Noise machine – Near the end

The bearded postman just delivered the tinny 8R speaker I ordered few days ago. And it works good, in spite of its small 1W it makes serious noise with a square wave! (I had previously tested the circuit with  a 15W car speaker with massive sonic result)

I encountered a design issue recently. My original entention was to use a lm386 for its ridicule price (and there’s nothing else actually) But it is set for an internal gain of 20 by default, and feeding 4V p-p the only result you’ll get is a MASSIVE CRAP! In the same time, I want to keep a (Mark) strong signal for the “line out” output (which switches off the internal speaker) to be able to plug that thing in other devices without needing large additional gain.

So here’s the result, bassically the signal (by default) goes into a resistor divider to output 400mV (0.4*20 = 4V) which gives us the maximun output available at the LM386 output with a 9V power supply (4V PP). The bipolar transistor is just use as a buffer. I haven’t done the math (actually I have built a small program to calculate those resistor dividers) but it could also be subsituted for a trimmers.

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Fortunately enough, the current remains relatively small unless you go very high, but the 4046 VCO has a maximun limit of 6Khz (for the values I used). Also the waform becomes really ugly above 1Khz, so trust the datasheet use the filter (silly me). Another cool addition could be the bass boost R/C filter shown in the datasheet. This will improve the bass response of the miniscule speaker. (I may need to add a blocking cap before the LM386 input)

the project is near its end (it doesn’t happen every time). Here’s the visual. I got the dimensions wrong but it gives the idea. This is now more question of dimensions, layout and remaining orders to do than anything else.Then I’ll find a shop for the engraved panel. (the knobs won’t be the ones in the pictures, they’re ugly)

 

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