I've nearly finished designing the circuit.
I'm going to connect a small speaker so I've decided to move most of the gain to the LM386 because the op-amps dont't have enough bandwidth over the audio range.
The diode produces about 250:mu:V of noise which will be amplified by two op-amps, one with an optional filter selected by a switch to produce pink noise.
The circuit is pretty similar to the one below, except for the following:
- The op-amps have a much lower gain.
- They are both configured as single supply amiplifiers.
- I've put a DC bias on the capacitors so they don't need to be non-polarised.
- The noise source is a zener rather than a transistor.
**broken link removed**
**broken link removed**
I have a question regarding the LM386 circuit: what value coupleing capacitor should I use?
The datasheet says 250:mu:F but with an 8
hm: speaker the lower cut-off frequency will be:
[latex]F_{C}= \frac{1}{2 \pi RC}= \frac{1}{2\pi 8 \times 250 \times 10^{-6}} = 80Hz[/latex]
I've worked out that for 20Hz I need a 1000:mu:F capacitor but is it really worth it as my cheap speaker will probably also roll-off at 80Hz.
Should I bother with 1000:mu:F or go for 470:mu:F which will give a roll-off of just over 40Hz and might be a good enough compromise?
Another question is, I'm tempted to go for a larger capacitor than 50:mu:F for the ripple by pass capacitor; is it worth it? I'm going to use unregulated supply and ams worried that a 47:mu:F capacitor at worst case might be only 37.6:mu:F (-20% tolerance) so it might not be good enough. I'm tempted to go for 68:mu:F or even 100:mu:F.
To be honest, I've never used an LM386 before and I want reasonable performance but I don't want to use too oversized components.