Hi All,
Thought I'd share some interesting evolving news regarding our LDR based passive preamp design.
To optimally transfer the audio signal (voltage!) from source to load, contemporary designs take an impedance bridging approach whereby low Z sources (<1k - CD, DAC etc.) are connected ("bridged") to relatively high Z loads (preamp/amp > 15k) rather than trying to actually match the Z's as was done back in the all-tube-all-the-time days. If this ratio gets too low, the sound can begin to lose dynamics and sound flat.
Historically, implementing an LDR based attenuator by conventional analog means (using a stereo pot to regulate the LDRs) results in a suboptimal impedance (Z) profile wherein the Z can get quite low (<1k) in the middle of normal listening range; quite the opposite of what you want. A considerable body of online discussion regarding the merits of LDR based preamps touches on this subject of "impedance mismatch". To mitigate this, LDR based passive preamp designers (including Tortuga Audio) have recommended a load to source Z ratio of 100:1 or greater. Even so, there are instances where an LDR passive preamp is not a great fit between certain sources and amps. And although Tortuga Audio's LDR preamp design utilizes digital control, to date we have largely emulated the LDR control achieved by a conventional pot. So while our focus to date has been on taming the variable and nonlinear LDRs and perfecting channel matching via software based control (which we have definitely achieved!), the impedance mismatch issue has largely remained unresolved.
That is about to change! Using test data, post testing data manipulation, and a revised software control design, we have now achieved an effectively fixed input impedance of ~20k over 90-95% of the attenuation range with impedance increasing above this level near zero and max volume (in order to achieve a nominal 0-60dB attenuation range).
What this means is while our LDR passive preamps already sounded great and we have a growing base of very satisfied customers, our LDR passive preamps are going to get even better going forward. Raising and fixing the input impedance to roughly the equivalent of a 20k potentiometer will both improve the audio dynamics and open up the application of our LDR passive preamp products to a much wider range of source-amp combinations including both tube and solid state types.
More on this as we work out the final details but we hope to begin shipping units with this upgraded design (all done in software - same hardware) within the next 30 days or so.
Cheers,
Morten
