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filter like my finger

C

ChronoFish

Jan 1, 1970
0
Okay, here is another ultra-newby question. I have rigged up (somehow successfully) a series of amplifiers and "filters" that
produce a pretty clear signal. However after fiddling with the circuit (physically touching it with my hands) I've moved parts
around which has re-introduced a small amount of noise. If I touch the battery, the noise goes away.

Other than the obvious - like "you have no clue what you're doing" - why does touching the battery (it's a 9V - and I just have to
touch the casing - not the leads) filter out the noise? I've tried adding a capacitor across the leads but haven't been able to
detect in significant change when doing this.

-CF
 
R

Roger Hamlett

Jan 1, 1970
0
ChronoFish said:
Okay, here is another ultra-newby question. I have rigged up (somehow
successfully) a series of amplifiers and "filters" that
produce a pretty clear signal. However after fiddling with the circuit
(physically touching it with my hands) I've moved parts
around which has re-introduced a small amount of noise. If I touch the battery, the noise goes away.

Other than the obvious - like "you have no clue what you're doing" - why
does touching the battery (it's a 9V - and I just have to
touch the casing - not the leads) filter out the noise? I've tried adding
a capacitor across the leads but haven't been able to
detect in significant change when doing this.

-CF
One obvious suggestion is that it is you that is making the difference. Try
earthing yourself (not touching the battery, but an earth wire back to a
radiator or something). If this has the same effect, you have identified the
problem.

Best Wishes
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that ChronoFish <[email protected]>
Other than the obvious - like "you have no clue what you're doing" - why
does touching the battery (it's a 9V - and I just have to touch the
casing - not the leads) filter out the noise? I've tried adding a
capacitor across the leads but haven't been able to detect in
significant change when doing this.

You are acting as an antenna - picking up the noise and re-radiating it
to sensitive bits of your circuit. When you 'ground' yourself to the
battery, you become less efficient as an antenna. You could be replaced
by a big sheet of tin or a tall column of salt water, with the same
effect.
 
B

Ban

Jan 1, 1970
0
ChronoFish said:
Okay, here is another ultra-newby question. I have rigged up
(somehow successfully) a series of amplifiers and "filters" that
produce a pretty clear signal. However after fiddling with the
circuit (physically touching it with my hands) I've moved parts
around which has re-introduced a small amount of noise. If I touch
the battery, the noise goes away.

Other than the obvious - like "you have no clue what you're doing" -
why does touching the battery (it's a 9V - and I just have to touch
the casing - not the leads) filter out the noise? I've tried adding
a capacitor across the leads but haven't been able to detect in
significant change when doing this.

-CF

I bet your circuit is oscillating. Try the following: measure the current
intake at the battery, and then come close to the circuit touching the
ground line or supply and when the consumption suddenly goes down and the
sound is clean- then it was oscillating at a very high frequency.
capacitor across the supply as near to the opamp supply pins as possible, a
small 100R in series with the output, there are more possibilities
depending on the circuitry.
good luck
oscillators do'nt, amplifiers do
 
J

John Popelish

Jan 1, 1970
0
ChronoFish said:
Okay, here is another ultra-newby question. I have rigged up (somehow successfully) a series of amplifiers and "filters" that
produce a pretty clear signal. However after fiddling with the circuit (physically touching it with my hands) I've moved parts
around which has re-introduced a small amount of noise. If I touch the battery, the noise goes away.

Other than the obvious - like "you have no clue what you're doing" - why does touching the battery (it's a 9V - and I just have to
touch the casing - not the leads) filter out the noise? I've tried adding a capacitor across the leads but haven't been able to
detect in significant change when doing this.

-CF

You are sitting in a room filled with electric fields from the power
wiring in the walls and from nearby radio stations. Your body has a
complex voltage on its surface as a result of being exposed to these
fields. Your circuit and its battery also have a voltage but since
its size and location in space is a bit different, its voltage
waveform is different from the one on your body. When you touch a
signal node of your circuit, you pass a small AC current to that node
as the voltage on your body equalizes with that of the circuit. But
if you also touch a common point, like a battery terminal, most of
that exchange current passes through that low impedance node, so much
less passes through the other contact point, so there is less effect
on the operation of the circuit from the total exchange current.

To put this into a physical analogy, imagine that you are riding in a
life raft on a rough sea (the waves represent the electrical fields
surrounding you and your circuit). Someone else is riding nearby on
another raft (that is your circuit). You reach out to them and try to
pick something out of their eye (a sensitive contact point). You
disturb the operation of that sensitive system because or your
relative motion. Then you wrap your other arm around them and repeat
the operation. This time it is more successful, because that other
arm is coupling the two rafts together by transmitting most of the
physical force between you two at a more rugged reference point, so
the finger in the eye doesn't have to deal with as much relative
motion.
 
K

Kevin McMurtrie

Jan 1, 1970
0
ChronoFish said:
Okay, here is another ultra-newby question. I have rigged up (somehow
successfully) a series of amplifiers and "filters" that
produce a pretty clear signal. However after fiddling with the circuit
(physically touching it with my hands) I've moved parts
around which has re-introduced a small amount of noise. If I touch the
battery, the noise goes away.

Other than the obvious - like "you have no clue what you're doing" - why does
touching the battery (it's a 9V - and I just have to
touch the casing - not the leads) filter out the noise? I've tried adding a
capacitor across the leads but haven't been able to
detect in significant change when doing this.

-CF

It sounds like an erratic oscillation. Putting your finger on the
battery is probably stablizing the oscillation or injecting a local
radio station signal.

Check your feedback loops. If they're anything more then a few nearby
passive components, they're causing too much delay. The amplifiers will
oscillate because they're over-reacting to compensate for feedback that
is arriving late. You can fix this using a small capacitor to provide a
short feedback path for rapid signal changes.

Here's a very simple example. The same holds true whether you're using
op-amps or single transistors.


complex
+-feedback-+
| filter |
| |
+----||----+ <- small feedback capacitor
| | near amplifier
| | \ |
+--|- \ |
| >---+---->
-----|+ /
| /
 
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