Hi vs. Lo Impedance

[DingoPepper11]

Kind of a newbie question.

I recently purchased some cheap Nady mics for live drum mixing. The mics work incredibly well for the price, but they come with a Hi Z plug on the end of the cable.

Our board has inputs for both lo and hi and I'm wondering if I would gain anything by changing to a Lo Z cable or replacing the plug on these cables with Lo Z.

What is the real advantage of Lo Z anyway? Greater gain, less noise, or both? And are there other advantages, such as frequency response, etc.? Also, since the mics have Lo Z connectors, is it simply the wiring/plug on the opposite end of the cable that makes the difference, or is there something unique in the mic that would dictate the use of a Hi Z plug on the end of the cable?

 

[Big Kenny]

A high impedance microphone or guitar will usually output a greater signal (voltage) than a low impedance microphone. This high impedance signal works fine and even has some advantages in a sound system as the mixer or amplifier doesn't need to boost the signal as much. Therefore, any noise on the line is also not amplified as much and this results in an improved signal to noise ratio.

Keep in mind however, that the impedance of the transmission line (or cable) is affected by the impedances of the devices that are connected to it. A low impedance microphone will lower the impedance of the entire line connected to it. Similarly, if you connect a high impedance microphone, you will have a higher impedance line all the way from the microphone to the mixer. This can become a problem as the length of the cable increases.

High impedance lines are more adversely affected by the inherent capacitance that is present in the cable itself. This capacitance combines with the impedances of the source and destination to set up a filter. As the impedance increases and/or the capacitance per foot increases, the active frequency at which the filter comes into play gets lower. The frequencies above this point actually begin to "short out" across the cable's conductors before they ever get to their intended destination. Keeping impedance low and using quality cables can be important issues for maintaining wide frequency response in long lines.

A high impedance line that is interacting with outside electrical interference will act more like an "antenna" than a low impedance line. This problem can get worse as the cable gets longer. This effect is usually insignificant for a guitar or high-Z microphone plugged into an amp with a 15' cord but it can have a big effect if that same signal is sent 100' down a snake. These are reasons why a high impedance signal is almost always converted to low impedance with the use of a Direct Box (DI) before being sent long distances.

 

[crazydoc]

The mics work incredibly well for the price, but they come with a Hi Z plug on the end of the cable.

Our board has inputs for both lo and hi and I'm wondering if I would gain anything by changing to a Lo Z cable or replacing the plug on these cables with Lo Z.

The plug doesn't determine the impedance - it's the microphone. You could connect an XLR plug (I assume this is what you mean by "low impedance") to the mic and it will still be a high impedance mic and treated as such at the input of your board. Because of the impedance mismatch, the volume and equalization will be different, and likely will sound like crap. If you're satisfied with the sound you're getting through the hi z inputs, leave it like it is.

If you want to try converting the hi z to low z, or especially if you have long cable runs of over 15 or 20 feet (which can become noisy if unbalanced/hi z), you can get an impedance matching transformer that also balances the line for use with your low z board inputs.
http://www.radioshack.com/product.asp?catalog_name=CTLG&product_id=274-017

 

[bleyrad]

Does that thing actually have a transformer in it? It would have to in order to do everything it says it does. Which really just makes it a really cheap and really small DI box.

 

[boingoman]

Does that thing actually have a transformer in it? It would have to in order to do everything it says it does. Which really just makes it a really cheap and really small DI box.

Yup, with no pad or ground lift, and you can't use it as a wheel chock or doorstop or amp tilter, like a regular DI. Also generally can't handle anything much hotter than a mic signal.

 

[DingoPepper11]

Thanks guys, great info. The mic actually has 3-pin (xlr) plug, and the cord has a 3-pin socket connecting to the mic, with a 1/4" plug going to the board. Since the cable runs are short and it seems to be working well as is, I'll probably leave it that way.

I have used transformers in the past, but don't think I need to go that route at this point.

So if the mic has an xlr output, does that mean the mic is balanced (lo z), or not necessarily? Just looking for clarification on that point. It would easily connect to an xlr cable (on both ends), but would that be a low z circuit?

 

[sile2001]

So if the mic has an xlr output, does that mean the mic is balanced (lo z), or not necessarily? Just looking for clarification on that point. It would easily connect to an xlr cable (on both ends), but would that be a low z circuit?

That's what you need to watch out for. Balance and impedance are not the same thing, although many people make the mistake of equating them. Generally, balanced and low Z go together and unbalanced and high Z go together, but not always. Balanced is the signal being carried over 3 conductors while unbalanced is the signal being carried over 2 conductors. This actually has absolutely nothing to do with the impedance.

If a mic has an XLR out, then it is most likely balanced, and also most likely low Z, but once again, not necessarily. As someone already said, the connectors have nothing to do with whether it operates in low Z or high Z. In this case, if you use an XLR on both ends, then the mixer board is expecting a balanced, low Z signal (which is what the mic is putting out). If you use the 1/4" connector on the mixer end, then the mixer is expecting an unbalanced, high Z signal, which is NOT what the mic is putting out. This results in an impedance mismatch, and can wreak havoc with a sound signal.

 

[DingoPepper11]

I appreciate all the great responses, but I'm getting a little cornfused. The mic has a 3-pin xlr plug (probably lo z) and the cord that came with it has a 3-pin female and a 1/4" male. I don't expect it to be a problem, but we are considering using a snake and locating the board remotely, in which case it could be come a problem (besides, now I just really want to understand this).

Follow up questions:
1. Since balance and impedence are not related, how can a 1/4" plug be balanced, if it requires 3 conductors? It seems like it would have to have an xlr plug to be balanced.

2. Is there a simple way to test for balance and impedence by looking at the cable terminations, or mapping out the conductors with an ohm meter, or should I try to research specs on the mic?

3. Will using the impedence matching transformer be affected by whether it's balanced or not (I'm guessing not, if they are not related).

4. What are the standard values for Hi and Lo Z? I know some cable (like coax for radio and tv use) is marked with specs, including impedence (I would check this, but of course all the hardware is in the trailer at another location).

5. Should I avoid using a cable with xlr's on both ends with these mics until I determine impedence and balance? I haven't tried this yet and don't want to do any damage to the mixer or the mic's.

Sorry to be a pain, but here's a chance for you guys to show your stuff and learn me a little something in the process.

Thanks

 

[Blue Bear Sound]

Balanced is the signal being carried over 3 conductors while unbalanced is the signal being carried over 2 conductors.

No... balanced signals are carried over 2 conductors, where unbalanced signals are carried over a single conductor. The ground is not called a "conductor".....

 

[DingoPepper11]

Eureka!

Ok, I have done some web homework and found some info that clears it all up for me. For future reference, here's what I found: http://www.audiotechnica.com/using/mphones/guide/charact.html

So I see now that a 1/4 plug can not be balanced. I also understand the impedence thing. I was thinking of radio and television where they are at a fixed impedence of 50 and 75 ohms respectively.

It seems clear to me now that mic's don't work this way. I suppose each mic has different characteristics and they are not subject to a standard impedence in the way I was thinking.

So I should be able to tell if the mic is truly balanced by checking the pinout of the xlr with an ohm meter, after which I can decide whether to change to an xlr plug on the cord. I will also research the specs as to the impedence.

Thanks again for all the info.

 

[Blue Bear Sound]

So I see now that a 1/4 plug can not be balanced.

Sure it can - using a TRS connector (looks like a stereo headphone plug) - which is a 2-conductor plug.... a TS connector (like the end of a guitar plug) would be an unbalanced example of a single conductor plug.

 

[paddyponchero]

The nady mics are low impedance -
DM90 - 150Ohms
DM80 - 250Ohms
DM70 - 600Ohms

The source impedance of the mic and the capacitance of the cable form a low pass filter, with a high impedance mic you can easily hear the high frequencies disappear as you lengthen the cable.