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Old 2nd April 2004, 11:16 PM   #1
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Join Date: Nov 2003
Location: flyover country
Default EE-Zee & Chee-Zee bias and offset adjustment idea

I think I just hit on a kewl plan to tweak up the bias and offset on my 6 channel hybrid HT amplifier project without having to take the cover off or use any equipment but a screwdriver.

I posted a photo of the prototype chassis which has a big chunk of heatsink for each channel module. The first part of the idea was to drill and tap holes through the extrusion bases so that I could screw slotted shaft miniature pots directly into the heatsink itself and have them accessible through the fins with a small screwdriver for adjustment but safe from wandering fingers. Of course a slight downside is that the pot will be subject to being at whatever temperature the heatsink is at - this might cause me to mount the pot on a bracket or pcb instead, but that's a detail.

Secondly, how to get feedback without a voltmeter? Well, it occurred to me that a dual opamp configured with one half as an active ideal fullwave rectifier driving the other half set up as an active current source would be helpful. Just hook up a LED (they have brightness roughly proportional to forward current), and, voila, an instant visual feedback circuit where zero input voltage extinguishes the LED. That makes offset trimming (with no input signal) a gimme, but bias trim takes just a little more circuitry. My idea is to sum the voltage across the source resistor(s) of the channel's output stage with a weighted fixed voltage in the opposite direction from a simple forward biased diode conducting a couple milliamps to the output which makes it simple to set up the circuit to sum to zero volts at the desired bias current range. Add a selector switch to choose the channel you want to adjust; you probably can see where this is headed.
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