View Full Version : Power Switch II circuit modification for transistor switching T-amp

2007-12-01, 01:01
I've just purchased both a Squeezebox 3 and a Sonic Impact T-amp. I'd like to switch the T-amp off and on with the Squeezebox and the Power Switch II plugin but I'm thinking the Power Switch II circuit can be improved on for switching the T-amp as it uses a DC supply. I'm hoping to avoid the relay and be able to switch the DC directly using a transistor and though I'm fairly experienced with some electronics transistors are something I'm weak on so I'd appreciate some help in refining the circuit. Hopefully I'll learn something about transistor circuits in the process.

My understanding of the existing Power Switch circuit is the 100 ohm resistor is a current limiting resistor for the collector/emitter side of the transistor, the 100k ohm resistor is for current protection on the base side of the transistor and the 330 ohm resistor is to reduce the voltage on the base of the transistor which doesn't need the full 3.3v to saturate. Also, by pulling the data sheet on the BC547 I see it has an Ic(max) value of 100mA and a hFE(min) value of 110 and that these are the only serious considerations for switching such low voltage DC. Please correct me if I'm wrong.

So looking at the power supply for the T-amp I see it is 12vdc @ 800mA so if I understand things correctly I can't simply use the BC547 as it maxes out at 100mA. I also see that transistors with an Ic(max) of 1A and a hFE(min) of 110 are not readily available but I'm thinking that it would be possible to reduce R(B) from 330 ohms to something less and then use a transistor with a lower hFE which could switch the entire 12v 800mA. I could use some help in selecting the correct R(B) value and a suitable transistor, if you can explain how you arrive at the R(B) and hFE values all the better.

2007-12-01, 19:50
My el cheapo original Sonic Impact T-Amp "thumps" (not very loud, and mainly mid/high frequency noise) when DC power is applied to it, and the old wall wart switched power supply that feeds it draws next to no power when the Squeezebox is off, so I opted to leave the T-Amp "on" all the time. You might experiment with the T-Amp and your speakers to see if it thumps, and, if so, how much that thump bothers you.