Elmo ST1200 - loud hum for about 5 seconds after tuning amp off

[Xander van der Merwe]

I've just purchased an ST1200 magnetic only (not the HD version) as a backup to my GS1200 and noticed that when you turn the amp knob off there is a loud hum type sound that lasts for about 5 seconds or so before dropping away. I also think that the sound quality on this projector is not great, but that could be since I'm used to the GS1200.

Any thoughts as to what the cause might be or should I just ignore it?

[Steve Klare]

That’s normal: mine has been doing that exact thing for about 11 years now!

Basically what it's about is there is energy stored in the amplifier's capacitors and when you turn off the power it gets released.

[Pat DAlessio]

HI Steve! How you doing?


If you unplugged the projector then turned on the power switch , would that drain the capacitors like a cold boot on a computer? Thereby eliminating the hum?

[Steve Klare]

Hi Pat,

I am well and hope you are too! It's nice to see you here again.

I think you'd hear the same turn-off transient, or maybe a different, bigger one, as soon as you unplugged the machine (Now you'd add in the energy stored in the main power transformer and the motor power supply and whatever else.)

Unintentionally, I rarely hear this burst myself. I operate through an external amp and when I wrap it up for the night, I almost always turn off the amp before the projectors and mixer. (You should hear what it sounds like when I get that wrong!). I'm sure my turn-off transients are just as spectacular as always, it's just when it happens it's got no speakers to express itself through!

[Xander van der Merwe]

Thanks for the advice, I will just ignore it then

I do have some other ST1200 questions to get this one up and running nicely, but will create a separate thread for that.