[sdiy] Shielding plastic cases

[Tom Wiltshire]

On 18 Oct 2007, at 15:46, Mike wrote:

> I get quite a laugh by the antics of some guitar effects friends. They
> carefully shield the box, and then hang a long aerial, (cheap  
> guitar lead),
> and a great magnetic pickup on the end! One of the lads was quite  
> amazed by
> how much extra top end he got, when I made and fitted  him one of  
> the Till
> FET preamps into his guitar. He thought it was a treble booster -  
> it was
> actually the low output impedence of the Till cancelling out the  
> effect of
> the Hi Z pickup and the 300pF/Metre of his guitar lead. I added  
> some tiny
> caps and a ferrite core - at one of his gigs he regularly got a  
> local taxi
> radio coming out of his amp. More baffling was a  mystery regular  
> clicking
> noise. He's had an injury to his left arm, and temporarily moved  
> his watch
> to his right wrist. The mechanism is a quartz driven tiny stepper  
> motor -
> the pickups were truthfully receiving the magnetic pulses

Great story!
Reminds me of a overdrive pedal I built when I was fourteen. In the  
search for more overdrive, I designed it with a series of 741 opamps  
and an overall maximum gain of around 100,000. It was almost  
impossible to make the feedback shut up, and even then, you could  
still hear Radio Moscow!

> Which neatly brings me to another point: just shielding the box is  
> often not
> enough.

That's what I learned from the above episode!

Tom