File:As-built diamond-core audio power amp.png

English: As-built audio power amp schematic built around a diamond-buffer core, with Sziklai-type current boosters. This is my take on a classic 1987 circuit by V.Ageev, with modern parts and added output protection components. Practical output power from +/-40V rails: 75 WRMS into 8 Ohm (if the rails are stiff enough). The bootstrapped opamp has a gain of 3, and thus requires a full +/-12 Vpk input signal (another opamp).

• Output transistors must be of sustained-beta type. Although single transistor per rail is able to supply the rated power, paralled devices are preferred for lesser distortion at high currents. A pair of MT-100 devices or a single MT-200 Sanken is a reasonable minimum.
• Output transistors and diamond core transistors must be mounted on TWO different, thermally uncoupled heatsinks. The signal current through the diamond core equals the base current of the output transistors (plus some idle currents, see below) - so the dissipation in the core at full power is not negligible at all. Ideally, T101 and T102 must be mounted directly on top of T103+104 and T105+106, respectively.
• Diamond core transistors shown in the schematic have Vce rating of 80V. At higher rail voltages, use MJE340+MJE350 and adjust R115, R117 for optimal bias. Note that the beta of MJE340+MJE350 (ca.70) is much less than that of BD139+BD140 (200+) and standing currents through the input transistors of the core might need an increase.
• According to Douglas Self, the optimal bias point for the Sziklai output stage corresponds to 2 mV across each of R127, R128. The idle current of around 12..14mA flowing through these resistors is split in two roughly equal parts - 1/2 flowing through the output transistors, and 1/2 through the diamond core. The latter stabilizes at around 5..6 mA, causing 500..600mV drops across each of R121, R122; these bias the output transistors so that all excess current flows through output transistors.
• Diamond core transistors T103+T104, T105+T106 are paired to obtain desired current replication ratio. In my setup, this also means R116, R117 had to be shorted. Alternatively, T104 and T106 can be omitted, but then R116, R117 must be set at around 2...3 Ohm (trial and error).
• PNP and NPN transistors of the diamond core (red block) must come from same batches, and screened for matching Vbe.
• The JFET+R107 is a 3mA current source that shifts the opamp into class A operation. Whether it's a worthwhile addition or not - only your ears will tell. I have no precision tools to verify its effects (if any).
• See books by Douglas Self and Bob Cordell for the discussion of output protection components (L1, C109+C129, surge diodes) and feeback-over-the-relay arrangement. Nothing new here.
• The two resistors in the lower leg of the NFB network (R103, R104) are there for the planned implantation of wire-impedance-compensation feedback. R103 would remain connected to the clean ground, R104 would connect to a virtual-ground node of the added feedback opamp. Their combined resistance-to-ground will remain at 2 kOhm.