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Old Jun 14, 2011, 10:50 AM
turboready is offline
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ON, Canada
Joined Nov 2007
178 Posts
Originally Posted by timetec View Post
Hello all

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.

1 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.

2 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.

3 A size comparision of the 1.5uf capacitor against a peppercorn and a grain of sugar.

4 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.

7 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.


Thanks for the info and time you took to do this Richard, its priceless.

It's very so much more rewarding and interesting when we can take the solutions to RC issues to a higher notch than just give up on a product.

Your bench testing, mods and info is very much appreciated.

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