Technology has brought to fruition several choices for amplifying speakers in our car. Last month we took a peek at four very distinct amplifiers on our bench and noted that each was very unique in its own way. However, knowing that numbers don't always tell the story, we put the Alto Mobile ADP102, Genesis Profile Two Ultra, Tru Technology C.7AT, and a Zapco C2K-3.0X through the wringer once more--this time on a pair of $10,000 home speakers. Could we tell the difference? Let's find out.
The Speakers
We used a pair of Edgarhorn fullrange speakers (on loan from Dr. Bruce Edgar, himself) for this listening test. These 8-ohm drivers have an ungodly efficiency rating of around 105dB at 1watt/1meter, meaning that they will put out 105dB of sound with 1 watt of input power. The fact that we are testing amps ranging from 26 watts up to 400 watts is not a problem when you are only going to be using a few watts for the test. "But wait," you say, "I've heard that all amplifiers sound the same when operated within their limitations. Why aren't you running the amps to clipping?" Good question. First of all, I have never bought into the "sound the same" line of thought. Can you tell what something sounds like simply by looking at the specs? There are so many different measurements that make up accurate sound reproduction and several of them can be manipulated in the design. As an example, THD specs can be greatly reduced (less is better) by increasing the amount of negative feedback in an amplifier. Too much negative feedback, however, causes the amp to sound "restrained." No, I don't know how to measure "restraint" in amplifiers, but that's exactly my point here. The numbers don't tell the entire story. Secondly, back to your question, the clipping characteristics of tubes vs. solid state have been well-documented over the years in an attempt to explain the sonic differences. Tubes clip "softly," causing a warmer, smoother sound, while transistor designs clip harder, sounding cold and harsh. Besides being efficient, the Edgarhorns are also extremely accurate and revealing, so any differences between the amps should show up quite clearly.
The Setup
The good guys at the Mobile Dynamics installation school once again granted us access to their facility for this listening test. This time we set up in the Theatre room. The Theatre room comes complete with a huge projector, screen and stadium-style seating for about 20 people. More importantly, there are acoustic panels on the side walls and the dimensions of the room are very nearly ideal for spreading out any standing waves that may occur. StreetWires provided enough speaker cable, power cable, fused terminal block and even a 1-farad capacitor to connect all of the amps and the speakers to the switcher.
The Edgarhorns were painstakingly measured to place them equal distances from the back and side walls, and the amps were all set to identical output levels using a 1kHz tone and voltmeter, and cross-checked with pink noise. If you're playing along at home, the pink noise will result in an up-and-down volt reading on the meter. You have to watch the range to see that everything is equal. If all of the amps are at the same voltage with the 1kHz tone, but there are wide variances with pink noise, it can be an indication of a crossover turned on or some EQ or boost present in the system. There was a small dB meter in the room, so we checked everything with that as well. After listening to some tracks and switching through all four amps to make sure they were at the same level, Tech Editor Casey Thorson noted that one of the amps had the left and right channels reversed. After that was corrected and left/right orientation was verified on the other amps, we were finally ready to begin.
The Suspects
For a listening test like this one you have to round up some really well-trained pairs of ears. One bonus to living in the Phoenix area is that it is home to several audio companies and even more seasoned audiophiles. Here is the starting lineup:
 Mark Lowe --former director...  Mark Lowe--former director of RTTI for something like 15 years, IASCA trained SQ judge and currently working on new curriculum for Mobile Dynamics. |  Rick Jones --Installer extraordinaire...  Rick Jones--Installer extraordinaire from 20 years back, Precision Power veteran and Team Gates crew chief for a couple of years. |  Casey Thorson --IASCA trained...  Casey Thorson--IASCA trained SQ judge, audio industry veteran (installer and product developer) and tenured technical editor for CA&E magazine. |
 And finally me, Bob Norvelle...  And finally me, Bob Norvelle--IASCA trained SQ judge, audio industry veteran (installer and product developer) and CA&E feature electronics evaluator. | | |
The System
While considering the judging criteria for this listening test, I found myself going back to the speaker listening test from a previous edition. Unable to think of anything else, I adopted the same criteria: bass, midrange, treble, clarity, tonal balance, staging and imaging. There was also a line for initial impression and final impression, and an extra page for notes. Each listener was given complete control over the listening experience. The listener chose the tracks to play, when to switch amps, and was given the remote to control the volume. There were no rules. Listen to whatever you want as long as you want (remember how listening longer affected the speaker rankings? See "Blind Listening Test" in the February 2005 issue). The only catch was that the listeners only identified the amplifiers as 1, 2, 3 and 4.
The Scores
Our first impression: "Oh crap! Don't tell me they all sound the same!" Well, they were all at the same volume and none of them were clipping. We were listening to the circuitry, not the power. OK, that was after about 30 seconds of quality listening time, but it was a little scary. After a few minutes the differences began to appear.
First of all, none of the amps sounded warm or "tube-like;" none of them sounded harsh or "solid-state." There were very slight differences recorded between each amp on bass, mids, highs and tonal balance. On a scale of one to 10, the biggest difference any listener recorded in these categories was only three points. The trends were that Amps 3 and 4 had slightly stronger lows, Amp 2 had the edge in the midrange and Amp 1 did the best job in the upper ranges. The differences were very slight, though. Do you remember the frequency response chart for the Alto Mobile ADP102 from last issue (August 2005)? If the graph told the whole story, the Alto amp should have failed miserably in the high-frequency range. Here's the deal on the Alto. The outputs use a filter consisting of coils and capacitors, just like a passive crossover. In fact it is just that, a lowpass crossover, but not tuned exactly like you would a subwoofer or tweeter crossover. I ran my frequency response sweep using a 4-ohm resistor as a load, causing the truncated high end. The Alto engineers tuned the filters to work with a 4-ohm speaker (novel idea), which is partly resistive and partly reactive. The actual impedance of the speaker will vary throughout the frequency spectrum because of the reactive component, and this results in a more acceptable measured frequency response. If you look at the graph comparing the resistive load to the speaker load, you will see that there is a hump in the 15kHz region. This is because I used an 8-ohm speaker this time, instead of a 4-ohm resistor. The curve for a 4-ohm speaker will right between the two curves. I used an 8-ohm speaker to get a picture of what we were actually listening to in the test, since the Edgarhorns were 8-ohm. Another interesting fact from my discussion with Alto: The power supply does indeed run at 125kHz, but there is a frequency divider that bumps the output speed up to 250kHz, all clocked by a 4-Meg oscillator. Cool...
OK, back to the listening. If the different frequency ranges and tonal balance were that close, what did make the difference, if any? Let's move on to clarity. Amp 1 garnered three of four first place scores, but again by a narrow margin with a couple of ties with other amps.
Imaging is what you can hear happening in front of you, across the space between the speakers. In some cases you can actually hear things wider than the speakers, but that was not the case in this test. You should always hear the solo voice or instrument clearly defined and pretty well centered between the speakers. The height of this apparent image can be higher or lower than the two speakers.
Staging is judged by your ability to identify particular instruments and voices in clearly defined areas of the soundstage. This can be left to right, up and down, and in some cases front to back. It helps to close your eyes, but with well-recorded and well-reproduced tracks you can hear everything clearly and determine its location, even when one instrument is in front of another. This whole three-dimensional soundstage is what audiophiles live for--to close your eyes and imagine that you are right there at the live performance any time you want.
Assuming there are no frequency response problems, imaging and staging are what sets amplifiers apart. Using a higher-grade capacitor or better op-amp, faster transistors or tubes, lower resistance connections--all of these elements conspire to improve or destroy the realism of the soundstage.
That said, the amplifiers had pretty distinct soundstage signatures. Amp 1 received highest marks for imaging. The solo voice/instrument was always well centered and about 18" higher than the other amplifiers. The left-right soundstage extended all the way to the speakers. Amps 3 and 4 tied for second place in this category, with a strong center image, though not as high as Amp 1. In staging Amp 1 again scooped up the most first place marks, with solid front-to-back locations for each instrument/vocal. Amp 4 followed closely with almost identical placement, but not quite the entire depth of Amp 1.
Conclusion
We're asking two questions here: Can you tell the difference between classes of amplifiers and which one sounds the best? I think most people would have expected the tube amp to have a very distinct sound to it, but as you can see, that was not the case at all. None of the amps really sounded either tube-like or solid-state. Let's remove the blindfolds:
Strangely enough, the final impression scoring tends to match the pricing on these units. The TRU Technology amp received three of four first place votes, beating out the Zapco by a couple of points. The Genesis and Zapco offerings were close. Zapco took second place on low and mid frequency and imaging scores, while the Genesis had overall better staging scores. The Alto Mobile Class D amplifier brought up the rear with good low and mid frequency scores, as well as overall tonal balance. The imaging was pretty well centered with this amp, though not very wide, and there was no depth to the stage.
What does all this mean to you, the car audio enthusiast? In my opinion it is extremely difficult to accomplish a soundstage in the average vehicle. You have to use really good speakers and experiment with placement and crossovers. It can be done, I assure you, but not with an amplifier that won't reproduce that soundstage in an ideal environment. You have to step up to a higher-quality, higher-priced amplifier. If that soundstage doesn't turn you on, life will certainly be easier and less expensive for you. You can choose an amplifier based on your criteria--reliability, size, efficiency, appearance or features. The number of channels and power per channel are fairly easy choices as well. If you're after the Holy Grail of Car Audio, however, there is no substitute for quality.
 AMP1-TRU C7.2AT, 2X13W TUBE...  AMP1-TRU C7.2AT, 2X13W TUBE AMPLIFIER, MSRP $3,200 |  AMP2- ALTO MOBILE ADP102,...  AMP2- ALTO MOBILE ADP102, 2X 50W CLASS D, MSRP $260 |  AMP3-ZAPCO C2K3.0, 2 X 75W...  AMP3-ZAPCO C2K3.0, 2 X 75W CLASS AB, MSRP $949 |
 AMP4- GENESIS PROFILE TWO...  AMP4- GENESIS PROFILE TWO ULTRA, 2X 150W, MSRP $599 | | |
Special thanks to:
Tom Gazda and crew from Mobile Dynamics; Hilary Moore and Eileen Prather from Mitek (StreetWires); Matt Borgardt of Image Dynamics and Dr. Bruce Edgar of Edgarhorn
Omission from previous article:
Genesis Maximum Current Consumption is 38 amperes at 400 watts total power.