[sdiy] Microphonic PCB

[Tom Bugs]

Thanks guys

I already did quite a lot of back-tracking, scoping and the suchlike 
trying to nail down where the problem could be.
- wouldn't do to come to the list without having done at least some 
preliminary hunting!

Thanks for the reassurance on the limiting caps such as 22pF.

But!
I think I found out the culprit
(maybe something to do with posting to the list gave the angels watching 
some reason to shine the answer to me?!)

I'm using a standard resistive divider + opamp buffer to generate Vcc/2
I had a cap from the middle of the resistive divider to ground and now 
that I've removed that the microphonics are gone.

If it is at all instructive to others looking at such problems - I 
eliminated all the audio stages where I thought the problem could be so 
from there I had to think what else could be an issue - a little 
thinking brought me to look at power and there lay the answer!

Cheers all - now back to finishing up the project..
Tom

On 04/02/2011 11:21, karl dalen wrote:
> Ah, sorry, no i misunderstood i thought you had caps in there
> already and you did the ceramics, current caps! hehe :)
>
> if the caps are just Bw limiting (10-30pF) they should
> not cause any micro phonics.
>
> Kd
>>>
>>>
>>>> Hi all,
>>>>
>>>> I'm scratching my head and wondering if anyone has
>> some
>>>> insight to help.
>>>>
>>>> I have a prototype made up using mainly standard
>> SMD parts
>>>> (the main components used are: 0805 resistors,
>> 0805&
>>>> 1206 ceramic multilayer caps, MLL34 silicon diodes
>> and some
>>>> dual shottkys, and SOIC TL074s)
>>>>
>>>> I am able to tap the PCB and hear it!
>>>> (obviously not very good)
>>>>
>>>> Recently I was informed by Mr.G.Richter that SMD
>> ceramic
>>>> caps tend to be microphonic - you won't tend to
>> notice it
>>>> unless there is some amplification going on.
>>>> Well, as the first stage of the PCB offers
>> amplification I
>>>> figured I should swap out the current caps and
>> solder in
>>>> some poly-box ones (no, it doesn't look all that
>> neat!)
>>>> The microphonics still remain..
>>>>
>>>> Are other parts microphonic?
>>>>
>>>> Cheers for any insight!
>>>> Tom
>>>>
>>>> -- BugBrand LTD
>>>> UK company No. 07199808
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>>>
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>> BRISTOL BS1 3SB
>> United Kingdom
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-- 
BugBrand LTD
UK company No. 07199808
VAT No. GB 988 2629 57
1 Ninetree Hill
BRISTOL BS1 3SB
United Kingdom
www.bugbrand.co.uk