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 kind B class design. AB class design for high voltage amp makes much heat even for modest biasing. This is a
big loss of efficiency in there. Depending on listening level AB class biasing can contribute to more energy loss than actual
music signal energy loss. 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 near by 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 velocity feedback amplifiers(velocity amplifier feedback is proportional to
back EMF from the driver). Voltage drive measures best at resonance and current drive everywhere else. Velocity feedback amp
measures worse than current drive, but sound is very interesting subjectively speaking. Needs more research and it is not for
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
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.
Noise floor of an amp depends on the impedance of the load. Because it is current not a voltage amp. So noise floor is higher than it could be with voltage amp. It's fundamental and can't be avoided for true current amp. Some cheats maybe possible, but not worth it as anyway i see the noise floor ridiculously low on simulations.
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.
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 off 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, ant this type of signalling 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.