Bias adjusting idle current class AB amp?

[Bobby Baird]

Hello, please help me understand how to bias this amp through the emitter resistors. I had to replace all output transistors which I matched based on hFE & Vbe as one gave up the ghost, also I replaced a few .47 ohm, 100 ohm emitter resistors and a few 4.7 ohm collector resistors . The amp I have is a Cerwin-Vega CV-1800 which uses the same board as the Samson SX-1800 which I found the schematic for posted here. My understanding is at idle the 2SC5200 2SA1943 pair should conduct some current even when no sound is present. Thus eliminating crossover distortion. I do not own a distortion analyzer. I see the bias adjustment pot is VR101 in the schematic. Would I use my volt meter across say R140 & R158 emitters to check mV current flow? As I only repaired one channel. I could use other channel to check it's bias current.

 

[JohnRoberts]

Class AB amplifiers typically want to see around 25mA of class A bias in the outputs.

Measure the voltage drop across one of the emitter degeneration resistors (carefully) until it indicates 25 mA, around 8 mV DC.

JR

 

[Bobby Baird]

Well I'm getting 5.5 to 5.7 mV across the .47 ohm resistors on both channels. That would imply the amp is biased cold? So, about 12mA of current flowing through the transistors. I'm curious to inject sine wave and view on a scope for crossover distortion. Would I need to hook up a dummy load 8 or 4 ohm resistor on the output to hook up my scope? Thanks.

 

[JohnRoberts]

Well I'm getting 5.5 to 5.7 mV across the .47 ohm resistors on both channels. That would imply the amp is biased cold? So, about 12mA of current flowing through the transistors. I'm curious to inject sine wave and view on a scope for crossover distortion. Would I need to hook up a dummy load 8 or 4 ohm resistor on the output to hook up my scope? Thanks.

12 mA is not bad to start... I would begin testing by feeding in a low level sine wave, maybe 1V p-p at 10 kHz, with or without a dummy load and look for crossover distortion. It takes time for the pull up and pull down devices to trade off. It is most obvious at low level, high frequency.

If it's under biased you will see a difference from increasing the class A bias but don't over do it, too much class A bias can generate a bunch of heat.

JR