The firmware for the DAC input selector has been debugged extensively, and now seems ready for real life testing. The source is here: SPDIF input selector source. The firmware has a manual, and an automatic selection mode.
- If the button is pushed for less than about 2 seconds, the selector enters manual mode, and skip to the next input.
- If the button is held for longer that 2 seconds, the firmware scans each input for one second and selects the first input that has data (audio).
The firmware works by sniffing the I2S data line to the DAC, to see if anything is going on there. To prevent the DAC from playing anything, before an input is selected, a relay has been added to the DAC board, to only enable the I2S data line to the DAC, when a signal has been selected.
When the project is finished the compiled firmware will be made available along with the rest of the design files.
I replaced the PSU capacitors for the power amplifier (C508/C509) with some 15000uF/50V parts I had salvaged somewhere else. This may be a little overboard, since the value of the capacitors in the original may very well, serve as protection against overheating the amp. The big idea being, that as the current draw rises, as the amp is turned up, the capacitors can not maintain their charge, thus lowering the supply voltage to power amp (and adding distortion). There is a possibility, that the amp can now overheat at high volumes, due to the larger reserve.
While I had the amp opened, I adjusted the bias (idle current) as well I upped it to 100mA, measuring 100mV across (R455/R456). This means the amp delivers a little more power before leaving class A, and heats up a bit more. I therefore added an extra heat sink, to the one already installed.
The bass of the amp now sound more controlled, and thereby somewhat less excessive, leaving a little more room for the midrange and treble to shine. This is consistent with what others have heard, changing the PSU capacitors. The increased bias current probably does not improve things to the same degree as the capacitors, and I will not recommend this mod unless you are prepared to keep an eye on the amp, checking it is not overheating. But change those capacitors.
All in all this is a really nice amplifier, and due to the simplicity of the circuit very mod-able. I won't regret the money I spend for a nanosecond.
There was an error in the last PCB posted, here comes the correct version.
I have just assembled and tested the prototype shunt regulator for the DAC. Inpsite of long wires and veroboard it performs a little better than the LM317+CSS+diode shunt I've been using so far. When they arrive I will try replacing the transistors with some low noise BC560 types, to see if they perform better.
The peaks at 50Hz, 100Hz and so forth comes from the SPDIF -> I2S converter board, where the PSU has not yet been optimized.
Here's a list of things to be done before the final product is posted:
New PCB layout At least testing a better power supply, I'm thinking TL431 shunts
- Try some BC560's in the shunt regulator.
- Testing some bypass tricks, around 22pf from supply to ground, and the same value from Iref to ground
Low noise BC550/BC560 transistors instead of BC547/BC557
Testing a better power supply for the CS8412, again I'm thinking TL431 shunts The SPDIF input circuit is bypassed, figure out why it does not work
- Possibly a new PCB layout
uC input selector:
- Install some bilateral switches to avoid switching the actual signal to the DAC before we know there is a signal
- Ponder on SPDIF signal detection, do I a need to program a PLL?
I know that nothing has been posted about the uC input selector yet. Basically I'm not the least satisfied with the way I have solved the problem up until now.
A PIC16F628 switches the input relays, until a signal shows up on the DATA line of the CS8412. meaning that the actual sound is output from the DAC as well. I have decided to try measuring at the SPDIF inputs by using bilateral switches instead. I may have to lock on to the SPDIF signal to detect audio data. the solution that I'm hoping will do, is to establish the length of the pulses on the SPDIF line, if there is data, some pulses should be half the length. I'm yet uncertain as to whether the preambles will screw this up. If so I will have to detect these.
http://members.chello.nl/~m.heijligers/DAChtml/Analogue/IV.html). Besides presenting a better impedance to the current output of the TDA1545, the active I/V stage also brings the output to about 2Vpp, in it's current configuration. The I/V stage was designed by "rbroer" of diyAudio, the original thread is here: Single rail, active I/V for TDA1543, TDA1545A.
The sound with the active I/V is definitely better. My test setup is a long way away from my stereo, therefore the DAC has to drive a 20 meter long cable, which has always led to a "muffled" sound. Since the active I/V has better current and voltage driving capabilities, this is now gone. This is a wonderful experiences, no magic, simple logic, that manifests itself, in the way I would have expected. Everything is firm and in control. As far as I can see this can only improve things, even when the DAC is back in place, with a half meter of cabling.
I will post a the relevant layout files, when I have finished testing and tweaking.
Generated on 2018-05-03 01:14:21.838435