need help setting bias current...

[SkinnyBoy]

apparently...

I have a car amplifier that I recently acquired...

at one stage in its life, the headunit to which it was connected committed a murder suicide, and applied 12volts to the amplifiers RCA input...

this fried the preamps, and input circuits, etc, but the previous owner replaced these...

now, for some reason, the bias current wasn't correct (I'm stupid when it comes to amps) so the guy rigged up a trimpot to each channel to adjust the bias, but the question is HOW do I set it? and, whats it supposed to be??

ANY help would be GREAT!!!

thanx 🙂

 

[Frank Berry]

Why do you need to adjust the bias?
Are you sure that the idling current isn't correct?
You have not provided enough information.

 

[SkinnyBoy]

thats all I know... thats all I was told when I got the amp...

I'm not sure why the original components wouldn't have yielded the correct bias current...

but like I said, one each channel, one resistor has been replaced with a trimpot and 2 resistors... thats if it was even the bias current.. but I believe it was..

 

[SkinnyBoy]

http://img87.echo.cx/img87/8601/amplifier6ny.jpg

he said if you set the bias too high, the amp would be running in class B all the time.... is that correct?

 

[SkinnyBoy]

excuse me while I cry...

no help? I wanted to use this amp to run my frone speakers, till I could afford something better, but I didn't want to use it like this...

 

[rpapps]

Hi SkinnyBoy

From the picture, it appears that only the centre leg of the trimpot is connected to anything or have I missed something?Cheers
Rob

 

[SkinnyBoy]

umm... when the turn the trimpot, the resistance across those 2 legs varies...

 

[pinkmouse]

Before anyone can help, you need to trace out he schematic of the anp, so we know what it is doing. Do you have access to a scope?

 

[SkinnyBoy]

Before anyone can help, you need to trace out he schematic of the anp, so we know what it is doing. Do you have access to a scope?

my cro stopped crowing...

how much of the circuit do you need? 😛 I guess the whole amp of one side... it doesn't look toooo complicated 😛 lol

 

[SkinnyBoy]

meh, its a crappy amp anyway... I'll leave it like it is 😀

 

[cunningham]

http://img87.echo.cx/img87/8601/amplifier6ny.jpg

he said if you set the bias too high, the amp would be running in class B all the time.... is that correct?

If the output arrangement is a complimentary emitter follower circuit, the bias servo transistor C & E pins pull the two bases together, turning them off, by saturating the servo transistor. The base of the servo transistor is biased by a voltage divider, sometimes with a pot from the base to emitter on the servo to adjust the bias. Saturating the servo turns off the output bias, making it class B, and lots of crossover distortion. The idea is that when the outputs get hot, Vbe turn-on will decrease. The outputs heat the servo, moving it closer to saturation and lowering the Vbe bias on the outputs maintaining a temperature coefficient of about 1 so the output bias isn't temperature dependent. Turning off the servo turns up the output bias.

Too much bias will burn the outputs up because the DC load line of the complementary emitter follower circuit is almost strait vertical. (graph Vce vs Ic) IOW when the bias is turned up just a little too much above cutt-off, lots of current flows. You want them both on but just barely. This makes the operation class AB. The AC load line is across the load impeadence instead of a "short" as with the DC load line. Measure the voltage drop across the output's emitter resistor and by Ohms law, you can see what the bias current is and set it.

Of course not all SS amps are complementary emitter followers....


Chris