I'm planning to use STA540 in a guitar combo, because it's one of a few chip amps that can give me about 15W with 12V supply and 4ohm speaker.
My problem is that the chip has four amplifiers and I will be using only two of them (bridged). What should I do with the other two?
My ideas:
- Ground the inputs and leave the outputs hanging.
- Use the second pair in parallel with the first one. This probably won't help me push more power to the speaker because I think I will be limited by input voltage and not the IC itself and I don't see any other advantages of this
- Find a smaller IC — but I already tried that and didn't succeed….
- Use them to drive headphones. Is this realistic, or would the headphones explode?
Best Answer
Since the inputs are designed for capacitive coupling, floating the inputs (and outputs) is okay because the chip provides internal resistors that establish the DC biasing point at the inputs.
Grounding the inputs will drive the unused outputs to the supply rails, which may increase (or decrease) supply current. Grounding inputs will likely trip the clipping detection (pin 10) even when the active channels are not distorted. Grounding inputs may also shift the SVR voltage, which could undesirably shift the quiescent DC voltage of the active outputs.
You could try both floating and grounding the inputs, and measure which results in the lowest supply current (to minimize wasted energy). If grounding inputs, check for significant shifting of the DC output voltage.
Paralleling outputs likely won't work here because the voltage gains of the amplifier channels will not exactly match. Because the active feedback of each amplifier tries to correct any output-voltage error, the amplifier channels will just fight each other causing excessive power consumption and likely distortion.