From: Pete Brunelli Date: Thu, 12 Sep 1996 11:23:45 -0400 (EDT) Subject: microphones and impedance
At 12:22 PM 9/11/96 -0800, John Frazer wrote: >At 2:47 PM 9/11/96, Pete Brunelli wrote: >>Impedance is abbreviated as "Z". >>As long as you match High-Z to High-Z you will get the sound that both sides >>were meant to produce. > >up to this point you were ok.
My goal was to clear up a few technical issues with someone who needed a SIMPLIFIED explanation.
As a point of reference, the Shure company bulletin on impedance matching: http://www.shure.com/app-impedance.html and one on transformers: http://www.shure.com/app-transform.html
> XLR jacks are typically used on high impedance >>devices, like crystal and dynamic mic's. Guitar amps are set up for low-Z >>devices like inductive guitar pickups. Adapters for high to low impedance >>(XLR to 1/4" plug), and low to high (1/4" jack to XLR), are inexpensive and >>solve these problems. > >XLR jacks may be used on some hiZ mics, but are much more typically found >on loZ mics. >my understanding is that guitar inputs are not loZ
relatively speaking, i stick to my characterization of an inductive guitar pickup as a low impedance device, maybe not as low as the voice coil of a speaker (another form of transducer) but probably under 1kohm.
>1/4"phone jacks are more likely to be a hiZ input.
On modern mixing consoles you typically have high-Z XLR inputs, and Low-Z 1/4" inputs.
Again, to reference the Shure impedance bulletin, a mismatch of low-Z source (mic) to high-Z destination (preamp stage of board/amp) gives a gain boost.
snip >>Impedance is resistance in an AC circuit (Mike C. will kill me for this). >>The issue is impedance matching. If a guitar amp is looking for a low (8 >>ohm- 1K ohm) input device, and you connect a 5 Meg-ohm device, you have >>created an impedance mismatch and the frequency response of the circuit has >>been altered. > >Not only frequency, but input sensitivity as well
See Above.
> You never know, it may sound better to you. The real problem >>is that the impedance of that 5 Meg device may be 1K at SOME frequency >>(probably not audio range), and the circuit will try to process it. This >>can cause problems with feedback and unwanted overtones. > >or not enough output. > > >>It sould match your system. A SM58, with an impedance adapter (Hi to Lo), >>into a guitar amp = a matched system. > >SM58 is a loZ source. you would normally use a lo-hi adapter to change it >to hiZ for a guitar amp input.
A SM-58 has an impedance step-up transformer as an internal device, and you will be raising the impedance further by doing this.
> A GB, internally switched to low Z, >>played directly into a guitar amp should also be well matched. > >The GB would probably be less feedback prone if switched to hiZ
I will have to take your word for this, as i am not a firm believer in this type of reasoning. What may be happening is a decrease in gain due to the relative impedances of the mic and the preamp, hence less feedback. This is dependant on the preamp, and will vary widely depending on the device. A 12AX7 vacuum tube in a Fender may present a much different load than an OP-AMP or other small-signal solid state device found in a mixing console or PA/Mixer.
I do a fair amount of work with home studio equipment, as well as holding an Amateur Extra Class amateur radio license (highest license class), and homebrewing amps and preamps. While i am learning new stuff all of the time, i feel that the advice i gave was technically OK.
Happy Harpin' /-----------------------------------\ |--/=====================================\--| ||| a | h | a | r | m | o | n | i | c | a ||| ||| i | n | m | y | p | o | c | k | e | t ||| |--\=====================================/--| \-----------------------------------/ Pete Brunelli p~onnix.com http://www.connix.com/~pcb
