What is the pupose of this?

[Tage]

So while I was working on a JCM 900 recently I came across this interesting ground section and was wondering if any one could shed some light on the purpose of this design. I am puzzled as to the purpose of the back to back diodes and the 22ohm and the 100N cap.

http://www.drtube.com/schematics/mar...2100-60-02.gif

Any takers?

[markr14850]

Some good reading here: Earthing (Grounding) Your Hi-Fi - Tricks and Techniques

[soundguruman]

This is to couple the audio ground to the earth ground.
It has become popular to leave a small resistance between the earth and audio ground.
This actually reduces the 60 cycle hum in the audio output of the preamp.
In the old days, the power supply was grounded at one and of the chassis.
The preamp was grounded to the opposite end of the chassis.
The small resistance in between was from the resistance of the steel chassis. Usually 2 ohms or more.
This lowers the hum in the preamp, although it may seem like a good idea to connect all the grounds to one central point, (star) it's really noisier that way. The ripple being rejected by the power supply shows up in the preamp...
Now days, a resistor is used, to deliberately separate the grounds. The diode(s) is across it to rectify a small AC voltage which may show up on earth. OR (opposite) a small AC voltage that may show up on preamp ground.
Using that little tweak, the hum is reduced.
This separation trick will work in a number of tube amps, although you don't see everybody doing it.

[Tage]

Thanks for the very concise response, I will have to keep this design in mind if I'm ever having issues with my preamp noise.

[Wombaticus]

Thanks for the very concise response, I will have to keep this design in mind if I'm ever having issues with my preamp noise.

Soundguruman gets the story wrong in a couple of important regards.

First, I believe you will find that both the preamp and the power amp are connected to a common star point, and it is this star point which is connected to the arrangement shown. The intention, therefore, is not primarily to separate the grounds of the power section and the preamp section. The actual purpose of this is to prevent ground loops between the amplifier and any other device that may be plugged into its inputs or into the effects loop. The modest resistance effectively breaks the loop through the circuit earth. This is why this particular approach is seen also in units like the reissue of the Fender standalone reverb unit, which is essentially an amp that must plug into a different amp -- If such a unit is plugged into a different receptacle than the amp itself, there can be a difference in earth potentials and a current will be established between the two resulting in hum.

The second point that SGM gets wrong is the purpose of the diodes. These are not, as he proposes, "to rectify a small AC voltage which may show up on earth. OR (opposite) a small AC voltage that may show up on preamp ground". You will note that these are 1N5408 diodes... certainly not your grandfather's small signal diode. These parts are rated at a beefy 3A/1000V PIV! No, these diodes are present to ensure, that in a fault condition, that a suitable ground path always exists to the chassis. The concern is that a fault condition could very easily burn out and open circuit the (probably only 1/4 or 1/2W) 22R resistor, which could result in the audio ground being left at a dangerously high potential. Since cables plugged into the amp will be at this potential, this represents a very serious concern. The diodes ensure that this cannot happen.

The capacitor is simply to shunt any HF noise to ground.

[Tage]

Alright, well then should this be a worth while setup to employ in future builds? seems to only have advantages and it's just 4 components...