DX Blame Group Buy

[AndrewT]

The output bias and the VAS bias and the LTP bias can all change with changes in supply voltage.

If one tries to set up the output bias via a rail dropping protective resistor, then one must recheck the biases of all stages after removing the protective resistor.
If necessary one must properly bias the stages with the normal Rail voltage applied.

BTW,
some amplifiers go way out of acceptable bias current tolerance when the mains voltage changes.

[Rudi_Ratlos]

John,

do it as Andrew said.

Remove the resistors across the fuseholders and insert the fuses.
Since you know that your AMP is running, you do not need this resistor any more and you avoid the voltage that drops over this resistor.

Do not connect a speaker to the AMP; short the INPUT (INPUT + = INPUT -).

Switch on the PSU and measure the voltage that drops across the emitter-resistors (0R22).
Depending on the type of output-transistors that you use and the size of your heatsink, adjust the quiescent current so that about 60 - 70mA flows through an emitter resistor, resulting in a voltage drop of about 13 - 15mV over an emitter resistor.
The huge MJL4xxxx transistors can (should) be adjusted to nearly 70mA, a smaller transistor (f.e. NJW0xxx) to 60mA, but this is only my listening impression.


Best regards - Rudi