Passive Audio Attenuation

[Caskré]

Hey!

I expect to shortly find myself in need of a set of switchable pads to put between the line-level output of mic preamps and the line-level inputs of an audio interface. I'd much rather put it together myself, mainly as a learning exercise.

I have a few of questions about how to go about this:

• How concerned should I be concerned about impedance matching and, assuming I should be a little concerned, how should I take these concerns into account while choosing resistor values
• What type of attenuation circuit would be a good choice and what are the differences in a case like this
• Should I use transformers to unbalance & balance the signal at input and output and use an unbalanced attenuation circuit or should I use a balanced  circuit?
• If I use a balanced circuit, should I still consider using a transformer at the input and output stages?
• If I wanted to include filtering in the circuit would I be tackling that separately or would I be piggybacking an inductor/capacitor onto the resistive elements already used for attenuation[li/]
  

If there is any other information that I could include to help make better informed decisions please let me know.

Also if this is in the wrong place, I apologise and will post in an appropriate place.

Thanks!

[gbaddeley]

In your case, you don’t need to be concerned about impedance matching. The simplest solution is unbalanced resistor pads in T or PI configuration, designed to work with the input impedance of the next amplifier, and don’t overload the driving amp.

[magic]

You need to match impedance in the sense that if your interface has 10kΩ input impedance (i.e. a 10kΩ resistor from line to ground) then your divider cannot be 1M/100k. Or if the output has 100Ω resistance, 100Ω divider isn't a good idea either.

Output impedance of a divider = the parallel combination of the two resistances (909Ω for 10k/1k, for example).
Input impedance of a divider = the sum of the two resistors

[gbaddeley]

To lead on, what is the approx output impedance of the mic preamp? Post the schematic of its output stage if you are not sure.
What is the input impedance of the next stage?
What switchable padding do you want? Eg. Rotary 12 position, or a series of fixed pads that can be switched in or out, such as 1 dbV, 2, 4, 8, 16, 32.
Lastly, can it be unbalanced, or is balanced a must-have?

[Caskré]

To lead on, what is the approx output impedance of the mic preamp? Post the schematic of its output stage if you are not sure.
What is the input impedance of the next stage?

As far as I know, the output impedance will be somewhere between 100-600Ω. I know that is a bit of a range but I can't find exact figures (schematic bellow) As for the input impedance, I believe it is 10kΩ

What switchable padding do you want? Eg. Rotary 12 position, or a series of fixed pads that can be switched in or out, such as 1 dbV, 2, 4, 8, 16, 32.
Lastly, can it be unbalanced, or is balanced a must-have?

I'm thinking a simple 1 pole 4 throw switch, 0 (bypass), 6, 18 & 24 dB, I'm not terribly concerned about meeting those values exactly but in that ballpark.

I know balancing in general in audio is good practice but I don't know if the circuit itself being balanced is necessary.. Is there any benifit to leaving the whole thing unbalanced vs. balanced? Is it effectively just a case of more resistors if it's balanced?

You need to match impedance in the sense that if your interface has 10kΩ input impedance (i.e. a 10kΩ resistor from line to ground) then your divider cannot be 1M/100k. Or if the output has 100Ω resistance, 100Ω divider isn't a good idea either.

Output impedance of a divider = the parallel combination of the two resistances (909Ω for 10k/1k, for example).
Input impedance of a divider = the sum of the two resistors

So... would I need to aim bellow the lowest impedance or would I need to tailor the divider (in the case of a pi or t-pad) to have >100Ω input and <10kΩ output..?