Calibration And Installation

Usage

I had made a similar indicator for the Engineer’s Thumb compressor.

As I describe there in detail, the principle goes back to an application note from THAT. It works well for the Engineer’s Thumb with minor changes, but the principle cannot be transferred to the RCC without substantial redesign, because its front-end is meant to monitor a (relatively slow changing) DC value as typically present at a genuine side-chain’s control signal, which is representative for the degree of compression, whereas the RCC basically controls compression with a copy of the input signal as control voltage, while the time constants are mostly governed by the LDR’s properties.

Some additional signal conditioning components would have been needed for the front-end, which would have increased its complexity unduly in relation to the simplicity of the bare-bone unit it is supposed to supplement.

The beauty of THAT’s circuit is their method of current-steering between two LED’s, which prevents the massive current-surges that would appear otherwise during on- and off cycles, were only a single LED employed.

Those surges can create audible artifacts and are well feared and loathed in LFO circuits and the like, because they are a pain to get rid of.

THAT use a comparator that detects the onset of compression (or, if you want, the „above resp. below threshold condition“) using cross-connected transistors for current-steering. Their unit "looks" at a DC signal and compares it to a fixed value set by a pot. Adding more components to make it work would of course have been possible, but the component count was already high. So a superior method came to mind.

Audio pioneer John Linsley-Hood had used CMOS gates throughout his developments for detecting analog signals, a ground breaking work that I have described here. He utilized a peculiarity their inputs exhibit, namely a very precise voltage detection window around half the supply. So a spontaneous voltage level seen on the output of the LED driver can be compared to this center value to trigger the respective gate. Subsequent gates wired as inverters can then be utilized to provide the current-steering function.

Download the CMOS indicator for RCC schematic V1.0 By downloading the file you agree to the Terms of Use.

The input of the indicator assembly connects directly to the output of the OPA that drives the LEDs (note: before the electrolytic cap). It thus „sees“ some AC riding on top of the bias voltage. The latter is irrelevant since it is cancelled out by setting the threshold pot accordingly. (see later).

One four-input NOR gate does it all. The series diode works as a peak detector, the other two diodes are there for protection only. There is no storage capacitor for the peak detector (which is usually there for visual persistence), since the first gate works akin to a switch de-bounce circuit that creates a dead time through R1/C1. This method requires a non inverting OR to work, hence the second gate is configured as inverter. G1 followed G2 constitute an OR then.

When G1 registers a peak ("1"), the output of G2 charges the capacitor rapidly and feeds back a "1" to the respective input of G1, until R1 has depleted C1 again. G1 will drop off afterwards, unless the stimulus on the peak detector still exists. The peak triggered this machinery, but may have gone again meanwhile. This is the typical behavior of a de-bounce circuit.

The two inverters following turn on the LEDs alternatively. The green LED (lights up when no signal is present, when the signal is below the detection threshold, and when the unit is turned on and idle) and changes to the red one when the threshold is crossed. A CMOS gate can easily drive a LED directly.

Akin to THAT’s circuit, there is always only one LED on. Although the transition is extremely fast (a CMOS gate switches in microseconds), a small current notch is inevitable. This transit can potentially be audible as a faint click, but the supply of this auxiliary board is designed to decouple from the main board’s supply via its own RC combination. In this case, a 100µ electrolytic in conjunction with a 100R resistor. This is crucial, since it kills the spike stone dead, so don’t skimp on this.

This supply filter will produce an overall voltage drop of slightly less than a volt, but since the gate’s threshold changes ratiometric with its supply, the absolute value of the CMOS supply voltage  is of no concern once calibrated. CMOS gates run with a 3-18V supply voltage.

The detection circuitry is of  very high impedance and does not irritate the driving circuit. Its application is not restricted to the RCC, but can be applied to any unit in order to inform you about a certain level being crossed. I used it with success as a clipping warning in a certain build, replacing the normal "effect on" indicator LED for a behavior akin to the one described above.

Back To Index

---

Calibration And Installation

For calibration an oscilloscope and a frequency generator are needed. Connect the indicator sub-assembly. Set the input voltage to about 100mVp-p. Set „sustain“ to minimum. The green LED should light up. If not, turn the trim pot, until the green LED turns on. Turn a little further well into the green.

Monitor the output of the OPA that is wired to the LDR with the voltage probe. Start increasing sustain until you see the output being affected. Turn the trim pot until the green LED just changes to red. Verify this setting by riding the sustain pot over this area and seeing the LED change when it should. Done. This is my personal approach of calibration.

A small auxiliary board can be fabricated that augments an existing build. The power connections are planned to go to an already present node after a protection diode. Otherwise you may want to add such a diode. The LED’s common cathode can conveniently be wired to the ground switch that likely exists for a typical true bypass switching scheme, where normally the cathode of single LED would go. Using a multi-turn cermet trim pot for adjustment is recommended.

Back To Index

---

Usage

Note that this is functionally another „above threshold indicator“ (or below, dependent how you view this), and it tells you nothing about the depth of compression or the current state of release. Due to its slow nature the LDR needs a fairly long exposure to the light to react, but then persists when the light is long gone. The compressor might thus well be in its release phase when the excitation is long gone. Since a transit from the „side chain“ might be too short to behold, the dead time has been chosen fairly long with the R1/C1 values (1M/330n, = 330ms rise time). Those values were found empirically, trying to avoid nervous detection chatter.

Unless a compression gauge can reliably display the true amount of reduction, it has to be taken with a grain of salt what it tells you. However, as with all compressors, you should see the compressor go in and out of compression frequently, otherwise  you remain in static compression, which is just another word for: attenuation.

But this the here depicted „above threshold indicator“ can do. A visual clue helps a lot when the ear has gotten tired. And, in accordance with THAT’s quote (p.2), adapting it to the present situation:

"As one might expect, the complexity and functionality of this circuit is roughly proportional to the one it supplements."    - The Author

• Oct. 13, 2025: small update
  
• Oct. 12, 2025: first release