The Aurora Compressor is a Ross Compressor or MXR Dyna Comp clone with a few modifications: a treble switch to control the amount of high-end attenuation after the compression; an attack/release control that can be wired either as a knob or a switch; and an internal trimmer that allows attenuation of the input.
This project uses the CA3080 OTA, which was obsoleted in 2005. There was still plenty of old stock available, but then a company called Rochester Electronics purchased the fabrication equipment from Intersil and began producing it again, primarily for the hobby & repair industry. These days, if you buy a CA3080 from Small Bear Electronics, you will get a new-production CA3080 from Rochester Electronics.
At $4.95 each, they aren’t cheap, but if you need them in any kind of quantity, I’ve had very good luck buying them from Chinese sellers on eBay in lots of 50 or 100. These are usually pulled from old equipment. You can get them for pretty close to $1.00 each. The usual disclaimer applies that there are a lot of counterfeit NOS parts on eBay, so check the seller’s ratings before ordering.
The LM13700 is a current-production OTA that can be used in place of a CA3080 in the Dyna circuit with no sonic difference, and in fact is the chip of choice in the Ibanez CP-9. The only trouble is that the LM13700 is a dual OTA in a 14-pin DIP package, so the PCB must be designed around it—it’s not a drop-in replacement.
1970s: MXR Dyna Comp and Ross Compressor
The MXR Dyna Comp was first released in 1972. The CA3080 was relatively new at the time, and this was the first guitar compressor that utilized it. The squish of the compressor paired very nicely with the twang and treble of a Telecaster, and the Dyna Comp quickly became a favorite among the Nashville crowd, where it’s still rooted in their traditions.
Surprisingly, the core circuit has evolved very little since that time. Ross, a sister company of Kustom, produced a line of “me-too” clones whose main three pedals (a phaser, a distortion and a compressor) were direct rip-offs of MXR’s classic offerings—even the Ross logo used on these pedals was designed to resemble MXR’s script era. They released their compressor in 1978, making a few small updates to the Dyna Comp circuit in the process, mostly related to power filtering and noise reduction.
1980s: Boss CS-2 and more
In 1981, Boss released their CS-2, which was based on the Dyna Comp circuit but—in typical Boss fashion—pretty significantly overhauled by their engineers so as to not be a 1:1 clone. In the process, they added an Attack knob, which technically controls the release of the compressor circuit, but the name stuck. Around this time, a number of other compressors were released as well, based around the classic OTA circuit in some form or another: the Ibanez CP-9, Pearl CO-04 and DOD FX-80B, among others.
1990s: Saffron Squeeze and Ross trace
In 1996, Way Huge released the Saffron Squeeze, which was the first Ross compressor clone. It largely went unnoticed in the DIY scene, with many suspecting it was based on the Orange Squeezer but leaving it at that.
In the mid-1990s, the DIY scene took notice of the fact that Trey Anastasio of Phish was using a Ross Compressor, which he had owned since high school. This circuit was dissected in 1999, and R. G. Keen released a schematic, which he updated with a couple of his own tweaks for even more noise reduction and power filtering.
2000s: Beginning of the boutique clones
In 2000, Analogman took apart a Ross compressor with the help of Alfonso Hermida (who later achieved a great deal of success with the Zendrive) and released the second clone, his “CompROSSor”.
A little later, in 2001, Robert Keeley built his Keeley Compressor, which started out based on R. G. Keen’s schematic and gradually added featured throughout the years: an input trim for high-output instruments, and a knob to control the attack (release) of the circuit, which was adapted from the Boss CS-2. This became Keeley’s flagship pedal and has sold well over 40,000 units. The original two-knob version was discontinued at the end of 2014 in favor of the updated four-knob version (Keeley C-4) with the input trim and attack knobs on the outside of the enclosure.
Roger McGuinn and the JangleBox
The JangleBox also deserves mention in this section. In 1988, Rickenbacker designed a signature Ric 370 12-string guitar for Roger McGuinn (The Byrds) which had an onboard compressor based on the Dyna Comp. There were only 1000 of these guitars made and the compressor was never used in any other models.
In the early 2000’s, the JangleBox was designed by Steve Lasko as a stompbox version of the onboard Rickenbacker compressor. Aside from some component value differences, the main addition to the classic Dyna circuit was a switch that removed the treble bleed capacitor immediately after the OTA, resulting in what is perceived as a treble boost. Roger McGuinn loves the JangleBox and says it’s a spot-on recreation of the compressor inside his signature guitar.
In the past 40 years, many new compressors have been designed using more modern technology… but there’s still something that endures about the old Dyna Comp circuit and its descendents.
This circuit has a particular interest for me as well, as it was the first pedal I built entirely from scratch after doing only mods and repairs for a couple of years. (Most people cut their teeth on Fuzz Faces, Big Muffs or Tube Screamers, but not me.) After no less than two years of iterating versions, designing and redesigning PCBs, and building prototypes, I finally released the Aion Compressor for sale in early 2013—the seventh major revision to the circuit.
The Aion Compressor is based on the classic Ross compressor circuit, incorporating the input trim control from Keeley and a switch implementation of the attack (release) control from the Boss CS-2, along with a switch that controls how much treble is attenuated after the CA3080 does its squeezing. It uses the highest-quality components I could find, and it’s built to last. I’ve had a number of people A/B it with a Keeley and come away preferring mine.
Is the Aurora Compressor project a DIY Aion Compressor? Mostly, but not quite. The core circuit is largely the same: it has the Treble switch, the input trim and the attack switch. The main difference in the circuit is that the Aion Compressor has a completely different output section that cuts down on some of the inherent harmonic distortion in the original Ross circuit. The Aion Compressor also has soft-touch true bypass via a relay driver, and it has clear knobs that are illuminated by LEDs, neither of which would be very easy to DIY using this project PCB.
- Last of the OTA’s — Don Tillman’s blog ↩
- Based on the pot codes in eBay photos. The earliest one I could find was dated 1978. ↩
- Bossarea: Boss CS-2 ↩
- Based on photos of unit #14 which has an inspection sticker dated November 10, 1996. ↩
- Analogman compressor pedals and mods ↩
- Keeley 2-knob compressor product page ↩
- 2008 Premier Guitar interview with Roger McGuinn ↩