To provide attenuation for balanced input of my power amplifier I have to use constant impedance logarythmic attenuator. I decide to use relay switched resistors for best performance.
In the Internet there are a lot of relay attenuators, but they aren’t logarythmic or with constant impedance. After hours of thinking I found the solution of the problem and I made simple calculator for my attenuator.
Here is the result for 6 bit 600 ohm attenuator with smallest step of 1dB:
The main problem of the circuit is that signal passes through several relay contacts. This leads to usage of relays with maximum possible wuality (and price) to provide best performance.