Hi folks,
We have an amplifier circuit that uses a lm386m-3 to a little 0.1 watt 8 ohm speaker. The board is completely through hole and has worked for years. Recently the components were changed to SMT, swtiching to a lm386m-1. The layout hasn't changed significantly. The circuit is used to amplify the signals of neurons in insects. We are looking at signals up to 1300hz. The circuit is powered from a 9v battery and the entire setup is not grounded.
The issue is that when the SMT board is setup the same way as the through hole, just starts screaming. Its possible to reduce it somewhat by grounding the board, either by putting a hand on the battery or battery terminal. Plugging in an external speaker disconnects the input to the lm386 and the board works with no noise.
So far have tried several boards, rf filter ahead of the ad623, different gains and filters ahead of the lm386, and changing the gain on the lm386 itself. We tried boards with ground pour, no ground pour, round corner traces, 4 layers, etc. Different boards have different behavior, but they are all noisy or need to be grounded. Sometimes the noise is just screaming, sometimes there is a voltage spike, like a capacitor discharging and the signal is completely lost. I'm hoping there is something simple that I'm missing, or if the smt version of these parts are that much different from through hole. The original through hole board uses ceramic capacitors and one electrolytic in series with the speaker. The smt board uses mlcc capacitors with an smt electrolytic in series with the speaker.
We have an amplifier circuit that uses a lm386m-3 to a little 0.1 watt 8 ohm speaker. The board is completely through hole and has worked for years. Recently the components were changed to SMT, swtiching to a lm386m-1. The layout hasn't changed significantly. The circuit is used to amplify the signals of neurons in insects. We are looking at signals up to 1300hz. The circuit is powered from a 9v battery and the entire setup is not grounded.
The issue is that when the SMT board is setup the same way as the through hole, just starts screaming. Its possible to reduce it somewhat by grounding the board, either by putting a hand on the battery or battery terminal. Plugging in an external speaker disconnects the input to the lm386 and the board works with no noise.
So far have tried several boards, rf filter ahead of the ad623, different gains and filters ahead of the lm386, and changing the gain on the lm386 itself. We tried boards with ground pour, no ground pour, round corner traces, 4 layers, etc. Different boards have different behavior, but they are all noisy or need to be grounded. Sometimes the noise is just screaming, sometimes there is a voltage spike, like a capacitor discharging and the signal is completely lost. I'm hoping there is something simple that I'm missing, or if the smt version of these parts are that much different from through hole. The original through hole board uses ceramic capacitors and one electrolytic in series with the speaker. The smt board uses mlcc capacitors with an smt electrolytic in series with the speaker.
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