Input shielding: Trainwreck Express

[dai h.]

that isn't anything exclusive to Trainwrecks. It's the signals interacting with the alu foil or sheet (more capacitive coupling = less interaction with ea. other which can equal solving a squealing problem). Keeping this in mind you might find it helps to make sure some wires are flat against the chassis. Plus when you see glue holding down wires in wrecks, that could be the reason.

[hasserl]

We've seen that input shield connected to the plate, an discussed it, a couple of times here on Ampage over the years. It's been noted that it is an effective means of shielding, but most people are uncomfortable with it due to possible safety problems.

[Satamax]

I'm pretty sure i've seen this on at least on another amp, but thinking of having B+ there is it that bad? The ring of the input jack is still grounded? And the shield of the cable is at B+ but not connected to anything? Where's the problem then? Would it still be usefull if one would put a cap between B+ and shield? Could someone explain what it does wired as per the schemo?

Thanks a lot guys.

Bye.

Max.

[dai h.]

just think of it as a cap from output to input like the small (10-51pF or so) cap you see in opamps. Helps deal with oscillation. With signal a bit of highs couple back into the input from output. You can use an actual cap as well. As mentioned, it's used on around mid 70s to early 80s Marshalls.

[Satamax]

just think of it as a cap from output to input like the small (10-51pF or so) cap you see in opamps. Helps deal with oscillation. With signal a bit of highs couple back into the input from output. You can use an actual cap as well. As mentioned, it's used on around mid 70s to early 80s Marshalls.

Thanks a lot Dai. Well, true, it's like a local nfb from plate to input, daft me! Huh, just a question, does it have the same time ofset characteristics as a real cap or may be it's a bit different?

Thanks a lot.

Max.

[dai h.]

I don't know but what I remember from old discussions was that it would be the same IIRC. (I vaguely recall Steve.A asking if the same thing done with a PCB trace was done due to some special properties versus using a cap, and someone answered no--perhaps kg??.)