Audio power amp grounding

Thread Starter


Joined Jan 3, 2011
In looking at a schematic for an audio amp I'm trying to fix, I found a grounding system I don't quite understand. Here's a low-quality schematic image of the amp I'm working on:

Attached is a higher quality PDF of a different version of the same base model. Supposedly the only difference is in the power amp section. The one I'm working on (above) uses MOSFET output transistors instead of bipolar, and so the driving circuitry is also different. But the PDF is much easier to read.

I noticed that the power amp (upper right area of the image) uses chassis ground as a reference, while the lower voltage circuits in the preamp section use signal common. In the bipolar version of the amp (NOT the one I have), the schematic shows a 10 ohm resistor connecting chassis ground to common. I didn't see any similar thing in the MOSFET schematic, but the amp on my bench has a direct short from circuit common to chassis/safety ground, less than 1 ohm.

Two questions:

1. Why is the output referenced to chassis/safety ground instead of circuit common? Is it because of the high output power (350W max), and the fact that the speaker connectors a person grabs on to are connected to the chassis? Or just because the high currents involved can create voltage differences in the ground connection, which could result in noise if introduced into the preamp circuits?

2. Is this a safety issue I need to worry about in my own power amp design? I'm building a 200W keyboard amp, using a bridge design. I had planned on using a 2-conductor (TS) cable to the speakers, in which both conductors would carry signal current but neither would be connected to ground. Now I'm thinking this is a bad idea. I could possibly, instead, use a TRS connector on the amp end so that the outer part (sleeve) that I grab on to is grounded to chassis/safety, but use a TS plug on the speaker end. The output section of my amp design is the second attachment.