I have built this audio amp circuit on a ground plane board and have taken care to limit trace lengths and provide as clean a layout as I could come up with. I'll try to attach pictures of the board layout later but I'm posting the circuit now. The amp works really well I think but I am interested in further reducing the noise (hiss). This amp has an unusually high gain because my goal was to detect very faint sounds. This project was started to give me an excuse to learn about analog circuit design. I put this together from months of casual research on amps, filters, drivers and AGC. The final design has a pretty good signal to noise ratio on the 120 ohm output with headphones but both outputs have some noticeable noise. I don't have suitable equipment to make accurate assessments regarding the actual noise level or S/N ratio. I'd like some pointers on further reducing the noise. I'm of course limited to through hole components but I am using low noise op amps and metal film resistors throughout. The input stage is a diff amp because I wanted to be able to experiment with noise cancelation. Currently I'm using only the one microphone on the board as shown on the schematic.
Please let me know what you think. My initial research included some low noise design principals but I am a novice so any suggestions are appreciated.
thanks
skip
P.S. Scematic Correction: The resistors R11, R12 and R13 are shown with incorrect values for a 300hz high pass filter. The correct values are R11 = 1k, R12 = 390 and R13 = 6.8k.
Please let me know what you think. My initial research included some low noise design principals but I am a novice so any suggestions are appreciated.
thanks
skip
P.S. Scematic Correction: The resistors R11, R12 and R13 are shown with incorrect values for a 300hz high pass filter. The correct values are R11 = 1k, R12 = 390 and R13 = 6.8k.
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