Merren Audio

Sometimes I encounter situations where clients are apparently not getting expected voltages in thier amplifiers or test equipment.

This is primarily with respect to DC voltages that have been rectified and filtered..

Just about every case involves the quality of the power in the MAINS outlet.

In the US the mains are standardized at 120V with a +/- variation of 5% ...

This would be a range of 114 to 126 volts AC....

 The main issue is the Emissions in the mains voltage... Harmonics in the mains are not very well regulated nor enforced in the US as they are in parts of Europe.

In the US the harmonic limits for mains power are defined in document IEEE 519-1992

Photo below is  AC mains measurement in Bergenfield, NJ 07621

Very High Harmonic content....primarily 3rd harmonic off the charts...

Notice the clipping of the SINE wave !!

Notice the RMS value is 120V but the real dilemma is the Peak-to-Peak measurement is at 324V instead of 339V where it should be at.

Rectifier/capacitive filter circuits will now have altered conduction angle and reduced charging currents, thus a difference in the expected DC output voltage.

So in this case the final DC voltage was showing up about 20 Volts short of the expected DC voltage of 450V DC in a guitar amp but the 6.3 V AC Heaters and 5V filament windings are showing up correct with TRUE RMS meter.

Another problem is when a TUBE TESTER is being used... The LINE TEST for calibration is showing up correctly and the HEATER voltage is showing up correctly but the DC voltages sent to the tube are low and all readings for the tube under test are erroneous !!! 

Older model curve tracers such as the Tektronix 570 and 575 rely on the mains peak voltage for Plate and Collector sweep voltage...while no harm is done to tracer ciruit, the peak sweep voltage will fall short of what is expressed on the dials

We did notify the power company QC engineering Dept and expressed our concerns and they did look at this issue, but they could not be bothered... still not fixed..

 Below is a more typical 60Hz SINE waveform from the mains outlet.

120V RMS at 339V AC Peak-to-Peak is more reasonable.

Location is 90245

The Quality of your Mains power is critical in the performance of your equipment. Further reading would include: Power Quality, Electrical Mains Harmonics, IEEE 519-1992, Power Factor Correction.