A/C Current testing

[konabass]

I think given a situation where I had multiple circuits, like in the past, at least I can pick the "healthiest" circuit for the larger wattage amps. In a situation where I was limited to just one outlet/circuit, and it was a must-play, last minute scenario, I would use a lower wattage setup...or refuse to play citing possible equipment damage. So far, in the last year or two I have had instances where the house was a fair distance from the power company's cable (transformer) AND we had to lay some 50' and 100'extensions. In those cases, I noted a significant drop in power and a horrible lack of tone. These were the reason for this post. I don't want this to occur again. Now, I am almost always pre-checking the venue about a week (if possible) before and pre-judging the current needs. For you guys who gig more often this would seem arduous or impossible. For me, it's survival of the equipment, one, and tone/volume, two, that drive me to do this type of testing. Being vigilant/cautious can't be a bad thing. Oh, and then there's those generator gigs!!!!!!

 

[Bob Lee (QSC)]

AmpegInsider, your test results are different from anything I've ever experienced. Maybe that particular amp does something that sucks power when it clips? I don't know. Were you measuring actual RMS current? Most amps present an overall resistive load (albeit not wholly resistive, and not linear, either) to the AC line and do not compensate for low mains voltage by drawing more current.

The most efficient operating point for an amplifier is right at maximum power--right at the threshold of clipping. That's where (Power out)/(AC Power in) is at its highest. That's fine for peaks, but in music, most of the time the signal level is well below max, so the vast majority of the time, the amp is operating at very low efficiency.

That's why design engineers came up with amp concepts like class G and class H, in which the amp has essentially two or more maximum power points, shifting automatically between the lower and higher powers automatically.

Lowering the mains voltage makes the supply rails sag in about the same proportion, but it doesn't change the amp's gain in its region of linear operation. This lowers headroom but brings the maximum power point closer to the signal's average power level. The ratio of (Power out)/(AC power in) increases, so the efficiency of the amp itself goes up. This may sound like a benefit, but losing headroom is overall a bad thing. The efficiency of the overall system has not improved, because when the mains voltage drops, it's because more power than normal is being lost in the wiring and distribution.

To the original question, I would meter the AC outlet without a load. If it's not up around 120V, ±5%, to begin with, you're already starting at a disadvantage. You can't fully test the AC outlet without a load, either, and as Nightbass pointed out, if you want to see how the outlet holds up with a 1000-watt load, you'll need something that will dissipate that much power, such as a lamp (like you've been using already}, a big resistor, etc.

If there is a relatively large amount of resistance in the mains wiring, the voltage will drop significantly under such a load. Check other outlets. Sometimes it's poor wiring to or a bad connection at a single outlet. This is potentially dangerous; whenever I find a faulty outlet, I tape over it to discourage its use and notify someone in charge of the venue that they should get it fixed.

Another tip: whenever you use an extension cord, it should be as short as possible and as large a wire gauge as practical. In addition to the 100-foot extension cords, I would suggest maybe one or two 25-foot cords.

 

[Jerrold Tiers]

Bob, I know what you are saying, it does seem counterintuitive. Power and current just should go down with line voltage.

I checked another different amp, a small competitor's PA amp we had handy. Unit has a conventional transformer power supply, and is a conventional linear amplifier.

Set for full power, measured, reduced voltage measured. Visible clipping on either reduced voltage.

Here is the data

at 120V RMS line
24.6V Vrms out signal (not clipped)
1.79A RMS line current
174W draw indicated

At 110V RMS line
23.9 Vrms out signal (visibly clipped)
1.78A RMS line current
158W draw indicated

At 100V RMS line
22.5 Vrms output signal (more visibly clipped)
1.75A RMS line current
134 watts indicated draw

Equipment: Staco 15A variable transformer, Valhalla 2000 true RMS line monitor, Fluke 8010A , 1% 250W load resistors.
The Valhalla measures line voltage, RMS current, and true power (not VA). Line voltage is always visible, so it can be monitored while in either current or power mode, no surprises.

This particular unit didn't have an increase, but the line current decrease was clearly not even remotely in proportion to the amount of line voltage decrease.

 

[Bob Lee (QSC)]

Interesting data! Thanks for the details. Good setup, too.

I haven't done such a test with a sine wave dipping further and further into clipping, just pink noise and music from a CD source.

I'll have to try your test sometime, and maybe have a look at the current waveform. It would be interesting to see if the instantaneous current or the conduction angle changes, or both.

Thanks again!

 

[Petebass]

My head hurts now!

 

[konabass]

From Bob Lee:
Another tip: whenever you use an extension cord, it should be as short as possible and as large a wire gauge as practical. In addition to the 100-foot extension cords, I would suggest maybe one or two 25-foot cords.

Thanks Bob, funny you should mention shorter (25') extension cables--I was on that same thought while reading these. I've read Bob Lee's "column" here a lot lately. I think I'll have to subscribe.