Audio attenuator -- Project 112 (ETI/TEST GEAR, 1977)


This useful audio attenuator project for the experimenter provides 0-59dB attenuation in one dB steps.

Fig. 1. Circuit diagram of the attenuator.



59dB 1dB ± - 0.3dB dc to 100kHz 600 S - 2 nominal 10k switched (+30dB attenuation)

600 SZ nominal 15 volt termination resistor for use with high


ACCURATE attenuators are required in a multitude of design, service, testing and measuring situations. These units are designed with varying degrees of accuracy and as many steps of attenuation as the designer feels necessary. They may be balanced or unbalanced and have whatever input and output impedances the designer requires.

There are three common types of attenuator configuration, P1, T or L. The latter is mainly employed where the output impedance is not required to be constant.

We have chosen P1 type sections for our unit. We could have connected the various sections in tandem to form a ladder attenuator, but this would have made more complex rotary switches necessary. Instead, we chose to employ a separate section for each step of attenuation, making only simple rotary switches necessary.

The input and output resistances of the unit remain relatively constant at 600 ohms over the full attenuation range. The input impedance can be changed to 10k by SW1 but an additional 30dB of attenuation is added. The output can also be terminated internally by SW4 when using a high impedance load such as a meter.



SW1 Single pole change over miniature toggle switch

SW2 2 pole 11 position rotary switch

SW3 2 pole 11 position rotary switch

SW4 Single pole change over miniature toggle switch

Diecast box 4 3 / 4 x 3 3/4 x 2 4

Terminals type L1568/1S or similar 2 Knobs


Fig. 2. Drilling details for the die cast box.

The maximum attenuation when the input and output resistances are set at 600 ohms is 59dB. There are ten 1dB steps from 0dB to 9dB, via a 10 position rotary witch, and a further six 10dB steps from 0 dB to 50dB via a six position rotary switch, giving a total of 60 steps from 0dB to 59dB. This range of attenuation is adequate for most purposes. Although further sections could be added, noise becomes a limiting factor in a simple attenuator such as this.

Fig. 3. Lettering and front panel artwork - full size.


It is advisable to employ separate wafers for each switch pole. If the type of switch that has two poles on one wafer is employed, there may be problems at the high frequency end due to stray capacitance. This would be evident as spikes on the leading edges of high frequency square waves.

The common rail for each switch is a length of 18 gauge tinned copper wire formed into a ring to allow termination of the shunt resistors ( R4, R23, R7 and so on). The series resistors are connected directly between the relevant switch contacts.

Layout of the unit may be seen by the accompanying photographs.


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