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Bboard buffer 2.0 (yet another version "2")

Posted 13th May 2016 at 08:22 AM by rjm
Updated 27th May 2016 at 12:47 AM by rjm

A little bit of the sapphire headphone amp output stage, and a little bit of the LSK489 application note of all things (scroll down to Fig 10.).

Yes, this is probably the third "version 2" of this line buffer I've posted... I keep tossing it out and starting over. This variant looks pretty good: though the transistor count is a little high for such a basic function the performance is definitely there.

The main innovation re. the sapphire circuit is to replace the bias set resistors with diodes made out of the Vbe of transistors Q9 and Q10. This generates more voltage than is ideal, but can be handled by using largish values for the emitter resistors R13 and R14. Since this is a line stage buffer and not a headphone amplifier the output impedance of about 30 ohms and the limited output current swing are not critical flaws. It will drive 600 ohms at 0 dB with 0.001% THD. The whole circuit draws just 150 mW. The input impedance is a very high ~15 Mohms simulated (is that right?), while the PSRR is a usefully high 70 dB 0-20 kHz.

*****

rev. 2.1 ties the collectors of Q1,2 to the emitters of Q7,8 as seen in the LSK489 app note schematic. The distortion decreases slightly, with no other changes to the PSRR etc. or downsides that I can see.
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Attached Files
File Type: asc bboard buffer 20.asc (4.6 KB, 179 views)
File Type: asc bboard buffer 21.asc (4.6 KB, 169 views)
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