Thursday, June 19, 2008

DIY Impedance Matcher--Pt. 1

Gonna jump in and attempt to build an effects box that will provide a better match up between the impedance level of my harmonica microphones with the input of any amplifier with the goal of bringing out the best tonal quality of each mic's element--in particular with crystal cartridges such as the Astatic MC151, but with possible improvements with controlled reluctance, controlled magnetic and ceramic versions also.

My buddy, Stephen Schneider is once again helping out the harmonica community by sharing just exactly what this little gizmo will do when it is plugged in between the mic and the amp. Go over to and click on the bulletin boards to get to the harp discussions and follow Professor Schneider's posts, especially the one on the Supro sound clips to hear the box in action. I was aware of just what this box would do from hanging out with him at Harmonica Organization of Texas (HOOT) meetings. He would bringing the box along and plug it into amps brought in by members and that he had never played before and manage to dial in a fatter sound with his mic that just didn't exist without the matcher. I was always amazed at just what a difference it could make. Many harp players have modified their amp's input resistance to achieve the same goal, but this allows for a plug and play with a tweak of a knob.

Now Stephen has provided us all with the schematic of what is called the Jayphat impedance matcher and it can be found by follow his posts over at the Weber harp board. I have started the process by ordering all the parts needed from which are:

Hammond box

#512-MPF102-- 12 cents

2-Switchcraft Phone Jacks

2-Battery Snaps
#121-0422/0--82 cents

2-Battery Holders
#12BH071-GR--56 cents

10K Audio Potentiometer

Resistors were 1/2 watts & 10 cents each:
2-470 ohm resistors-#660-CF1/2C471J
10 ohm resistor-#660-CF1/2C103J
2-330 ohm resistors-#660-CF1/2C331J
2-1.2K resistors-#660-CF1/2L122J
2-2.2M resistors-#660-CF1/2C225J

Mallory DC film cap
.68uF 100v
#539-150684J100FC--77 cents

Xicon Radial Electrolytic
100uF 25v
#140-XRL25V100-RC--9 cents

2-Xicon Radial Electrolytics
4.7uF 25v
#140-XRL25V4.7-RC--12 cents

So, if you want to tackle this little box and want to go through Mouser, then these order numbers should work. Several folks have responded over at the Weber harp board about this project with at least one harpman building it already and offering mod suggestions. I foresee that those posts will be very informative. I haven't received my parts yet, but Mouser is pretty darn fast, so I'll either have to put up or shut up. I post my progress as I go--maybe. I hate to expose my hacking skills and let them hang out for all to see.

P.S.--The good Professor Schneider reminded me that I didn't list isolation washers for the jacks that are needed to combat grounding loop hums and such. I didn't order the washers because I have some on hand from somewhere. I meant to mention that and knew that I needed to get back here and post it. He had sent me parts numbers for the washers a few days ago so here they are: 502-S1028 & 502-s1029--one of each for each jack. I also didn't order a switch, which I should have mention. Tons of choices there. Anyway--


Stud Terrapin said...

I've built one of the earlier impedence matcher/buffers and used it successfully with my harp mics through a modded Princeton circuit amp built to Bruce C's 5F2H design. It works great with crystals (vintage MC-151, and cheaper replacements). It also seems to add something to the Shure elements (CR Black label and CM's). What interests me, is that the ceramics I have don't work through it. I can barely get a ceramic signal through the matcher/buffer.

Don't know why and haven't checked it out further. It makes the vintage 151 into more of a monster than it was.


HTownFess said...

Which of the earlier matchers was it? That's just curiosity, may not bear on the ceramic issue at all. I would say that most hi-Z mic elements that were designed before the late 1950s stand a good chance of picking up something from the Jayphat box; those elements were meant to be used with earlier tube PAs that usually had substantial input impedance (usually at least 5 Meg, up to 18 Meg in Newcombs) as part of the grid leak biasing of their input stages. Greg Heumann explained it once, think he said such elements require a lot of kickback voltage to work properly so there needs to be a lot of resistance to ground (impedance).

Thus CMs and CRs, a lot of the EV dynamics in the 6-- series will pick up a bit (sounds more like upper mids to highs on the Supro Supreme clip I put up, listen to 4 draw with the box/CM compared to straight in). Obviously MC-151s are dramatically different; the Shure Slim-X is a whole lot better, to name another crystal; and supposedly the Shure bullet crystals (R7?) benefit substantially (mine didn't but it's clearly not healthy).

Ceramics, I've never been struck by the difference with the Jayphat box, a Harp Commander's crystal setting, or a grid leak input tube PA. I have a couple relatively recent MC-127s built up and I will do a clip with them to see whether we can hear anything much. The ceramic problem with your buffer is odd, can't think of any explanation offhand. Ceramics are happy if they've got 1 Meg of overall input impedance and that's not hard to achieve without resorting to an outboard matcher. But build a Jayphat box if you want something that works with Astatic ceramics, at least.

Ricky Bush said...

Thanks for the comment there, Stud and Professor. I just hope that my works after my first attempt. I've heard what the Jayphat does with a MC-151 and that is my main mic, but I'm also interest in hearing what it does to my 1940s CR. Stud--I like the "more of a monster" statement. Anyway--

See ya--

DinoSS100 said...


Could you tell me where you bought your perf board?

Thanks for all the information!

John DiNatale (Boston, Ma)

Ricky Bush said...

Hey John--

The perfboard came from the local Radio Shack. Good luck! Anyway--