What is setting BIAS and DC offset ?
- Thread starter CavScout
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"Bias" is the idling current for the output transistors in a solid state amp or receiver. This current allows for proper conduction of the outputs at low volume. The settings vary from model and manufacturer. Some like Sansui measure actual current, while most measure voltage across a resistor on the emitter of an output transistor. Setting in millivolts can be 5 and above.
"DC Offset" or DC Balance is the ability to limit the amount of DC voltage at the output to the speaker. The optimal setting is "0.00 volts"
"DC Offset" or DC Balance is the ability to limit the amount of DC voltage at the output to the speaker. The optimal setting is "0.00 volts"
Can someone please explain this to me like a 5 year old without slang or use of acronyms?
Please and thanks.......I am a novice
I'll take on the BIAS part of your question.
Transistors, in general, can operate as either switches (on/off) or as amplifiers (more input gets you more output). In order to amplify, the transistor must first be switched ON. This takes approx. .6V. You don't want the first .6V of your precious music signal "wasted" just to turn the transistor ON, so you set up a biasing circuit that keeps the transistor ON at all times, waiting to amplify even the smallest music signal. The BIAS adjustment sets the voltage/current to turn the transistor ON (put it in amplifier mode) even when there is no music input signal present. You might think adjusting BIAS for the transistor to be just barely ON would be the best setting (least "wasted" power, less heat, longer transistor life, etc.), but the amplifying characteristics of the transistor are better with it a bit more ON than just barely. This property is called linearity and has to do with operating with a (near) constant amplification ratio (called gain) of the output signal to the input signal (i.e. x% increase/decrease in input gets you almost exactly x% increase/decrease in output). Transistor manufacturers publish technical datasheets showing the linearity of their products, which the amplifier designers use in determining the exact specification for their BIAS settings.
Last edited:
Scott75s1974
Well-Known Member
My understanding (for which I would welcome correction) is that on A/B amplifiers (which includes most classic amps) bias also sets the point where the two output transistors divide up their amplification of the positive and negative half of signal. The bias adjustment is a way of minimizing the distortion that this handoff point introduces into the signal. Is this correct?
On some amps, there is also a second adjustment potentiometer for setting DC offset. But on many, this can only be adjusted by replacing the differential transistor pair with two that are more closely matched for gain.
On some amps, there is also a second adjustment potentiometer for setting DC offset. But on many, this can only be adjusted by replacing the differential transistor pair with two that are more closely matched for gain.
Yes, that is correct.
Think of output stage as hot and cold faucets. And you want to be able to turn either one on very quickly. However, to turn them completely off, you need to turn them hard, and it is slower to turn them back on. And, when you don't want hot or cold water, you want it to be at room temperature. So you turn on each faucet so they are at a slow dribble, This is bias. And you adjust them so that the temperature is at room temp. That would be DC offset. You want the offset to be zero. You want a small amount of water going through each faucet so you can turn them on quicker.
To extend the analogy further, Suppose your faucets, if they got to cold or to warm, would start to let more water through. This is thermal runaway. To compensate this, there is a valve which reduces flow in the same amount as it would increase. This is called the bias transistor. Anything beyond this is too advanced for 5 year olds ….
