Microphone preamplifier circuit

guangshen

Microphone preamplifier circuit [Copy link]

According to the following schematic diagram, the microphone preamplifier circuit is designed. When the microphone is not connected, the pins 6 and 1 of the OPA2604AU have a sinusoidal periodic signal. The same problem occurs on both the PCB and the breadboard. Is there a problem with the circuit schematic?

guangshen

Among them, SWA, SWB, SWC are used to connect switches and set gains

guangshen

I am a novice and I feel that it is not a PCB problem, but I don’t know why the noise signal is introduced. Please give me some advice.

scxuchenglong

What does "When no microphone is connected, there is a sinusoidal periodic signal on pins 6 and 1 of OPA2604AU" mean? I don't understand

chineseboyzxy

Self-excitation? When there is no input to the MIC preamplifier circuit, does the input end have to be grounded? The microphone socket of the power amplifier seems to be like this. When the microphone is unplugged, the input end is short-circuited to the ground.

guangshen

chineseboyzxy posted on 2020-8-22 08:58 Self-excitation? When there is no input to the MIC preamplifier circuit, does the input end have to be grounded? The microphone socket of the power amplifier seems to be like this, the microphone...

The microphone has two input interfaces, one for input signal and one for ground. When not connected (VinA), it is in floating state.

guangshen

scxuchenglong posted on 2020-8-22 08:35 What does "When no microphone is connected, there is a sinusoidal periodic signal on pins 6 and 1 of OPA2604AU" mean? I don't understand

Pin 6 is the feedback input, and pin 1 is the output of the amplified signal. There is no input signal connected. There should be no signal on these two pins. Does the circuit generate its own signal?

chunyang

Let's first clarify the amplitude and frequency of the output waveform when the input is floating.

guangshen

chunyang posted on 2020-8-22 20:03 First, let’s clarify the amplitude and frequency of the output waveform when the input is suspended.

When the microphone is not connected, the waveform of the sinusoidal signal fed back from pin 6 is as follows: The waveforms of pins 7, 1, and 2 are as follows:

chunyang

guangshen posted on 2020-8-23 12:24 When the microphone is not connected, the waveform of the sinusoidal signal fed back from pin 6 is as follows: The waveforms of pins 7, 1, and 2 are as follows

The frequency is in the MHz range, and the amplitude is not high. It is suspected that it is induced by an oscillator in a nearby circuit, such as a crystal oscillator circuit. It may also come from power supply conduction noise, such as a switching power supply that works at a MHz frequency. For further testing, you can temporarily remove C5 and test again. In addition, you can try connecting a small capacitor in parallel at both ends of R7.

jiguangshen

The problem has been solved. To sum up:

The 5 input pins of the microphone preamplifier circuit always have ripples as shown in the figure above, which is caused by the 24V to 18V power converter used. After trying to replace the power supply, the ripple becomes very small at around 2mv.