Electronic – Reducing noise in audio circuit (optical pickup + op amp)


I am building an optical pickup using a photodiode connected to a LM741 op-amp. My circuit is similar to this one:

photodiode circuit

Except that I've added a passive high-pass filter after the output of the op-amp, to eliminate DC (since I am using 0V and +12V as my V- and V+, respectively). I use Rf=500K Ohm (is this way too much?). In addition, I have an LED adjacent to the photodiode that serves as a light source. The LED is powered by 5V and the op-amp is powered by 12V, both from a PC power supply. The photodiode and LED are connected to the circuit using a 2m long guitar cable ("PL").

The circuit works and produces audio signal when I modulate the intensity of the light shining on the photodiode, but my problem is that the signal is very noisy. I can hear/see two types of noise:

  1. Electrical noise similar to a noisy electric guitar pickup. I suspect that it originates in the long cable (or the tip of it, where the photodiode and the LED are conected) collecting ambient electromagnetic noise. This noise is present all the time, even when no light is shining on the photodiode.
  2. Another noise is present only when a signal is generated, i.e only when I modulate the light intensity. I suspect it is a result of amplifying thermal noise, since my gain is very high.

I would like to know what is the best approach, or in other words where to start in my effort to eliminate the noise:

  • Improving the signal to noise ratio at the source, i.e by optimizing the physical conditions (ambient light, precision of the position of the photodiode, etc.).
  • Using a different circuit – I've seen many suggestions on the web and started with the simplest.
  • Using a different op-amp, one that is more suitable as an audio pre-amplifier.
  • Improving the shielding of the pickup itself, to eliminate electromagnetic ambient noise.
  • Using batteries as power source instead of the PC power supply (I am thinking maybe part of the noise is coming form the mains).
  • If none of the above, What would be your suggestion?

Best Answer

Several things:

  1. A long cable carrying the most sensitive node in the whole system is a bad idea. Carefully shield and then tightly couple the amplifier to the pickup. Then you can send the higher level lower impedance signal over the long cable.

  2. A 741 is a joke in this application. Look for a low noise opamp. There are amplifiers specifically for audio applications where noise matters a lot. Even a general purpose device like a TL07x will be lots better than a 741, both in gain and bandwidth.

  3. Applications that need good lineariry and frequecy response usually use a photodiode in reverse bias configuration. Think of it as a diode that leaks proprtional to light.

  4. Don't try to get all the gain in a single stage, particularly the first one. The first stage should take the little input signal and make is stronger and lower impedance such that it is much less susceptible to noise. The first stage amp can do this better if not run close to its gain-bandwidth product. You don't have to care about offset voltage since audio can be AC coupled between stages. The second stage can then make a nice strong signal that can be sent over 2m wire.

Audio circuit design is mostly about thinking carefully about noise every step of the way. The bandwidth may be low, but the signal to noise ratio needs to be very high.