This is a follow-up to post #4371
on page 292
addressing the problems many users have had with 'clipping' audio (sharp clicks and pops) when recording with the 808 #11.
It is caused due to variations in low-frequency air pressure, and is normally encountered in a moving car enviroment or caused by wind (in flight on a RC plane for example).
I'll split this into two posts, starting with a description of the nine pictures shown in the collage attached to this post.
This is a diagram of the electret microphone circuit, showing the biasing, decoupling and supply splitting arrangement.
You will notice the very high value of the microphone decoupling capacitor : 1.5uf (circled in red) - this is what we need to change.
A close-up of the PCB showing the location of the components depicted in the circuit diagram, figure 1.
These are outlined in the same colours for easy reference / identification.
A size comparision of the 1.5uf capacitor against a peppercorn and a grain of sugar.
Measuring the value of the audio decoupling capacitor using a digital capacitance meter - coming in at a huge 1.504uf.
This value is excessive in the extreme and will allow very low frequency and sub-sonic audio to be amplified and digitised, causing clipping.
5 & 6
The 1.5uf capacitor was removed and two single wire stands were seperated from an off-cut of hookup wire.
Holding the PVC sleeving, these were then soldered to points A and B (see figure 2), so I could test different values.
This shows a test capacitor soldered in place - in this instance, a 470nf was been 'auditioned'.
8 & 9
Showing the finally selected value of 220nf soldered in place on the PCB - plus the addition of a small jumper wire.
I decided to use a phsically larger capacitor than the original type, because I couldn't find one that small and even if I could find one, I wouldn't be able to solder it in place !
In the next post is a description of the test, the different capacitor values tested and the results, plus two videos - before and after.