So here’s the thing. LOL how many times have i started a new blog post with So. You know what it means though, that something did not go to plan, did not work and just became an exercise in learning. Yes, this is one of those times when I learned a lot and still have nothing functional and working at the end of it. 🙂
So JLCPcb were running a special of 30% off on boards. I had been tinkering with an audio amp and filter board for a while but it was no where near ready. I really had been lazy and just not put any time into finishing it. So while searching around for something else I came across this Indonesian site that contained downloads of Gerbers for some of their projects. So i downloaded one and looked over the schematic, it looked interesting so i then rushed into getting my audio board ready to make and sent off for the boards.
Now I did do somethings very wrong here, I played the main components like the relays, IC’s etc and locked them in place and used the autoplacer and autorouter to place all the passives and route it up. Obviously not the best idea, I also should have put a via fence around the negative voltage inverter and the same to isolate the 2 audio filters. So anyway, i got the board populated and started checking voltages. No 12v rail to the 2nd opamp, audio preamp or the audio amp. Seems that the 12v net did not get autorouted in 2 places and I did no checking to ensure things were right. Noob mistake, but i was rushing. Bust out the MOD WIRE.
So does it work? As pictured NO. The relay below the voltage inverter on the right hand side of the board is acting as an antenna for the voltage inverter oscillator frequency, which in turn is causing the audio amp to oscillate like crazy. Removing that relay and bridging the pads fixed that problem. The second major issue was the values I used for the filters.
Normally i would simulate in LTSpice just to confirm things are ball park. Turns out that the calculator i used, uses the frequency supplied as the -6db point. Kind of useless for a CW filter where you want 600hz peaked not attenuated by 3db LOL. So i will suck out the parts and replace them with something more appropriate now I have redesigned the filter response and simulated it.
These are not going to be the sharpest filters in the shed. They just have Salen-Key responses. I did not want brick wall filters, I just wanted something to make listening more pleasant. Eventually I will do the aggressive filtering in the IF using DSP. I have a partial design for this happening already, but it is a long way off being ready. Crystal filters it is for now, but DSP IF is coming, it just takes time.