Over-compression easily happens, particularly with stealth devices whose presence you don't notice until you turn them off.

To avoid that, any indicator that gives you a good clue on what your compressor truly does at any given moment, is welcome; because...

"The most common use for compressors is to ruin the sound." - Rod Elliott, VCA Techniques Investigated, https://sound-au.com/articles/vca-techniques.html

Having such a device for dialing in the right amount of compression on the recently made SMASH-2 dual-mode-compressor (a.k.a. SM-2, a.k.a. up/downwards compressor, a.k.a. up/downwards sustainer) is particularly helpful.

While a full blown gain reduction meter, gauged in discrete dB steps or so, may be indispensable for a studio unit, it appears an overkill for a stomp box unit like this, besides inflating complexity unduly. In fact, it would be distracting.

For a compromise, I decided to use the brightness of a LED as an indicator for the depth of compression, while its fluctuation,  the speed thereof and the frequency of extinguishing completely inform you about the signal dynamics - a more-dimensional LED information so to speak.

A unipolar version of such a gain reduction indicator  has already been presented for an earlier (downward) compressor unit (SOMA), but this can only display the depth of compression.

The newly developed  version for bipolar control voltages is an extension thereof. To add visual information on the magnitude of boost too, it uses an RGB LED, where the red LED indicates the depth of compression (in tradition of our use in SOMA), and the green LED indicates the magnitude of boost.

At idle, the green LED is lit, reflecting the VCA’s attempt to boost as much as the upwards ratio pot allows for. This is attributed to the side-chain's design. If the upwards-ratio is turned up too much, you increasingly hear system noise.

Upon application of a signal the green LED gets dimmer (i.e. less boost is necessary to lift the signal up to threshold level), and eventually the red LED turns on and compresses according to the setting of the upwards ratio pot. When the signal becomes weaker, the red LED becomes weaker too,  until it turns into green again.

This simple yet multi-dimensional indicator turns out an invaluable and indispensable tool for setting this unit’s controls properly.

Since many of the concepts used have been explained in the description of the  unipolar gain reduction indicator, there is no need to repeat them here. I suggest you read that first.

Bipolar Gain Reduction Indicator LED driver: (click on the picture to load larger image) A common anode RGB connects to the PCB with wires (not shown). The large IC is a quad OPA, which separates positive and negative voltages from the bipolar CV and feeds them to the small IC, which in turn drives the RED and GREEN LEDs to the right brightness. The CV's magnitude is reflected by the brightness of the LEDs.

Akin to the control signal path (side chain) in SM-2, IC1 separates both directions using precision half-wave rectifiers again (Boscorelli calls them augmented diodes).

The negative voltage is then flipped upwards again by an inverter, whose input resistor presents a load by design, but the positive voltage needs a dedicated resistor to ground.

Both signals feed the proven precision current sources that work on (now) positive voltages and must not be connected to the negative rail. Find more information on the unipolar version.

As different, the LEDs have been relocated towards the upper rail. By doing so, a common anode RGB LED can be used (the blue LED is not used). For functional backwards compatibility, red is used for the CV going positive (downwards compression) and green for CV going negative.

Since both LEDs’ control signals are derived from the very CV, only one can be on at a given point in time, indicating the CV’s magnitude by a corresponding brightness level.

The LEDs used were made by HHE (Hanhua LED); the data sheet specifies an average brightness of 4000 mcd for green and 2500 for red. To compensate for the perceived brightness difference (since brightness is directly related to the amount of gain resp. gain reduction), the sense resistor for RED RsR has initially been chosen about half (27R) of RsG (47R) as in the previous project.

A negative rail is needed for IC1 to be able to process the bipolar CV, but the precision current sources themselves are again comprised of rail-to-rail single supply op-amps to simplify the LED driver. The OPA chosen for IC4 is a TL0x4 (for high input impedance), but any other j-fet input op-amp will do.

Supply is derived directly from the battery node with a minimal supply decoupling to counteract any audible repercussion inflicted by the LED's current changes. However, this is subsonic and not to be expected.

• Mar. 11, 2026: first release