Parasound HCA-800ii

The HCA-800ii was manufactured sometime in the 90s. Her rated output is 100W into 8Ω and 150W into 4Ω. Over the years, she has gained a reputation for one of the better sounding amplifiers. Let’s take a closer look at her circuit.

Parasound HCA-800iiFig 1 – Parasound HCA- 800ii Amplifier Schematic

Fig 1 is the HCA-800ii schematic. She employs a double differential typology in that her input is a differential (Red line) as well as her VAS. No Constant Current Source is use in the input stage. Instead a regulator is installed in the positive supply rail and tail current is determined by a 33K resistor. This translates to 0.66mA, meaning 0.33mA per leg in the differential. It appears the collector load is 3.3kΩ leading to a 1V at the base of the VAS differentials. Less one diode drop of 0.6V, we have 0.4/150 = 2.6mA, which means 1.3mA per leg. This is awfully lean for a VAS.

After this voltage stage is the driver, a 2SC2168 driving the Sanken 2SC 3519 output transistor. Negative feedback (Pink line) is via a 56K resistor with 1K5 and a 220uF capacitor to ground. That works out to roughly 37dB.

Summary

Although the Parasound HCA-800ii circuit is a proven one, her biasing particularly in the VAS, is questionable. Perhaps it is due to using 2SC2705 which are 800mW transistors. Nonetheless, she is a successful amplifier in the business sense in that she managed to carve herself a place in a very competitive HiFi market.