File:As-built diamond-core audio power amp (v3) complete with PSU.png

English: The (almost) complete audio power amp built around the diamond core - a development of a 1987 design by V. Ageyev with modern components. The size and power were dictated by the parts which I already had lying around - most importantly, power transformers and heat sinks. As built, the amp clips at 60..75 W into 8 Ohms depending on line voltage. Overall gain, excluding effects of wire compensation feedback - 30x (10x in first opamp stage, 3x in second opamp stage). Overall idle power consumption, including losses in transformer, rectifiers and protection circuit - 18..22 W (and up to 25..27W if R24, R25 are increased to 47 Ohms, see below).
Idle operating currents are 40mA through T1, 40mA through T2, and around same 40 mA through the output quad (with 10-Ohm R24, R25; higher R24, R25 values increase current). At high output currents, currents through T1, T2 increase up to threefold due to current-sensing feedback. Transistor triplets T1+T3.1+T3.2 and T2+T4.1+T4.2 must be matched by Vbe (ideally, at operating I and U to no more than 5 mV spread), and placed closely together on a common heatsink. T5, T6 also need heatsinking (same as T1-T4 or different). As shown, there is no provision for cancelling voltage shift caused by bias current through R09. This causes around -30mV negative offset at the output. Later I added a passive fix to trim the offset to around 2 mV max.
HF stability: the second-stage opamp is sensitive to component placement and stray capacitances. Taming oscillation without sacrificing loop gain at treble is quite tricky. I tested some configurations for NE5534 and LME49710 - and the optimal adjustments were quite different. The simplest crudest option is to add shunt capacitor (33pf to 100pF) immediately between the output and the inverting input of IC2. Increasing base resistors R24, R25 is even better (won't compromise HF gain), but it also increases output stage idle current (the DC voltage drop across R24, R25 is not insignificant) - 30 Ohms seems like a reasonable maximum.
Not shown: 0.47 uF film capacitors right at the power pins of each opamp IC. Bleeder resistors across PSU capacitors.
Protection as in File:UPC1237 speaker protection 3. Add delayed muting relay.png. Overload pin is normally shorted to ground with a normally-closed thermal fuse. Both relays shown in ON (energized) position.