Dinosaural OPA-101 Overdriven Preamp

The Dinosaural OPA-101 Overdriven Preamp is a rare overdrive and preamp pedal designed by Dan Coggins, formerly of Lovetone, and hand-built in small quantities between 2012 and 2016, with only a few hundred estimated to have been made¹. It is now discontinued². The pedal was sold direct only, either through Dinosaural's website or Facebook page¹.

The circuit: op-amp stages around a two-transistor clipper

The OPA-101's circuit is built around an op-amp input gain stage and an op-amp output buffer, with a middle two-transistor amplifier stage between them. That middle stage is what Aion FX describes as "responsible for the unique clipping characteristics," and it is a diode-less clipping arrangement: an overdriven two-transistor amplifier that serves as the clipping element itself, rather than a pair of clipping diodes¹³.

Aion FX's 2020 trace of the circuit turned up a few details worth knowing for anyone looking at a used unit¹:

• The op-amp gain stage at the output has been hard-wired as a unity gain buffer by jumpering the feedback resistor, which Aion FX reads as a late-stage change to the design³.
• A resistor and capacitor going to ground near the volume control form a filter that cuts the treble frequencies after the clipping stage. This treble-cut network is present on all known units and appears to be a late design change as well³.
• Component values were not entirely fixed: of the units Aion FX examined, "two of them used 4k7 for R13 and one used 2k2"³.

There is no tone control. In its place is that fixed treble cut after the clipping stage¹. Aion FX notes the lack of a tone control is a common complaint among owners, and that two known owners swapped the fixed resistor for a trimmer to make the treble cut variable³.

From the Tube Bender to the OPA-101

The OPA-101 was intended as an update to the [Dinosaural] Tube Bender, Coggins's original design from 2003¹. The Tube Bender itself was similar to a silicon Tone Bender, based on a design Coggins did for Guitar and Bass Magazine, and used an all-discrete bipolar transistor topology⁴³. Before the 2016 Tube Bender reissue, Dinosaural had released two other commercial pedals: the OPA-101 Overdriven Pre-Amp and the OTC-201 Opticompressor⁵.

What changed between the two pedals is the gain staging. In the OPA-101, an op-amp input gain stage and an output buffer replace the Tube Bender's discrete stages, while the middle two-transistor clipping stage is kept the same¹. Aion FX puts it plainly: "the only thing in common is the unique clipping arrangement"³. Guitar.com's Richard Purvis, writing about the later Hypoid Drive, noted the lineage too: "That later turned into the OPA-101"⁶.

Bypass, and what replaced it

The OPA-101 offered a choice of buffered or "hardwire" bypass. The hardwire mode is not technically true bypass, because the signal is always fed into the input of the circuit even in bypass³.

In 2020, Aion FX traced the OPA-101 circuit and released its own adaptation as a DIY project, the Ixora Overdriven Preamp¹. The Ixora adds a tone control and a true bypass hardwire mode, addressing the two most common criticisms of the original³.

For readers weighing the OPA-101 against other circuits, it sits in the territory covered by Preamp Pedals and Overdrive Pedals: a gain stage designed to push an amp or act as its own low-gain drive, rather than a distortion box in the sense of Distortion Pedals.

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