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I'm trying to amplify my dynamic microphone's (SM57, 1.6 mV OCV, 150 Ω) differential signal to about instrument pickup level (-20 dB) to drive effect pedals.

Currently, I'm testing with an AD620AN to amplify it with G around 1000 (47 Ω gain resistor) to line level for now, but I'm having a lot of trouble with noise.

circuit

I used a balanced low-pass filter from the TI Analog Engineer's Pocket Reference with a differential cut-off at ~23 kHz since I have quite a bit of radio interference, from the equation

$$f_{D} = \frac{1}{2\pi (2R_2)(C_1 + 1/2C_3)}.$$

I also attached a 10 μF blocking cap at the output for a HP filter with the cut-off at ~10 Hz given a large enough load.

When I connect to an LM386 to test the audio, it does not pick up a lot of lower-volume input. The output is extremely fuzzy and distorted. How would I improve the response? Should I add some voltage buffering as well for my supply rails?

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  • \$\begingroup\$ I have no experience with audio equipment, but I imagine the microphone expects to see some load impedance that is comparable to its output impedance. Instrumentation amplifiers however are purposely built to have very high input impedances. You're probably better off with another amplifier. Try placing a resistor (several 100s ohms) across the microphone input to see if things are improved. \$\endgroup\$
    – polwel
    Jan 24 at 19:39
  • \$\begingroup\$ Please show your PCB layout and note that a high input impedance will be fine for this microphone and is not causing the problem. The 386 could easily be generating that noise all by itself if PCB layout is poor or supply wiring/tracking is inappropriate. \$\endgroup\$
    – Andy aka
    Jan 24 at 20:45
  • \$\begingroup\$ Needs decoupling capacitors. Deriving the split supply this way will not work. The reference input requires a low impedance connection to ground reference. \$\endgroup\$
    – RussellH
    Jan 24 at 20:57
  • \$\begingroup\$ This is fun, but you're re-inventing the wheel. All you need is a (VERY cheap) level converter such as: pyleaudio.com/sku/PDC21/… These can be found all over the 'net for sub- $20 \$\endgroup\$
    – Kyle B
    Jan 24 at 20:59
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    \$\begingroup\$ Check whether the AD620 can give the output swing required for a 1.6 mV RMS signal with a gain of 1000. Also note that the 1.6 mV is for the reference input of a 1000 Khz, 94 dB (SPL). Dynamic microphones can take almost any input sound pressure level. Also make sure the AD620 is OK with a 10 μF capacitor on its output. \$\endgroup\$
    – Theodore
    Jan 24 at 21:16

2 Answers 2

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I've been in this situation in the past. Amplifying microvolt-level signals with good SNR is a nightmare.

I can spot two mistakes in your circuit.

The first mistake: R2 and R5 are way, way too big. The thermal noise voltage of a (combined) 44k Ohm resistor over a 20kHz spectrum at 25°C is 3.8µV RMS, which is already on the same order of magnitude as low-volume audio signals from a dynamic microphone. The noise from these resistors alone will be louder than some of your audio signals. You can calculate the noise power density of a resistor using the formula for Johnson-Nyquist (thermal) noise.

The noise contributions in your system are (referred to the input):

  • Microphone (150 Ohms): 1.6 nV / sqrt(Hz)
  • AD620AN: 8 nV / sqrt(Hz)
  • Low-pass resistors (44k Ohms): 27 nV / sqrt(Hz)

Multiply this with the square root of the bandwidth and you get the RMS noise voltage. As you can see, the bigger the resistor, the bigger the voltage noise. To fix this, you can remove those 22k resistors entirely (replace them with wire links) and place the capacitor of your RC lowpass directly across the mic's output. Then you're just using the mic's well-defined internal resistance (150 Ohms) as the RC filter's R. Alternatively, you can also use 100 Ohm resistors without significantly degrading the noise performance.

One way or another, the 22k resistors have to go, though.

The second mistake is in your rail splitter: The AD620's REF input is not high impedance. In fact, it's got an input impedance of about 20k Ohms. As a result, your virtual ground (generated by those two 18k Ohm resistors) is swinging all over the place and causing the AD620 to oscillate and malfunction in all kinds of exciting ways. You have to use an OpAmp to buffer the virtual ground.

Additionally, the AD620AN itself has a rather large input equivalent noise voltage (it's about an order of magnitude noisier than the mic). Consider using a lower noise instrumentation amp, i.e. the AD8429, which is an order of magnitude less noisy than the AD620AN.

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  • \$\begingroup\$ I'm really new to this so this is all extremely helpful! One question though--how did you do the noise calculation? The AD620AN has 8nV/sqrt(Hz) input noise. For audio, we have 8nV * sqrt(20000-20) = 1.13 uV. Are you referring to before or after amplification? \$\endgroup\$
    – Andrew Li
    Jan 24 at 22:29
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    \$\begingroup\$ These values are all referred to the input (before the amp). The amp's 8nV figure is also referred to its inputs. In general, you should be suspicious of any resistor bigger than a couple hundred Ohms in any ultra-low-noise circuit. You often see feedback resistors down in the tens of Ohms in such circuits for noise reasons. \$\endgroup\$
    – Jonathan S.
    Jan 24 at 22:40
  • \$\begingroup\$ Ahh, I see. In that case, would I also want to swap out the 18k resistors for something smaller? I imagine the supply noise might also contribute to noise in op-amp performance. Or a capacitor across the source to decrease ripple? \$\endgroup\$
    – Andrew Li
    Jan 24 at 22:43
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    \$\begingroup\$ A pair of large-ish capacitors (1µF or so) in parallel to R3 and R4 will be enough to reduce their noise. One cap from GND to +6V, another one from GND to -6V. Keep in mind, though, that you still need an OpAmp buffer (voltage follower) to drive the REF input. \$\endgroup\$
    – Jonathan S.
    Jan 24 at 22:45
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There is no power supply decoupling. Add a 0.1 uf ceramic and a 10 uF electrolytic cap in parallel from pin 7 to pin 4. Make sure the ceramic cap is as close as possible to the device pins with the shortest possible leads.

Your signal "ground" is actually a 9 K impedance, the Thevenin equivalent of R3 and R4. Add another capacitor pair across R4. Select the electrolytic cap value such that the corner frequency of the R3-R4-cap network is at least one octave below the lowest frequency of interest.

Note that your lowpass filter corner frequency is so high that it has no effect inside the audio passband.

IIRC the output of a SM57 is floating with respect to ground. See page 17 of the 620 datasheet for how to handle this.

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  • \$\begingroup\$ Thanks for the answer! I'm really new to this so everything is really helpful, but I also want to understand why the values are selected. Why a electrolytic and ceramic in series and why doesn't the 0.1uF suffice? And why that value for the ac decoupling? \$\endgroup\$
    – Andrew Li
    Jan 24 at 21:51
  • \$\begingroup\$ Also, I was under the impression of filtering so that only audio is outputted, so first a low pass stage, then a high pass stage after the op amp with the overall effect of just 20Hz-20kHz (and so low radio noise). \$\endgroup\$
    – Andrew Li
    Jan 24 at 21:53
  • \$\begingroup\$ Why does your microphone have 4 wires instead of the 2 wires coil plus a ground for the shield wire in the cable? Oh, you have two microphones? \$\endgroup\$
    – Audioguru
    Jan 25 at 1:19
  • \$\begingroup\$ Usually, you do the high pass filter first, followed by the lowpass. In this way, the lowpass filter is filtering whatever high frequency noise generated by the highpass filter. \$\endgroup\$
    – AnalogKid
    Jan 25 at 18:24

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