Getting Together with the PreSonus ADL 600

Plus, An Exclusive Interview with designer Anthony DeMaria

As the story goes, Anthony DeMaria (legendary tube circuit designer and President of DeMaria Labs) was chatting with Jim Odom (President of PreSonus) at an AES show in the Big Apple. Noticing a prototype preamp with a beefy design in the back of DeMaria’s booth, Odom was blown away by the wide dynamic range and extreme low noise – to the point where he pursued a collaborative effort between the two. Well, the end result is the two-channel ADL 600 – designed by DeMaria and manufactured by PreSonus.

Here it is... a lean, mean, preampliflyin' machine.

Keep it Simple The ADL 600’s front panel layout is quite straightforward and simple. The split design is centered around dual analog output VU and eight-segment LED meters, under which sit the Power, VU METER –6dB switches and a blue LED that lights up when the unit is powered on. Each duplicate side then features a military-grade, brushed- aluminum INPUT rotary switch with Instrument, Line and Mic selections of 1500, 900, 300 and 150 Ohms. Next to that sits a HP FILT (Filter) with selectable 40, 80 and 120 Hz filtering (1st order @ 6dB per octave/roll-off) and an 8-position rotary Grayhill GAIN switch, incrementing from 30 to 70 in 5dB steps. Then there is a conductive plastic variable TRIM pot for fine adjustments of the output level +/- 10 dBs. The front bottom row features a 100KOhm instrument input and large chrome toggle switches for +48V phantom, POL INV (polarity inverse), -20dB PAD and HP FILTER – to switch it in and out of the signal path. The dual left to right layout also makes things easy when working two independent channels, or just dialing up a quick sound. The back of the unit features your standard IEC connector, above which sits a Chassis Ground Link, from which the grounds can be isolated by removing the shorting clip. Aside of that sits a large heat sink, followed by a pair of Neutrik +4dBu balanced Line Outputs. Then there is a pair of Inputs for Channel Two and Channel One - each having a balanced Line and Mic Input connector. That’s it – a nice clean design.

Yes, that's Anthony DeMaria's signature gracng every ADL 600.

Inside the Box Once you pop the top on this baby, you get a better idea of why it sounds like it does. The first thing that catches your eye is the huge Mu-metal shielded toroidal input transformer, delivering clean Class A power and exceptional noise characteristics. Following the neatly wrapped wires, you are drawn to the power supply, which features fast switching diodes and overrated power regulation. That then leads to the custom designed output transformer, above (looking down) which sits three military-grade vacuum tubes per channel, each consisting of a high-gain 12AT7 for the first stage and a pair of 6922 ‘audiophile’ tubes for the second tube stage. These run on +/- 320 volt wide power supply rails, which translates to massive tonality and headroom. At the bottom of the unit sit the sealed Mechanical relays. Located exactly at the point where the switching is required, it keeps the signal path short and helps minimize noise. The ADL 600 specs are quite good with a Noise floor of –95dBu, a microphone EIN (Equivalent Input Noise) or –125dBu, an overall frequency response of 10Hz to 45kHz +/- 1dB and a total preamplifier gain of over 72dBs. There are no IC’s (integrated circuits) or op-amps in the signal path, which provides a super-clean design. The attention to detail in every component is evident in the most important thing of all – the sound.

A closer look at the front panel, with input impedance selector, a high pass filter, gain and trim controls, an instrument input, and switches to engage/disengange phantom power, polarity invert, pad, the high pass filter, and meter calibration. Also clean: the rear panel, with line and mic inputs for each channel as well as line outputs.

More than a Preamp I like to think of this unit as much more than just a preamp. The addition of both instrument and line inputs makes it a flexible ‘tube’ tool in today’s world of DAW’s. Hooking it up to my Pro Tools HD rig on a pair of analog inserts, my first test was inserting it on the Master Fader of a finished mix. Clicking both INPUT dials to LINE, I set the GAIN at its lowest setting and the HP FILT to 40. I immediately noticed a warm, tube-like sound that seemed a bit low in dB’s. However, I tweaked the TRIM knob on each side to provide a few dB of gain, and flipped up the HP FILT switch, lighting the LED above it. The overall effect wasn’t dramatic, but enough that I noticed a difference for the better, especially in smoothing out some high end of the mix. It almost sounded like a slight, warm compression was taking place, with no added hiss or unwanted sonics. After getting used to the Line input sound, I plugged my trusty Les Paul directly into the Instrument jack and set the switch to…. well…INSTRUMENT. I was greeted with a toasty warm jazz-like tone, surprisingly devoid of those nasty mids that most DI’s deliver. I dialed up a few Amp plug-ins and was pleased by the results. In comparison to my non-tube DI that I’ve used hundreds of times, I was impressed by the ‘creamy’ sound and gentle tube feel. Next I check a 72’ Fender Precision and found the same applied to bass – warm and spongy. I was truly psyched because the sound was so good I didn’t need to add a Sans Amp plug-in, which is my standard bass ‘go-to’. You could clearly hear the tubes were glowing - and I was smiling. Then I took the direct recording output of my Mesa Boogie MK IV head and plugged in a Fender Telecaster. Same results, but even more ferocious - since the Boogie has some great tubes in it as well. The waveforms were thick on the screen, and the sound could best be described as ‘beefy’. After capturing some cool tracks, I again switched both INPUTS back to LINE and reset the unit to condition the two-mix. Very cool.

Under the hood... obviously a well-designed piece of gear.

The Old "Sample the Radio" Test The last test turned out to be my favorite and the highlight of this preamp. I set a radio up in a room and miked it with an Earthworks QTC-1, a high-quality omni I know very well. Then I recorded a sample at each impedance to see if I could hear a difference. Well, it was quite obvious that variable impedance does alter the sonics of a microphone. 1500-ohms was the most open, with 150-ohms resulting in a ‘closed’ sound. When I tracked the same experiment on a vintage jumbo Guild acoustic, the same results applied, regardless of microphone. Certain mics, such as good-old dynamics, had the least change. But the higher quality the mic, the more change I heard in the overall tonality and presence. One of my favorite vocal mics, the AT 4060 tube, had a nice, deep presence running at both 900 and 300-ohms (which sounded like analog tape). I tended to prefer the warm sound of 900-ohms, which was pleasing to the ear - regardless of mic used. If I was doing drum overheads or cymbals, I would probably use 1500-ohms to open the mics up. For a tape saturation sound, I got some cool results with 300-ohms. On acoustic guitar, 300-ohms sounded like Jimmy Page's Martin sounds on early Zeppelin records. Overall, the ADL 600 is a super quiet, high-quality tube mic preamp with a ton of headroom and a warm, spongy sound. The fact that it doubles (and triples!) as a DI and a LINE conditioner makes it that much more valuable in todays studio environment. Check one out for yourself – I bet you’ll feel the same as I do. ADL 600 Quick-hit list: - Highly flexible - can be used as a preamp, line input or DI. - Uses military-grade components. - Super low noise (-100dB S/N ratio). - High voltage Class A operation. - Over-rated toroidal power transformer. - Three dual-triode vacuum tubes per channel. - Both Analog VU and fast-acting LED metering. - Sounds awesome!

Interview: Hanging with Mr. Anthony DeMaria So there I am having a tasty tap beer at Barfungul, a private pub at the comfortable Millbrook Sound Studios in upstate New York. As studio owner/host Paul Orfonio was pouring St. Bernardus Prior 12, my buddy Chip introduces me to the guy next to him - who turns out to be Anthony DiMaria. After a few ‘oh shit’ moments, I mention I just reviewed his ADL 600 preamp. As it turns out, Anthony has a few killer pieces inside the studio, so we sat in the control room and rapped about gear for the next hour. A few days later, I rolled some tape on him to give the GCPRO.com readers a little extra insight into his designs. - RT Q: How does high-voltage affect the sound of the preamp? A: What happens is the tubes operate on something that looks like an ‘S-Curve’ on a graph. The middle of this “S” has a long section that we call a ‘load line’ or ‘quiescent point’. It is this long section that - as a designer - we are the most interested in. We want to have the tube operate along this line because it is most liner or consistent there. Simply put, the high voltage has allowed me to stretch that line. Granted, everything is physics, and it’s not like your going to cheat the laws. There’s no magical thing here. You simply look at the tube, pick its placement and the resistors around it, and tweak until it sounds great. Q: What made you choose the tubes in the ‘600? A. Typically, most designers would choose a 12AX7 because of the high mu rating of 100. I knew that based on the design, this preamp would develop a healthy amount of gain - around 72dB worth. Thus, I could use a tube with less gain that would be quieter in operation. That’s why I went with a 12AT7 from Sovtek, which has a mu of 70. The other two tubes are Sovtek 6922’s. They are driver tubes and part of what is called a cascade design. 6922’s are solid, well-built tubes that are similar to the front-end 12AT7’s and are very available - words many clients love to hear. Also, I chose to operate the 12AT7 with a bipolar power supply, which allows the tube the maximum amount of gain. There are always trade-offs, more gain with more noise or less gain less with noise. As a designer, you must pick and choose. Q: What does variable impedance do when you plug in a mic? A. Each position on the preamp will load the mic differently. For instance, when you have the impedance set at 150-ohms, you’ve loaded the mic more then if you have it set at 2,500 ohms. As an engineer, the 150 ohm setting may make the mic sound very ‘closed’. With the 2,500-ohm setting, you will hear a more ‘open’ sound, since the load is much less. This simply gives the client more sonic options.