I have built myself an Ambika with all the sharp documentation that Olivier Gillet shared (thanks so much) but I am facing an issue with the 2 SMR4 boards that I have: They both have the same problem:
I have no audio coming out of the board and the note is triggered only by some of my midi notes.
The audio signal is suppress just after the input AC coupling capacitor of the filter.
By bypassing the analog filter I have verified that the audio is heard.
How can I debug the filter?
What about the issue of notes non-triggering?
Thanks in advance
Debugging the filter: Check the schematics and trace the signal at the output of each darlington buffer (pin 8 and 9 of the LM13700).
Have you checked the power supply? That both the -8V and +8V rails are present.
About the notes non-triggering… The motherboard doesn’t know how many voicecards are connected to it, so it’s maybe sending notes to non-existant voicecards. Have you correctly configured the voice allocation?
Thanks Olivier for your guidance,
I have the power rails ok, although the -8 is around 7.8V
Audio signal is attenuated a lot at the input of the first stage of the filter:
LM13700N_IC3.P4 is a sine of 60mVpkpk centered on -0.6V (for a sine of 2Vpkpk on C21 with a C3 note and a value for freq of 79) and at pin5 I probe a DC signal at -470mV
Pin 8 and 9 of both LM13700 IC2&3 is 0.6Vrms centered on 4.2Vdc.
I observed I_CUTOFF voltage and it is stuck to -7.0V, whatever the value of the freq parameter (0-127).
V-REF is at -3.3V (which I guess is ok)
For the non-triggering of the note I am ashamed… you are perfectly right, it was sending notes to 3 voicecards so only 1/3 of the note - this part is solved!
Still couldn’t find what is the problem with the filter.
I have tested the LM13700 and TL074 amps with a 4P board and they are working well.
I have checked the impedance of the power lines up to the IC socket and they are all below 0.5ohms.
No short between the IC socket pins.
The two junctions of 2N3906 transistors seems fine (~0.7V for each) and they are placed in the good direction toward silkscreen.
Could it be a parasitic ohmic bridging? I_cutoff stuck to -7V seems strange to me but I coudn’t tell…