parasound hca-3500 bias adjustment

Does anyone have experience with doing this? Can you point me to where I can find the output resitors and TVR 1 and 3

Special Bias Adjustment Procedures for the Parasound HCA-3500

The bias tracking circuit of the HCA-3500 requires being set with no signal applied and again with a signal
applied. When the HCA-3500 has an input signal connected, The bias rises to its high level setting.
1. Connect a DV voltmeter (in the millivolt range) across any of the emitter resistors.
2. With no signal applied, adjust TVR 1 until the bias level is 3 mV +/- 1 mV
3. Apply a 1 kHz sine wave to both inputs of the HCA-3500 Do not connect a load to the output.
4. Adjust TVR 3 until the bias level is 20 mV +/- 2 mV.
5. Confirm that the bias has remained at 20 mV +/- 2 mV.

This is picture I found online But I can take closer pictures of a particular side if needed.


  • SeleniumFalcon
    SeleniumFalcon Posts: 2,169
    edited December 2021
    According to the schematic the emitter resistors are 0.22 ohm 5 watt resistors (probably the white colored cermet type resistors R63 for example) connecting any of the output transistor's emitters to the output. TVR1 connects resistor R139 (470 ohm) to the base of Q30, TVR3 should be located close to transistor Q1's base.