Tuesday, February 22, 2011

Velleman HPS50 Portable Oscilloscope for Testing Amp Output Power

I finally got a chance to test the Velleman HPS50 today. It was pretty simple to setup, even for the o'scope novice like myself.

I don't have my dummy load resistor bank yet, but I do have a 6.5" Realistic Dual Cone car speaker I was willing to donate to the cause. I measured it's resistance to be 4.2 ohms.

I used a 100Hz test tone for my tests and calibrated the HPS50 per the instructions. First, ensure the switch on the probe is set to 10x, also check the display to ensure 10x. Then use the tip of the probe to touch the calibration point on the side of the scope. Lastly, view the waveform, and adjust the small screw on the probe until the wave form has even square waves. (I was wondering what the small, plastic screwdriver was for in the box, now I know!)

You have a few choices for measuring power output. I initially tested Peak watts, then decided RMS was a better test. In the HPS50, it is displayed as W^4Ω. You can also set the ohms (Ω) to 2, 4, 8, 16 or 32. Since my speaker measured 4Ω, I set the meter to "W^4Ω" for RMS wattage at 4 ohms. I'll eventually put together a demo video of the unit, with setting up the unit for power output being the main objective.

I already had a Rockford Fosgate Punch 4020DSM on my test bench, so I decided it would be a good first test. The "old school" amp is rated at 20 watts x 4 @ 4 ohms. My test signal was a 100Hz tone and my power source was my Audio Authority Model 978 100A 13.8V power supply.

Here are my results:

76.65 watts RMS per channel (only 1 of the 4 channels driven) @ 4ohms, 100Hz @ 13.8v. Not bad for an amp rated at 20 watts per channel! The main difference to note is I can't measure THD, so Rockford's measurement (although still way underrated) was tested at 0.1% or less THD while mine could be up to 10%. They also usually test at 1000Hz, my test was limited to 100Hz (mainly to save my eardrums!). Once I get my resistors, I'll be able to test at various frequencies.

I also had a Rockford Fosgate Punch 30HD close by and knew it would be a good test. The amp is rated at 15 watts x 2 @ 4 ohms. I measured 24.75 watts RMS (from 1 channel) @ 4ohms, 100Hz @ 13.8v.

The HPS50 will connect to a PC via USB and capture real-time data as well. I attempted to test with my Windows 7 32-bit laptop and was unable to get the o'scope to communicate with the Velleman software. From reading the forms about this unit, many others are experiencing issues as well with Windows Vista. I'll attempt to install the software on a Windows XP computer to see if I can get the device communicating for real-time capture.

So far, the Velleman HPS50 is a fun tool! More tests coming soon...I'll have my dummy load setup complete and will begin testing the output power of my old school amps. Stay tuned in...


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