Tuesday, September 15, 2009

A new Audio Amp for DC Receiver

With my new found abilities of home PCB construction (see previous PCB posts), I have put together the circuit layout for a Audio Amp for my Direct Conversion QRSS Receiver. It has not tested yet, but I am hopeful, . . . (that it does not turn out to be an oscillator?).

 Audio Band Pass Amp using LM324N

Yes, there are four jumpers on the board, plus one under the IC, I did not want to make this a two sided board.

And if you look very close, you can see I forgot one two solder joints on the capacitors to the right of pin 8, and 14.

Forgot Solder Near Pin 8 - This is an Easy Fix!
Note: These are 1210 pads with 1206 parts, also note that there are traces or Ground Plane under many of the parts, it is easy to create solder paste shorts under the parts - as I have found out!

Now for some testing and circuit check out!

UPDATE 1:  Well, . . It is, . . . . an Oscillator! . . . . . I will have to do some more checking on the layout, and maybe some "LTSpice" simulations. Maybe it is just a short, or a miss mounted part - alot more checking is needed.

UPDATE 2:  I think I need to look for another circuit for my "Band Pass" audio amplifier, the circuit that I used was cobbled together without much thought. What I need is about 60-80db of gain with; 200 to 2000Hz Band Pass.

UPDATE 3:   A very old Navy friend of mine, Richard Davis, after seeing the photo, offered this bit of advice on FaceBook: "Don't use Taiwanese electrolytics! ;-)". He must be very observant, as I used the Caps that I had in the junk box. Maybe, I should look at the selection of Caps closer, or maybe the International friends will also have comments.

I found the following Active Band Pass Filter Calculators:

UPDATE 4:   I found one layout error, the output feedback cap of the third stage op-amp was connected to the bias line and not the the inverting input cap as it should. That would make the op-amp oscillate!

I may change the component values used to implement a 6 pole Chebyshes Filter as the Filter Calculator above suggests, the schematic and layout stays the same (after I fix the layout error found above).


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