mark@mips.COM (Mark G. Johnson) (04/09/88)
In reply to a posting of mine containing the schematic of the phono stage of the Mark Levinson audio preamp model JC-2, Jan Postma found a typographical error: I incorrectly typed `63' when I should have typed `20'. The schematic as originally presented was missing a "solder dot" connecting the two together --- this shows that proofreading errors can occur with text as well as with drawn figures. A corrected revision in included below. The JC-2 circuit (designed by John Curl) is rather interesting in its biasing of differential BJT pairs. The original posting _was_ correct in that regard: the emitters of the diff pairs ARE directly connected together, and then tied to the rails (supplies) by a resistor. In both cases this resistor is actually implemented as two 249 ohm resistors wired in parallel. Jan points out that this unusual for the NPN current mirror. It *is* unusual but this is what's in the Mark Levinson preamp. Finally, Jan writes: > BTW, Mark, do you have any other Mark Levinson or High-End > diagrams :-)? The answer is, yes, and I'll try to post them between now and Tax Day. ! ########################################################################## ! ########## REVISION 2: corrects a typo. Node 63 == node 20 ! ! Mark Levinson audio preamplifier model JC-2 ! Phono amp section (including RIAA equalization) ! ! here is the active amplifier circuit; feedback network comes last ! positive input = node 10, negative input = node 20, output = node 50 ! system ground = node 0 ! ! Power Supplies & local bypass capacitors Vplus 80 0 13.0 ! Volts Vneg 90 0 -13.0 ! Volts C80 80 0 4.7U ! (micro)Farads C90 90 0 4.7U ! (micro)Farads ! ! Input differential amp with resistor loads (lowest possible noise) J10 D=21 G=10 S=23 Nch TYPE=E110 ! Siliconix N-channel JFET J20 D=22 G=20 S=23 Nch TYPE=E110 R21 21 80 100.0 ! Ohms R22 22 80 100.0 ! Tail current source for input diff amp J23 D=23 G=25 S=24 Nch TYPE=E110 R24 24 90 249 R25 25 80 27.4K ! (Kilo)Ohms R23 25 90 10K C25 25 90 10.0U ! ! Second stage: fully complementary push-pull w/ differential inputs R31a 31 80 249 R31b 31 80 249 ! Note: 2 R's in parallel (!!) Q32P C=32 B=22 E=31 PNP TYPE=2N4403 ! Motorola bipolar PNP Q33P C=33 B=21 E=31 PNP TYPE=2N4403 Q32N C=32 B=32 E=34 NPN TYPE=2N4401 ! Motorola bipolar NPN Q33N C=33 B=32 E=34 NPN TYPE=2N4401 R34a 34 90 249 R34b 34 90 249 ! 2 more R's in parallel ! stabilization network (NOT the RIAA phono equalization) C33 32 0 39P ! (pico)Farads C35 21 35 220P R35 35 33 10.0 ! ! Output stage: complementary parallel-JFET push-pull (!!) J41 D=80 G=33 S=41 Nch TYPE=E212 ! Siliconix N-channel JFET J42 D=80 G=33 S=42 Nch TYPE=E212 R41 41 43 60.4 R42 42 43 60.4 J45 D=90 G=33 S=45 Pch TYPE=E175 ! Siliconix P-channel JFET J46 D=90 G=33 S=46 Pch TYPE=E175 R45 45 44 60.4 R46 46 44 60.4 ! 100 ohm trimpot, adjust for zero volts DC output R50a 43 50 49.0 R50b 44 50 51.0 ! ! ! Now the overall topology in which the active amplifier is embedded ! Phono input = node 1, phono output (to selector switch) = node 70 ! ! ! input network (to provide correct RC load on phono cartridge) R10b 1 10 100.0 ! Ohms R10c 10 0 49.2K ! (Kilo)Ohms C10b 10 0 50P ! (pico)Farads ! ! RIAA equalization curve (2 pole, one zero) R63a 50 20 50K C62a 50 62 0.06U ! (micro)Farads C62b 62 20 0.02U R62 62 20 3.75K ! low frequency pole R64 20 64 110.0 C64 64 0 1000U ! (micro)Farads ! high frequency pole R61 60 61 10.0 C61 61 0 470P ! ! output network (connects phono amp to selector switch) C60 50 60 2.0U ! hi quality polypropylene R60 60 0 51.1K R70 60 70 60.0 ! ! ! that's all -- -Mark Johnson *** DISCLAIMER: Any opinions above are personal. *** UUCP: {decvax,ucbvax,ihnp4}!decwrl!mips!mark TEL: 408-991-0208 US mail: MIPS Computer Systems, 930 E. Arques, Sunnyvale, CA 94086