Marshall DSL input observations ? (Diodes, cap to ground, bead)

[Iplayloud]

See the schematic at the bottom left:
http://www.drtube.com/schematics/mar...61-02-iss1.pdf

In a Marshall DSL, just before the first gain stage right at the input, there's a pair of diodes to ground, are they clipping diodes like a distorion pedal ? And that capacitor to ground must ground a bunch of higher frequencies ? And what does that bead do ?

[Iplayloud]

Well how about just the bead, the component itself looks like a small bullet, it's probably a winded coil. What job would that do before hitting the first gain stage ?

[raiken]

The diodes/resistor/cap circuit is a ground loop breaker to prevent hum. Read more about it here (shown using a bridge instead of individual diodes):

Earthing (Grounding) Your Hi-Fi - Tricks and Techniques

If you look closely, you'll see the diodes are on the ground side of the circuit, so they can't function as signal clippers. They are there only to limit the voltage difference between grounds to approximately 0.7V for safety reasons.

From the article: "The loop breaker works by adding a resistance in the earth return circuit. This reduces circulating loop currents to a very small value, and thus breaks the loop. The capacitor in parallel ensures that the electronics are connected to the chassis for radio frequency signals, and helps to prevent radio frequency interference. Finally, the diode bridge provides the path for fault currents. The use of a large chassis mounting (35A) type is suggested, since this will be able to handle the possibly very high fault currents that may occur without becoming open circuit. Note the way the bridge is wired, with the two AC terminals shorted, and the two DC terminals shorted. Other connection possibilities are dangerous, and must be avoided."

The ferrite bead and 47pF capacitor is for RFI suppression to keep the amp from picking up radio stations.

Randall Aiken

[Iplayloud]

Wow Mr. Aiken from Aiken Amplification ! Thanks for the reply, I hope those EU RoHS regulations aren't giving you too much trouble. Do you think these extra parts affect the tone in any way ? I've never had ground loop problems except when splitting my guitar signal to 2 amps and if they affect the tone I prefer removing them.

[raiken]

No, they won't affect the tone, unless you consider extra hum or radio stations playing along with you as a tonal benefit. Leave them in there.

RA

[melvin]

So is the only earth (chassis) connection at the input jack? Is this the safest way to do it? I thought the earth connection needed to be near the pwr transformer secondaries.

[raiken]

So is the only earth (chassis) connection at the input jack? Is this the safest way to do it? I thought the earth connection needed to be near the pwr transformer secondaries.

The best grounding approach is to ground the circuit only at one point, preferably at the input jack for best RFI and noise immunity, and don't depend on the jack nut to make the ground connection!

Note that this doesn't include the power line safety ground, it must be connected right at the power entry point with a short wire well bonded to the chassis.

RA