plugging a mic into an unbalanced input

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antispatula

antispatula

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I need to be able to run a mic into an unbalanced TS input. To do that I can simply solder the cold signal to the ground wire, and as long as the cable isnt long, there is no problem, correct?
 
since ocnor isn't feeling gregarious right now I'll expound.
You need to convert the low impedance of the mic into a higher impedance that an unbalanced in can handle.
 
Lt. Bob said:
since ocnor isn't feeling gregarious right now I'll expound.
You need to convert the low impedance of the mic into a higher impedance that an unbalanced in can handle.
Click to expand...

Thanks Bob I'm a little drowsy and didn't feel like trying to explain as it would require too much thinking.
 
Impedance isn"t really going to be the Issue as there is No problem running a Low impedance signal into a high impedance input (high impedance signal into a low impedance input would be a Big Problem) accept that it would be a Bit noisyer but all high inpedance inputs are going to be noisyer, The problem is going to be that generally a Unballanced input is not going to be a Mic Preamp input but a Line amp input which will not have nearly enough gain to get the mic signal to a useable level , typical mic preamps are going to have 10,000x Gain were a Line amp input could be anywhere from Unity gain to 10x gain....... Pluss an unballanced input is going to have no common mode rejection and is going to be quite noisey with a Mic.....

To fix this get a mic preamp and plug the output into the Unballanced input .....


Cheers
 
Minion said:
Impedance isn"t really going to be the Issue as there is No problem running a Low impedance signal into a high impedance input (high impedance signal into a low impedance input would be a Big Problem) accept that it would be a Bit noisyer but all high inpedance inputs are going to be noisyer, The problem is going to be that generally a Unballanced input is not going to be a Mic Preamp input but a Line amp input which will not have nearly enough gain to get the mic signal to a useable level , typical mic preamps are going to have 10,000x Gain were a Line amp input could be anywhere from Unity gain to 10x gain....... Pluss an unballanced input is going to have no common mode rejection and is going to be quite noisey with a Mic.....

To fix this get a mic preamp and plug the output into the Unballanced input
.....


Cheers
Click to expand...
in it's simplest form a mic preamp in a mixer is often nothing more than an impedance matching transformer.
 
It is true that you can use Transformers for some gain there is No way you can get Say 40db to 60db of gain from just a transformer (maybe 20db with a few Drawbacks like excesseively high output impedance) , If you could everyone would do it ....

If Said person was electronicly inclined they could easilly build a ballanced to Unballanced converter with a Gain stage with just a couple opamps that would retain a Ballanced connection and CMR, have enough gain , but why bother when you can get a cheap Mic preamp for 30 Bucks.....


Cheers
 
1.Hey guys thanks for the help. I do not plan on using the preamps on the mbox 2 mini, I already have an excellent preamp. The XLR (input 1) includes a preamp, but I would like to not use it. I want to go mic>external preamp>ch1 mbox. Does that work?

2. So to plug a mic into the unbalanced input (ch2) I simply need to run it though a preamp, then into the input of the mbox?
 
I have an EV ND757 that's so hot output it will drive a guitar input with some useability, but normally:

You need to use a pre-amp. The cheapest one is that Radio Shack $20 thing that ocnorz mentioned. They work fine except by the 3rd gig someone usually runs into it because it sticks out so far and bends the bejesus out of it, and I don't think they sell those separately. :(
 
Lt. Bob said:
in it's simplest form a mic preamp in a mixer is often nothing more than an impedance matching transformer.
Click to expand...

This thread is really confusing terms. A transformer can never be an amplifier, period. Amplifiers have the ability to add power, a transformer can never do that. So if a mixer has a transformer input for a mic into its line input stage, that only works *because* the line input has a high-input impedance amplifier.

Minion is also correct in that high input impedances don't mind low source impedances. Impedance "mismatches" are the other way around.

Impedance is also totally independent of signal level. You can have a high source impedance signal that is 1V or 1uV, either way it is still high impedance.

That said, we have come to use impedance as shorthand for various signal types. So "high impedance" means an instrument-level signal (say -20dBV) with a high source impedance; "low impedance" means a mic-level signal (say -50dBV) with a low source impedance.

The problem there is a line input impedance is usually 10K, but a line output impedance is typically 100. And there are many people so confused about impedance that they think that is a "mismatch". It's not.

OK, back to OP's problem. 1/4" inputs come in two flavors: instrument and line. Instrument should be very high input impedance, let's say 500K minimum. Line level is only going to be 10K.

This has a significant effect on what we can get away with in a transformer. Transformers "reflect" impedance across their windings, according to the square of their turns ratio. The change in level is the turns ratio.

Be careful with using these XLR-TS transformer adaptors with line inputs. Most are designed for instrument inputs. They are usually 150:50K impedance, which is a turns ratio of 18, for gain of +25dB (20 * log(18)).

This works great with a 500K instrument input: the input impedance as it appears to the mic becomes a reasonable 1.5K; the source impedance as seen by the line input is 50K (note that is far too high of an impedance to send down any cable, so always use these at the amplifier end, as they are designed).

What happens with the same device into a line input? The mic sees a 30 ohm load, the amp sees a 50K source. That is a severe mismatch, you will drop a large amount of the gain you expected as a result, and the complex impedance of the system will cause very undesirable consequences for frequency response.

So you really want a transformer with no more than a 1:4 turns ratio. That gives you gain of +12dB and keeps all of the impedances within range. But that's not what you will get with these inline adaptors. And +12dB of gain is nowhere near enough from a dynamic mic into a line input.

Now of course in the interim we've learned that OP is really just trying to connect a mic preamp output (line level) to a line input. And yes, that will work just fine. But we still have the issue of going from balanced XLR to unbalanced TS. This is not an impedance or level issue (well, there could be a -10dBV/+4dBu issue, but I'll ignore that for now), just a format issue.

Balanced outputs can be one of three types:

- transformer out
- "active" or electrically balanced out
- impedance balanced out

Each needs to interface with an unbalanced input on its own terms. The transformer out MUST have pins 3 and 1 shorted, or it won't work. Most active balanced outs do NOT want their pins 3 shorted. A few are designed to compensate for that, but many others will potentially distort if you do that.

Impedance balanced outs don't care either way, since their pins 3 aren't really doing any heavy lifting.

In summary:

- transformers are never amplifiers.

- you can use a transformer to go from instrument to mic (or vice versa), but not mic to line if you need any reasonable amount of gain.

- you need to check your gear to determine how to go from balanced to unbalanced.
 
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