Analog current drive amplifier

This is a stereo board with balanced inputs. It suits full-range systems especially well. You can run it without DSP, but I would not: the onboard analog input filter is less precise and takes real work to tune. Input sensitivity and filter values are set by soldering resistors.

The job of current drive is to cut distortion in the loudspeaker driver. The idea is old, but outside DIY it is still rare. Current drive is not ideal at the driver’s resonance, so this amplifier is mixed-mode by design: voltage drive at low frequency, current drive at high frequency. A jumper selects voltage amp, mixed mode 1, or mixed mode 2.

In current mode, voltage controls output current, so the output voltage depends on the load. To keep mixed-mode response reasonably flat, the front end trims gain and filter shape for the driver — again by soldering, with no DSP. The board is meant for a naked driver in an active system, with no passive filters. Multi-driver passive speakers will work, but you lose some of the current-drive benefit.

Turn-on and turn-off are silent: the board comes up and down with the supply. The same circuit also flags a supply fault or a sudden voltage sag.

The power stage is an LM3886 in a composite setup, with a second layer of overcurrent and over-temperature protection. Some of those functions can be jumpered. The board is designed for a switching supply. A toroidal supply can sag enough to trip protection; you can disable some of that, but this amplifier was not designed around a transformer PSU.

The complete user manual, with all technical information, is here.

Equalised to the same response, voltage drive and current drive still sound different. Fair A/B is hard, so I built a DEV amp that switches modes from a phone. The sound opens up: more real, more involving. Most of the difference is in small, subtle details. On voltage drive those sounds are there, but damped, as if behind a thin filter. On current drive they simply appear. It is a more relaxed, natural listen — like taking the back off a box speaker and hearing open baffle.

Current drive is not a drop-in upgrade for speakers designed around voltage drive. For a new system I would not build it any other way. I have used this for 15 years. Tube amplifiers sit somewhere between voltage and current; some Nelson Pass designs also run high output impedance.

The measurements agree: 10 dB less THD is common, often more. Good drivers already sit around −50 to −60 dB, so that is a large step, and it hits the distortion you actually hear. Another interesting paper about current drive here.