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Old 9th July 2007, 03:45 PM   #1
snax is offline snax  United Kingdom
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Default Help with PSU for Tri-Amp System

Guys and girls (trying to be PC),

This is my first post, my brain is hurting and I need to share a problem. I had the intention of making a pair of tri-amplified active speakers but I am having trouble with the power supplies.

I have two big 600VA transformer each with three secondary’s at 36-0-36.

These are to run six digital amplifiers (three per transformer) rated at 133Watts per amplifier. So far so good…

I have an AC-DC design which splits each of the 36v rails, rectifying each side of the secondary sharing the center tap as the negative. (see PDF doc as It gets tricky to describe with my limited electronic vocabulary)

Particular concern is the splitting of the centre tap for each of the bridges. I am currently building a Heath Robinson prototype to see if I’m going to have problems with common mode errors or burning components but would like some advice before I flip the power switch.

The amplifier company recommends a transformer of 200VA per channel which for three amplifiers I have taken as 600VA. This is where I'm having problems as I have calculated a VA rating of 1600 for three amps @ 133watts and was wondering if this is correct?

My third and final quandary is that 36 volts into an 8 ohm load should be 133 Watts output from the amplifier. My problem is the fuse rating. Calculating this I have the main fuse F1 rated at 14 Amps. Due to the power supply configuration I can divide the 4 amps per channel supply by 2 for each rail making a 2 amp fuse for F2 and F3. However all the calculations are for 8 ohms but if I put a 4 ohm load or less to the amplifier then the fuse ratings double which makes the main fuse F1 somewhat high. Is this correct or am I just not seeing the wood for the trees.

Any advice would be much appreciated.
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File Type: pdf power_supply_3_channel.pdf (29.9 KB, 51 views)
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Old 9th July 2007, 05:32 PM   #2
sreten is online now sreten  United Kingdom
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Hi,

Fuses on the wrong side of a rectifier bridge will not last long.

36-0-36 is two secondaries. Why do you have two bridges per amplifier ?

Click the image to open in full size.

/sreten.

I'd have a good read of other projects supplies and fusing issues.
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Old 9th July 2007, 05:43 PM   #3
sreten is online now sreten  United Kingdom
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Hi,

Fuses on the wrong side of a rectifier bridge will not last long at all .....

36-0-36 is two secondaries. Why do you have two bridges per amplifier ?

Click the image to open in full size.

/sreten.

I'd have a good read of other projects supplies and fusing issues.
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Old 9th July 2007, 07:01 PM   #4
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sretens circuit will work but the one you posted initially won't because it short circuits the transformer. When the point marked +36v ac is negative and -36v ac is positive the transformer is shorted through the upper left diode of the upper bridge and the lower left diode of the lower bridge.

That circuit is brought up here pretty often with tread titles like "My diodes blew", "Humming transformer and blown fuse", "Will this work" etc...

The answer is: NO, center tap and dual bridges don't mix
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Old 9th July 2007, 07:32 PM   #5
snax is offline snax  United Kingdom
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Thanks for posting Sreten,

The PDF shows three secondry’s all centre tapped, 36-0-36, 36-0-36, 36-0-36 so that’s one for each amplifier. The theory for two bridges per channel was to reduce the load per rail (2 amps in stead of 4 amps per rail) and to get a smoother response.

I had done a fair amount of research and have been following Dejan Veselinovic from TNT.

I will have to look at the fuse issue and which side of the bridge i put them.

Cheers

Snax
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Old 9th July 2007, 07:32 PM   #6
snax is offline snax  United Kingdom
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So what’s the answer megajocke I believe every one is getting their tuition on the TNT website. I have to admit that his figures make sense even if his wire diagrams don't.


http://www.tnt-audio.com/clinica/ssps1_e.html (link to the page)

Have I read it incorrectly? Should I be using two centre taps rather that a shared common?

many thaks for posting guys.

snax
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Old 9th July 2007, 07:58 PM   #7
snax is offline snax  United Kingdom
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Doh, I had searched and searched this board but never TNT… It seems like an in joke.

“Not quite, TNT drew the bottom bridge the right way round”

So yes I’m a fool for taking diagrams as perfect.

So if I correct the bridge will a shared centre tap hold or will I have common mode problems?

There’s nothing like the smell of burning components in the morning. lol
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Old 16th July 2007, 02:17 PM   #8
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If you use a center tapped tranformer together with separate negative and positive rectifiers it will blow up. The TNT website does not show any schematics of them combined. If you "split" your center tap you make a short circuit.
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Old 16th July 2007, 03:17 PM   #9
AndrewT is offline AndrewT  Scotland
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Hi Snax,
have another look at that TNT page you linked to.
The diagram at the third from bottom is exactly as you need.
A centre tapped transformer with a single rectifier, smoothing caps, and finally a fuse in each supply rail.

You need one primary side as drawn plus three secondary sides, just repeat the TNT diagram.

Forget fusing between transformer and smoothing caps and do not try to dual rectify a centre tapped secondary.

You can only dual rectify a dual secondary.

133W into 8ohms should operate with F3.1A fuses in the two supplies. If any of your drivers are 4ohm you will need to increase the fuses to F5A or F6A.
Repeat these fuses for each amplifier.
Check your amplifier is safe with one supply fuse blown. A single supply can make an amp offset to the rail voltage and take out a speaker.

The 600VA toroid will probably need a soft start, otherwise you'll need a 7A or 10A fuse in your plug top. If you go with 50r and a bypass relay timed at about 0.5seconds the mains fuse can be reduced to about 3A. You might even get away with T2.5A without nuisance blowing.
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Old 16th July 2007, 08:17 PM   #10
snax is offline snax  United Kingdom
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Hi Andrew & Megajocke,

Andrew thanks for the advice. I found another thread that you featured quite heavily in, “Bob Cordell Interview: Power Supplies”. I have to admit that I have still a lot to learn and rather than a quick fix I have started at the beginning again and got an analogue electronics book. Although having done electronics as part of a BSC Product Design degree 9 years ago I can see that I am way off understanding even the most basic arguments you where supporting.

Thanks again

Snax
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