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Attenuation from Adam’s Amps

L-Pad

 

An L-Pad is a very simple, purely resistive, voltage divider attenuator. It consists of 2 resistors, one in series with the speaker, and one in parallel with it. The ratio between the 2 forms a voltage divider. There’s a usefulL-Pad calculator which you can plug an impedance and desired dB attenuation and it will spit out values for R1 and R2. Or, I’ve included a table below with the values necessary for -3 through -15 dB in 3 dB increments at 4 and 8 ohms.

I’ve heard it mentioned that purely resistive attenuation seems to sound fine for low power amps, but higher powered amps need a reactive load to sound more natural under heavy attenuation.
I built an L-Pad into the 8” speaker cabinet I built for my Moonlight. It has a single 8”, 4 ohm Jensen reissue speaker and a rotary switch allowing for the selection of no attenuation, -6dB, -9dB, -12dB, and -15dB. It’s quite effective. The higher attenuation settings do sound fairly weak to the ear, but that’s due in part to the fact that, when driven by a 1W amp, the SPL at -15dB is extremely low. I’ve excellent results close-miking the speaker for recording at -9dB and -12dB.

dB Z R1 (ohms) R2 (ohms)
-3 4 1.17 9.70
-6 4 2.00 4.02
-9 4 2.58 2.20
-12 4 3.00 1.34
-15 4 3.29 0.87
-3 8 2.34 19.39
-6 8 3.99 8.04
-9 8 5.16 4.40
-12 8 5.99 2.68
-15 8 6.58 1.73

Bridged-T

 

A bridged-T attenuator is another purely resistive design, but one which reportedly has superior sonic characteristics to the L-Pad.

FM Systems, Inc. has a page about the bridged-T. I found the link on the Ampage BBS. In this circuit, the values of R3 and R4 are always equal to the characteristic impedance of the whole thing. I.e. if you’re building one for an 8 ohm load, R3 and R4 are both 8 ohms.

FM Systems’ page describes the math for figuring R1 and R2. I’ve included the same table below for the Bridged-T as above for the L-Pad.

dB Z R1 (ohms) R2 (ohms)
-3 4 9.70 1.65
-6 4 4.02 3.98
-9 4 2.20 7.27
-12 4 1.34 11.92
-15 4 0.87 18.49
-3 8 19.39 3.30
-6 8 8.04 7.96
-9 8 4.40 14.55
-12 8 2.68 23.85
-15 8 1.73 36.99

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