It's probably a good thing I didn't give values for a practical design like 25K, forcing any of us to check the values several times before building.
To hopefully belabor the obvious, so people can check me, my method is this: Fix R0 to be the desired input impedance. Pick R1 so the pair R0, R1 gives 1 attenuation step, here 2.5 dB. Pick R2 so the circuit here to the right has the same impedance R0 with this switch up or down.
Move left one stage. Pick R3 so the pair R0, R3 gives 2 attenuation steps, here 5 dB. Pick R4 so the circuit here to the right has the same impedance R0 with this switch up or down.
This recursive step requires some thought: Since the circuit to the right has impedance R0 in all switch positons, we can simplify the current stage so it looks like the rightmost stage. The output voltage is then the input voltage to the next stage, so the attenuations add.
The remaining stages go the same: Move left one stage. Pick R5 so the pair R0, R5 gives 4 attenuation steps, here 10 dB. Pick R6 so the circuit here to the right has the same impedance R0 with this switch up or down. Move left one stage. Pick R7 so the pair R0, R7 gives 8 attenuation steps, here 20 dB. Pick R8 so the circuit here to the right has the same impedance R0 with this switch up or down.
Quote:
Originally Posted by rickcr42
.I can see the potential for the accidental full volume out scenario
|
Yup, this did cross my mind, too. The nicest slide switches might be better suited for this than toggles.
On the other hand, in my personal experience with commercial products I've seen digital controls go to to full on, far more often than mechanical controls. (I have tape to this day over the infrared eye on my Adcom preamp, for example.) If I made momentary up/down controls for a digital attenuator, my fear would be that a circuit malfunction would start it racing up on its own.
Quote:
Originally Posted by dsavitsk
Last, I think the natural evolution of this is to use transistors as the switches allowing for a very very inexpensive digitally controlled rotary stepped attenuator.
|
Quote:
Originally Posted by rickcr42
and why not cmos analog switches ? Just a prepackaged FET switch that uses little power and you can even get "clickless" versions plus they are a plug and play solution.
|
This is a very appealing idea to me. Are you saying that an FET switch can be cleaner than the best gold or silver contact mechanical switch? We're going through 4 to 6 of them at once, if they add distortion it defeats any advantages to a stepped attenuator.