Bias Voltage Resistors

[John C]

Has anyone had trouble getting correct bias voltage (about -33) using the Kelly '90 scheme? I had to move the 47k resistor to the other side of the bias pot to get a voltage high enough. It's now between the 20k pot and ground, which seems to make sense mathematically in terms of a voltage divider.

Anyone else experience this?

[Firestorm]

Really? Seems like that would make the problem worse. In the Kelly schematic, the wiper on the 25K pot is grounded, so nothing at the bottom of the pot is ever in the circuit. In fact, that end of the pot doesn't even really need to be grounded. As I see it, the way to get more negative voltage would be to increase the size of the 47K until you get into a range you can use.

[John C]

Umm... Maybe I'm missing something. The "Kelly '90" scheme I'm looking at does not have the wiper grounded. The scheme I see has one outer lug grounded, the other outer lug going to the two (220k) bias resistors, and the wiper connected to the 47k resistor.

[Firestorm]

Ahh, the David Hicks Kelly schematic. Take a look at Dana's Francesca schematic in the "Files" section. It was revised to reflect the actual wiring of an actual Express.

[John C]

Oh, wow, that answers it. The bias voltage is taken "above" the 47k resistor, just as I had "discovered". All this time I hadn't noticed that about the Franchesca...

Thanks