Audio Research LS15 circuit and description
Until recently i owned this and seeing there is little info at ardb.ws decided to draw and share the circuit. Not sure if it is worth cloning but may come handy for repairs.
I've only drawn the actual amplification stage, so here is a short description of the rest. All inputs pass through Zettler relays and a source/monitor/direct switches before going to two DS1267 digital pots. Outputs pass through a DG409 multiplexer before getting to the active circuitry. The DG409 seems almost unnecessary as all it achieves is some extra attenuation at the two lowest volume settings. This may be useful in other preamps but the LS15 has very moderate voltage gain and this extra attenualtion is just, in my opinion, a silly complication, compromising sound quality. The active circuit uses unmarked constant current devices which i have replaced with 2SK170BL. Input cathode CCS is set to 5.46mA, anode loads at 6mA and cathode followers at 4.85mA. Remarkably, the output 200ohm resistors are 5W.
The PS is very straightforward with a miniscule EI transformer followed by rectifiers supplying raw dc of 144v for the anode supply regulator, 20v for the heaters regulator and -49v for the input cathodes regulator. Both anode and negative regulators are very simple using just a source follower Mosfet and a fet ccs feeding a resistor at the source follower gate. No zeners. Raw anode voltage is filtered by 2x470uf/200v SMH Chemicon (i think) in parallel with 47uF/250 Nichicon VX, 2uF/250 Relcap PPMF and 0.22/160 Wima MKP10. After the regulator there is just a single 10nf Wima.