Electronic – Driving LM386 with signal referenced to virtual ground

amplifieraudiolm386virtual-ground

I'm working with a chip that outputs audio signals to headphones that are referenced to a virtual ground. Since I'm connecting the audio signal to a speaker instead of headphones, I'd like to implement a basic audio amplifier on the same board (sharing the same power source and ground).

I'm thinking of using an LM386 since they're a jellybean part.

However, I'm not sure how I would go about doing this. I've read the data sheet of the LM386 but the application notes don't really go into handling a virtual ground. The virtual ground is roughly 1.2V when referenced to actual ground and can source/sink about 150mA before it goes into short circuit protection mode.

All the circuits I've come across just ground both the inverting input of the amplifier and the input signal ground (why?). Obviously, if I did this, I would short the virtual ground to actual ground.

The schematic I am specifically referencing is the one below (source):

enter image description here

Is it just a simple matter of connecting the virtual signal ground (instead of actual ground) to the inverting input of the amplifier or would I have to look into building in a isolated power supply to power the LM386?

Best Answer

Connect your "virtual ground" (or as I would call it an AC ground) via a capacitor to real ground. This prevents the DC levels on your virtual ground getting upset by a conducting connection. Make sure the capacitor value is reasonably high; in your circuit you are using 1 kohm input resistors so, at 20 Hz, the capacitors impedance should not be higher than 1 kohm. Sounds like 10 uF should do the job. I'd use a non-polarized one although it will probably be OK with an elecrolytic.

You may need to put a 10 uF capacitor in series with each 1 kohm resistor too.