:
Originally posted by EWorkshop1708


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).

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Old 9th October 2005, 05:49 AM   #10
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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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