Power Amp input shield ground

[rperry]

I've looked at some of the threads related to input shield grounding (for unbalanced inputs) and the consensus seems to be don't connect the shield to chassis ground. I've been working on an ARC D100B and while the input shields are not connected directly to the chassis, they do connect to the ground trace on the driver board which connects to the main star ground and chassis. My question arises because my amp is not wired like the schematic shows on the ARCDB site which shows a 2.7 ohm resistor and .1 uF cap connecting the shield to ground (see attached). My PCB has no place for those components. During testing, I got some hum when connecting to my preamp (Mac MX113) which has no earth ground so I'm wondering if the resistor/cap combo was a later addition for hum reduction. Any recommendations?

thanks,
Russ

 

[rayma]

I got some hum when connecting to my preamp (Mac MX113) which has no earth
ground so I'm wondering if the resistor/cap combo was a later addition for hum reduction.

Yes, that would be useful if you have hum. If it's bad use up to 10 ohms,
usually for both channels.

 

[sgrossklass]

Looks like the D-100B has its ground tied to mains PE, right? Then I would suspect the hum came in via something attached to the MX113 that has a ground connection of its own - either a signal source or, since this appears to be a tuner pre, the antenna. Welcome to the wonderful world of unbalanced audio connections and the joy of ground loops.

 

[rperry]

FYI, I cut the trace for the input grounding and added the cap/resistor as in the schematic above and it did wonders for the hum. Now I have an AM-2 module that shorted out that I'm dealing with... This is/was a great sounding amp - very dynamic - so I'm building some new AM-1 and AM-2 modules. I've already recapped it and replaced the out of spec resistors.



These first ARC solid state amps are an interesting story... The initial Stereophile review was heading for a Class A recommended status except for reader reviews when driving a particular speaker (Fulton J-Modular). The TAS review was all negative but I think that was mostly in response to the possibility of ARC abandoning tubes.



Russ