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Voltage Monitoring Discrepancy

MetalDaze

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Hello all,

I am in the U.S. & have a 2.2 / 5.2 home theater set up, running off a single 20amp circuit. The equipment work flow is listed in my signature. I have a seriously heavy duty splitter connected to a short run of 12AWG extension cord from the outlet, which is mounted directly next to my electrical distribution panel (my set up is now in the 'man cave' of the house we moved into). This makes my equipment rack connection cleaner and more convenient. My A21+ occupies one slot on the splitter, connected to an Emotiva CMX-2 (which reduces the amperage from 20 to 15). But hasn't been a real world issue. The other slot is occupied by my Panamax MR5100, which also takes the available current down to 15amps if I'm not mistaken. All of my downstream components not listed above are connected to the MR5100 receptacles. I have another CMX-2 connected to one of the "high current" receptacles on the Panamax which has my XPA-4 Gen3 on it for the surrounds and center.

On another high current receptacle I have my APC UPS 1100 for my PC, Xbox, TV etc in the event of power loss. Neither the Panamax or the APC UPS regulate voltage. They only display the voltage on the front screen.

As the title states, consistently the APC UPS which is plugged into the Panamax displays a voltage of 2-3 volts higher than what the Panamax display says. So if the Panamax "Automatic Voltage Monitoring" shows 115 volts, the UPS will display 117 or sometimes 118 volts.

To get a fair idea of what my available line voltage is at any given time, I usually average the two. I dislike using the system during times of heavy utility demand, or the like, ergo mid summer during the days when the voltage sag becomes noticeable. If my average available voltage is less than 115 volts I opt to not turn on the big amps, and just use the pass through audio to my TV with my receiver in standby, or switch to my headphones.

Outside the summer months, this isn't much to pay attention to. My normal incoming voltage from the utility company is anywhere between 118 - 123 volts (the latter usually seen very late in the evening when power demand is minimal)

My only completely baseless guess as to why this occurs is that the Panamax is displaying the line voltage available after all else has been distributed to the utilized receptacles, within the unit itself. I've done some internet sleuthing but came up with very little when trying to research a possible answer.

As another bit of background I've had the system set up this way, through multiple dwellings. And the differences in voltage monitoring has always been there, at the same magnitude.

This has been a bit of a head scratcher for me, any one else experience something similar or have any ideas?
 

DVDdoug

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The current (Amperage) depends on the load - Ohm's Law says Current = Voltage/Resistance. We usually don't know the resistance of an AC load (but we can calculate it if we want) and with something like an audio amplifier it can very moment-to-moment depending on how much power it's putting-out.

Power is calculated as Watts = Voltage X Current.

If nothing is plugged-in the voltage is there but no current flows. In the U.S (120V "nominal") A 100W light bub takes a little less than 1A. A 1500W toaster is about 12.5A and if you plug-in two toasters you draw excess current, the circuit breaker blows and voltage drops to zero.

There is also a voltage drop though the wiring which is proportional to wire resistance and current (Ohm's Law again). So with a high current load it's likely that that the voltage at the outlet (at the "far end" of the wires) is lower than at the breaker box. With no load there is no voltage drop and with a light load it will be very small and maybe unmeasurable.

As the title states, consistently the APC UPS which is plugged into the Panamax displays a voltage of 2-3 volts higher than what the Panamax display says. So if the Panamax "Automatic Voltage Monitoring" shows 115 volts, the UPS will display 117 or sometimes 118 volts.
No measurement is perfect. I just checked the specs for my multimeter and on the AC range it says +/-1.5%, plus "3 digits". (On this range it has 0.01V resolution so 3 digits means a possibility of 30mV more error), If I had 2 of these meters, one could read a little more than 1.5V high and the other a little more than 1.5V low for a difference of more than 3V!

Almost all modern audio equipment has a regulated power supply that holds the internal DC voltage constant under "normal conditions". The voltage regular provides "line regulation" to hold the DC voltage constant when the AC varies and "load regulation" that holds the voltage constant when the load varies (i.e. depending on how much power the amplifier puts-out to the speakers). Most audio equipment is probably good down to 100V or so.

If you have an unregulated power supply, a small drop in power line voltage would mean the amplifier will clip at lower output power (so you couldn't get 100W out of a 100W amplifier) but otherwise it would be OK up to a point, where things just "stop working properly".

With a regulated power supply the voltage regular also helps to filter-out AC hum so you might start to hear hum if the voltage falls too low and the regulator "drops out of regulation".

There are some "universal" switch mode power supplies (and some LED light bulbs) rated for something like 90-260VAC so they work anywhere in the world. (It's easier to do that with a switch mode power supply than with a traditional "linear" power supply).
 
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Blumlein 88

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My first guess is one meter just reads higher than the other. Use a multi meter and check voltages. You otherwise might be chasing a ghost that is just a meter difference.
 
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MetalDaze

MetalDaze

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Power is calculated as Watts = Voltage X Current; There is also a voltage drop though the wiring; Ohm's Law says Current = Voltage/Resistance; In the U.S (120V "nominal") A 100W light bub takes a little less than 1A. A 1500W toaster is about 12.5A and if you plug-in two toasters you draw excess current, the circuit breaker blows and voltage drops to zero;

I guess I should have added I have a background which allows me to be familiar with some electrical engineering, am an avid audiophile (for better or worse), have a BS in science, am a licensed home inspector which requires a thorough understanding of a structure's electrical system (and many other areas), work for an engineering company etc, etc. So through self evaluation I believe my knowledge to be apt on all these things outlined above :)

No measurement is perfect; I just checked the specs for my multimeter and on the AC range

This is a great point which I failed to look into. How accurate are the pieces of equipment themselves. And my own meter to check them which Is rather inexpensive.

Almost all modern audio equipment has a regulated power supply; probably good down to 100V or so.

Probably the smaller pieces of equipment which use an SMPS like my DAC, or even the supply for the phono preamps are regulated. But of the larger gear in the rack, the only one I know which has a regulated power supply is the XPA amp, which uses a large SMPS in a class H output configuration. In those cases yes probably down to around 100 volts before the equipment would switch into protection mode, if it has one. Otherwise (if an amp) as you said it will clip at lower output, or rather have less headroom during demanding dynamic passages.

IME most modern equipment is rated around 110volts as minimum. However with an actual line voltage of say 115, just turning the A21+ on soaks up about a volt or two. It then stabilizes quickly after the soft start but idles around 160watts / 1.45amps. Then if I turn on the rest of the equipment in the rack and crank up some multichannel music, I'm riding the border of 112volts at the tap. Still probably fine but when I can have 120volts nominal +/- 2 volts I feel better about turning the volume up.

some "universal" so they work anywhere in the world. Easier to do that with a switch mode power supply than with a traditional "linear" power supply

This has always been my experience as well. If I see a regulated linear power supply, it is always rather large/heavy by design and consequently costs some coin.

one meter just reads higher than the other, Use a multi meter - otherwise might be chasing a ghost.

I guess this is what I will have to do. Confirm the accuracy of the gauge I'm using, and take readings myself. For science!
 

Doodski

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IME most modern equipment is rated around 110volts as minimum. However with an actual line voltage of say 115, just turning the A21+ on soaks up about a volt or two. It then stabilizes quickly after the soft start but idles around 160watts / 1.45amps. Then if I turn on the rest of the equipment in the rack and crank up some multichannel music, I'm riding the border of 112volts at the tap. Still probably fine but when I can have 120volts nominal +/- 2 volts I feel better about turning the volume up.
I monitored the AC mains voltage for one electronics service bench that I operated. The voltage varied up and down throughout the day as the commercial industrial area where I was located experienced differing loads on the AC mains power supply. The voltage varied down to ~105VAC and everything operated. Another example that I can offer is when servicing audio amplifiers with linear power supplies. In order to safely power the units ON after a circuitry rebuild I used a variac with an ammeter in series and wound up the voltage till the relays engaged and the sound came ON. I have seen amps output sound when the AC variac supply was as low as ~70VAC. So the gear can usually handle a fairly large variation in the AC mains. :D
 
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MC_RME

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My only completely baseless guess as to why this occurs is that the Panamax is displaying the line voltage available after all else has been distributed to the utilized receptacles, within the unit itself. I've done some internet sleuthing but came up with very little when trying to research a possible answer.
There are two easy explanations. One you mentioned already, check the accuracy of both displays by using a better multimeter. The other one could be that one unit is measuring RMS while the other one does not. With a perfect sine both would measure the same. But the AC waveform normally is not a real sine but shows capped peaks, then they will show different values. I have that situation (with other gear) and use the 'difference' to know how much the power grid is currently under load (the sine deformed).
 

restorer-john

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Your UPS is a line interactive UPS. I just had a look at the datasheet for your unit. They usually have a range of taps which they deliberately switch in order keep the output voltage within their own specified range. That is before they switch to the inverter derived output.

In other words, they regulate (coarsley in some cases, fine in others) the output voltage over a smallish range, not just provide battery generated modified sine when there is a power dropout/brownout

My UPS is also a line interactive. When I first got the unit, I opened it up, saw the relays and multitaps and proceeded to look for the thresholds of primary voltage required to trip a tap up or down. IIRC, mine had a tap either side (+/-) to allow for around +/-5-10V (on 220V) before going to inverter drive.

Voltages in a typical house vary significantly depending on the length (and gauge) or your service drop, the layout of your circuits and what is pulling what around the house. Press the toaster down and watch your line voltage. Do the same with the electric oven/range, heaters and or tumble dryers/air conditioners. Just for fun, you can do a bit of maths and work out the source impedance of your power supply. :)
 
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mansr

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As the title states, consistently the APC UPS which is plugged into the Panamax displays a voltage of 2-3 volts higher than what the Panamax display says. So if the Panamax "Automatic Voltage Monitoring" shows 115 volts, the UPS will display 117 or sometimes 118 volts.
My APC UPS always displays a few volts higher than actual. It's not a precision instrument.
 

Speedskater

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Most DMM don't actually read RMS voltage (they convert their reading to RMS). In fact only true RMS volt meters read RMS voltage. Also the AC power line has lots of harmonics (many generated by our power amplifiers), different meters may measure the harmonics differently.

A 20 Amp AC power circuit will deliver 200 Amps for a short period of time (and some circuits often do).
If a UPS or power conditioner can't deliver 5 to 10 times it's rated current for a fraction of a second, it will have problems with big power amplifiers.
 
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