15 Apr
2008
15 Apr
'08
1:47 a.m.
On Monday 14 April 2008 21:03, Jim Leonard wrote:
I have a need to record the output of a (5150)
speaker. Although I
thought that I could just alligator-clip a positive lead to one speaker
terminal and the negative to the case/ground, the output was decidedly
"buzzy" (I assumed it was too "hot" and overmodulating). I routed
it
into a mixer and turned it down (speaker is 5v, not sure what line input
is) but it still didn't sound right.
"Line input" is typically no more than a volt or so...
The output being buzzy -- do you mean while the speaker was sounding or all
the time? It's quite possible that the mixer was picking up other stuff from
inside the case. Or if it's only when sounding then you're probably
overdriving the mixer input which could have all sorts of effects depending
on the circuitry there.
I found some old directions on hooking up a PC speaker
to a line input,
and was confused by the use of a capacitor -- I would have thought that
a resistor would have been more appropriate, to limit the signal
perhaps? In any case, here are the instructions, followed by my question:
Parts required:
- 6' to 12' shielded cable with RCA plug (male) on one end
- Two alligator clips
- One 4.7 uf capacitor
1. Connect one alligator clip to the shielded portion of the cable.
2. Connect the (-) minus side of the capacitor to the center conductor
of the cable and then connect the (+) side of the capacitor to the
second alligator clip.
3. Attach the clip with the capacitor to one of the wires going to your
computer's speaker. Attach the other clip to the metal case (ground)
somewhere (such as a screw or bolt connection).
4. Connect the RCA plug to the auxiliary input on your stereo system or
boom box.
While I have read the wikipedia entry on capacitors, I'm missing
something obvious. My question: Why the 4.7uf capacitor? Does it
serve to limit the signal? Reduce it's voltage? (or increase it?)
Filter the signal in some way?
That's a rather odd way of doing things. I had thought iniially when I read
this that you were looking at perhaps blocking DC with a capacitor, which
would probably be a good thing anyhow.
I could be mistaken about this, but my recollection is that they drove that
speaker with one wire tied to +5V and the other coupled to ground through an
open-collector device such as one section of a 7405 inverter, or similar.
What I would suggest is that you use both resistance and capacitance there,
you want to attenuate that signal downwards some (probably by at least a
factor of 5-10) and you also likely want to block DC from the input of your
mixer, though there may possibly already be a DC blocking capacitor at that
input.
I'd try say a 10K resistor connected to the speaker, a 1K resistor connected
to ground, and the mixer connected to the common junction of the two. Maybe
even make that 1K a trimpot, if you want to be able to adjust the level
downward further.
Is there some reason you don't want to just use a microphone?
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, ?a critter that can
be killed but can't be tamed. ?--Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin