Can I use Mosfets to disconnect a 3-Watt speaker from an amplifier circuit without any loss of sound quality?
I'm using a microcontroller controlled MP3 player called a Flyron FN-M16P, which features both terminals for powering headphones (terminals 4& 5, with GND terminal = 7), as well as an amplifier circuit for driving a 3-watt speaker (SPK+ = terminal 6, SPK- = terminal 8).
I want to have my device play MP3 files to the speakers under normal circumstances, but disconnect the speakers and play to the headphones when headphones are plugged in.
Please see the diagram here enter link description here
I cannot ground SPK+ and SPK-, the manufacturer tells me that this would blow the amplifier.
My initial idea was to use an SMD 3.5mm stereo jack with isolated NC (normally connected) switch, to disconnect SPK+ (Please see my attached diagram), but I cannot find an SMD 3.5mm stereo jack with isolated NC (normally closed isolated switch). I want to avoid PCB parts. I have found an SMD 3.5mm audio jack with an isolated NO (normally open) switch from Technik.HK. Please see the link here enter link description here
Can I use a mosfet based circuit to disconnect the 3-watt speakers from the amplifier pins when this switch gets closed without having it introduce any loss in audio quality? How would I do that (I'm a mechanical engineer not an electrical engineer).
Best Answer
A single FET won't do the job properly because the signal is AC, and the FET has a body diode that conducts when voltage on the Drain is negative with respect to the Source. To fix this you need two FETs, connected back-to-back.
Another problem is that to keep the FET fully turned on the Gate-Source voltage must be higher than its threshold voltage. Since the amplifier output swings (almost) from rail to rail, the Gate drive voltage must be a few volts higher than Vcc. If your circuit is powered from a higher voltage that is regulated down to 3.3~5V then you may be able to tap into the unregulated supply, otherwise you might need even more parts to generate the required Gate voltage.
Assuming you have a sufficiently high voltage available, the circuit could look something like this:-