First Project Sanity Check: A Simple Attenuator

[SubSpec]

TL;DR -> Does this schematic appear to fit the need of switchable 10db/20db attenuation targeting an output impedance of 11.2k?

Well, been lurking for a bit but outside of very minimal mods and plenty of cable construction I'm about as green as they come. So I'm starting simple.

I'm looking to create an attenuation box that will take DB25 in from my True Precision 8, have switchable 10db/20db attenuation for each channel, then back out over DB25 to my interface (Antelope Zen Studio+).

While I couldn't find input impedance for the Zen Studio, I did find the Orion from the same time frame is 11.2k and I'd be surprised if the ZS+ isn't the same or close. So that's the figure I'm working with. The output impedance has been prioritized, but fwiw the input is 100 Ω.

I've spent a good bit of time on the Uneeda Audio "All About Pads" page and worked through a bunch of match to get to a combo that I *think* does what I want in terms of the circuit of one channel. The schematic itself is from the L2P with R values updated to reflect where I ended up with my calculations.

So as a sanity check, does this even look like it's in the ballpark of what I'm aiming for? It's highly likely I have misinterpreted things at some point, so just want to check before I go any further with the design.

Thanks!

 

[ruffrecords]

First thing to say is I think you mean targeting a load impedance of 11.2K because the Antelope input impedance is the load for your attenuator.

The second thing is that in the 20dB position, the load your attenuator puts on the driving source is about 2.5K. This may be too low for many sources and could lead to increased distortion at high signal levels.

As a rule, for line level signals, you should aim for a design which has an input impedance of 10K and assume a normal load of 10K. In addition, if the attenuator is going to drive a significant length of cables, the output impedance needs to be a lot lower than 10K to avoid high frequency loss due to cable capacitance.

All the above means designing a 10dB attenuator is a challenge. 10dB is about one third so you might start the design with three 3K3 resistors arranged as in your schematic. Input impedance would be about 9K9 and the output impedance would be less than 3K3. Probably OK for short cable runs. If you then switch in a 750 ohm instead of the 620 ohms in your diagram you get 20dB attenuation and the output impedance drops to around 600 ohms.

Cheers

Ian