3875 - RF is the enemy. & Last and final grounding. - diyAudio
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Old 9th September 2011, 12:02 AM   #1
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Default 3875 - RF is the enemy. & Last and final grounding.

I have battled RF since I have had a couple 3875 amps. At times it is so quiet you can not tell what noise it actually happens to be.

Here is what I can tell you now.

The 300pf (actually 330pf is about the only size you are going to find, film) capacitor across pin 7 and 8 fixes most RF. However last time I did I was not entirely made a believer. I since have built an amplifier, and tried all sorts of grounding re-arrangements to prevent RF. I then tried ferrite bead filters on all lines going into the unit. Lastly, disappointed with the ferrite beads and grounding, I added the 330pf capacitors again. The noise now is so low it is inaudible unless you get your ear just about touching the speaker.

My recommendations. If your noise floor is not undetectable do both ferrite beads and the capacitors. The sonic improvements are great on top of the removal of annoying noise. Everything is better, not just a lack of extra radio stations in my room. I think due to the nature of the 3875 chip you get a loss of quality when you have any RF due to the speed of the 3875 chip.

Grounding: I have been over this many times reading all the different stuff from PD and elsewhere... No amount of it will remove RF if you have it. RF comes directly from the lines going into the equipment.

How to Ground:

Earth can be connected to chassis for safety. I do this very close to the power inlet and recommend it that way to keep from having charges crossing toward the amplifier boards if earth ground is near them.

There are two PG- and PG+, all four should connect to star ground. From star ground you will have four wires leaving. One for each binding post, and one for each amplifier board. There is no good reason to connect more than one ground wire to each amplifier board. The only other thing to mention is that the signal ground connects directly to amplifier boards, and is isolated. You can connect the wires from star ground to each amplifier board in any of the following: PG+, PG-, CHG, or OG. They are all on the same plane if the PCB board drawing is correct. OG might be the best choice in my opinion to keep the lowest noise in the signal area because the electron draw will be directly off starground and not potentially from anything in the adjacent area. (That is my anal opinion, and probably excessive and unnecessary, but given the choice..)

I like to star ground by using a screw and a bunch of crimp on eyelet connectors so I can easily take it all apart or re-arrange wires to one side or the other etc. Mine is just an extra long screw off the rectifier board. Soldering is fine but it sure is kind of messy to get them all in the same spot. This could ground to your chassis (no need to make the effort, it might just happen), which one could say eliminates a technical need for earth to chassis, but for safety reasons I keep the earth ground.
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