[JimB]

I've built my 100n from the ground up. The testing process has yielded fairly acceptable results: Rails all at 50.1 volts, across R25/R26 was 6.2 volts, earth to speaker output before P1 adjustment was left channel -40mV and right channel -56mV. I then turned P1 clockwise to adjust bias and reached 55mV at test points 1 (56mV at test points 2). I then measured the voltage between speaker output and earth and came up with left channel of -65mV and right channel of -76mV. These voltages are outside of the 50mV limit mentioned in the instructions, and well outside of the preferable voltage of 15mV. My question is which components should I change, and by how much?

One more tidbit I can add is that I had to turn P1 almost to the end to achieve the 55mV at test point 1.

Any and all help will be greatly appreciated.

Thanks,

Jim

[AKSA]

Hi Jim,

In the instructions there are detailed directions on how to remedy this situation.  Have a gander, check it out!  To decrease the offset, you'll need to increase the value of R3 by either adding another resistor or removing the present one and inserting one of larger value.  There are spare resistors in the Nirvana kit for this.  This course of action is predicated on the offset at the speaker terminal being negative with respect to ground;  if it is positive, then the value of R3 will need to be reduced.  I'd say increasing it by around 3K3 would be about right.

The adjustment of the bias is easy too.  Replace the resistor right near the adjusting pot, R12B (150R from memory) with one with 50R less resistance, namely 100R.  This will pull up the adjustment to the half way point.

There are five semis controlling bias.  Any and all will influence this setting, and the design prevents overbiasing to thermonuclear levels during initial setup.  Bipolar output stages are noted for their tetchiness during biasing, and this configuration keeps you on the straight an narrow.

Hope this helps,

Cheers,

Hugh