Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!wuarchive!zaphod.mps.ohio-state.edu!unix.cis.pitt.edu!fmgst
From: fmgst@unix.cis.pitt.edu (Filip Gieszczykiewicz)
Newsgroups: sci.electronics
Subject: Re: DC motor noise in audio circuit
Message-ID: <31302@unix.cis.pitt.edu>
Date: 24 Aug 90 06:08:04 GMT
Reply-To: fmgst@unix.cis.pitt.edu (Filip Gieszczykiewicz)
Distribution: na
Organization: The Last Jedi
Lines: 61


<I'm using some cassette mechanisms with a circuit of my design in a
<language learning system, and have magnetic coupling of noise to the
<ground plane.

	Greetings. Hey, I'm not the only one.... :-)
<
<The motor is by necessity physically close to the circuit, and it is
<definitely a case of magnetic coupling only (not direct conduction).
<I can move the motor away to stop the noise, and it is being driven
<by a supply voltage which is completely separate from the audio circuit
<(there is a ground reserved only for the motors).  Problem is that there's
<no way to permanently move the motor far enough away.
<
<The motors have an integral speed control circuit, and it could be part
<of the problem, too.

	That depends on what type of control they use. Is it
	one of those centrifugal (sp?) ones or a transistor one?

	If it's the mechanical one, the best you can do is put
	a capacitor across the power leads. You could also
	convert it to an electronic control (if you really care :-)


	If it's an electronic control, read below.
<
<So how do manufacturers of portable cassette players keep this trash out

	Well, for one, they do keep the motor as far away
	from the pre-amp as possible. One could shield the
	pre-amp but that may not work... (well, for me...)

<of the audio?  They don't *seem* to do anything special as regards the
<motors themselves, so is there a black magic way to keep the power bus
<clean?

	Oh but the do _do_ something to the motors. Not only are
	the motors put in a metal container (with rubber vibration
	"cups" (top and bottom)) but they also wrap the motor with
	a metal "band". It provides additional shielding.

	(I'm talking about more expensive motors, not the $14.95
	cassette recorders :-)
<
<I suspect there's no easy answer to this, in keeping with the ways of magic.

	No magic - just a lot of wishfull thinking ;-)
<
<INTERNET:tindle@ms.uky.edu | "Could you please continue the petty bickering?
<BITNET: tindle@ukma.bitnet |  I find it most intriguing."   ---    Data, 
<Ken Tindle - Lexington, KY |  Star Trek, The Next Generation, "Haven"
					      ^
	Take care. (ST:TNG sucks beans! ------+)

	
-- 
_______________________________________________________________________________
"The Force will be with you, always." It _is_ with me and has been for 10 years
Filip Gieszczykiewicz    "A man without a dream is like a fish without water."
FMGST@PITTVMS  or  fmgst@unix.cis.pitt.edu "My ideas. ALL MINE!!"