New Amplifier Board
A 2021 development log. Not a product page.
New amp board is in progress.. 2021 02 03
The baffless main audio system is based on current drive amplifiers. They work almost 10 years now very well, but they need to be improved. The requirements for the new design are:
Very high performance galvanically isolated I2S input DAC should be integrated on amplifier board. This simplifies future multiway systems. Just add and multiply concept. Simple plug and play. DAC integration on board improves performance. No possibility for ground loops. No additional wiring.
Current drive amplifier should be floating load design. This means not to use improved Howland current pump design previously used. Main reason is the possibility of circuit linearisation with low current low voltage precision circuit. Power circuit can be quite bad and simple. Low power circuit will take care of that.
Power circuit should be simple bullet proof design. This means it has to be powerful and reliable with minimum parts involved. Power supply connection, standby power and isolated digital in connection.
Amplifier should be cold design. It should be class AB with minimal biasing — just enough to stay linear, not so much that idle current becomes the main loss. On a high-voltage amp that heat still has to leave through the heatsinks, so extra bias wastes both energy and radiator budget. High quality music has high crest factor and average listening is not so high in level and not so long in duration. Switching amps would be even more efficient, but EMC compatibility and pollution with noise of circuits nearby would be a very big development challenge for a very high performance for now, so not used.
Auto power on feature should be added. Amplifier should not be controlled by another system, it should be independent from other systems. It knows when to power on and when to shut down by himself.
Amplifier has to be current drive all the way except at resonant frequency of the driver. It is because voltage drive has better THD at resonance at least for tested high quality drivers and provides electrical damping in case it needed. This can be done in DSP also.
Amplifier has to be high performance first, price will be good anyway as no alternatives are existing.
What have been done so far...
Some progress have been done so far.
Assembled and measured some concept of proof amplifiers on bread board. Measured current drive, voltage drive and kind of negative impedance amplifiers (negative impedance amplifier feedback is proportional to back EMF from the driver). Voltage drive measures best at resonance and current drive everywhere else. Negative impedance amp measures worse than current drive, but sound is very interesting subjectively speaking. Needs more research and it is not for now.
Assembled voltage-current amp to see the concept is working. Voltage-current amp is voltage at resonance, current above that.
Made a simulation of a voltage-current amp. At least theoretically it looks promising. THD is 0.000x in simulations.
Soldered the DAC part of amplifier board.
DAC part of amplifier board
Has to be done: layout and manufacturing of PCB. And real test. If everything ok, than add additional protection features to the board and finish the board in 2–3 developing stages.
Challenges
Compared to voltage amplifier, THD of this amp also depends on the current sense resistor. So care should be taken here. Making amp current drive is very easy, but making it voltage at resonance and current elsewhere is very tricky. This was unexpected.
Update 2021 02 09
Amplifier board one channel layout is ready. It lacks some features, but is good to verify the concept, and to see the performance. It is a test board and is in manufacturing stage now.
Amplifier board layout
Update 2021 02 21
DAC part of the board works fine. Looks like performance of the DAC is perfect. Signal to noise ratio measures 118dBA. Dynamic range is better, but can't tell in numbers because of the measurement limits of my measurement gear. THD looks like better than 0.00075%, but it is also limited by measurement gear.
Now I have to make code for microcontroller which controls DAC chip. And improve my measurement gear for better specs. A small board which will allow to do that is already on the way. So I will check the real THD and DNR of the board. Anyway the performance of the DAC is good enough not to worry about it.
The amp part of the board is still in the production... Looking forward to get it faster.
Update 2021 02 24
LVDS transmitter board is made and in production. It is because my DAC/AMP board is I2S signal input, and this type of signalling is only good from board to board for small distances (example same enclosure). System has to be universal and robust so LVDS signalling has to be used.
LVDS transmitter
Update 2021 03 31
Looks promising. The concept works. Have to tune things a little bit more and to bake rev2 PCB with changes made. That will be very close to final design.
What came of it
The board that came out of this work is used in Single Speaker Stereo.
