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RE: Effects of Chassis material choice for input signal shielding ?


For this discussion, I am assuming a Single-Ended RCA input into your amplifier.

Your input to your amplifier will come from a "hot" wire, and a "cold" wire. In some setups, the "cold" wire is a shield around the "hot" wire, or in some cases, there are two wires, with a shield around both. In some cases, the two wires are both not connected to the shield, in other cases, one wire-- the "cold" wire, may also be connected to the shield.

Once your amplifier's input sees the "hot" and a "cold"-- you have your complete single-ended input signal into the amp.

Now, what? Assuming that you're using an RCA male to connect into your amplifier, once inside the amp-- in good amp design-- the "hot" WILL NOT be shielded inside the amp, rather, the "hot" lead from the input jack will go directly to the input grid on your first stage of amplification. In some amplifiers, you may wish to have a D.C.-blocking capacitor in series with this "hot" input. In this case, the cap becomes the short unshielded wire-- from the RCA jack "hot" pin-- to the input tube's grid.

This "hot" lead should-- ideally, be under one inch long-- that is, the input jack should be located very close to the input tube's grid.

In this way, your "shield" or "cold" side of your input jack can, and should, go to the amplifier's central, or "star" grounding system, BUT this ground wire from the RCA jack should have an inch or two of length-- a little separation-- from the "star" system before it connects to it. This is your input tube's ground reference point, relative to that tube's grid. It needs a little separation from the other grounds in the amplifier.

The primary reason that shielding should be avoided inside an amplifier is because shielding acts as a pickup device and injects distortions found inside the amplifier's chassis into the shielded wire's "hot" lead from induced radiation found within the wire's shield.

The basic rule is easier to understand-- shields go OUTSIDE the amp-- NO shielding for inside the amp.

Amp chassis materials are ideally chosen for their PHYSICAL sonic properties-- that is-- whether they "ring" or "deaden" sound. Ideally, they absorb very little energy (so they do not dull the life out of music), but they also do not ring like a Bell (and so, do not add colorations to music).

Aluminum adds a "whitish" coloration that is evident no matter where or how it is used in an amplifier.-- or even in wire.

Copper attends to a dulling, slowing down effect on music, but it does not lose introspective or depth of image if it is single-crystal, and oxygen-free. Copper is greatly improved by cryogenic treatment.

Copper use in chassis can provide a decent ground-plane for circuit designers to build upon, but it will also dull sound quality. Large-gauge silver wire, placed exactly and only where it belongs, outperforms copper ground plates in spades-- as a grounding system.

The use of woods and plastics in amplifier design is fraught with problems-- unwanted resonances, and sounds that are not found on your record or CD! With that said, it is still possible to use these materials wisely and come up with decent (but not the best possible) sound quality.

Steel is also not without its problems. It is magnetic, and therefore, layout and mounting of components is critical with steel, as is the steel's thickness..

For steel chassis, brass should be used for all fasteners, and all magnetic components mounts (transformers, chokes, etc., which are also steel should be mounted on brass standoffs).

Brass is THE best material for working with steel chassis.

If you are considering working with steel chassis, its thickness is critical, as this determines how it manifests its magnetic properties, and this also determines the actual physical, sonic signature of the completed chassis.

If a steel chassis is painted, or Powder-Coated outside, then it must receive the same treatment inside.

Truly advanced amplifier builders will choose to work with steel chassis because if its thickness and dimensions are correct, and if component layout and mounting are all correct, the finished piece will not dull music, and further-- it will not add colorations either.

Steel is hard to work with at the top level of performance, but when done right, nothing else can touch it sonically.

---Dennis---




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