Riverside and Sunset

Although both Riverside Drive and Sunset Dual Overdrive share similarities, they are two very different beasts.

Riverside‘s sound, dynamics, and feel are like that of a multistage tube preamp with cascading gain stages. The clipping in these gain stages is based on the clipping characteristics of preamp tubes. Riverside can be truly thought of as an “amp in a box” pedal.

Sunset‘s overdrive circuits are based on diode clipping circuit types found in classic overdrive pedals. We carefully voiced each circuit to deliver a superlative tone and playing experience.  You can select from six different drive circuits, and stack two drives at once, essentially giving you two overdrive pedals in one unit.

In the White Paper below, Strymon Co-Founder and DSP Engineer Pete Celi provides an in-depth look at single stage and multistage gain topologies, and how they create the wide range of drive tones that we know and love.

Single Stage and Multistage Gain Topologies White Paper

Introduction

Many classic pedal gain circuits are a single-stage gain topology, meaning the gain is achieved in one section in the signal path. These circuits may have an input buffer stage whose job is to provide a high input impedance and condition the signal, and/or an output buffer following the gain stage for driving long cables and subsequent pedals, but the actual gain and clipping is achieved in one stage. Sunset features six different classic topologies to cover a wide range of boost and overdrive pedal styles.

Cascading multistage gain structures are found in the preamp sections of high gain tube amps. In contrast to single stage circuits, they create gain and clipping by connecting several lower gain stages in series with each other. By spreading the gain over several stages, a varying harmonic structure is achieved as the stages go in and out of clipping. The interstage circuitry is a critical part of the design of this type of structure. Riverside uses a cascading multistage topology to achieve its wide array of preamp drive tones.

FET Boosts

A boost adds gain with little, if any, clipping of the guitar signal. It’s a volume boost that allows you to drive the front end of your amp harder. Boosts commonly use a JFET transistor for the gain stage to provide the high impedance, lower gain, and mild clipping characteristics that are desirable in a boost. Any clipping that happens is a result of the amplified signal exceeding the linear portion of the gain stage as determined by the input signal level, the amount of gain, and the power rails.

Fig 1. JFET Boost Circuit

Boosts can be used at the front of the signal chain to buffer and drive subsequent pedals, or at the end to act as a volume boost or a solo boost, or to drive an already overdriven amp even harder. The Strymon Sunset ‘JFET’ type is based on this topology. It adds warmth and dimension, while adding just a hint of mild breakup at high Drive settings. The ‘Treble’ type is a higher headroom boost that can remove the low frequencies for a sharp tone to tighten up and push amps or other pedals.

Soft Clippers

The most common circuit topology for overdrive pedals is known as a ‘soft clipper’. Typically, an op amp or high gain BJT (Bipolar Junction Transistor) is used to achieve the gain, and clipping is achieved with diodes connected in the feedback path of the gain stage. Low frequencies are usually filtered before the gain stage, and high frequencies are filtered after the gain for a responsive mid-pushed character with a smooth top end.

Fig 2. Feedback Soft Clipper Circuit

The properties of this configuration are such that the input transient dynamics are preserved at the output. As you dig in harder, the clipped portion of the signal creates more harmonics but does not get ‘louder’, while the input signal is passed through with its associated dynamic peaks, retaining clarity. This is great for situations where the amp is already working fairly hard, and you are looking to get a dynamic push with the pedal in front. The Strymon Sunset ‘Texas’ type is based on this topology.

Another way to produce a soft clipped result is with a circuit topology that uses a hard clipper topology (more on that soon) with a separate parallel path where the input signal is explicitly mixed in with clipped signal. This manner allows for a tailored approach to the dynamic response as the Drive is increased. The Strymon Sunset ‘Ge’ type is based on this topology, voiced for a strong response in the low-mid frequencies.

Fig 3. Parallel Path Soft Clipper Circuit

Hard Clippers

Hard clippers are found in many distortion pedals. The clipping diodes are connected to the output of the gain stage so that they ‘chop off’ the peaks of the gained up input signal. They typically have more gain than overdrive pedals, but a more fundamental difference is how the hard clipper reacts to input dynamics. Instead of passing transient peaks to the output, the hard clipper will create more harmonics in response to higher input levels. This translate to a different feel from the soft clipper. You can still ‘work’ the dynamics by digging in and varying your playing intensity, but it’s about fattening up and driving harder as opposed to retaining the original clarity of the input dynamics.

Fig 4. Hard Clipper Circuit

The same filtering philosophy as overdrive pedals generally applies to distortion pedals as well. The low frequency filter may be more aggressive to keep the low end tight at high gain settings, while the increased harmonics can create a broad top end response that has some sizzle. A more relaxed low frequency filter results in a looser, ‘fuzzier’ response. The higher gain range lends itself to using the guitar volume knob to control the amount of drive. Hard clippers work well with cleaner amp tones, allowing the harmonics to be generated by the pedal. The Strymon Sunset’s ‘Hard’ type is based on this style of circuit, with a wide range of gain available from mild to heavy.

Two-Stage Circuits

Another topology for overdrive and distortion combines the soft clipper with the hard clipper. A traditional soft clipper stage creates the transient clarity and dynamics, and is then passed to a hard clipper through a network. There is still only a single gain stage, but there are two clipping stages. Depending on the network and diode configuration, the two-stage configuration can be tuned from a soft clipper to a hard clipper to anywhere in between, allowing some of the soft clip dynamics through before being hard clipped at a second stage. The Strymon Sunset’s ‘2-Stg’ type is based on this versatile topology with a fair amount of gain on tap.

Fig 5. Two stage Clipper Circuit

Multistage Gain

The op-amps and transistors used in the single gain stages are capable of producing gain well beyond what is needed for a gain pedal. The gain stages use resistive feedback to set the gain to the range that’s needed. For reference, a typical boost pedal may have 20dB of gain, an overdrive typically has 40dB or 50dB, and a distortion pedal may have 60dB or 70dB of gain.

Preamp tubes have far less gain than op amps, so several tubes stages need to be cascaded to create high gain. This was the approach used in the designs of the first high-gain tube amps, which started by ‘hot-rodding’ some of the traditional designs. The cascading tube stages create a harmonically complex signal that changes as the individual stages go in and out of clipping. The resistors and capacitors form networks that determine the gain, frequency response and output levels of each stage as the signal passes from one section to the next, and play an important role in this aspect of performance.

Fig 6. Multistage Tube Gain Circuit

The clipping characteristics of the preamp tubes are asymmetrical, meaning one half of the waveform gets clipped harder than the other half. Each stage inverts the polarity of the signal which, combined with the asymmetrical clipping, results in shifting bias points in response to transients dynamics and sustained signal decay. The dynamic response and feel is different from either a hard clipper or soft clipper due to the interaction of the multiple stages.

Stymon’s Riverside Drive uses a multistage topology that consists of four cascading (series) gain stages to create harmonics and varying levels of drive. The first gain stage is a discrete Class A JFET gain stage, and the following three gain stages are based on the asymmetric clipping characteristics of preamp vacuum tubes configured as shown above. Riverside’s continuously variable circuit tuning adjusts the values of the interstage circuitry as the Drive knob is turned to allow for optimal tuning of these components from clean to saturated gain.

Conclusion

Traditional single stage gain topologies are used for a diverse array of classic boost, overdrive and distortion pedal tones. Multiple cascaded gain stages are used to create tube-style preamp tones ranging from clean to heavy distortion.