Electric and Magnetic Shielding

1. Electric Shielding

Shielding against electric fields is relatively easy. Any bit metal will do, as long as it is connected to ground. The chassis provides the ultimate shield against external electrostatic noise, but shielding may still be required internally, either to protect sensitive wiring against noise coming from other areas, or to enclose noisy conditions and prevent their electric fields from permeating the air. The wires leading from the input jack to the first tube often need to be shielded to prevent them picking up larger AC voltages present at the far end of the amplifier, which would cause parasitic feedback.

Shielded cable can be bought specifically, but the DIY alternative is to scrounge it from cheap audio interconnects.

Noisy wiring should run close to the chassis wall or, better still be pushed into the corners to maximize the capacitance to earth. The earthed chassis will then have a similar effect to a Faraday cage and will tend to draw the electric field towards itself, this often eliminates any need to completely shield the wiring, especially filament wiring.

Shielding cans may be used for small tubes, and if the tube socket has a central spigot, then this can be grounded to to some convenient point, as it will offer some shielding between opposing pins. Some tubes like 6922 include an internal shield or screening cage that can also be grounded. Another technique used in some hi-fi circuits - which offers electric screening while leaving the valve on display - is to use a pentode as a triode. But instead of connecting the screen and anode together as would normally be the case, the anode is instead grounded while the screen alone is used as the anode. The real anode thus serves as a shield.

Large power supply smoothing capacitors can sometimes be placed to provide some screening between different parts of a circuit since the metal can is normally connected to the negative terminal, which is often grounded. Likewise, other grounded metal objects like smoothing chokes or reinforcing brakets can be used to advantage. in extreme cases the chassis may be partitioned into sections with bulkheads to create isolated chambers that are shielded from one another.

Take a note that shielding only attacks the symptoms of the problem. The more direct route is to reduce the source of the electric filed by minimizing AC voltages. In this sense, amplifiers operating from lower supply voltages have an advantage. In particular, it is much easier to avoid parasitic oscillation in high-gain designs by keeping supply voltages to a minimum - less than 300 Volt. After all, headroom is not a top priority in a high-gain design, sot there is no need to have large signal amplitudes in the preamplifier or headphone amplifier.

2. Magnetic Shielding

Shielding against magnetic fields is much more difficult than shielding electric fields for two reasons. Firstly, at low frequencies the metal used for shielding must have high magnetic permeability, which limits us to expensive materials like mu-metal (which si at least one hundred times better at magnetic shielding than steel). At high frequencies ordinary metals can be used for magnetic shielding because they allow eddy currents to be set up, which generate opposing magnetic fields to the ones creating them, but this is not much use for audio. Secondly, the shielding must form a completely unbroken band or box since the idea is to encourage the interfering flux to flow in the shield rather than in the thing being screened. Any breaks or seams will prevent magnetic flux from flowing in a complete loop in the shield, so the physical construction of the shield is critical. Signal transformers and inductors are extremely sensitive to picking up magnetic hum and often need multilayer shielding, so they are avoided wherever possible.

Explicit magnetic shielding is therefore rarely found in audio amplifiers. A mild steel chassis may have a slight magnetic advantage over stainless steel or aluminum, but it is hardly noteworthy. Instead, the most practical techniques for minimizing the effects of magnetic fields are to adopt a tight audio layout so that the area of any loops is minimized, and to keep transformers and high-current carrying wires well away from sensitive circuitry.

Join Us All Products My Account Site Map
Twitter New Products Shopping Cart Shipping & Returns
Facebook Featured Products Checkout FAQ
Google+ Specials Register Advanced Search