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Dirty-South Blues Harp forum: wail on! >
MIC Impendence ? Help
MIC Impendence ? Help
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jibberjazz
8 posts
Apr 11, 2012
10:51 AM
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Just getting my first amplified rig together and have a question as to what I would need as far as impendence transformers/converters go.
My mics are:
Bottle o Blues
Astatic 335H- 48
Monarch MC-24 copy of the Shure Slim-X
My amp:
Roland Cube 30x now, but I think I'm gonna get a VHT Special 6 this week.
Other stuff donated by friends:
Art Pre Amp
Line 6 DL4 Delay
I have yet to plug in either the delay or the preamp. Just wanted to make sure I would have everything I need as far as impendence goes. Also, any weird cables I would need (Currently just have a few reg pointy guitar cords)Or reccs for effects to ad etc
Any help would be appreciated...
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Kingley
1978 posts
Apr 11, 2012
11:03 AM
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As far as I know all that stuff is Hi-Z so you should be ok. Hopefully Greg Heumann will see this thread and confirm that for you.
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HawkeyeKane
862 posts
Apr 11, 2012
11:13 AM
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I agree with Kingley. All hi-Z. They should work fine through your preamps and both your Roland and the VHT.
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Hawkeye Kane
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Greg Heumann
1563 posts
Apr 11, 2012
11:37 AM
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Yep. PLug and play.
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/Greg
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tookatooka
2905 posts
Apr 11, 2012
12:28 PM
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Bottle o Blues - High Impedance? At 500 Ohms I always considered Dynamic Mics to be low impedance.
At what resistance or Z does the transition become from low to high. I mean most crystals are way up in the Kohms area?
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Last Edited by on Apr 11, 2012 12:29 PM
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jibberjazz
9 posts
Apr 11, 2012
12:35 PM
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Thanks for the input everyone.
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5F6H
1141 posts
Apr 11, 2012
1:04 PM
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@ Tooka "At what resistance or Z does the transition become from low to high. I mean most crystals are way up in the Kohms area?"
Dynamic mics, including CM/CR, at 1-2kohms (typically 1000 to 2000ohms) generally have a very much lower dc resistance than crystals). DC resistance is not "impedance", impedance is an AC term - AC signal sees "turns" on a transformer/winding rather than the dc resistance of the wire used to make those turns.
Crystals tend not to be measured in ohms, many recommend NOT trying an "ohm" test as putting a voltage accross the element (as happens in a resistance test) can damage it. Astatic used to refer to their crystals as having an impedance equivalent to a capacitance value (390pf from memory?), probably hundreds of megaohnms if you want to think in those terms, but ultimately pretty meaningless ohms-wise. A few hundred kohms to several megaohms (typical high gain guitar amp inputs) tends to "work" with crystals. Very high loads (5-10meg)do help the dynamics of the instrument (e.g. harp, accordian), but are not as intrinsically linked to element type as people often think.
500/600ohms is sometimes called "medium" impedance & an unbalanced dynamic will often work into either high or lo impedance unbalanced inputs.
Real world answer? You get a problem when your mic cannot produce a decent signal into a hi-Z input (in the thousands of ohms, at least tens of thousands, usually millions of ohms)... its the input load's relationship with the source/mic impedance that is critical.
Technical answer? Refer to the "real world" answer ;-)...it doesn't matter what the designer says (e.g. a CM/CR has a dc resistance of ~1-2Kohms, but is described as "100,000ohms/100kohms impedance" by Shure & often plugged into 1megaohm+, or at least 250-500kohms if fitted with a pot), what "works" works, it just helps to have an idea why something might not be working & what then to do about it.
Real problems occur when trying to mate lo impedance mics (150-200ohms) to hi impedance inputs (10,000ohms/10kohms plus), or high impedance devices (generating small AC signal voltages expecting to be supported by high impedances) to low impedance inputs...whichever way round whatever causes the actual problem is the lowest common denominator.
Most common issue is hooking up a balanced lo-z mic (XLR) to an unbalanced hi-z input (like on a tube amp) just by connecting one of the hot pins of the mic, without boosting/matching the z with a transformer (e.g. just by using connectors that would appear to work in each socket)
Signal sources & line outs tend to have lower dc resistance & impedance loads than following inputs because low input loads affect the potential signal available if too low, acting like a volume pot that can't be turned up to full, cutting the signal level that the source/mic is trying to provide.
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www.myspace.com/markburness
Last Edited by on Apr 11, 2012 1:20 PM
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HarpNinja
2328 posts
Apr 11, 2012
1:19 PM
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Say I have a SM57 with impedence transformer. What sort of impedence value do I want to plug into?
Thanks...I ask as I know I can just plug into an amp, but I have an adjustable impedence input on my POD HD500 and never know where to set it.
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Mike
VHT Special 6 Mods
Quicksilver Custom Harmonicas - When it needs to come from the soul...
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5F6H
1142 posts
Apr 11, 2012
1:24 PM
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@Harpninja "Say I have a SM57 with impedence transformer. What sort of impedence value do I want to plug into?"
High. Actual value in ohms might realistically be 50kohms (50,000ohms) to 5Meg (5,000,000ohms) depending on what is available on the unit you are plugging into. Take a leap of faith & use what works best for you within those perameters. I would err on the lower side if I was singing through the mic too, on the higher side if just playing harp through it and if it didn't cause any unwanted harshness.
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www.myspace.com/markburness
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tookatooka
2906 posts
Apr 11, 2012
2:16 PM
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@5F6H. Well, I asked for that one didn't I? I read your response but I'm gonna need to read it a couple more times for it to sink in. I've always had a problem with impedance and the like. I've always (rightly or wrongly) considered impedance to be similar to resistance except impedance works with (reduces) the current in AC using inductance whereas resistance works with voltage in DC using resistors.
Analog has never really been my thing, with digital electronics there is so much less to worry about except when interfacing with the analog world.
I used to think your 5F6H handle was a radio hams call sign, Now I think it probably is due to your knowledge of this subject.
The best thing I've learnt tonight, and thankyou, is that there is far more latitude for matching mics to amps than I appreciated. I will be copying and pasting your response for my future educational training cos I really do want to try and understand it properly.
Thanks very much indeed. :o)
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5F6H
1143 posts
Apr 11, 2012
3:36 PM
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@ Willspear "iirc the inline transformers for balanced xlr to 1/4 inch jump to 50k from 150-200"
OK, let's break down what all those numbers really mean, hopefully simplifying the issue. I'll use an AKG D19C as an example...just because I have one in my hand...
It's a "200ohm" lo-z mic. That is it's rated AC impedance & the impedance of the circuit that it should be used in.
The actual element has dc resistance (DCR) of 50ohms.
A Shure A95UF in line transformer takes that 50ohm DCR & "200ohm impedance" and converts it to 3Kohms DCR and "high" impedance (where the "50kohms" comes in in your example Will...that 50Kohms is more of a ideal/minimum target, you would typically plug into a guitar amp input or a hi-z instrument input on a PA/preamp that might actually be from 50kohms to a few megaohms). The in-line transformer may also be rated in terms of a gain boost which may be expressed as "+29db" for example.
Wired lo-z the mic puts out 0.04 volts AC (40 millivolts) @ 50ohms DCR/200ohms impedance. Now plug that into a tube amp's input and you bring down the overall impedance of a circuit that is expected to be 50,000 to 1,000,000 ohms, or to put it in perspective 1/250th of the lowest expected impedance. The signal level in that much reduced impedance circuit is also 1/26th of the signal level that you get with the transformer.
Wired hi-z, via the Shure in-line transformer, the same mic now makes 1 volt AC (1000 milivolts), a much bigger signal (x25), supported by an impedance of at least 250 times that you get in a low Z circuit and a DCR 60 times bigger.
Conversely, plug the hi-z mic (1 volt AC @ 3kohms DCR/50kohms impedance minimum) into a lo-z circuit (200ohms impedance load) and that 1 volt AC is now subject to the 200ohm circuit acting like a 16:1 resistive voltage divider reducing that 1 volt @ 3kohms DCR/50kohms hi-z to a "barely audible" 62 millivolts @ 200ohms.
All this is even without considering the factors of unbalanced vs balanced operation, where the balanced lo-Z mic has 2 signals (3 wires to the element) V+ & V- and exploits the difference in potential between them which an unbalanced circuit cannot do.
You can see that the magnitudes of signal level are quite extreme, but your amp might amplify a signal by several thousand times so you might get away with some mismatch...as long as your sound isn't too loose & lacking in dynamics. So if you are gigging a certain amp & mic, getting the job done & wondering whether your impedances are "right", then you probably don't need to wonder. On the other hand, if several mics work in a certain amp, some stronger than others, but all useable...then you try a known working mic that puts out no/very faint signal through that amp, then you may need to investigate impedance matching (after ensuring the connectors & cables have a matching "pin-out" - e.g. some XLR connector Astatics were wired "pin 2 hot" while others were "pin 3 hot", using a cord wired the wrong way turned the mic off!)
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www.myspace.com/markburness
Last Edited by on Apr 11, 2012 4:02 PM
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