Electric Guitar Interface
- High-impedance input for electric instruments
- Preamp drive circuit with germanium diodes
- Amp simulator with analog tone shaping controls
- Envelope follower and gate voltage extractors
- Dedicated eurorack-level FX loop
- Eurorack and line-level audio outputs
Direct instrumental injection
Injectr is a fully-equipped electric guitar and bass interface for eurorack synthesizers. The module features an overdrive-capable preamp, versatile amp simulator, and control voltage extractors designed to inject stringed instrument signals directly into the modular environment.
Injectr’s high-impedance instrument input, INST IN, feeds straight into a variable gain circuit which can produce a range of tonal responses, from clean to heavily overdriven. The smooth character of the DRIVE parameter is made possible by a soft-clipping topology using a classic 4558 op amp and four diodes, two silicon and two germanium.
A post-drive LEVEL control is provided to trim the output of the preamp stage, which is accompanied by a RED LED, indicating that the signal is reaching the typical eurorack audio level of 10 Vpp. Because stringed instruments tend to have strong transients, it is suggested to adjust the LEVEL control so that the LED is illuminated with moderate to heavy playing.
Buffered directly from the instrument input is a responsive envelope follower circuit, which produces a control voltage signal at the ENV output jack based upon the input’s volume dynamics. The ENVELOPE parameter adjusts the sensitivity of the circuit, depending on how the instrument is being played. The adjacent WHITE LED is a visual indicator of the envelope follower’s output. The switch labeled SLOW MED FAST selects the follower’s response time, affecting both the rise (attack) and fall (release). An inverted copy (negative going voltage) is present at the ENV- output jack.
Additionally, a 0-5 V gate control signal is present at the GATE output jack, which is derived from the envelope signal and is indicated by the GREEN LED. The sensitivity of the gate extractor circuit is adjusted using the GATE parameter, and essentially sets the threshold at which the gate signal is triggered on and off.
One of Injectr’s most useful features is the Amp Simulator circuit. Using a complex arrangement of analog filters, it imparts an amplifier-like quality to the signal.
The CHARACTER parameter alone can dramatically alter the sound, giving the user combined control over the middle “dip” frequency and bandwidth. The PRESENCE parameter adjusts the low-end response, thickening or thinning the overall tone, whereas the BRIGHT switch enhances high frequencies. In combination with pickup selection and tone controls of the guitar itself, these parameters provide a wide range of tones all while staying within the eurorack system. If preferred, the Amp Simulator can also be toggled off by disengaging the SIM ON switch.
The module is equipped with a eurorack-level effects loop, FX SND and FX RTN, serving as an access point to process the signal after the Preamp/Drive circuit but before the Amp Simulator. It is also useful to process the signal with eurorack modules before taking the signal back out using the Line Out.
High-gain modules such as overdrive/distortion and intense compression may cause unwanted feedback in the FX Loop, and so are best patched after Injectr from the OUT jack.
The OUT jack provides a post-simulator eurorack-level audio signal, which is useful when it is to remain in the system to be processed or mixed with other signals.
The LINE OUT jack provides the same signal at line-level, which is suitable for sending the signal out to external gear, interfaces and amplifiers.
PEAK LED INDICATOR
Using a 1/4-inch headphone adaptor, plug a 1 kHz sine wave or similar source signal into the INST IN jack. Turn the DRIVE knob down to about 1. Use an oscilloscope to measure the peak-to-peak voltage from the OUT jack, and set the LEVEL knob so the signal output is exactly 10 Vpp. Now adjust the PEAK LED trimpot on the module’s right side until the RED LED just barely turns on.
ENVELOPE ZERO VOLTAGE
Unplug or mute your instrument from the INST IN jack. Adjust the ENV ZERO trimpot on the module’s left side until the WHITE LED just barely turns off. You can also precisely measure the voltage from the ENV jack and adjust the trimpot until it outputs exactly 0 V.
Width: 11 HP
Depth: 1.06 in (27 mm)
Weight: 4.5 oz (128 g)
Input impedance: 1 M ohms
Power: +12 V 38 mA, -12 V 30 mA
FOLLOWER FILTER – Patch an analog filter into the FX Loop and control the cutoff frequency using the ENV signal. Use the SLOW MED FAST switch to adjust the filter’s response time. This is basically the classic “envelope follower filter” effect, and it can often be even more interesting when using the inverted envelope signal.
GATED VCA EMPHASIS – For a heavy gated effect, patch a VCA into the FX Loop and use the GATE signal to open and close it. This can chop your signal off abruptly, or can add emphasis if the VCA initial setting is already partially open. Tempo-synced modulations of the VCA are also very fun in this configuration.
DYNAMIC BLENDED TONE – Use the FX SND jack to take the “dry” signal (without the Amp Simulator) and mix/crossfade it with the “wet” OUT signal (with Amp Simulator). The mix level(s) can then be modulated with the ENV or ENV- signals. Pan the two signals away from one another in the stereo field for an added spatial effect.