View Full Version : Just bought a Realistic APM-500 power meter. How to use it?

ToTo Man
05-04-2006, 05:09 AM
I'm still waiting on it arriving but figured I should try and find out how to use it before it arrives :D. Seems pretty simple at first glance, - hook it upto set B on the amp and there you have it. But....

What's the impedence of the APM-500?
Is it safe to run it while running your speakers on set A, or do you crank your speakers to the level you want, turn set A off then turn set B on so that the speakers and the meter are never on at the same time?

Or am I worrying about nothing as usual? :scratch2: :D

05-04-2006, 07:39 AM
Great message header! I myself have several devices around that I don't know how to use.


05-04-2006, 09:03 AM
I have an APM-200 that I haven't used yet so I can't tell you much, but it has a 4/8 ohm speaker impedance switch on the back. I assume the APM-500 is similar.

05-04-2006, 09:49 AM
The meter itself has no effect on system impedance, and can be left running. The impedance selector on some of them is to ensure an accurate reading, as the wattage at a given output voltage is going to be different into 4 ohms or 8. The 500 IIRC had nice analog meters. I had one, forgot the model, that just had LED's. I enjoyed it for a while then sold it to Thor to add to his Shrine to the Holy Phenomenon of Bouncy Lights.. .:D

Todd in Beerbratistan

ToTo Man
05-13-2006, 10:09 AM
It has arrived, finally! Turns out the apm-500 has the red LEDs (not analog).
I'm still finding it a bit hard to get my head round how to use it, safely. :scratch2:

You say it has no effect on system impedance, so does that mean the meter has zero impedance or infinite impedance or what? If it has zero impedance, then that means I can't run it on its own as it will short circuit my amp, right?

The complication arises as I want to use the meter on my HT amp. The amp has discrete amplifiers for each channel, so playing set B along with set A does not make it parralel, they remain seperate. So, if I hook the meter upto set B, will I be ok?

ToTo Man
05-13-2006, 10:19 AM
Sorry, maybe 'Front Set A' and 'Front Set B' aren't discrete after all.

I just checked my HT amp's manual and it quotes the following specs:

8ohm load:
Front L&R = 105wpc
Surround L&R = 105wpc
Center = 105wpc
Surround Back = 105wpc

It does not differentiate between 'Front Set A' and 'Front Set B', so it probably follows the same parralel format as vintage stereo amps.

05-13-2006, 12:50 PM
YOu can parallel it with the A speakers if you like, since that's what you want to measure. The meter itself does not load the amplifier.

Todd in Beerbratistan

05-14-2006, 06:46 AM
The power meter impedance is very high so it presents virtually no load to the ampliifer. COnnecting in parallel with the "A" speakers will work for sure, connecting to the "B" speaker output will probably work too.

You can use thin wire to connect it and be sure to set the meter impedance switch to the setting that most closely matches your main speakers.

05-14-2006, 09:27 AM
These meters are DESIGNED to work in parallel with the speakers you use - just hook the APM to the amps main speaker outs and then connect the speakers to the APM. That's how it's designed to work. It will present no load, no impeadance, and no problems whatsoever so long as you run it within the APM's power handling capability. If you open the APM up you'll see that the left and right sides are totally seperate from each other and thus will not present a problem to amplifiers that are descrete, balanced, grounded, etc. It's just like connecting an extra speaker except it's invisible to the amp. It's a fool-proof connection - I call it the "Monkey Connection". So simple even a monkey can make the connection. :dunno:


05-14-2006, 10:54 AM
:lmao: Crap, I forgot I had one of these collecting dust in the basement!

05-14-2006, 11:32 AM
I had a 200 wpc watt meter once and a 100 watt analogue.

The 200 was a digi and that's how I knew the SX-1010 driving 4 ohm subwoofers would crank out 200 watts thermal no problem. Can't believe I sold that monster. :tears:

Anyways, I liked the analogue meter better. It had great ballistics for a cheap rat shack meter :thmbsp:

ToTo Man
05-14-2006, 01:17 PM
Well I hooked it upto set B and it works. At my loudest listening levels the 3rd LED is pretty constant and the 4th LED flickers from time to time, so I guess thats in the 15wpc range for 8ohms. Incidently I just measured my speakers by hooking up a multimeter to the binding posts and I'm getting a reading of about 3.9ohms. My speakers are quoted as '4 to 8 ohms', and I was kinda hoping they'd be in the upper of the that range but they're obviously not :thumbsdn: . According to the power meter manual you've to double the value to get the wpc for a 4ohm load, which would put me in the 30wpc range for 4ohms.

My speakers are rated at 200w max so I guess I'm still well within my limits.
My amp is quoted as 105wpc 8ohms and 130wpc 6ohms (obviously not brave enough to quote values for 4ohms :D ), so again I guess I'm still well within my limits unless the wpc quoted is BPC wpc!.

As well as preserving my speakers, I wanted to hook the meter up to confirm that I do in fact listen to Concert DVDs at higher volumes than normal stereo music on my vintage rig, and it confirms that I do :D. The power needles on my vintage Yamaha never dance any higher than 10wpc 8ohm (20wpc 4ohm). But I don't know why I do this. Perhaps my subconcieous is telling me that concerts deserved to be cranked more than normal studio recordings in order to achieve 'concert atmosphere' :D.

Incidentally, when you run set A and set B at the same time on your amp, aside from impedance, does it half the total power the amp can output before clipping or does it not get affected?

05-14-2006, 02:10 PM
Incidentally, when you run set A and set B at the same time on your amp, aside from impedance, does it half the total power the amp can output before clipping or does it not get affected?

Yes, it does half the power since the outputs are the same and you are just adding set B in parallel with set A. But, since the APM has no "load", per se, then it *should not* present a load on B and you should still retain full output power - UNLESS your reciever/amp automatically halves the power when you select "A+B" on the selector - most amps work this way. You should assume that you will halve the power anytime you select speaker A+B regardless if you have an actual speaker connected to set B. That's why you would want to connect the APM to speaker A and then to your main speakers leaving set B unselected. The APM is *invisible* anyway and you should not hear it nor should it cause any abnormalities in circuit.

ToTo Man
05-16-2006, 11:48 AM
thanks for clearing that up JN. one more Q: if i just run Set B on its own (i.e. just the meter without any other speakers playing) will this harm the amp? I'd only do this for a few seconds to see how many watts my loudest listening level is, then I'd switch the meter off. If what I suggest is not safe then I'll follow you advice and hook it upto Set A....