Hi Mark, correct. I have attached a schematic of the preamp with the modification. The preamp is the same as in earlier posts with some possible component variations that essentially do not alter the performance. The 30 VDC power source can be nearly anything. I used a wall wart. It need not be clean and really any value from about 28 VDC to 35 VDC should be fine. The design does not permit charging while in use. This is intentional as you would need to filter it well. Since I estimate battery life at over 200 hours continuously this is really not an issue. I used 18500 batteries, but the 18600 are more common and would extend the playing time by about 30%. Folks building this mod should be aware that when fully charged the batteries can deliver a lot of current. I smoked a number 20 wire when building this unit. They can deliver several amperes for a short while. The setting of the regulator is somewhat unusual as the terminal voltage on the battery string desired is about 23.4 volts. Running them to 4.0 volts is not recommended by the battery companies. You would expect that setting the regulator for that value would get them there eventually. I observed that it would not. A no load regulator output of nearly 27 volts was needed to trickle charge the string at 25 ma. BTW for those of you that would say check your meter ....The one I use is accurate to 5 places when compared to a known voltage precision reference. Now the important part. Does it sound differently? In short no. The S/N of the original AC powered design was in the -100db range, essentially at the limits of what I have confidence in measuring. This one is the same. The biggest advantage is that it is portable.
Please excuse the hodge podge in the schematic. I just pasted the mods on an earlier AC powered version.
Good listening
Bruce
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