The Presonus MP20 Preamplifier Conundrum
Technical Support Lead
Dealer RA Support Supervisor
For 13 years the core of my service shop had been my Audio Precision ATS1, the coolest piece of test equipment I’ve played with since I had constant access to the AP System I at QSC. When I was doing regular studio maintenance, I carried this great piece of equipment along on almost every service call. I had customized an old biomed ICD case to fit the AP and that case and the equipment it contained flew across country several times, travelled in the back of my nasty old Ford Escort through heatwave and blizzard, and suffered the slings and arrows of living in my basement shop for more than a decade between trips. AP knocked it out of the park with this series of test gear and I learned more from using mine than I did from four years of electrical engineering classes.
You can get to many of the same places with regular test equipment, but you get there mu ch faster with a great piece of specialized gear. Frequency sweeps with a variety of resolutions, speeds, and steps, which are documented and printable, provides a lot more immediate information than an otherwise perfectly functional sweep generator and oscilloscope. The same goes THD, phase, IMD, input impedance, and power measurements. A variety of otherwise painful, complicated, and troublesome system checks can be almost automated with the ATS1 and that makes troubleshooting a pro console in the studio environment more consistent, professional, and documentable. Again, that’s also true for multi-track tape decks, external gear, and studio wiring.
A few years ago, I more or less planned on letting my estate sort out the value of things like the ATS1 and a few other indispensable pieces of electronic test equipment I’ve relied on for most of my career. When I retired last year, I began to reconsider that plan. I have felt no compulsion to return to audio engineering, even on a hobby basis. So, last week I put the ATS1 on eBay (http://www.ebay.com/itm/301350546710?ssPageName=STRK:MESOX:IT&_trksid=p3984.m1561.l2649) for a very reasonable price and a rental company snapped it up. I’d rather see it used to death than have it rot in my basement shop unappreciated and unused. Electronic equipment, like mechanical equipment, needs occasional use to keep electrolytic capacitors and other parts operational. I’d say I hated seeing it go, but I didn’t. We’ll use the money on our retirement home and being mortgage-free is more important at this point in my life than having the capability to do work I no longer want to do. With that in mind, this is my odd way of saying “so long and good luck” to a great piece of equipment and some great years of my career.
Without the ATS1 in my toolbox, I have one more excuse to turn down crawling around under a studio console or banging my head against some moronic piece of poor design work or swabbing out Coke, coke, or some other crap from broken effects boxes.
30 years ago, Pat Quilter handed me a piece of junk that QSC had purchased to help make objective evaluations of audio gear; an ABX box that had been sold by some audiophile company a few years earlier. The original box had goofy firmware that occasionally retained incorrect test history, lousy circuitry for switching, and limited current capabilities. Since we would be testing everything from line-level signals to power amp output levels into a common speaker system, I pretty much dumped everything except the remote control and redesigned the whole box. From a previous engineering project, I had a few mercury-wetted, silver contact low signal relays in my parts bin, which I used for the line-level switching components. From the design work I’d done on the “award winning” Wirebender Musician’s Preamplifier, I incorporated a hard-wired parallel shielded signal path that minimized crosstalk (to 85dB unweighted), wiring capacitance and inductance, and kept the connectors-to-relay-to-connectors signal path as clean as possible. For the high power signal path, I used gold-plated dual banana connectors and some silver-plated-contact motor controller relays, also from my previous design engineering life.
From there, we took every opportunity to drag engineers, musicians, recording professionals, live sound professionals, and office staff into the test lab, out in the field, at music stores, and in recording studios where we could test our and competitor’s designs against the discriminating hearing of anyone who would submit themselves to testing. Mostly, we learned that hardly anyone could hear the difference between great gear and crap. Years after I left, QSC turned the whole ABX concept into a product and discovered that proving your customers have mediocre hearing is not a great marketing tactic. On average, the “ABX test” established something that an audiologist discovered at a 1980’s AES, most audio professionals are “functionally deaf” and hate having that demonstrated in public. After a study that categorized the most arrogant audio professionals (live sound “engineers”) as being as deaf as their product consistently demonstrates, the AES banned audiologists from AES shows until the late 90’s. The HEAR organization began providing “confidential” testing at AES conventions in 1997 (“The results of the free hearing tests are kept confidential and maintained in a database that tracks hearing screenings performed for AES members since 1997.”) and even their by-profession data is kept secret.
A recent Pro Audio Review article, “Why Gear Doesn’t Matter,” has similarly fired up everyone from audio amateurs to professionals to, especially, boutique gear manufacturers. Even though any good design engineer will admit that getting a simple audio signal, 20Hz to 20kHz, through modern electronics is a pretty remedial design task, the fact that most modern records sound like crap is easier blamed on not having the right gear rather than recording mediocre musicians playing lifeless music using techniques that are guaranteed to squeeze the life out of any song ever played.
On the other hand, a good number of those functionally deaf recording engineers have managed to record some amazing music over the years. Knowing your limitations and compensating for them intelligently, along with possessing musical knowledge and technical skills, will carry you a long way. When I first sat down with my new toy, the company ABX tester, I discovered a lot of painful things about myself. Listening through high-end headphones, to studio monitors, to audiophile speakers, and decent home stereo speakers, I first discovered an old bias of mine—bipolar output transistors vs FET transistors—was an illusion, in my case. For years I had been convinced that I could clearly hear the difference between bipolar and FET output components and my ABX tester proved that was bullshit. Next, I experimented with slew-limiting, another characteristic I was certain made a big difference. In my case and many other listening test subjects, I learned that a slew of over 4V/μS was reasonably clearly “superior” to a slew under 1V/μS. Going for higher slew improvements didn’t seem to produce an audible advantage. Harmonic distortion turned out to be considerably less apparent than I’d expected, too. In fact, lots of professionals failed to detect clipping distortion until it exceeded 0.5% and as much as 2%. Put a low-pass filter on the clipping and detection got even worse.
It has been 20-some years since I played with an ABX box and I’m sure my 66-year-old ears are considerably less discerning than my 30-year-old ears. Time is not kind to the hearing mechanism. However, when I hear young or old people make ridiculous claims about the difference in cable “sounds” or insisting on having some over-priced preamp coupled with an even more overpriced microphone before recording pop “music” is possible, I instantly blow off that person’s opinion as bullshit. I’m not buying any of it.
A couple of years ago, a friend (Aaron Hodgson) and I built an ABX test box to use with our school’s AES chapter and various equipment experiments at the school. Since I retired last fall, I am not in a position to be part of that testing, but Aaron is “ready and willing” to let anyone who is interested test their hearing at the school or locally. We went to the same extremes with this tester that I’d used on the original ABX box in the 80’s, including using the same mercury-wetted relay. If you really want to know if you can hear the difference between a Presonus preamp and a Great River (or any other brand), this is your chance.
My good friend, Rob Schlette, wrote an article about digital ABX self-testing for the Pro Audio Files, “Audio Perception and ABX Testing.” Not only does he provide readers with links for ABX software and some history, Rob has written an excellent step-by-step testing procedure that should produce consistent results and provide the testee with a lot to consider. At the digital signal level, there are several ABX test programs you can play with for free. Try more than one to be sure your results are consistent.
On the other hand, if you don’t want to know what you can’t hear, you’re not alone. My experience demonstrated that most “professional audio engineers” were happier not knowing their limitations and became downright hostile when they discovered their golden ears were tin. If that’s you, have fun blowing your money on overpriced gear and get used to knowing that the music you play and record will be something different than your illusions.