Tuesday, March 1, 2011

Old School Cheater Amps - RMS Power Output Test Results

I'm sure many of you have been eager to see my test results from my previous posting on early 90's "cheater amps". Well, I've finally assembled the pieces needed to test RMS power output from the amps. See my previous post for details on my testing methods.

Did I have any favorites? I must say, I thought the Orion 225HCCA would take top honors, but envisioned the Phoenix Gold MPS-2240 would be close behind. Although the Rockford Fosgate Punch 40i DSM was not included in my original "cheater amps" posting or the video I created, I felt it was a worthy opponent to the similar amp models. I put together a separate video on the Punch 40i DSM a while back and it nearly blew me away with it's power and clarity....I guess I was unaware of just how underrated this little amp is!

The Precision Power Pro Mos 25 was no slouch either. It wouldn't win the price/performance ratio, but it outputted 1.5x or greater it's rated output at all test loads. Since this is not a sound quality test, the only comparisons were RMS output power. The PPI Pro Mos 25 would probably gain points to some people for it's "Art" design, but I was always a bigger fan of the black series PPI amps. Also, in the old school, lacking bass and/or treble controls would be another minus for me.

The venerable Punch 45HD was a favorite back in the early 90's and for good reasons. It was reasonably priced ($279 US) and put out at least 2x it's rated power at 2 and 4 ohms. As I've stated in the past, Rockford didn't rate this amp to handle 1 ohm stereo or 2 ohm mono loads; however I've had a Punch 45HD since December 1991 and used it for several years in the 2 ohm mono configuration with no issues whatsoever. It was interesting to see the power output lower in the 2 ohm mono test than the 4 ohm mono test with the Punch 45HD. The limiting circuitry must have come into play to protect the amp from overload.

The Orion 225HCCA and Phoenix Gold MPS-2240 were the only two amps in my test rated to handle 1 ohm mono or 0.5 ohms stereo loads. Unfortunately my current setup provides loads down to 2 ohms and no 1 ohm or 0.5 ohm configuration. I'll add more resistors in the future and test these amps again to find out their full output potential. I have little doubt the Orion 225HCCA will output over 500 watts but have less optimism in the Phoenix Gold MPS-2240...especially when comparing the 4 ohm mono vs. 2 ohm mono loads with this amp.

I hear ya, enough with the jibber-jabber, on with the results! See the embedded spreadsheet below or Click HERE to see the full size image:

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....