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power transformer ratings

PostPosted: Tue Jun 28, 2005 12:26 am
by harry
hi all,

i am building 2x 1176's in one 2ru box. Apparently they draw 500ma each so i was wondering if i should get a 1amp transformer - or if i should get one that is a bit bigger ? 1.5 amp. I figured that it might be better to have a bit more juice, but on the other hand this will mean a bigger transformer - will the circuitry be more prone to picking up hum and inteference from the larger transformer??

sorry if this is a rudimentry question everybody, i am good at joining the dots, but i would like to know more about the why's and why nots... of how things work.....

thanks in advance...

PostPosted: Tue Jun 28, 2005 9:58 am
by chris p
Like you I'm a "join the dots" constructor with no real technical background, but whenever I have used (on the basis of bad information) an underpowered transformer, it has lasted about 100 microseconds. Definately go for at least 1.5 amp in my (very uninformed) opinion.

Remember that the number is what the transformer is rated to bear - what it actually draws depends on the great V=IR ohms law. Putting in a higher amps rated trannie (still of the correct step down voltage, yes?) will not make the circuit draw any more power, it just makes it less likely to blow. Therefore, I don't think (and I really reaching the end of my knowledge here) that it should produce any greater hum.

PostPosted: Tue Jun 28, 2005 10:22 am
by mfdu

i believe you can use an over-rated power trannie, as long as the actual step-down is ok. viz a viz a 24-0-24 toroidal with 3.2Amps should be fine - check on the prodigy forum tho - go into the meta on the 1176 build - might be the thread you need.

re. hum, i used the 2RU case from jaycar, and it has adjustable internal mounts. i was able to totally isolate the power trannie and remove *all* hum (well, i dont want it crystal clear - otherwise i would'a made a GSSL instead) by having the trannie mounted to the rails and raised up off the floor of the case - no direct hum transmission.


PostPosted: Tue Jun 28, 2005 1:44 pm
by rick
more amps in audio will always be a better option , but it depends on the design of the power supply for the speed you get acess to that power
, the bottom line is i doubt the hum level would change between a 1 and a 1.5 amp trannie if the transformers are made by the same mob.

in my experience higher rating trannies are usually made a bit better so the hum field can actually be less

you are using a torodial right ?, they are pretty quiet, if not i bet you could line up 10 transformers with the same amp ratings and get 10 different levels of hum.

PostPosted: Tue Jun 28, 2005 7:41 pm
by harry
thanks for your ideas all.

m.f.d.u, i printed out the entire meta thread to read through, must have missed this page, will stop being so pedantic.......

start the build 2morrow...

i guess i might need the extra juice, especially if i add relays for hard bypass (at a later date..)

now i just need to find out how to make a diode bridge?? to make my crap vu's usable... unless my tv repair friend has a bunch of real vu's sitting in his workshop (established 1963) will check 2morrow...

thanks again...

PostPosted: Tue Jun 28, 2005 8:06 pm
by rob
beware of hanging a VU with a diode bridge directly off the ouput. It causes distortion!

PostPosted: Thu Jun 30, 2005 3:45 pm
by mfdu
good point rob. but harry is using the mnats 1176 boards, and the signal send to VU is totally out of the signal path, so that's all good. i have zero hum with my 24-0-24 3.2A torroidal and $20 altronics VU meter.

harry - be a pedant. it's ok. after all, 240VAC can kill.

all the best with the build!!!!