[Harp-L] Re: JAYPHAT impedance matcher explained a bit (long)

[HTownFess <Spschndr@xxxxxxx>]

Okay, due to maddening HTML issues in the digest I normally read, I
will post this using the Google Groups page interface and see what
happens:

The JAYPHAT2 picture file at http://groups.google.com/group/harp-l/files/
is a ~provisional~ schematic of the JAYPHAT outboard impedance
matching box my friend Greg Schlacter designed/built around 2001.
It’s provisional because I drew the schematic from the prototype and
eventually realized that Greg must have built it with odd parts he had
around at the time, things like chains of resistors to get particular
values, etc.  Simplified versions have already been built and I will
post a revised schematic soon & mention that onlist, but in the
meantime this file will get you started if you want to build one.  It
is really quite simple as electronic projects go and the parts cost is
in the $20-35 range depending on what kind of enclosure you use, etc.
Greg said he didn’t mind people building this box as he had no plans
of going commercial with it (he was engineering for the oil industry,
he’s OK for $) and besides it is simply his adaptation of a public
domain design for a buffer box--the second schematic at
http://www.muzique.com/lab/buffers.htm is this kind of JFET buffer,
and for all I know is the design Greg adapted.  One useful feature is
that this circuit provides a slight overall boost in gain (perhaps 3
dB) which is regulated at the output by a level knob.  I usually run
mine at around unity (1:1) gain, but the boost can be used to pump up
a weak mic or otherwise drive what’s downstream harder.

The design uses two 9v batteries in series as its 18v power source.
It does not use batteries quickly--I run mine on rechargeable--and
apparently will still work as voltage drops; you just compensate with
the level knob.  We are still waiting for someone to try an 18v wall
wart with one--it should work fine.

It could easily be built with parallel outputs to feed two amps at
once.   The multi-amp sound clips should convince you that’s a good
idea.  Building it in a metal box will shield it from outside noise
better.  Layout photos of one build can be found at Rick Bush's blog
(www.bushdogblues.blogspot.com); there's more than one way to lay it
out, but you want to isolate the jacks from the enclosure, and give
the components a common grounding buss that ties to the sleeves of the
jacks and grounds to the metal enclosure away from the input jack.

35v cap ratings are a good minimum on the electrolytics, higher than
50v on all the caps mainly increases their physical size; ½ w
resistors should be sufficient.

Schematic simplification/mods: the pair of 2.2Meg resistors as the
grid load on the input can be a single resistor of 4.7-6.8 Meg; this
is the loading your mic will see.  The pair of 470 ohms can be a
single 1K resistor.  The 1.2K/1.2K/330 ohm chain can be a single 2.7K
resistor.  The polarized 5 mfd electrolytic cap on the output may
cause a little level pot noise in operation and can be replaced with a
non-polarized one, or can simply be replaced with whatever coupling
cap brand you like to use, value between 0.1 mfd and 1.0 mfd (0.68’s a
commonly available one in guitar circles).

Some anticipated FAQ:

Your hi-Z microphone plugs into your JAYPHAT box, and everything else
in your signal chain goes after the  JAYPHAT box.

Yes, the archives will show a public domain design for an op amp
device touted to do the same impedance matching job as the JAYPHAT
box; however, it will be hard to find a reliable report of said device
being built by anyone.  The JAYPHAT has already been successfully
built by at least one person who’d never built an electronic project
before.

Yes, this circuit is something like what I believe was sold in Europe
as the Marble Max; I don’t know if they still sell it or whether they
sold very many.  Until recently I thought of the JAYPHAT box as mainly
for crystal mics, but the recordings made me realize how it helps
other kinds of mics significantly.

Yes, this circuit is something like the modification done to the
inputs of delay pedals by Ron Holmes and others, just isolated in a
box for one kind of convenience; building a buffered input into the
pedal itself is another kind of convenience--your call.

Yes, many harp-specific outboard boxes incorporate proper impedance
matching already, such as the Kinder AFB+, the Holmes Harp Commander,
the Lone Wolf Harp Delay and Harp Tone+ pedals.  The Jayphat won’t
help on either side of those.

Yes, the JAYPHAT box ought to let us run an MC-151 crystal mic
directly into a modern SS PA without the usual loss of tone/volume; I
just keep forgetting to try that one.  It should work something like
running a CM Green Bullet into a PA--not so shabby if you know how to
play--but the CM would be better in the PA with the JAYPHAT too.

No, the JAYPHAT is probably not going to help matters with a vintage
tube PA--their inputs are already optimized for hi-Z mics, in most
cases.

Yes, this device could be built into a belt-clip-sized box for use
with a wireless transmitter or even a long cord (say 50’); no one we
know has tried that yet, but I would definitely give it a shot if you
play a bullet mic wireless.  I would use a detented pot on the level
control or something else that wouldn’t get knocked around
accidentally.

Yes, the JAYPHAT can be used as an outboard master volume or remote
volume control; for the former, turning the box’s level down a little
and the amp’s level up a bit can sometimes hold the mic’s tone
together pretty well while allowing you to get more volume before
feedback, sometimes.  As a remote volume control, put the JAYPHAT near
you and run a cord from it to wherever your amp is.

No, I don't recall what the JAYPHAT box does for the current Blues
Blaster mic; do not recall it magically redeeming it.  However, I do
remember the current crystal element being noticeably better when used
without a volume pot--not the worst thing I ever used into some amps.

Yes, there is an electronic explanation for why the JAYPHAT helps,
which Greg Heumann or Steve Warner will perhaps be kind enough to
provide.

Stephen Schneider