Ross Compressor

The Ross Compressor is a two-knob compressor pedal from the 1970s that has become one of the most copied compressor circuits in guitar history. General Guitar Gadgets calls it "one of the all time 'classic' compressors" and notes that it "is treasured by many players as being 'the one' compressor"¹. Its circuit is a close relative of the MXR Dyna Comp, built around an operational transconductance amplifier (OTA) chip, and its reputation today rests as much on the long line of pedals and kits derived from it as on the original units themselves.

The pedal's modern fame has a specific starting point. In the mid-1990s, the DIY pedal-building scene noticed that Trey Anastasio of Phish was using a Ross Compressor, which he had owned since high school². That attention led to the circuit being dissected in 1999, when R. G. Keen released a schematic, updated with a couple of his own tweaks for more noise reduction and power filtering². From there the circuit spread through boutique builders and kit makers, and it is still being produced in various forms today.

Where the Ross Compressor came from

The Ross Compressor was produced in the 1970s, and Stomp Under Foot refers to it simply as "the original 1970s Ross Compressor" when describing the circuit its own pedal is based on⁵. Ross also made other effects in the same era: General Guitar Gadgets documents a Ross Distortion that is "very similar to the MXR Distortion Plus pedal," suggesting the company followed MXR's designs across more than one effect type⁹.

The pedal's afterlife is more eventful than its original run. According to Aion FX, the Way Huge Saffron Squeeze was the first Ross compressor clone². The circuit itself was dissected in 1999, when R. G. Keen released a schematic that he updated with a couple of his own tweaks for more noise reduction and power filtering². The DIY scene had taken notice in the mid-1990s, after Trey Anastasio of Phish was seen using a Ross Compressor².

The gray finish of the original is part of its identity. Retro-Sonic describes its own compressor as based on "the famous gray Ross Compressor known for is smooth compression and attack response"¹⁰, and the gray unit is the version most builders mean when they say "Ross Compressor."

How it works: an OTA compressor

The heart of the Ross circuit is an operational transconductance amplifier, or OTA. Early Ross units used the RCA CA3080E OTA⁵, the same chip found in the MXR Dyna Comp. An OTA is a type of op-amp whose gain is set by a control current rather than fixed resistors, which is what makes it useful for compression: the circuit measures the input signal and feeds a control current back to the OTA, so the gain falls as the signal gets louder and rises as it gets quieter. The result is a smoother, more even output level and longer sustain.

The rest of the original circuit used 2SC1849 transistors, which General Guitar Gadgets notes are now obsolete¹. Modern replicas substitute available parts while keeping the topology. The General Guitar Gadgets replica, for example, uses the CA3080 as IC1 and 1N914 diodes for D1, D2 and D3, with an LED at D4¹.

The controls are simple:

GuitarPCB's compressor documentation describes the Sustain knob's role plainly: "The Sustain knob controls the compression level perfect for country, blues, and Southern Fried rock"¹¹. Two knobs is the classic format, and several Ross-derived pedals keep it, including JAM Pedals' Dyna-ssoR with its "simple two-knob design"¹².

The OTA approach has proven durable. Jackson Audio builds its Hour Glass dual compressor on "dual OTA analog circuits"¹³, and HTJ Works describes its Vintage Analog Compressor Core as "an OTA-based circuit that reproduces smooth sustain and tonal balance while preserving the dynamics of the instrument"¹⁴. Both are part of the design tradition the Ross shares with the Dyna Comp.

Ross and the MXR Dyna Comp

The Ross Compressor's relationship to the MXR Dyna Comp is the most discussed thing about it. General Guitar Gadgets states it directly: "The Ross Compressor was copied from this compressor," referring to the MXR Dynacomp⁶. JHS Pedals puts it more strongly still, calling the Ross "a mildly modded Dyna Comp circuit"⁷.

What the mods amount to, according to Aion FX, is mostly in the power section: the Ross "has better power supply filtering and thus lower noise, but is otherwise very similar to the Dyna Comp"². JHS notes that Ross performed these same mods on the Dyna Comps of the 70s, and offers a service that performs "the exact same mods that Ross performed on the Dyna Comps of the 70s," turning a Dyna Comp into a Ross Compressor for less than buying a vintage original⁷.

The Dyna Comp's own influence extends well beyond Ross. A GuitarPCB technical analysis notes that its "electronic circuit was also used as a reference design on many other compressor effects like the Ross Compressor, T-Rex CompNova, Ibanez/Maxon CP5/CP9/CP10 and boutique pedals"¹⁵. MXR, now under Jim Dunlop, continues to sell the line in several forms, including the Script Dyna Comp, the Dyna Comp Deluxe, the Dyna Comp Mini, the Super Comp (which updates the Dyna Comp with an Attack Level control) and the Dyna Comp Bass Compressor¹⁶¹⁷¹⁸.

The closeness of the two circuits shows up in the parts bin too. General Guitar Gadgets' Ross Compressor replica and its Dynacomp replica are so similar that "either can be built on the same PCB or with the same Kit"¹⁶. Griffin Effects sells the conversion in both directions, offering a Dyna Comp Ross Mod Service that "converts the Dyna Comp to the same circuit as the Ross compressor"¹⁹, and describes the Ross mod as "the holy grail of compressor mods"²⁰.

Original production and the Taiwan units

Documented details of the original production run are limited. The pedal was made in the 1970s⁵, early units used the RCA CA3080E OTA⁵, and the original units used the now obsolete 2SC1849 transistors¹. The gray enclosure is the version most commonly referenced by builders making replicas¹⁰. Vintage originals now sell for $300 to $400 on eBay and forums, a price JHS cites when explaining why players might prefer a modded Dyna Comp or a clone⁷.

Pedals based on the Ross circuit

The list of pedals that trace back to the Ross circuit is long, and it spans boutique builders, small makers and DIY projects⁸¹⁵.

Analog Man's CompROSSor is a particularly direct descendant. The company's manual describes the product as "based on an improved version of the Ross compressor"²¹, and its Ross mod page explains the philosophy: "We still use the Ross circuit, we just improved the output section to fix these problems"²⁴. The Keeley Compressor belongs to the same family tree; its documentation traces the design through circuits that "are, in turn, based on the classic Ross OTA compressor"²⁵.

Wilson Effects' Compressor V.2 shows how builders extend the original design: it keeps the Ross circuit but adds a clean blend and other features for shaping the compression²². Stomp Under Foot's Divided Drive takes a different route, embedding a Ross-based compressor section alongside drive in one pedal⁵.

DIY kits and replicas

The Ross circuit is a staple of DIY pedal building, largely because the 1999 schematic made it accessible². Several kit and PCB projects are available:

• General Guitar Gadgets Ross Compressor Replica: a DIY effects project kit and PCB, a "Vintage Ross Compressor Replica project" whose complete kit "includes all the parts and materials you need to build one, including the PCB"¹. The project uses a CA3080 IC¹.
• General Guitar Gadgets Ross Compressor Replica Kit: a DIY stompbox kit that "includes all the parts and PCB to build this great compressor"²⁶.
• GuitarPCB MoRC: a DIY compressor PCB project described as "our modified version of the Ross Compressor," which is "also extremely similar to the vintage Dynacomp as well but enhanced for sonic clarity and sustain"²⁷.
• Aion FX Aurora Compressor / Sustainer: an independent project based on the Ross / Dyna Compressor design².

Because the Ross and Dyna Comp circuits are so close, the General Guitar Gadgets projects for the two pedals share a PCB and kit, so a builder can construct either version from the same set of parts¹⁶.

Pedals on this page

MXR Distortion Plus

MXR · Distortion

JAM Pedals Dyna-ssoR

Jam Pedals · Compressor

MXR M282

MXR · Bass

$159.99 list price

Analog Man Comprossor

Analog Man · Compressor

Keeley Compressor

Keeley Electronics · Compressor

Ross Compressor Replica Kit

ROSS Electronics · Compressor

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