That would change the voltage drop across the resistor and depending on the circuit, the output voltage/current. In many pcs of equipment there is also an RC filter network that is part of the output. Some have an emitter follower circuit, and others use this resistor as part of a negative feedback network, changing the resistor opens up a whole can of worms. Designing it with low output impedance in the first place is a much better idea. If by just changing the resistor Sherwood(the manufacturer of the 990) could have lowered the o/p impedance, don't you think they would have done this to keep it low, we both agree lower is better? It is not as simplistic as you make it out to be. You just can't change one component and have no worries about other effects.

Tim

Depending on the 990/997 'differential' output design, one might adapt to find a very low impedance output available, even if some might stand aghast at such a suggestion.

I've been involved in the design of audio equipment for over 30 years, so, yes, I am very familiar with what the circuit path of home theater preamps looks like, and know what the ramifications of these circuit changes would be.

I think you would find the 990 circuitry as I described, and any competent engineer could change the value of these resistors without potential problems. Any RC type filtering like you describe would be performed within the feedback loop of the op amp, and not passively after the output pin. I haven't seen an emitter follower used in any modern piece of equipment except for very high end designs which use discrete components.

So, if you don't like the output impedance of the 990, change it, or find somebody who knows how to do it. It's not as big a deal as you seem to think it is.

Even in a differential output stage, there will still very likely be an isolation resistor in each leg of the signal. This is just good design practice.