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Power Attenuator Project

August 11, 2010
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I’VE NEVER SEEN PLANS OR KITS FOR building a power attenuator. How would I go about making one myself? —Dave K. via email

gp0710_gearfixdiag1A power attenuator is a device that’s placed in the signal path between the amplifier and the speakers so that you can turn up the amp until it sounds its best, and then attenuate the signal going to the speakers to control the volume. Not only does this allow for a great overdriven tone at an appropriate volume level, but, when used in recording situations, an attenuator will remove most, if not all, of the hiss and hum from your pickups and amp. When you attenuate a few db, the first thing to go is the noise.

Here are the plans for making a two-stage power attenuator at a cost of about $85. Besides the true-bypass configuration, this design features two toggle switches that each attenuate the output signal by 6db or 12db total attenuation with both switches on. And since the load impedance is kept constant regardless of which switches are on, your amp will “think” it is operating normally.

PARTS LIST

SW1 and SW2 – DPDT toggle switch. Mouser part # 691-2BL62-73
J1 — Input jack. Mouser part# 502–11
J2 — Output jack. Mouser part# 502–12A
Chassis — Metal enclosure. Mouser part# 537–138-Pv Heat sink compound — Mouser part #5878-CT40-5
Hook-up wire — About 2 feet or less. Use 18 gauge.

For 4Ω version:
R1 and R3 — 2Ω 100-watt wirewound resistor. Mouser part# 284-HS100-2.0F
R2 and R4 — 4Ω 50-watt wirewound resistor. Mouser part# 284-HS50-4.0F

For 8Ω version:
R1 and R3 — 4Ω 100-watt wirewound resistor. Mouser part# 284-HS100-4.0F
R2 and R4 — 8Ω 50-watt wirewound resistor. Mouser part# 284-HS50-8.0F

For 16Ω version:
R1 and R3 — 8Ω 100-watt wirewound resistor. Mouser part# 284-HS100-8.0F
R2 and R4 — 16Ω 50-watt wirewound resistor. Mouser part# 71-RH50-16 Mouser can be reached at (800) 346- 6873, or you can order online at mouser.com

BUILDING IT

gp0710_gearfixdiag2Refer to Fig. 1 to see the suggested parts layout. Look at the parts and invest a few minutes planning before you drill any holes. Remember the amp-builder’s rule: “Plan it twice and drill once.”

Notice that the wirewound resistors are heatsinkable. We will mount them inside the chassis, leaving enough room for the jacks and switches. The heatsink compound is applied between the resistor and the chassis, which allows for superior heat transfer from the resistor to the chassis. You must have adequate heat sinking for the resistors to actually perform at the specified wattage rating! The component values shown will work for amps up to 100 watts and still have a margin of safety. If you would like to use the power attenuator only with a 50-watt amp, you may safely cut the wattage values of all the components to half of the listed values.

Also, notice that the output jack has a shorting switch and the input jack does not. The switch is a safety device in the event you happen to turn on the amp without a speaker plugged in to the attenuator. The switch lead on the jack simply connects to the ground lead on the same jack.

You can place the attenuator in the back of your amp and leave it there, or you could also mount it inside the lower back panel for easy access to the toggle switches. When the toggle switches are set in the positions shown on Fig. 2, then maximum attenuation occurs. But when both toggle switches are switched to the bypass position, the circuit returns to stock. Note: If you want to be able to change the attenuation level for different songs while you’re playing, use DPDT (double pole/double throw) footswitches instead of toggle switches. —Gerald Weber, Kendrick Amplifiers

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