25v transformer to run 2 '200w' amps.

[ricardo electric]

i have a slight problem in the way of :
i have got a toroidal transformer coming through the post, which is around 300+ va.
i have got 2 200w (400w total power at 4 ohms) amp kits rated at 120 va each (there are 4 leads off the transformer to connect to the 4 connectors on one amp kit)
if i were to put a screw connector block on the 4 individual output leads, this should give me 2 sets of four leads with 120 va feed per amp (240 va total), i don't know if this is safe or not as i thought it might over power something or even cause backfeed?? ???
i would like to fit switches to operate the amps independently but that would then mean having to tap off the 2 25v inputs, leaving the grounds working/coupled, is this switching idea safe? ???
i would appreciate any responses i get from this! :)

[audioguru]

A 200W amp uses about 300VA from a power supply. It is probably rated as 200 Whats, which is about only 80 Watts to 100 Watts and uses 120VA to 150VA from a power supply.

Are you making the Velleman amp kit that blows up? Two of them will draw a max of 240VA to 300VA from your transformer before they blow up. Make sure you connect the 4 wires with the correct polarity.

You should not switch the low voltage from the transformer because its peak current pulses are huge and might weld one of the switch contacts. Then both the amp and your speaker will blow up.

[ricardo electric]

thanks for that.
it is a velleman kit, the info you gave me made some sense.
i understand your point when you say that switching the power from the transformer to the amp may cause a 'pulse', the 4 leads are labelled in the booklet but i am not worried about colour coding at the minute.
would you say that it is safe to run 2 velleman kits from a 300 va transformer?
have you also got any good advice on how to connect 2 to the same transformer or will i more than likely have to search out another one?
i look forward to your reply, the first one was pretty useful!

[audioguru]

About 1 week ago a guy on this site said his new Velleman amp blew up.
On another electronics chat site we are joking about how weak it is because amps with similar power use paralleled output transistors to be reliable.
Its distortion is listed at only 10W output. What about near full power?
It uses a dual 25V or a dual 30V transformer. Its rated 100W into 4 ohms might be with it clipping its head off (distortion adds power) when it is powered with a dual 30V transformer. With your dual 25V transformer its ouput power at clipping might be only 60W.

I don't think it is safe to use these crappy amplifier kits with 4 ohm speakers. The amps will blow up and might take your speakers with them. Also maybe cause a nice fire???

[ricardo electric]

i will not slag you off, you were right with these amps blowing up!
the first time, the wiring was going the wring way and it frazzled the track on the board.
the second time, a resistor (nearly 2 got taken out) went on fire, rendering the board as scrap (nearly!).
the speaker that was coupled made a buzzing sound (had it on for no more than one second) and then tried to make a burning smell, i ended up with 25v ground running through it, no wonder it tried to go on fire.
maybe i should have listened to you before applying power?!?!
at least i've now learned to go to the shops and buy an approved amp!

[audioguru]

You have had very bad luck. :'(
I have made many powerful amplifiers that work fine.