Modified Sine Wave Inverter and Audio Noise

Recently, I was busy trying to create a screen capture video for a client with new software.  Thought I had everything worked out when I did my final capture.  Big surprise this time, as there was a very loud hum, hum, hum, in the background of the audio I had just captured.   What was the problem?  Earlier versions were fine.   As it turns out, my modified sine wave inverter was to blame.  I’ll explain where the noise comes from and some solutions I have for defeating the hum in my recordings.

Where does the noise come from?

The modified sine wave inverter is very common for off grid power systems because there used to be a substantial savings over the true sine wave inverter.   What the MSW does, is it approximates a sine wave by changing its output voltage in a series of discrete stair steps.  This is generally a useful and workable setup for most items you might want to run with two exceptions:  1) certain audio equipment,  2) low frequency Radio reception (AM, Amateur bands).

Two kinds of noise are created from the stair step waveform.  In real life, the vertical part of the steps is not truely vertical, but has a slight slope to it.  It is this steep slope that creates the noise.   The noise finds its way to your ear in an indirect fashion, coming through your radio, or recording equipment.

Where You hear it

Using a MSW inverter for over ten years, we can list the places we hear the noise.   It rarely causes any problem, but it can be heard.  Here are the common sources:

• The inverter itself will have a low 60Hz hum in its normal operation for battery mode.  When the inverter/charger is charging,  there is no noise.
• ceiling fan.  When we run it on inverter power ,  it works fine, but makes a mild hum.
• Stereo equipment.  This can vary based on the equipment’s power supply design.   Normally its nothing more than a mild hum in the speakers on FM.  AM is very difficult to use.
• Amplified PC speakers.  Again, its mild but there, the hum.   You only hear it when you listen for it when there is no music or sound coming out the speakers.  There can be some variation on amplified speakers, as I got one par that was particularly bad (save your receipts!)
• Recording equipment.  Can be very bad if you don’t take precautions.  This is covered below.

My recording suggestions

The real problem with MSW inverter noise comes when you want to record something with a microphone.  If that recording device,  a PC or tape deck, or whatever,  receives its power the inverter, there will be hum.   The problem is that microphones work with very small electrical signals that are amplified greatly.  The noise jumps in the recording during this amplification.

Here are some suggestions for your off grid recording sessions:

• Use battery powered recording devices.  When I unplugged my netbook from the power and did the recording it worked much better.
• Record while on generator power.  Your generator will produce something much closer to true sine waves and will not add this source of hum to your recordings.
• Purchase a small, true sine wave inverter to power all the audio equipment used in the recording process.
• BONUS tip:  Bluetooth.  I found that I could always do simple, quality recordings on MSW power when I used my bluetooth phone adapter with my netbook.  It turns out that these devices digitize the audio before sending it wirelessly to your cell phone, or PC in my case.

What kind of hum do you have in your life?  How do you adapt or eliminate it?  Tell us in the comments below!

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