advice for first hi-Z build for a guy who has built lo-Z

isaac · advice for first hi-Z build for a guy who has built lo-Z

Hi all, I'm a fairly handy guy, and I've made several nifty (and I like to think, pretty nice) harp mics out of various low Z dynamic elements (basically following the I-mic design, with a couple of circuit modifications).
I recently got my hands on some high Z elements, which, although not identified, look a lot like shure elements. I got 'em on ebay from guy who says he got a surplus lot of 1000 of the things from some manufacturer. I tested them, and they are indeed very hot! Using test leads, I connected one to my amp, and it definately had a huge sound...
My question for this list is: Are there any specific circuits that I should consider for a high Z Harp mic? I know that I should probably use a 1meg pot for the volume control, and that I could use a capacitor (suggested values?) to bypass the vol control so that I retain trebles at low volumes. I seem to recall that a cap can be placed from signal to ground right up at the element, and this can help prevent hum... Is this correct/desired? What values of capacitors would be good for this? Also, is a DC blocking cap at the output jack a good idea? I'm think about protecting the element from damage if I accidentally plug into some phantom power... I don't do this with my dynamic mics b/c I'm pretty sure they won't be damaged by short exposure to low amounts of DC, but I have no idea about high Z elements (ceramic, crystal, etc.)
Anyway, any advice/help would be greatly appreciated!!!

gheumann

The vast majority of high-Z elements should be wired very simply. If they are crystal or ceramic, then a 1M or 5M pot is right. Furthermore crystal and ceramic elements are amazingly sensitive to input impedance and ANYTHING you put in the way of the signal on its way to the amp is likely to rob tone.

Magnetic elements (dynamic, controlled magnetic or controlled reluctant) are much more tolerant of a little extra load. A 1M pot is overkill and results in too sensitive a control. Use 100 - 250K - you'll be happier.

Phantom power is ONLY present in low-z, balanced wiring systems. The output of your mic should terminate in a 1/4" plug that will be plugged into the high-Z input of a PA or a guitar amp - and there is no chance that phantom voltage will ever be present. So don't solve a problem that doesn't exist.

I recommend against the "tone cap" - I think it is a LIE. The design robs some of the treble that could be there, only to add it back as you reduce the volume, thereby theoretically keeping the response a little more even. Frankly,, when I turn down, if I lose a little edge it is OK with me because this means I'm comping anyway. But I don't notice a change in frequency response in my mics with lowered volume anyway - good elements with properly matched VC's.

This mod is sometimes called the "Rod Piazza" mod, although it was found in some of the Hohner Blues Blasters. I had a customer's mic in the shop with such a mod and because I was curious and had the opportunity, I played through the mic while disconnecting/reconnecting the cap in the circuit. I could not hear ANY difference at any volume setting.

That's my 2 cents - but there are a lot of other experts here and I'm interested to here their thoughts.

P.S. - if you post a photo of the elements you have someone here might be able to identify them....

isaac

gheumann, Thanks for the great reply! First off, as I should have done in the original post, here is the link to the e-bay auction: http://cgi.ebay.com/MICROPHONE-EL...PageNameZWDVWQQrdZ1QQcmdZViewItem
I did the "buy it now" option, so essentially I bought two of these elements for a total of ~$10. That price point is low enough for me to feel comfortable with fooling around with these elements (ie, trying out several differnt circuit configurations) and not be too heartbroken if I broke one, especially since I got two at that price! Anyway, I am fairly certain that they are of the controlled reluctance type, and not crystal or ceramic (although the same e-bay vendor does have some crystal elements at a slightly higher price).
I actually tried to measure the resistance of the element with my multimeter-- A method I find to work very well for low Z dynamic elements. However, it does not seem to work with these elements (I get the generic value of "1" which is what the mm reads when it get confused). My multimeter can only read resistances of up to 2 megs, so it could be that the element has an impedance higher than that...
One thing I'm curious to hear about is the relationship between specific impedance values and the value of the VC pot.. One thing you mention in you reply is "good elements with properly matched VC's". Do you mean that you use some sort of ratio to pick the VC pot value based on the specific impedance of a particular element?
I have played around a bit with different pot values in my dynamic mics... I've tried 10k, 25k, 50k, and even 100k, and these are with elements that have impedences varying from 16 up to ~400 Ohms... (everything from small speakers to actual mic elements)... I have also used the "better volume control" circuit found over at the ESP website ( http://sound.westhost.com/project01.htm ), which apparently modifies the taper of the pot in order to offer more "usable" volume levels across the entire rotation of the pot. I definately notice a LARGE (and good!) difference with this mod...
I find that, using this particular volume control circuit, a dynamic element with impedance of ~300ohms, seemed to be best paired with the 25K ohm VC pot. A 10k pot seemed to not have enough range of volumes for me, and I found that with a 50k pot I had to be very careful or I would quickly go past the "sweet spot".... Does this sound right to anyone else?
Anyway, I also want to say that I've really enjoyed reading through the archives here... I wish I would have found this site earlier on! It would have saved me a lot of time! But, I guess that time is part of the hobby, and I can't say it was lost time, since I gained valuable experience with it!

bacon-fat

Isaac,
Based on your meter reading and the photo of the element, as well as photos of similar items the seller is offering, I think what you have there is a crystal or ceramic element.

Since by their nature crystals and ceramics will appear to be an open circuit with infinite resistance, these elements cannot be measured for dc resistance with an ohmmeter, like the coil of a magnetic element, and using a meter could possibly damage the element.
A magnetic element would also be much thicker to accomodate the coil, magnets etc.

gheumann

Yup - I concur with Bacon-fat. I also sincerely doubt those were made by Shure, and I'm afraid you're going to find they don't sound very good. But you never know - and the price was right. Because they're unknown you'll want to test to see if you can get away with a 1Mohm pot for the VC, or whether you might even need 5M ohm. If the latter, and you're building it for yourself, then you might want to look into modifying your amp to a 5M input as well.

isaac

Hey thanks for the element ID and advice guys! i really appreciate it!
I'll give it a go with a 1M pot, and see how it sounds... I hope I didn't too too much damage with my multimeter, but like I mentioned before, that's why I got two of 'em! I also expect them to not be fantastic quality, and at 4.98 a pop, I'd be ecstatic with even halfway decent tone. Mainly i want to fool around with different VC types, enclosures, and preamp circuits without much risk of serious monetary loss. When I find a combo of these things that I like with these cheapie elements, and when I feel confident enough with my build techniques, then I will drop the more serious dough on a really quality element.
I'll keep you guy's posted on my progress with these things, and when I get a working mic, I'll record myself on YouTube, and post a link to the vid here!

Cheers!