The Diamond Compressor is a high performing compressor that is very popular for its tone enhancing abilities. I decided to make my own version as personal project for bass, calling it the "Under Pressure". Here is some analysis of the circuit and an explanation of the included mods. Shown below is the audio signal path.
This circuit uses a BJT buffer at the input. Using a technique that is relatively uncommon of pedal circuits, the standard common-collector buffer is enhanced with an active load. The emitter resistor is replaced with another BJT configured as a constant current source. Two diodes hold Q1 base at about 2 x ~0.7V, and the transistor's VBE drop puts the emitter at ~0.7V. The fixed voltage at the emitter creates a constant current through R5 and thus the Q2's collector. This improves the the amplifier's ability to source current while maintaining linearity. This is especially important because the buffer is directly driving the variable gain element, a vactrol.
An optocoupler, or vactrol, (shown here as R22 for simulation purposes) is used here to provide attenuation by shunting the signal directly to ground when its LED is supplied current. Because of this arrangement, the compression ratio is fixed by the properties of the vactrol, supposedly around 3:1. The Diamond Comp is a feedback compressor, meaning the gain reduction happens before the signal is fed to the envelope detector, creating a negative feedback loop. The feedback design, fixed ratio, and optical control make this comp suitable for gently smoothing out signals rather than clamping down on peaks. Following the vactrol attenuation is a gain stage using a high-quality audio op-amp, the OP275, which features a specialized input amplifier for exceptional audio performance. This op amp keeps the THD+N very low. The first op-amp stage provides a gain control which increases the signal level being fed to the envelope detector to increase the amount of compression. This stage also includes some low pass and high pass filtering at the edges of the audio range.
The audio signal is fed into another active-load buffer, and then into an interesting “Tilt” EQ. This EQ implements a high and low shelf filter that is flat in the middle and can simultaneously boost and cut +-6dB around a center frequency, 900Hz in the original. Additional capacitors can be added in parallel to adjust this center frequency.
Shown above is the schematic for the envelope detector. The envelope detector uses a RC4558 op amp to create a buffered and an inverted signal. This op amp doesn’t need to be as high performance as the OP275 since it is not in the audio path. This section employs two high pass filters on each side to sharply roll off frequencies below ~140Hz. Lower frequencies close to DC will cause the RMS level to fluctuate, which manifests as audible ripple. Filtering out these frequencies prior to the envelope generator creates a smoother control voltage. The buffered and inverted signals are fed into a BJT differential pair, which acts as a full wave rectifier. This rectified signal at the output is then smoothed out by a capacitor, creating the control voltage representing the RMS level of the audio. A resistor converts this voltage into a current, which is then used to drive the LED in the optocoupler. This type of rectifier is used in other compressor pedals such as the Ross/Dynacomp/Keeley as well as the Carlin Compressor, which is the basis for our first pedal, the Astronomer. The Under Pressure has been modified to include a variable resistance in the rectifier, which limits the speed at which the capacitor charges, slowing the attack to allow more transients through, a great addition for bass players who want a bit more punch.
The designers of the Diamond Comp took great care to maintain excellent signal quality. Overall, it’s is a relatively hifi circuit that offers tasteful compression that is perfect for an always on pedal, and can still give you the heavy squish or jangle that some guitarists love.
The Under Pressure Comp was a personal build and is based on the original diamond comp, but is modded to include elements from different iterations of the diamond to make it perfect for bass guitar:
- 9V to 18V charge pump. The BCP-1 used a reverse polarity 18V power supply. Boosting internally allows for greater headroom with the use of a standard 9V supply.
- Tilt EQ Frequency toggle. This was adopted from the Bass Comp reissue and can center the EQ around 500Hz or 250Hz.
- Attack knob adapted from the guitar-version reissue’s attack toggle.
- Larger coupling caps to preserve bass frequencies.
I made this for personal use, but the Mooer Yellow Comp is an affordable clone of the original, and uses the same hifi op amp. It should be more that enough for most people, especially if run off of >9V.
References:
Ambler Tilt EQ (Small Signal Audio Design, Douglas Self, p.429)