[sdiy] percusion synth design and problems.

[John L Marshall]

I used a SWTPC power amplifier as a studio monitor amplifier at an AM radio
station. The output transistors were running skin burning hot. I could not
figure it out until I put a scope on the amplifier output. The audio
amplifier was amplifing the RF. There was no rectification. Not a hint of
the AM station through the speakers. So much for clean, wide bandwidth,
audio amplifiers. Additional shielding and differential inputs solved the
problem.


Take care,
John
www.sound-photo.com
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----- Original Message -----
From: "harrybissell" <harrybissell at prodigy.net>
To: "Arun Bohm" <arun_bohm at hotmail.com>
Cc: <synth-diy at dropmix.xs4all.nl>
Sent: Sunday, April 13, 2003 5:10 PM
Subject: Re: [sdiy] percusion synth design and problems.


> Hi Arun...
>
> The LM386 amp is like walking around with a sign on your back that says
> "Kick ME !!!".   It is almost sure to pick up radio and even the pros
would
> have a tough time...  Check out the device datasheet and make sure you are
> using the AM noise reduction techniques... you might even be re-radiating
the
> noise to yourself....
>
> National Semiconductor Audio/Radio Application Book (1980) says
>
> AUDIO RECTIFICATION ELIMINATION TIPS
> (or how come my phono detects AM ?)
>
> a)  Reduce input impedance
> b) Place capacitor to ground close to input pin or base (~10-300pF)
> c) Use ceramic capacitors
> d) Put ferrite bead on input lead close to device input
> e) Use RF choke in series with input (~10uH)
> f) Pray
>
> H^) harry
>
> Arun Bohm wrote:
>
> > Hi, Was working on this here percusion synth idea and was wondering why,
> > instead of damped oscillations, I was getting so much noise. Because I
am a
> > novice, I am used to making the mixing stage from manuals I got from
radio
> > shack(don't laugh, please) the amp I use is a utility amp using a 386
chip.
> > The end result is all this RF transmission (I actually get some AM
station)
> > and a bunch of noise. What can I do to eliminate this problem? Any
> > suggestions would be appreciated.
> > arun_bohm at hotmail.com
> >
> > _________________________________________________________________
>