Noise Generator Circut Understanding

[geekygenius]

Hello,

I am new to electronics, and I found this circuit, and I want to make sure I'm understanding how it works right. I've attached an image with the schematic and what I think its parts are. The only things I don't understand is the resistor tied to ground at the output and the 2u2 cap near the amplifier. I think the resistor is a pull down, but I don't understand why you would want one inside a noise generator.

Source forum post: http://electro-music.com/forum/post-322518.html

Anyways, thanks for any help/advice!

 

[Harald Kapp]

The pull down at the output doesn't seem to be necessary. No type of opamp is given, so this is a wild guess: the opamp may operate best if some current is flowing through the output transistors. The 1Meg feedback resistor may be to large, so the designer of this circuit may have added the output resistor to provide some load.

The 2-2µF capacitor blocks DC. So the DC gain of the opamp is 1. For AC the capacitor acts as a short circuit seting the AC gain by the 5.1k and 1Meg resistor. Thus the DC operating point is set by the lower 470k/470k voltage divider, while AC gain is set by the feedback resistors.
Note that the capacitor in the feedback loop will limit the lower frequency range of the nise spectrum, thats why it is rather large

 

[geekygenius]

Thanks for the reply Harald. I've had trouble understanding capacitors and how they relate to AC and DC signals, but that really helped me to understand. I was reading the thread on the other forum and the 386 was being used. Does that require the 10k pull down?

 

[Harald Kapp]

The LM386 is an audio amplifier, not an operational amplifier. It is not particulary well suited for this application.
You can use any general purpose operational amplifier, e.g. ua741, TL081 etc. (note: the link is to TI's webpage. This doesn't mean I particularly endorse TI. Many other manufacturers produce similar chips).

 

[KrisBlueNZ]

Good answers Harald.

As Harald said, the pulldown resistor on the output isn't normally used. It can be used to avoid crossover distortion in the output stage if you're using a cheap op-amp such as an LM324, but my solution is to use a good op-amp that's designed for audio applications.

The 2.2 uF capacitor also makes the amplifier a high-pass filter, i.e. its gain is low for very low frequencies, but the main reason for it is as Harald explained.