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Old 24th January 2004, 03:46 PM   #1
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Default Logarythmic attenuator?

I've searched this site and found a lot of postings about attenuator for volume control with resistor dividers and relays.
But almost all of them are linear, including Nelson Pass's Aleph-p.
I found 2 or 3 logarithmic but they use a lot of relays (16 for one channel).
Do you know is it here a circuit like Aleph-P's but logarythmic?
I mean for example using 7 relays to make 127 steps each of 1dB attenuation.
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Old 24th January 2004, 04:03 PM   #2
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You can do so by cascading 7 constant impedance T or Pi attenuators. But then you have always 7 relay contacts in series. And you have always 6 dB attenuation at max. setting then, but this does not need to be a problem.

You don’t need a 127 dB span, 70 dB is more than sufficient. So you can do with 6 relays.

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Old 24th January 2004, 04:33 PM   #3
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Check this

Constant impedance relay-resistor logarithmic attenuator
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Old 24th January 2004, 09:04 PM   #4
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You could make a coarse ladder attenuator with 5 positions, 0dB, -15dB, -30dB, -45dB and -60dB. Selection of one of these outputs requires 5 relays. Then add a fine ladder attenuator with 5 positions, 0dB, -3dB, -6dB, -9dB and -12dB. This requires also 5 relays for selection of the required tapping. In total you need 10 relays to get a range of 0dB to -72dB in steps of 3dB. There are always (just) 2 relays in series. Probably you always have some additional attenuation in the 0dB posistion, but that makes no difference for the step size and range.

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