Dunwich Amps DA120 Overdrive: Boosted Matamp GT120 in a Box

The information contained within this blog post was compiled from several sources, but basically quotes information I found in various forum posts, Facebook Group posts, and through my own direct correspondence with Nick Williams at Dunwich Amplification. The DA120 Overdrive is an incredible achievement in tone, and I thought it worthwhile to share all of the info I’d gathered with you all. Enjoy!

Info From the Dunwich Amps Website – www.dunwichamps.com

“The Dunwich DA-120 Overdrive is a MOSFET Emulation of a boosted/overdriven amp with the overall block diagram of the design taken from my DA120 amp (which in turn was based on the 70s GT120). The idea with this pedal was to get the low-medium gain tones and texture of the amp and also achieve higher gain sounds of the amp being boosted.

The total amount of gain can be controlled using Boost, Drive, and Gain controls. Although each controls gain, they function differently and affect the gain at different locations in the circuit. The pedal shines best when a little bit of amp gain is combined with the gain from the pedal.

Frequency shaping is achieved using the Depth, Bass and Treble controls. These controls mirror those found on the DA120 amp where the Bass and Treble are a passive EQ (bax/james) and the Depth controls the RC low end cutoff early in the pedal.

The pedal can be run off a 9V center negative supply but if desired it can run up to 18V (center negative) for additional headroom and output swing.”

More Info from Nick At Dunwich

“It’s not exactly the preamp of a GT120; it’s more like an emulation of a pedal hitting a GT120 and it does have a phase invertor and power amp emulation section inside it as well. It’s got a lot more gain on tap than what you would get out of just a GT120.

In general, the pedal is designed to sound like Sleep’s Dopesmoker, or like the guys from Windhand, or any band who drives the front end of a GT120 with another pedal to achieve a really saturated sound.

It’s not really designed to stack with other pedals but that would depend upon how much gain you are using on it. You can dial it very clean if you want and it might be better with other pedals. I only use it into my super low gain amp with 150W on tap.

I would hazard to guess that none of the Matamp based pedals are made to go into a loop. To my understanding the loop is really only for time based effects (verb, delay, modulation), since you want to get all your clipping in before you add those effects (in some people’s ideal world).”

Circuit & Controls

The DA120 Overdrive circuit design follows the basic block diagram of a 70’s Matamp GT120, with the addition of a Post Phase-Invertor Master Volume followed by a midrange Boost:

Gain Stage – Depth – Drive – Gain (called Volume on a GT120) – Gain Stage – EQ – PI Driver – PI – Master – Boost (~700Hz)

Depth is similar to the original Depth control on the GT120 but it has different capacitor values. The control basically selects from among 6 different cap values that are connected to the output of first gain stage and those caps from a high pass filter with the circuitry they are connected to. Small cap value, higher cutoff frequency, less bass; large cap value, lower cutoff frequency, more bass.

Gain and Drive control the actual gain levels at two different points in the circuit in two different ways:

Gain is basically equivalent to the Volume control on the original GT-120, it is controlling the amplitude of the signal from the output of the first gain stage into the input of the second gain stage.

Drive sets the gain or amplification factor of the first gain stage from ~3dB to 30dB in total range.

Bass and Treble are identical to the original EQ on the GT120; it’s a passive James/Baxandall EQ which are ~shelf filters.

Master occurs after the Phase Invertor / Power Amp emulation and sets the overall volume of the pedal.

Boost (in the current revision) is a 700 Hz global mid range boost that comes very late in the signal chain (post all other controls actually). It ranges from 0dB to 20dB of boost centered around 700 Hz, with the idea being that the Boost is more like a Presence control on an amplifier, which typically happens in the power amp section and effects the global mid range. The Q factor tends to get sharper as you increase the Boost setting.

“Sleep Dopesmoker” Settings

Running into an amp set loud, and just on the edge of breakup:

Master – Just loud enough to drive the amp above unity
Boost – 10 o’clock
Treble – Min
Bass – Max
Drive – Max
Gain – Max
Depth – Max