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.
So for the receiver I am building I figured that some sort of filtering is going to be required in the audio stages. Not entirely sure yet on the final make up of things, but to start with I thought that I would design and evaluate a high pass filter to cut out the low frequencies as this is likely to be fixed weather i am receiving CW or SSB. Though for the low pass filter, i do want either variable bandwidth or select able widths. More on that later.
As with all filters more orders, more betters LOL. And so i jumped online to a calculator tool and quickly designed up a 3rd order Sallen-Key highpass with a cutoff of 350hz. The simulation looked reasonable so i then simulated it in LTSpice just to confirm things and check the OpAmp i had chosen was going to be ok.
Schematic of 3rd Order Sallen-Key Highpass Filter.
Simulation Bode Plot
Next the circuit was built on a solder-less breadboard, the OpAmp is an NE5532 and negative supply rail is an LM2662 Charge Pump. This gives the OpAmp plenty of room to swing when powered with + – 5v.
For the initial testing the OpAmp was fed with 1vpp 600hz sinewave.
Dumb people do dumb things and I spend 10mins wondering why I had 10x gain in a circuit that should have unity gain, then i noticed i had the scope probe on 1x not 10x where it usually lives. There is my 10x gain.
For final shits and gigles i busted out the bode plotter and swept the filter from 10hz to 5000hz to see just how it really looks, and other than a little noise down close to DC, which i think is just the frequency generator not liking being that low, the filter itself is pretty much as designed. -40db at 100hz should be good enough for the kinds of girls i go out with.
Next job will be to either add in a couple of low pass filters for typical CW and SSB filter widths or have a crack with switched capacitor lowpass filters and make it variable. Thats a job for another day.
So I have had this pile of Ebay sitting on my desk for sometime and today I have gotten excited enough to actually start taking a look at it all and seeing how it works. I got these frequency generator chips for like a buck and after setting them up with the test circuit, i could not get them to work, it happens, now and again you get Ebay’d in the butt.
So I figured next i should test out the TDA2822 audio amps. I for 50 for 2 bucks which is a lifetimes supply. Do they work, well, yes they do and here are the results.
Here is the test circuit straight out of the PDF. As you can see parts count is low. So I put the IC on the breadboard and used just 1 1/2 for a mono amp. I also used just 2 caps, pin8 to ground, 470uf as it was already on the breadboard and the input cap on pin 1. Powered with 8v as its a handy voltage i have on my breadboard. 12v would probably be a better option to allow for a larger voltage swing.
As you can see, nothing fancy here, just the IC and 2 caps and my signal gen and oscilloscope probes doing there thing allowing the pixies to in and out and display them in the screen.
So we stick in 0.1v 600hz sinewave and see what happens.
Well, we actually hit the voltage rails and clip somewhat. 0.1v in almost 8v out, that is the voltage gain there. And when i do some da finger poken, the IC itself is cool to the touch, not warm, not hot, but about the same as ambient temperature of the room. So i am thinking Bye Bye LM386, and hello life time supply of TDA2822. And being a stereo IC, I can also bridge the left and right for even greater output. Not that I think i would need it.