Transistor Amp-Why Two Grounds?

[CJ]

i see this a lot, preamp section has a different ground point than the power amp,

Stability? Noise?

Thanks for any help.

Pioneer SX-626 Receiver>

 

[ruffrecords]

Sinclair used to do this in his early transistor pre amps in the late 60s. I think the recommendation was to star them to the power supply output cap to avoid output current flowing in the sensistive input ciercuits. I still have a home brew stereo power amp using a couple of Z30s wired exactly this way.

Cheers

Ian

 

[ccaudle]

The power amp section is controlling a lot of current, so even with reasonably low resistance in the wiring you still have a measurable voltage drop proportional to current x resistance. If that current from the power amp flows along a path which is used as a reference by the preamp section then the preamp circuitry has no way to distinguish a change in the input signal from a change in the reference point. Besides looking like a change in gain with signal level (i.e. distortion) there is also a path delay, so you get a feedback path from output stage to input stage that can cause oscillations.

 

[Bo Deadly]

I used to think this was a good idea to separate pre amp and power but I have since changed my mind. I think the arrangement shown is probably not a good idea because the noise of power amp current will be "seen" by the pre amp. The pre should be relative to the local power amp ground so that it doesn't "see" that noise. Specifically, giving each channel (pre + power) it's own ground back to the PS supply cap would be a better arrangement.

In the ol'days an interstage transformer would provide a differential input. Then you could and would want separate pre amp and power amp grounds. An OT would also help a LOT in a number of ways. Later on op amps could have be used to implement a differential power amp input but with modern designs and chip amps being very quiet and linear from the get go, it just isn't necessary anymore.

 

[abbey road d enfer]

It's almost impossible to analyze this without a schemo of the power supply and a view of the layout, which would help figuring out current circulation.

 

[CJ]

grounding points for pwr amp are bottom two circles, preamp upper right

 

[CJ]

pwr supply

 

[abbey road d enfer]

I think the arrangement shown is probably not a good idea because the noise of power amp current will be "seen" by the pre amp.

Agreed.
It looks like the single smoothing cap is grounded to the power amp PCB, as it should, so the parasitic voltages between the power and preamp grounds should be quite low. However, I don't consider it good practice.

 

[moamps]

It seems to me that smoothing cap is grounded to the chassis with a short black wire. The chassis plays the role of a large low-resistance ground bus, so it seems to me that the grounding scheme is quite ok. Especially considering that a single power supply is used.

 

[abbey road d enfer]

It seems to me that smoothing cap is grounded to the chassis with a short black wire.

Correct. I didn't notice this wire.
My only gripe with this type of grounding is that one must make sure the chassis doesn't touch any other apparatus, and that magnetic fields can generate voltages in the loop, more so than a properly laid out wire, because of the flux concentration due to the ferromagnetic nature of the chassis.
Of course, this technique has been extensively used in radios and guitar amps, but they are not particularly renowned for their outstanding S/N.

 

[JohnRoberts]

grounding points for pwr amp are bottom two circles, preamp upper rightView attachment 92041

That grounding scheme may have been determined by trial and error, for best result, on an engineer's bench.

JR