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Old 12th March 2002, 01:33 AM   #1
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Default Transformer questions

Hello,

I have four 1000VA transformers w/37.5V secondaries (bought from apexjr), and I am going to use them with eight Marchand amps, PM224s. Four amps will be used per channel with an acitve XO. Should I buy four more transformers so each amp will have it's own dedicated power supply or will one transformer per pair of amplifiers provide enough high current output? Basically I want to get the most out of this project but I don't want to waste money. Will four transformers per channel pull too much AC mains current to blow a 15A-20A breaker? I would like to use inrush limiters on the mains input, so maybe I could get away with four transformers per channel, any thoughts on that? Can anyone recommend rectifier diodes for this spec transformer?

Any inputs are appreciated.

Thank You,

Digi
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Old 12th March 2002, 08:42 PM   #2
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Default Transformer Questions

Invariably one transformer per amplifer will sound better than sharing, so buy the extra if funds permit.

The inrush current is not caused by the transformers themselves, but by the reservoir capacitors that follow the bridge rectifier (and you don't say how much capacitance there is). The more capacitance, the more inrush current..... But the more reservoir capacitance the better the bottom is likely to be and the less hum you are likely to hear at the speakers.

As for bridge rectifiers, the price differences are not so great - get a 25 or 35 amp bridge, mount it on the metalwork of the case and it should last a long time (subject to the high inrush currents caused by the reservoir capacitance not causing premature failure).

Switching on all the amplifiers at the same time will collectively cause a large inrush current and should be avoided if you want to avoid resetting your circuit breakers.
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Old 12th March 2002, 11:31 PM   #3
haldor is offline haldor  United States
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Actually at least a part of the inrush current is caused by the transformers. Until the magnetic field is estabilished the transformer is going to act like a dead short on the AC lines. This is why toroidal transformer have more inrush current the EI transformers with the same VA rating.

Phil
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Old 13th March 2002, 01:32 AM   #4
PassFan is offline PassFan  United States
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Depending on where you live 4 transformers may be a problem. 4000 va equates to 4000 watts. Volt amps is volts times amps which gives you wattage. If you live in the US its 4000/120 = current. A 20 amp breaker at 120 volts = 2400 watts or 2400VA. If you are in europe then 20 amps at 230 volts= 4600 watts or 4600VA. Breakers will hold their full rating provided they are not old and worn (the heat flowing through the bi-mettalic element in them causes the element to weaken over time). Generally they will last almost forever at 80% of their rated load and as little as a month at their full load. At full load each one is different and your rolling the dice. Also one amp anywhere else in the house while your listening and TRIP, they will not hold any extra unless their GE. You can just about arc weld with GE breakers and they will not trip. Anyway, hope this helps and good luck
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Old 13th March 2002, 02:08 AM   #5
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Thanks guys,

Passfan

Your logic makes abosolute sense, but why can some manufacturers use 6000VA trannies, or others bi/tri-amping with big Pass or Krell amps?

Can anyone recommend a specific model number# of a bridge rectifier, 35A+? How about snubber caps for the rectifier, (film or ???)? Also, an inrush limiter, what resistance and rating?

Sorry for all of the questions, I have studied the theory but applying it isn't a simple.

Thank you,

Digi
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Old 13th March 2002, 04:45 AM   #6
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apart from inrush current, 4 or 8 x 1000VA transformers should not present any problems as i am assuming that the actual load will be well below the rating of the transformer.

having a 6000VA transformer in an amp in the same kinda deal ... the transformer is way over rated for the job so as long as there is an adequate soft start circuit you should not run into problems....
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