Beta Preamp

From the vendor

A single-channel pedal recreation of the Sunn Beta Lead and Beta Bass amplifiers, unique in using overdriven CMOS stages for their drive tone.

The Beta Preamp is based on the Sunn Beta Bass and Beta Lead ampifiers, first released in 1979. The guitar and bass models were identical except for a handful value changes that made the EQ more suitable for the frequency range of each instrument.

The Beta amps were unique in that they used overdriven CMOS gain stages for their drive tone. This had been previously pioneered by Craig Anderton for his Tube Sound Fuzz circuit in 1977, and Electro-Harmonix released their Hot Tubes pedal in 1978 with a similar topology, but the Beta was the first time this method had been used in an amplifier.

The Beta Preamp is a single-channel adaptation of the original amplifier. It can be built in either Bass or Lead configurations by swapping out a few components. While it can be used as a drive pedal, it works best as a true preamp driving a power amp directly.

The original Beta amplifiers ran on +/-15V, but in this adaptation the supply voltage has been reduced to +/-9V so it can be powered conveniently by a standard 9V supply. In the original amp, the drive tone is generated by CMOS hex inverters run on a reduced 8V single-supply, so the heart of the distortion tone remains exactly like in the original amplifier. All of the other active components will perform the same on the slightly reduced voltage since there is a great deal of of headroom either way.

Note: This project uses a type of capacitor that has been temporarily discontinued by WIMA. Please see our news article about the issue and the substitution spreadsheet for project-specific recommendations. The build document has not yet been updated to reflect this.