I found this circuit inside an old 1990's "Electronics and Wireless World" magazine.

I was curious about it - and decided to analyse it.

It's an Opamp output network that is meant to always deliver a constant 600 ohm output impedance (resistance) to a 600 ohm audio load.

It appears to "current sense" the output, but also adjust that value with some "voltage sensing" too - and I'm just talking about the Non-Inverting input.

I've been analyzing it, trying to understand how the output side monitoring is working.

In my LTspice file, I've redrawn it to make more sense to me.

But I look at the 4 resistor network and wonder whether he was considering a resistor BRIDGE network (see the right hand image). There is only one resistor value that really matters, and that is the 17k (in relationship to the 60R current sense resistor and 600R Load resistor).

But that 17k is also responsible for the 13.6dB gain too.

Possibly the designer has combined two separate network functions, which is making it confusing for me.

I'm trying to work out a mathematical solution to this network.

I also tried simulating - for simplicity - with the load set to 1k and the current sense resistor set to 100R.

It's still hard to work out a mathematical solution - as that 17k resistor changes to become such an unusual value.

Is anyone able to give a mathematical solution as to what the network is doing?