The hissing from my ma12070p board magically stopped. I don't know what fixed it. Based on testing this at 12V via battery, and observing that the hissing seemed proportional to supply voltage, I found a 15V supply to use (instead of the 24V I was initially using). I figured if the hissing was quieter, and I put the lid on the chassis, I could live with the hiss.
So I modified it to use the 15V supply, and let it run continuously for a day or two. Thus far, I'd only been using it with cheap speakers. But since it had been working predictably (except for the PCB hissing), I decided to change to my nicer speakers. I continued to let it run continuously.
And yesterday I noticed there was no more hissing! I don't know exactly when it stopped, actually, as at 15V with the case lid on, it was fairly easy to ignore. I did a power cycle, and the hissing did not return. Just now I reverted back to the original 24V PSU, and there is no hissing!
Now my problem is that it appears there is some noticeable latency/lag between issuing commands (start/stop playback, volume change) and hearing the result of those commands on the speaker. I don't know how to measure this, but it appears to be on the order of half to three-fourths of a second. Enough that, for example, AV sync on video playback is so bad as to be unwatchable.
Also, here's a little Python code for doing some basic interaction with the ma12070p via I2C. It's not complete or "production ready" by any means, but hopefully serves as a useful example.
Code:
#!/usr/bin/python3
from smbus import SMBus
def status(bus):
for offset in [ 0x1d, 0x40, 0x35, 0x36, 0x7e, 0x60, 0x61, 0x62, 0x64, 0x65, 0x66, 0x6d, 0x75, 0x7c ]:
b = bus.read_byte_data(address, offset)
print('offset {0}: value={1} / {2}'.format(hex(offset), hex(b), bin(b)))
# I2C address is determined by pins 28/AD0 and 29/AD1
# 0x20 is the default for the ma12070p reference board
address = 0x20
bus = SMBus(1) # using RPI GPIO physical pins 3 and 5
print('initial status:')
status(bus)
# enable limiter
# bit 0 set to 1 => default i2s_sck_pol [default]
# bit 6 set to 1 => use the limiter
bus.write_byte_data(address, 0x36, 0x41)
# enable processor AND use right-justified I2S
# bit 3 set to 1 => use the audio processor
# bits 0:2 all 0 => i2s standard / right-justified 20 bits
bus.write_byte_data(address, 0x35, 0x8)
# set volume
# default is 0x18 => 0 dB
# 0x19 => -1 dB
# 0x20 => -2 dB
# ...
# 0x3a => -34 dB
# 0x40 => -40 dB
# 0xfa => -50 dB
# ...
# 0xa8 => -144 dB
bus.write_byte_data(address, 0x40, 0x3a)
# set power mode profile 3 (PMP3)
bus.write_byte_data(address, 0x1d, 0x3)
print('status after writes:')
status(bus)
bus.close()
