Need help with Bias

croco64

Member
2015-12-29 5:14 pm
Hi everyone,


I have a Phillips PA5110B stereo amplifier that I'm trying to set the bias.
I've looked everywhere on internet, also called Phillips for the service manual but no luck.


So far, I've taken the measurements over the emitter resistor on both channels (left it ON for about 30mins, with CD selector on, volume pot at minimum, no load, and all the other functions off)
Readings over 1st and 2nd resistor were only about 4mV ( resistors values are .27 ohm) so that gives me only 14.8mA. When I'm turning the trim pots all the way up my multi-meter reads 6mV over each resistor.
Also DC offset on L channel 17mV and on R channel 21mV.


Bellow are the pictures with the points that I'm taking measurements from and also trim pots for adjustments.


Can someone please clear things up for me:
#1)am I taking measurements over the correct resistors?if so, am I using the correct pins?
#2)if I'm taking the right measurements then why am I getting only a max of 6mV !?
#3)what should be the bias settings?



Note: A while back the amp started to sound funny, like after 1/2 hr or so of playing(when would warm up) the amp would get fatigue, so I have replaced the two filter caps(63v 10000uf) and the problem disappeared but now I don't have the powerful rich sound anymore so I thought it might be the bias setting.

Thank you!
 

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I design my own amplifiers and had no guide to set the bias current.
So the sensible thing seemed to be was to use a signal generator and scope and increase bias until crossover distortion goes with a speaker connected.
I found I was getting around 10mA bias current when crossover distortion went so I would suggest your amp isn't far out where it is.

Some people seem hell bent on massive bias currents, in mine and Peaveys opinion its not needed. More current equals more heat.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
The DC offset is fine.

There is no way to know the recommended bias current... if we knew the circuit topology we could make an educated theoretical guess... but it would probably be far removed from the recommended value.

As Nigel says, you'll find crossover distortion audibly vanishes at very low bias currents, even a milliamp or two is enough for that.

To get a true reading make the measurement without any speakers connected. The small DC offset skews the result.