As we gain more runtime on the new LDR300x we've been refining not only the control firmware but have also been tweaking the SSPB.V2 gain/buffer board used in the active version of the preamp. One thing we've noted from day 1 is that the LDR300x has a noticeable bass boost especially in the active configuration. It's pleasant and not boomy but I've wondered about it. We've been using the LME49720 audio op amp in the active LDR300x all along and until recently haven't really taken the time to explore alternatives. Fortunately, the dual channel op amp is a plug-in PDIP8 type so it's easy enough to swap out the LME49720 for alternatives.
Today we swapped out the LME49720 and tried a good old TL072 JFET op amp. To our pleasant surprise the change was an overall positive mode. The base boost was dialed back while everything continued to sound marvelous if not actually a tad bit better. My tech thought so right away. Moreover, the TL072 exhibited very stable unity-gain operation - arguably better than the LM49720 which tended towards some low level motor-boating with low/zero gain.
Based on the success with the TL072, which was first released back in 1978 believe it or not, we are going to try a few other alternatives op amps and report back on results. As things stand this moment I'm going to switch over production units to the TL072 unless we find something better after trying a few others.
I'll report back on findings after we've tried some others.
The other tweak we've been testing is running the LDR300x with direct coupled output. Direct coupled means we don't use output coupled capacitors ..... at all!! For this to work we have to ensure that the DC offset on the output signals are near zero (just a few millivolts). We baked this capability into the design of the SSPB.V2 board but until recently haven't focused on it. The use of a split voltage power supply together with adjustable JFET buffer current allows us to literally dial in zero DC offset using trim pots. What we're finding is that once trimmed out, the DC offset remains quite stable and resumes near zero levels after power up each time. Running without output/blocking caps yields the best possible sound quality since the best coupling capacitor is no capacitor at all provided the DC offset is small and stable. Anyone with an existing unit can implement this themselves using jumper wires, a volt-ohm meter, and a thin bladed screwdriver.
Between the op amp optimization and the direct coupled output, the active version of the LDR300x is evolving into a truly great sounding preamp.
Cheers,
Morten
