# Built a electret preamp but poor low frequency response

I have built this preamp using the TL071 IC.

It works, but it does have a bad low frequency response. If I plug the mic directly to my PC i can hear low frequencies much better. I don't have any software equalizers active on my PC. What is the problem? Also, is this design good?

So let's analyze the low frequency cutoff:

At the input we have a high-pass filter made by C2, R2 and R3. For the audio signal R2 and R3 are in parallel so they are combined 50 k Ohm.

The cutcoff frequency is at

$$\f_c = \frac 1 {2\pi RC} \$$

For C = 0.1 uF and R = 50 k ohm that gives $$\f_c\$$ = 32 Hz

That's a bit high, I suggest that you replace C2 with a 1 uF capacitor which will lower $$\f_c\$$ to 3.2 Hz, that's low enough.

At the output something similar can happen depending on what you connect to the output. If you would connect to an input with 50 kohm input resistance then again you would get a cutoff point at 32 Hz.

However, most line inputs have a much lower input resistance though, in the order of 10 kohm. That will move the cutoff point to a much higher frequency and you will definitely hear that, the 32 Hz at the input is still " OK-ish" but much higher than that and sound will suffer (as you have experienced).

Again I suggest increasing the capacitor's value. I would make C4 10 uF, yes that's 100 times larger than you have now!

• C2, R2 and R3 have a cut-off of 31.8 Hz
• C5 and R5 are OK at about 3.9 Hz

But the killer might be your input impedance that connects to A_OUT2. If it is (say) 10 kohm then the cut-off due to C4 and the unknown impedance (assumed to be 10 kohm) is 159 Hz and will get worse such that if the input impedance is 1 kohm then the cut-off will be 1.6 kHz. Try upping C4 to 10 uF and see what happens.

Also, is this design good?

The TL071 op-amp isn't recommended at supplies lower than 10 volts - you have 9 volts and that will fall with battery life. Choose a more appropriate op-amp.

• Where do you read that 10V thing in the datasheets? Commented Oct 10, 2018 at 17:46
• @PlasmaHH In recommended operating conditions before the main tables on performance. It states +/- 5 volts. Commented Oct 10, 2018 at 18:19
• Ah, looks like I was looking at the ST micro datasheet and you at the TI ... Commented Oct 10, 2018 at 19:52