Over compression easily happens, particularly with stealth devices like the one I recently made (SOMA = SOM 2). The SOMA is a very unobtrusive and exceptionally easy to use compressor of very high quality and versatility.

"Auto variable attack and decay are easy to accept once the player learns to trust them. Their convenience tends to spoil one for manual modes."  - Nick Boscorelli, The Stomp Box Cookbook, 2nd ed., p.241

Thus, to avoid overdoing it by getting high on it, 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

Some people stubbornly advocate to "tune your compressor by ear". Now read this you folks:

"The nature of soft knee [compression] prompts the player to rethink compression control settings. Hard-knee threshold defines the level at which the control voltage first alters gain of the VCA. In a soft-knee device this point is impossible to hear. Ratio varies with signal level. Soft-knee compression sounds subtle enough that the player cranks the ratio so high as to defeat the point. Players seeking audible compression should stick to hard-knee transfer. Soft-knee settings are best fixed by meter, rather than boosting the ratio to the point of blatant compression."  - Nick Boscorelli, The Stomp Box Cookbook, 2nd ed., p.164-165

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.

Since people asked for it, THAT suggested a simple and low parts count below threshold indicator (DN113 and DN132). However, the input signal of a unit using such an indicator might have long returned above the threshold again (and the indicator with it), while the output truly still encounters severe attenuation due to the side-chain’s timing mechanisms. This is typical for say, LDR based compressors. Such a display just tells you, when the compressor gets active… Clearly, this indicator is of very limited usefulness. We can call it one-dimensional.

As different, the structure of SOMA lends itself readily to implementing a genuine gain reduction indicator, because the CV that governs the compression depth is directly related to the degree of compression and it is readily accessible.

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.

Although a LED’s brightness is not linearly correlated to its driving voltage – the connecting law involves logarithm –  compression works logarithmic too, so the same linear voltage that drives the VCA for a log compression can be used for driving a circuit  that exhibits log brightness. Note that this circuit is not related to Boscorelli. Indeed, I have not seen it being used in a similar context elsewhere.

Gain Reduction Indicator LED driver: (click on the picture to load larger image) The circuit is technically a precision current source, or at other times a constant current driver, dependent on its application. It comes out ridiculously trivial.

Both applications are well described here, so any further explanation is futile:

• https://www.allaboutcircuits.com/technical-articles/how-to-buffer-an-op-amp-output-for-higher-current-part-1/
• https://www.koheron.com/blog/2018/10/16/understanding-the-basics-of-laser-diode-drivers/

This circuit scales SOMA's 0-100mV CV to a 0-2mA LED current. Note that the LED starts to light up immediately after deviating from zero volts – its turn-on-voltage drop is „counted out“ by the OPA. Thusly, the exact type or color of the LED is inconsequential, however it should be ultra bright.

But how much current do we want through the LED? Using a super-bright LED, even 2mA will turn it on blindingly bright. Let this maximum brightness value be the setting that reflects deep compression resp. a large CV.

Our CV is 100mV at maximum (positive going), and since the OPA tries to make the voltages on both inputs equal, the expected voltage across the resistor must be the same. Ergo, the resistor must be R=U/I =100mV/2mA = 50 Ohms (47R chosen).

►	Note that this resistor does not have the function of a conventional LED series dropper resistor, as one might have expected: it is a current sense resistor.

Indeed this value works very well. You can discern shallow compression from deep compression by looking at the LED’s intensity, and you see how fast attack and decay are.

►	If you feel the LED is too bright towards deep compression, try changing the resistor to 56 Ohms.

Note that the transistor is used in a linear fashion, „class-A“ so to speak. It is subjected to roughly to the full voltage (9V in this case) minus the LED’s turn-on voltage. This amounts to some 40mW power dissipation, where the BC550 is rated for 500mW. Watch out for the maximum allowable dissipation, if you change anything!

Also, this circuit will only work if the OPA is a rail-to-rail unit like the TLC2262, since the signal drops to zero. The negative rail is not needed (and should not be used, requiring a different circuit without additional benefit)

Since the current through the LED is modulated in a linear fashion, no sudden current jumps are produced that could leak into audio paths.

Its input is high impedance so it cannot possibly annoy the CV going to the VCA.

Overall, the addition of this indicator is unproblematic and is really worth while. It is very useful when exploring different combinations of the compressor's settings, particularly when using the soft knee setting.

• Feb. 23, 2026: update on chp. "Know Your Compressor's Gain Reduction"
  
• Feb. 23, 2026: first release