Ajusting rail voltage on amps.


Gold Member
Username: Van_man

Boston South, MA

Post Number: 2300
Registered: Mar-06
I posted this question before, but never got an anwser, so here it is again.

On A Zapco Z100 C2 Compotition amp, there is a left and right bias adjustment.


According to Zapco
"Turn the bias controls fully counterclockwise. No input & no speakers attached. You need a current meter connected between the main power input lead and the amp, apply power and turn unit on. Take a reading from the current meter - should be around an amp of current. Adjust the left bias control slowly, watch the current meter, add 100ma or .1Amp to the original reading. The adjust the right channel bias control, add another 100ma or .1Amp to the reading after adjusting left channel. That should be it.
You cannot adjust bias without the outputs, etc mounted to the heatsink as the transistors heat quickly. When that happens, the bias goes up also - cascades into a shorting condition blowing the outputs and possibly the power supply Fets.

There is a more accurate way of doing this, but it involves speaker loads, oscilloscope, signal generator, heatup times, gets involved.

Mike Boronowsky
Service Manager
Arpa of America
209-577-4268 ext 119"

So what do they do if you turn them up? I have 2 amps, 1 amp had them turned about 3/4 up, the other is about all the way CClockwise down. If these amps were used in comps, what gain would they get by increasing rail voltage? Whould you need higher voltage in order to adjust these? like 17 volt systems?

I plan on adjusting them like Zapco suggests, but am courious as to why they would have that adjustment, and what is it used for.

Gold Member
Username: Nd4spd18

Southeast PA

Post Number: 1136
Registered: Jul-06
My US Amps AX 300TU has those adjustments, but I ain't gonna touch them.

Gold Member
Username: Hockeyknight

Post Number: 1018
Registered: Apr-06
bump i wanna learn too

Platinum Member
Username: Glasswolf

Wisteria, Lane USA

Post Number: 11728
Registered: Dec-03
the bias adjustment just balances the left and right channel input stage of the amplifier.
Don't turn them up beyond the recommended 100mA range, or you'll overdrive the output stage of the amplifier, ad send the amp into clipping, causing audible distortion, and damage to your speakers.

This feature is there to allow you to balance the left and right channels for equal output.

Gold Member
Username: Van_man

Boston South, MA

Post Number: 2311
Registered: Mar-06
So like an internal l/r balance control to tweek imaging problems.
Thanks. Great info, as always.

Platinum Member
Username: Glasswolf

Wisteria, Lane USA

Post Number: 11741
Registered: Dec-03
well, yes and no.. you really want L and R to be equal, and matched to the input voltage (using your input sensitivity adjustment/gain) of the head unit line outs.
but yes, it's just to tweak the input voltage levels.
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