Analog current drive amplifier
Overview
This amplifier board is stereo with balanced inputs. Very good choice for full range systems. It is possible to use without DSP, but not recommended. Analog onboard input filter will not be as precise as DSP and requires knowledge and effort to tune. Input sensitivity can be adjusted by soldering resistors. Input filter parameters also adjusted by soldering resistors.
This amplifier is made to be current drive amplifier to reduce distortion in loudspeaker driver. It is nothing new but almost not existing outside DIY world. But current drive is not optimal at drivers resonance. So this amplifier is mixed mode by design. Voltage drive at low frequency and current drive at high frequency. Actually board is universal and has 3 modes of operation: voltage amp, mixed mode 1, mixed mode 2. Modes are chosen by the jumper.
Current mode means voltage controls output current. So voltage output depends on the load. So for mixed mode to be relatively flat, amplifier has frontend which adjust gain and filter response for any given driver without the need of DSP. (adjusting done by soldering). This amp designed to work with the naked driver without passive filters, for active systems. Passive systems with multiple drivers and filters might not work as expected. It depends on the system. Current drive benefits can be reduced for such systems or DSP needed to flatten frequency response.
Amplifier has silent turn off/on feature. Amplifier turns on/off silently by supplying voltage from power supply or by controlling enable/disable input pin. This input pin is fully isolated, it doesn't add any ground loops or other noises. Silent turn on/off feature also will acts as power loss indicator in example due to fault in power supply or inadequate current . Sudden sag of power supply voltage will trigger it.
Amplifier has several protection features. One is clip protection. Because clipping is distorting signal a lot and should not happen anytime in high quality audio reproduction chain.
Second one is fast overcurrent protection. It also can act as oscillation protection.
Third is over temperature protection. All these three protections are controlled by onboard IC. Status of protection is outputted over onboard LED. Also there is fault output pin for external IC interface. It is also isolated.
The amp uses LM3886 chip as power driver in composite configuration, which itself has second layer of additional overcurrent and over temperature protections. The amp is safe to use. Some protection features can be disabled by soldering some jumpers. Amplifier is designed to work best with switching mode power supplies.
Advantages of current drive
Subjectively the sound is different voltage drive vs current drive equalised to exactly the same response. To accurately compare this is not an easy task. Differences can be short term and long term. To compare short term differences i had to develop DEV amp. Which allows to switch in between modes on the fly by pressing button on the phone. That helped me to conclude that the sound becomes more open, more real. Most difference is in small details. On voltage drive these subtle sounds kind of exist, just they are not noticed like in current drive. Kind of hidden or damped somehow. Like through some filter. On current drive these nuances just appear without the need to focus on them. It is more natural relaxed feel when listening to music. It is like removing some layer which block transparency. This feeling reminds me one when i do near field comparison between box and open baffle by opening back panel from box speaker. Someone who familiar with open baffle sound can get what i mean.
Of course current drive might be not the best fit for existing voltage drive designed speakers, but for new system designs this is no brainer for me. I use this technology for 15 years and happy with it. Tube amps also are kind of between voltage and current devices. And some Nelson Pass designs are also high output impedance.
Objective measurements also shows clearly THD reduction with current drive. 10dB reduction is not uncommon to see with current drive or more. Knowing that good drivers distort at minus 50-60dB range it is a huge improvement. Best is to tackle distortions where they are big and audible first. I am pretty sure static or sine sweep measurements doesn't reveal the true story. And Fast Subband Adaptive Filtering with real audio signal should be proper way to measure voltage vs current drive. But these are done by other people and can be looked at here
Below some measurements on Visaton BG20 fullrange driver. As expected 3rd harmonic is reduced the most. Around drivers resonance voltage drive looks slightly better also as expected. This is one of the reasons why mixed-mode is used in this amplifier. Measurements shown are full voltage and full current drive. No mixed-mode. Harmonics are shifted to its original places. So for 1kHz signal third harmonic level is at 3kHz mark. We can see that above 2kHz spectrum is a lot cleaner.
Pic.1 Voltage drive 94dB at 1m
Pic.2 Current drive 94dB at 1m
Pic.3 1kHz tone current(white) vs voltage(red) offset for visual purpose
Pic.4 2kHz tone current(white) vs voltage(red) offset for visual purpose
Pic.5 intermodulation 1k,2k,3k current(white) vs voltage(red) offset for visual purpose
Pic.6 intermodulation 1k,10.5k,20k current(white) vs voltage(red) offset for visual purpose
Board price
Board price is 250 eur. This is for assembled and tested board with mounting brackets. Shipping not included. Shipping from EU.
Other things needed for powering the board is heatsink and power supply.