Re: Pesky capacitor; Guitar example

[Bruce Steinberg <bruces@xxxxxxx>]

Nice ASCII diagrams, guys (and cool idea going to the guitar book
for a reference, Harv).

I think what we're looking at here (in either the mike or guitar
example) is a basically a simple, elegant way of using that
single C to make sure that the signal stays bright as the volume
goes down.

If you consider the circuit without the C, all overtone
frequencies generated by the crystal simply appear across the
total pot resistance, and the wiper picks off a proportion of the
voltage drop, resulting in a lower voltage signal (containing the
same proportion of all frequencies) being sent to the amp for a
desired lower volume -- your basic volume control.

                +-------------+
                |             |
                |              >  
             +--+--+       R1 <   |
             |     |           ><-|----------> Vout
             |  X  |          <   |
             |     |       R2  >  V ccw
             +--+--+          <
                |             |
                +-------------+--------------> Ground

However, when you add a C1 (where Rick and Harv have
independently corroborated it's supposed to be :), it offers
higher-frequency currents the possibility of a lower "resistance"
path (lower reactance, actually) than going through the R1 part
of the pot, depending on (1) the value of C1 (or more
specifically, the reactance it offers each frequency component),
(2) the mike volume setting (i.e., R1 in particular), and (3) the
input impedance of the amp (Zin).

                +-------------+---||---+
                |             |   C1   |
                |              >       |
             +--+--+       R1 <   |    |
             |     |           ><-|----+-----> Vout
             |  X  |          <   |
             |     |       R2  >  V ccw              Zin (to amp)
             +--+--+          <
                |             |
                +-------------+--------------> Ground

So as R1 increases (with decreasing volume setting), C1 looks
increasingly better to higher frequencies, and the higher the
frequencies, the better it looks (hmm, is there a blues lyric for
robots in there somewhere? :).

There are actually some pretty complex little current loops one
could analyze here (none for me, thanks, I'm trying to cut down :),
but the bottom line would appear to be that as the overall volume
goes down, the mike signal EQ gets proportionally brighter.  It's
conceivably all lip smacks and dog whistles just before it all
fades out to zip.  (Does this match anyone's actual experience
with this puppy?)

Of course, what specific effect C has on all this is still
completely (and critically) dependent on the value of C, which
directly determines the reactance (Xc) it offers at any given
frequency.  You can figure this out on your own (Xc = 1/6.28fC),
but some quick, sloppy number crunching tells me that, say,
C=.001mf, or 1000pf (i.e., 10^-9) results in such a high Xc that
it won't affect doodley at 100 Hz.  Make it more towards .01mf,
or even .1mf, and the EQ effect might start to get interesting to
listeners of the non-canine persuasion.

BTW, I don't get the referenced mention to this C somehow
fixing an "out of spec" pot -- sounds like double-talk.  The
reference to the C affecting the mike's impedance is arguably
true, but I would think negligible -- at any given frequency, the
major component of the mike's output impedance is R2 (another
argument for distortion increasing as R2 drops towards zero --
the amp input Zin, presumably pretty high, starts looking at an
approaching dead short).

Which brings up an interesting point -- which is the preferable
jack on one's amp to plug this baby into, HI or LO (impedance)?
The spec on my trusty Champ says that Input 1 is 1 megohm, and
Input 2 is 136K (borne out by the R values in its clever little
front end circuit).  Seems to me if you're going to play with
this 500K mike volume pot close to wide open most of the time, HI
is the ticket, and if you're running the mike pot generally low
(and running the volume up with the amp further down the line),
then LO is the better match.

Again, does this, uh, match anyone's experience?  

Whew, I'm out of here.  Hope this helps more than clogs.

Later, B*


P.S.  After all this tech talk, I feel kind of obligated to
voice agreement with a sentiment in Randy's earlier mail today:

        Date: Thu, 27 Oct 1994 12:49:12 -0700 (PDT)
        From: Randy Lilleston <randyl@xxxxxxxxxx>
        Subject: Re: CTI/caps/crystals/JT30s/TONE
        
                                <...>
        
         I've had several harp players come up to me between sets and take 
        careful inventory of what I'm using (and I ain't a Tone King, especially 
        by the very high standards of the numerous great harp players in the DC 
        area). I always tell 'em that the quest for tone starts with a whole lot 
        of practice, but nobody likes that answer ;). I consistently have found 
        that the more I practice and the better I get, the less the equipment 
        matters.

                                <...>

Right on.  Good equipment is important, and effects can be great,
but first learn to make 'em laugh and cry through a shitty house
PA (or even just unplugged), and I think everything else pretty
much falls into proper perspective.  B*