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Custom designed electronics at Plasma Music

Over the past couple of weeks, I’ve been designing a gadget to switch on a power amp when an audio signal is detected and to wait fifteen minutes to switch off the power amp when no signal is present on the input.

The audio signal comes straight from the sub bass output of an A/V amp and is therefore about -10dBV (approximately 310 mV) unbalanced. So apart from auto switch-on / switch-off, the circuit also amplifies the input signal to +4dBu (about 1.25 V) balanced so as to properly drive the power amp.

After testing a prototype for several months, this evening I ordered the PCB (the  design of which is pictured above) for the final version.


Well, the power amp in question, drives a passive sub-bass system on a home cinema. It's a big, professional amp and definitely wouldn't come with auto power-on / power-off which of course, the main (consumer) A/V amp does have and which 99.9% of sub-bass units have built-in.

When the home cinema system is switched on, the power amp comes on when it detects a signal. When the system is switched off, the power amp switches off a few minutes later... PERFECT!


I don't see the point of reinventing the wheel, especially if someone has already designed something really good that works. Hence, the switching circuitry of this gadget is based on a design by Rod Elliot of Elliot Sound Products(see project 38). Rod puts up some fantastic, in-depth explanation into how the circuit works and how it can be tweaked to your specific requirements.