Re: Mic Mods

[Mike Curtis <wd6ehr@xxxxxxxxxx>]

> 
> At  4:06 AM 1/19/95 -0800, Mike Curtis wrote:
> >4. Drill a hole in the threaded recess and mount a 50k miniature pot.  I
> >bought the best one I could find, and found a nice aluminum knob that
> >just fits.
> 
> Apparently a magnetic cartridge element is less load sensitive and can
> drive a bigger load than a crystal element which is why Astatic uses a 500K
> pot in the JT-30 and Blues Blaster. Is this true?

I suppose that's as accurate a way to express it as any.

Actually, a controlled magnetic mic (like the Bullets R47 cartridge) has 
a medium impedance.  The crystal (or ceramic) in the JT-30 is very high 
impedance.

As far as a 500k pot in the JT-30, this is actually too low impedance for 
the cartridge.  However, if you use a bigger one, its action will be poor 
on certain amplifiers.  The choice of 500k is a compromise.

What would be best is an internal preamp (i.e. op amp voltage follower) 
direct to the mic element, and run the volume control off the output.  
This would totally isolate the element, making sure it feeds a very high 
impedance at all times regardless of what you plug it into.

The only problem with this is that it needs batteries.  These would have 
to supply a voltage equal to or greater than the peak to peak maximum 
output of the cartridge.

> My experience says that every stage should have a high input impedance and
> a low output impedance . A 50K pot would greatly effect the load on the mic
> element.

What we want to do is either match loads, or make sure they're not 
mismatched into a lower impedance.  If you use very high input impedance, 
it will match anything - so you're correct.

> Since we beat this horse pretty bad last fall, I'm curious as to why you
> chose a value of 50K.

Because it matches the impedance of the R47 in high-Z mode.  A 500k would 
also work, but 50k is technically "more correct".  I'm sure some out 
there use 500k on their bullets, and will tell us these work just fine.

But if you use 50k on a crystal or ceramic, it will sound something like 
buzzard puke.  Even 500k is low for these.

In case anyone is curious, here's how to build a simple voltage follower 
circuit:


10 uF tantalum cap from input to pin 3 of 741 op amp
10 uF tantalum cap from pin 6 of 741 to output
short pin 6 to pin 2 on 741
power with 2 9 volt batteries (in series) connect negative to pin 4, 
positive to pin 7 of 741.

Connect the common connection between the batteries to ground.

To switch it on and off, put a switch somewhere in the battery line.  I 
use a 1/4" stereo jack, with the battery wired to sleeve and ring, but 
there's no reason you couldn't use a switch on your volume control (if 
you have room for all this in the mic :-)

Of course, you could build it all into an outboard box - but keep in mind 
that cable length affects sound and loading.

I highly recommend using an IC socket. 

Pin 1 has a dot by it.  They go in order down that side, and in reverse 
order back up the other.

741 op amps are under a buck at Radio shack, and if you shop around, you 
can find them even cheaper (but why??)

There are higher performance op amps that could be used, but the 741 is 
perfectly suited for this application.

 --  mike curtis