So I'm integrating the SpeakJet into my next project, and using the LM386 to try to boost the speakers. Unfortunately, the package didn't include any circuit (or information whatsoever, really) and Radio Shack has discontinued their databooks. So I went to the go-to info tool, THE GOOGLE! I've seen this circuit all over the place:
A few questions about this circuit:
1. Is the .05uf cap and 10 resistor recessary if it's just going to ground?
2. what if I used a 220uf cap instead of a 250? (can't find it anywhere).
3. Is the little circle on the far left the outpin from my sound source?
I’ve just done exactly this - built an LM386 based amp for a SpeakJet. the details are here. See the second paragraph of the Feb 3rd update.
The amp can be configured for different gains. The config that you have is the lowest (20x). See the Nat Semi spec sheet for configurations up to 200x gain. Personally, I couldn’t hear any difference.
To answer your questions:
1) It’s in everyone’s diagrams and the sample circuits in the spec sheet, so I assume that it’s necessary. I think it may have something to do with preventing the amp oscillating. I used a 1uF capacitor.
2) I used a 220uF cap - it’s a common value - and I’ve seen it in other circuit dags for the LM386
3) Yes, that’s the audio input. I wish I knew what the 10k trimpot is for. I assume it is some sort of volume control, but it seems to make no difference.
I think you need a common ground for the two chips; and the SpeakJet tolerates a maximum supply 5.5v.
1. The little cap and resistor appear to be balancing out the speaker on the other side of the bigger cap, the 250. You can probably omit them, but the circuit probably works better (more sound or less feedback) if you have it on there. Can probably sub a 0.047 uF for the 0.05 uF. Looks like they weren’t paying attention to standard values.
No difference between20x and 200x. to get the 200x, connect a 10uF between pins one and eight - +ve to pin 1. This bypasses the internal resistor that limits the gain.
Does the 1uF look like an electrolytic? If it is then polarity matters.
Yes, you should be keeping all those components that go directly to ground. The trimpot is a volume control, and is not critical. You can sub it with a different potentiometer or just hardwire a resistor in it’s place.
I played around with these before, when I got them free from my class.
http://www.youtube.com/watch?v=IS2mcsyfDR8 is an example I made. I had it on 20x gain, because if I used 200x gain the speaker would be playing odd noises even if I just touched the input with my finger.
