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Dirty-South Blues Harp forum: wail on! >
Impedance Matching options for low Z mics.
Impedance Matching options for low Z mics.
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Zhin
99 posts
Jan 17, 2009
6:27 PM
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Hello. Since my trip to Perth last month I have changed my amplified setup. I thought I'd share some of this info for others to use. Especially for those who live in regions that don't allow them to get the most recommended solutions...
Firstly, for those who don't know, when you plug into a guitar amp you need a high impedance mic or guitar. Though this does not limit your options for mics. You can use a low impedance mic. This is what I want to talk about since they are more widely available.
Typically, the recommended setup for any low impedance mic like the Shure SM57 or SM58 is to use it with an in-line impedance transformer. Shure makes these. And they really do work.
For me, in the past I used a different method involving a microphone preamp device. It's a relatively cheap Behringer MIC100 Ultragain mic preamp... I had no access to any kind of in-line transformer. It's the only thing I could use here in Malaysia. Even the local licensed Shure distributors wouldn't bring them in because I had to order them in bulk and they wanted to impose an exorbitant tax on it too...
The preamp method works when you really have no options left. But recently I discovered it's definately not as good as using a simple in-line transformer (It's a little adapter that typically goes in between your mic and whatever it's gonna plug into).
When I was on vacation in Perth I went equipment hunting and managed to find 2 different in-line transformers. One made by Hosa and one by Sound-king. The only places that had them by the way was this huge warehouse guitar store and a DJ equipment store.
I tried the the 2 transformers and the mic preamp when I got back home with my current mic, a Shure Beta57a and modded Epiphone Valve Special amp.
There's quite a difference. The Sound-king one sounds the best to my ears. The Hosa one is slightly muted because it converts 200ohm to 50Kohm, the mic is apparently 290ohm. The Sound-King transformer does 600ohm to 50Kohm. From my limited amount of understanding it's ok to plug something with a lower impedance into an input that expects a higher impedance... as long as it's not too much of a difference in value.
My conclusion is that you'll always be better off using an in-line impedance adapter even if it's not the original Shure or EV ones.
I did some checking and found out that even the Shure and EV ones basically convert the signal up to around 50Kohm too.
As for the mic preamp I now only use it with my computer for recording. I no longer use it for amplified jamming or gigging anymore.
Hope this helps and if I'm wrong about anything please let me know. Thanks.
Last Edited by on Jan 17, 2009 6:30 PM
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MrVerylongusername
119 posts
Jan 18, 2009
1:06 PM
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Spot on Zhin!
Incidently impedance matching is not a phenomenon restricted to low-z mics. The tone of a JT30 can be HUGELY improved by using a device known as a unity gain amplifier. The nature of crystal elements mean they have HUGE ouput impedances (1-5 MEGA Ohms), often even higher than the input impedance of a guitar amp. The mismatch causes cable capacitance, whereby elements of the signal 'bleed' off - particularly the bottom end. The UGA will match the output impedance of the mic so it can be fed into the amp input without mismatch.
A few years ago Mike "Ironman" Curtis posted a schematic to Harp-L. He called his UGA the MIcMojo. It's an easy circuit, even for a novice like me. It cost about £5 and took about an hour to build the circuit. Big difference in volume and tone on my JT30 (sadly underused now) it even beefed up my Shure 520 too. Here's a link to the Harp-L archived article:
http://www.harpl.com/2002101611.html
Last Edited by on Jan 18, 2009 5:30 PM
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Zhin
100 posts
Jan 18, 2009
9:47 PM
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Hey thanks for the additional info about that man.
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