Have one customer who bought one of our V1 controller boards second hand. After I updated it, customer hooked it up to his DAC and reported very poor bottom end. Which is unusual for our LDR preamp boards. After consulting with the DAC manufacture was advised that the DAC needs to see a minimum of 50k impedance downstream to perform properly or else it would lose bottom end. The V1 has a 10k input impedance and the V2 has 20k. In parallel with most any amp the effective impedance seen by the source will be less. This is a good example of where a passive may result in disappointing performance relative to an active if the source (a db Audio Labs Tranquility SE DAC in this instance) needs such a high impedance to perform properly. I've not run into this problem before with DACs I've used nor have most of our customers. But worth checking with your DAC manufacturer if you already have one or are considering buying one.
With our V2 boards, I can jack up with input impedance to say 50k with a simple software change. Upload that to the V2, rerun AutoCal, and you have higher input impedance. The only reluctance to setting the input impedance even higher is that is causes higher voltage drop across the series LDR and higher Vdrop across LDRs does raise their distortion level. Whether the distortion is sufficient to matter is questionable. We did beta testing at 50k last year with the V1's and nobody complained.
When I get some time to come up for air, I still intend to add a feature to the V2 software that will allow users to adjust its input impedance level between 1k and as high as 50k.
Best,
Morten
