Download Article Download Article DC offset (also known as DC bias) is a concept employed when dealing with electrical components, most notably audio equipment. These components send either power or audio signals using alternating current (AC), by which the signal reverses direction regularly. If the signal does not spend an equal amount of time traveling in both directions, however, the resulting imbalance is referred to as DC offset. A high level of DC offset can damage electrical components and degrade signal quality. The guide below will teach you how to measure DC offset using perhaps the classic application — a stereo receiver or amplifier.

Steps

    1. Image titled Measure DC Offset Step 1 1 Purchase a multimeter. A multimeter is a handheld device for measuring electrical current, resistance, or voltage. Multimeters are available in either digital or analog models, and can sport a variety of different features. The important consideration when choosing a meter to measure DC offset is the lowest available voltage range.[1]
      • Because DC offset readings will be quite small (usually under 100 mV), you need a meter with a small range (sensitivity) so that the readings will register correctly. A meter with a 200 or 400 mV range is ideal, but a range of 2 V will also work.
    2. Image titled Measure DC Offset Step 2 2 Prepare the amplifier to be measured. To achieve an accurate DC offset reading, you need to follow a few set-up steps.
      • Begin by disconnecting the speakers. As an alternative, you can take the readings from the «B» or «remote» terminals and set the speaker control switch accordingly.
      • Set the amplifier’s input selector switch to an unused position such as «Aux.» Do not set it to «Phono.»
      • Set the volume dial to minimum, the tone dials in the middle position, and the balance dial in the center.
      • Turn on the amplifier and let it sit for at least 10 minutes before taking a reading.

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    3. Image titled Measure DC Offset Step 3 3 Set the multimeter to the appropriate mode. The meter should be set to measure DC voltage (not current). Set the range of the meter to its smallest setting (200 mV is ideal); if you have an auto-ranging meter you won’t need to perform this step. Make sure the test leads are plugged into the jacks used for measuring voltage.[2]
    4. Image titled Measure DC Offset Step 4 4 Touch the multimeter’s test leads to the amplifier’s speaker terminals. To measure the amp’s DC offset, begin by touching the black test lead to the negative speaker terminal. Next, touch the red test lead to the positive speaker terminal. Hold both leads in place while looking at the reading on the multimeter’s face. After taking the reading, reconnect the speakers and set the controls back in their desired positions.[3]
    5. Image titled Measure DC Offset Step 5 5 Analyze the result of your reading. The ideal amount of DC offset is 0, which results from a perfectly balanced AC sine wave. In reality, an offset between 0 and 20 mV is very good. An offset between 20 and 50 mV is slightly higher than the ideal range, but the distortion caused will barely be audible. A range from 50 to 100 mV will begin to cause noticeable degradation of the audio quality, while an offset over 100 mV may cause permanent damage to your speakers.[4]

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  • QuestionWhen using tube amplifiers, it is recommended not to run the amplifier without a load (no speakers). Is there another method for checking if there is DC pollution in my AC power supply?Community Answer Since almost all tube type amps have transformers at the output, and transformers do NOT pass DC, there is none to measure. Do not bother with this test on tube type amplifiers.

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  • Adjusting DC offset is usually accomplished through replacing capacitors in the electrical component; this task should be left to a professional.As a small thank you, we’d like to offer you a $30 gift card (valid at GoNift.com). Use it to try out great new products and services nationwide without paying full price—wine, food delivery, clothing and more. Enjoy!

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  • Before using a multimeter, read the owner’s manual in its entirety. Improper use can lead to electric shock or damaged equipment.As a small thank you, we’d like to offer you a $30 gift card (valid at GoNift.com). Use it to try out great new products and services nationwide without paying full price—wine, food delivery, clothing and more. Enjoy!

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Things You’ll Need

  • Multimeter
  • Amplifier

References

About This Article

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\$\begingroup\$ I got a bit confused about how AC voltmeters on digital multi-meters work. I don’t want to know about for example sampling method or conversion of analog to digital data. I need to know whether it can only measure the sinusoidal wave with only zero offsets. I tried to compare the AC voltmeter and oscilloscope responses together in one of the electronic simulators programs (Proteus.) I know about the difference between the Vrms and Vpeak. When the AC signal has the DC offset the AC voltmeter shows an unacceptable value. For example, by using the signal generator, I applied a sine wave by Vp-p= 12 V and DC offset = 0 as you can see in the following picture. It shows the correct answer. Vp-p/2 = 6 V => Vrms= 6/sqrt(2) = 4.24 V Example01 Now I’m going to add the DC offset to my signal by changing the signal (from «bi to uni» in signal generator settings.) If you check the image you will see that the amplitude of the signal and even the frequency has remained constant while the AC voltmeter shows the value of 7.34 V. How does the voltmeter calculate this value? Example02 Shouldn’t the AC voltmeter remove the DC level of the signal automatically and only calculate the differential voltage between the maximum and minimum voltage? JRE's user avatar JRE 62k8 gold badges97 silver badges164 bronze badges asked Dec 12, 2021 at 21:34 Mohammad Reza Hasanpour's user avatar \$\endgroup\$ 3 \$\begingroup\$ Your meter is actually doing what it’s supposed to. It’s measuring the RMS value of its input. For the first signal, with no DC offset, the RMS value is for a sinewave of 12 VP-P which is 4.24 V. With a DC offset the RMS value is given by \$ \sqrt{ AC_{RMS}^2 + DC^2} \$ which is \$ \sqrt{4.24^2 + 6^2} \$ which is 7.34 V. If you don’t want the DC offset to be included, add a series capacitor as already suggested. Many meters (especially analog such as the Simpson 260 and Triplett 630) include an internal capacitor for this purpose. This mode is often called AC output as it was intended for measuring the output voltage of an AC amplifier without the DC bias voltage. Transistor's user avatar Transistor 164k12 gold badges181 silver badges373 bronze badges answered Dec 12, 2021 at 23:19 Barry's user avatar BarryBarry 15.3k1 gold badge25 silver badges27 bronze badges \$\endgroup\$ 1 \$\begingroup\$ Some general talk only, no comments about the meters in your simulation program. Most multimeters filter out DC offset and show in AC range only the AC component assuming the offset doesn’t lift the peak voltage too high. If you have say 100mV AC with 20V DC offset you must insert a highpass filter to remove the offset — that’s said already in a comment which says «use series capacitor». DC voltage ranges filter out the AC and show the DC-component (=the average of the voltage) Measured AC voltage in multimeters is most often actually the average of rectified AC-component, only scaled to the right RMS value by assuming the voltage is sinusoidal. True RMS meters exist, but an oscilloscope gives so much more info of electronic circuits than a multimeter that having one is a superior advantage over the situation where one must guess the waveform. The voltage math and finer signal analyze modes in modern oscilloscopes are a big bonus. Unfortunately the situation is that if one wants a good and low cost instrument he must buy both of them. That makes an oscilloscope unaffordable for many of us. answered Dec 12, 2021 at 22:12 \$\endgroup\$ 1


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