Tuesday, March 1, 2011

Testing Power Output from Old School Amps

Old School friends, have you ever wondered how underrated your treasured amps are? Well, I must say I've always wanted the ability to measure actual output from my amps to show we did actually get what we paid for.

My wishes have come true with the latest 2 items I've added to my test bench....an Oscilloscope and a "dummy load" (aka "Fred Sanford") resistor bank. The o'scope is a portable model by Velleman (HPS50) and the resistor bank is one I put together based on another project I found online. See the detailed layout here:

400W Dummy Load Project

I essentially took his design and doubled it. This gives me (2) 400W loads at 4ohms or (1) 800W load at 2ohms. These resistors can handle up to 3x their rated power for short periods of time. I had originally planned on using 32 instead of 16 resistors, but then decided I could add more in another enclosure as needed. Here is a pic of my creation:

So some of you may be wondering what the resistors are used for. Basically, they are designed to handle the output power from the amps without producing any sound. This allows you to reach max output without damaging your hearing. After I did a simple test with an old Realistic speaker, there was no doubt I needed to build a Fred Sanford. Not to mention, I can safely test many different frequencies, including 1000Hz, which is not pleasing to hear at high volumes!

Next, I needed an oscilloscope to monitor the waveform from the amp. I considered many models, including desktop and portable designs. I finally settled on the Velleman HPS50 portable oscilloscope. I purchased from Kitsusa.net and it was around $230US shipped. I also considered the HPS10SE, but it did not have USB output for monitoring / capturing via PC, which I thought may be important. Another selling point for me of the Velleman o'scopes is the built-in audio measurement tools. They essentially have a built-in True RMS Volt meter and taking the AC Volts x AC Volts and dividing by the ohm load you get max output power (The Velleman does the math for you and just shows power output). The HPS50 will also let you decide between MAX power and RMS power. I decided to perform all of my tests using the RMS power measurement.

Components used in my testing include:
  • 100A Audio Authority 978 13.8V Power Supply
  • iPod Nano (1st Gen) with test tones - 100Hz and 1000Hz
  • 1/8" to stereo RCA adapter
  • 12ga Speaker Wire to Connect Amp to Dummy Load
  • Windows XP laptop to monitor/capture live waveform from HPS50
  • Velleman HPS50 Portable Oscilloscope
  • 800W Dummy Load Resistor bank (AKA "Fred Sanford")

So, I bet you are curious about some of my findings....stay tuned as I'll post results very soon....



normalicy said...

I have made a similar dummy load myself, however, I used an old dead amplifier's heatsink (LA Sound - Hermosa in particular) with muffin fans on top to keep the resistors cool. It can double their continuous wattage. That said, nice job.

Unknown said...

Have you ever tested a U.S.Amps VLX400? Or could you?

dwillis said...

I don't currently have the "electrical" with my bench setup to test the VLX-400, but hope to in the future!

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