Switchable attenuator for both balanced and SE signals?

[abza]

I want to attenuate a signal before it hits my shiny new audio interface.


The idea is to have a single box that can attenuate both single-ended and balanced signals in -6dB steps. I put together this circuit and the simulation seems to check out:

Switch position 1 leaves the signal untouched, with each higher position cutting the signal in half, then a quarter, then an eighth, etc.


Is there any problem I'm not seeing in using this circuit for both balanced and single-ended signals? (i.e. single ended would have IN- as ground) My gut instinct is telling me that having one side of the signal (IN+) being the only side "exposed" to the resistor path may be a problem?


Thanks!

 

[Douglas Blake]

To maintain proper balance on differential signals you need to switch both sides against ground... Switching only one side is going to cause some rather strange behaviour in the next stage of your system.

 

[Mark Tillotson]

This kind of attenuation chain doesn't have good noise properties, the more attentuation the more noise (well only about double), but the lower the signal level the less noise you want competing with it. A standard attenuator has less noise with more attenuation.

 

[Monte McGuire]

To maintain proper balance on differential signals you need to switch both sides against ground.

Indeed. The other way to think about it is that this attenuator won't attenuate the common mode component of a signal, and that can cause problems if the downstream gear doesn't want to see any common mode component.

Two ganged single attenuators, side by side, will attenuate difference and common mode signals equally.

 

[abza]

Great, thanks everyone! Suspicions confirmed 🙂 I’ll have to wrangle another design - will check back once I’ve got a revision!