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+--- Thread: Automatic Drive Compensation (/Thread-Automatic-Drive-Compensation)
Automatic Drive Compensation - K O'Connor - 09-08-2026
Hi Guys
Since the introduction of Power Scaling back in 1990, I have occasionally used automatic drive compensation in some amps I built, eliminating the need for the second control. At that time, I referred to the control as 'Limit' and the automated feature as 'Auto-limit'.
Drive compensation is a necessity in a Power Scaled amplifier, but having a separate control for it is a turn-off for some players, and physically difficult in many installations. Some circuits are amenable to the "single-pot solution" using a dual pot to perform both functions (Power Scaling and drive compensation). However, a purely electronic approach is desired by many players and installers. To that end, I designed two kits that perform this function.
Mickey Corrieri of Soultone amps asked me for help with an idea he had to automate the drive control using LDRs (light-dependent resistors). The LDRs are actually more than just a light-dependent resistor, as they have a light source built in and are a unitised opto-coupler. The light source could be neon lamps or LEDs (light emitting diodes). LDRs were used in Fender amps to couple the tremolo/vibrato oscillator to the audio path as the actual modulation element. Fender, Rivera, Mesa and many others used LDRs for channel and feature switches.
As a drive compensator, there are two ways to wire the LDR output: in series with the signal or in parallel with it. The two situations require that the LDR resistance default be 'low' or to be 'high', respectively. This requires two different types of control circuit, hence two different resulting kits: ADC-L and ADC-H. The natural default for an unpowered LDR unit is to have a high-resistance output.
The Soultone application was specific inasmuch as the supply voltage is known and there was an auxiliary supply that could be used to power the LEDs of the LDRs. Standard LEDs can be current hogs, needing about 10mA to be lit to a level suitable as a front-panel or foot-switch box light. They can be driven with up to 25mA, some up to 50mA, for extra brightness. However, for the integrated LDR unit we only need a few milliamps for a useful range of output resistance. Mickey wanted to use the LDRs in series with the signal, for which the output resistance default had to 'low'. This means for the full-power condition of the amp, the LEDs in the LDR have to be 'on'. Using the auxiliary supply, there was no problem supplying LED current. We used current-steering (explained in TUT) to control the LED and thus the resistance imposed in the signal path. This was optimised for the LDRs he chose and for the voltage environment in the amp.
For a general purpose form of the custom application, I chose a different LDR with a more sensitive input. This keeps waste heat to a minimum and allows a major simplification for the series-signal approach.The circuit is able to work properly in voltage environments ranging from 250V to 550V. For the signal control approach used above, the ADC-L AutoDriveComp-Low-resistance default would be the up-to-date circuit choice. This board is 1" square (24.4mm square).
If you wanted to shunt the signal at the power amp input, for example, the approach has to revert back using current-steering and the board is larger at 1.3" square (31.9mm square). The ADC-H board defaults to a high LDR resistance and has to work against a fixed resistor in the audio circuit to form a voltage divider. Again, a supply voltage between 250V and 550V is accommodated. Both kits have trimpots so the control range can be calibrated to the supply voltage present. Both kits also have two LDRs with floating output resistances that can be used however they need to be.
The kits are not on the site yet.