Transformer location

[Witchdoctor]

Would it cause problems if the power and output transformers were mounted under the chassis?? I'm going to use the alarm panel cabinet for a small project, and I'd like to build a pair of 100 watt PPP el34 amps with a steel cabinet that measures 13x17x6.75 inches and has more than enough room for the trannies under the top of the chassis.

[anatech]

Hi Witchdoctor,
Two reasons to mount them on top. One is heat dissipation, the other is radiated hum. You don't want either beneath the chassis.

-Chris

[dr._sleep]

I usually mount OT's under the chassis without problems, just trial your power tranny under the chassis insuring good ventilation.

dr._sleep

[Witchdoctor]

I've been planning on installing a small 12VDC supply inside the chassis to run a muffin fan or two. The chassis has about 8 inches of grill area on the front and back, so I don't think heat build up will be much of a problem.

[anatech]

Give the fan a try. I'll warn you that you've just made a very high tech vacuum cleaner.

-Chris

[Witchdoctor]

Being an amateur radio operator who happens to like playing around with electronics, I don't think that will be much of a problem. I'll probably spend more time toob and cap rolling than listening to it...

[richwalters]

There's a big difference.......I get similiar questions form SE...these are way more sensitive as there is a gap in the core and magnetic circuit is then sensitive to stray mag fields. You don't mention if you are using NFB, this makes a massive difference as nfb contributes to cancelling hum by lowering effective output impedances. I have a sim amp stereo p-p with transformers 1cm apart side by side with mains tranny in middle and get roughly -75dB down hum level. In SE without NFB from secondary this would cause hum problems.

In fairness, some transformer manu (O/P and mains) do offer drop through versions of both. Sowter UK is an example. On question, a drop through chassis type with SE one has to be quite sure where the mains tranny is going to be located and alot of forward thinking is required.

richj

[rcavictim]

This wasn't mentioned but the use of a choke in the power supply can result in hum problems. The magnetic gap can emit an enormous magnetic field modulated with the hum that it is blocking that will induce AC mains hum in any signal carrying conductors in the vicinity. Low level audio lines or passive devices associated with those circuits like C's in the choke field will be especially affected.