The load impedance must match the attenuator impedance.

...AND the source impedance. Each attenuator pad input and output impedance must match the source and load impedances. Otherwise each attenuator stage looks back into an impedance which is modified by the source impedance.

You can design a

*single* T- or pi-attenuator of

*any* attenuation to match between

*any* source impedance and

*any* load impedance.

You can design a string of switched series T- or pi-attenuators to provide any desired attenuation steps between matched source and load impedances.

You

*cannot* design a string of switched series T- or pi-attenuators to provide any desired attenuation steps between

*unmatched* source and load impedances, as the second pad in the string may see the first pad as a source and

*vice versa*. To maintain the calculated attenuation each pad must match its source impedance. If this is not the same as the load (output impedance of the preceeding pad) then the attenuation is affected. Of course if the only mismatch is to the source and each pad that is interposed has equal input and output impedances then this pulls the seen impedance closer to the correct value. This is why pads are sometimes used in RF design to minimise the impact of any mismatch.

For instance, you can design 6, 12, and 24dB L-attenuators between 50k and 50k. You can now switch them into and out of circuit (using a DPDT switch per stage) to give 0, 6, 12, 18, 24, 30, 36 and 42 dB attenuation (using a binary switching pattern) and both source and load will see 50k looking into the attenuator (or looking into each other) regardless. Each DPDT switch is wired to switch in an attenuator or a short. Make the source and load impedances different and the whole thing falls apart. It may well be possible to design a complicated arrangement of switches and resistors to achieve the desired result, but each pad must be modified in the switching to change its output impedance to be identical to its input impedance and this will change its attenuation which much again be corrected for.

If the source and load impedances are different you need to design 7 different T- or pi- pads (to match the range given above) and switch just one into circuit. Or modify the source or load impedance (or both) with a series or parallel resistor to make them identical (and accept any losses) and match the attenuator to this.

In this case the power is irrelevant as it is small enough, being line level, to be ignored.

This is all a bit nit-picking, you may well get away with something like this, or look at the link for values:

Attenuators : AMPLIFIERS AND ACTIVE DEVICES
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