Impedance

[indiehouse Recording]

Does anyone know anything about impedance matching? I know the rule is that the mic pre input impedance should be about 10 times greater than the output impedance of the mic.

Here's my situation:

A while back, I built a set of EZ1290 pre's (Neve 1073 sans EQ), and I added an impedance selector switch to flip between 300 and 1200 ohms. The problem is that I've been second guessing myself for the last week about which position was 300 and which was 1200.

Example:

I was tracking a female vocal with my MicRehab CV4 through a 1290 pre. The "down" position is what I assumed was 1200 ohms, which is the recommended input impedance that Miktek says is optimal for the CV4. However, when I switched to the "up" position, I got a noticeably brighter/open tone, with an increase in volume, but a slight decrease in the low mids, almost like a HPF or something.

So last night, I took apart my pre's and removed the switch from one channel and compared it to another. The channel without the impedance selector switch matches the "impedance switch down" position on the other channel. So, unless I built these wrong and have them wired for 300 ohms to begin with, it appears that I was right, that 1200 ohms input impedance is the "down" position.

The problem:

I was always under the impression that the higher the mic pre input impedance, the louder and brighter the signal will be. If that's true, it doesn't match what I found last night. What (I think) I found is that my 1200 ohms position is noticeably darker and quieter than my 300 ohms position.


Questions:


1. What effect does mic pre input impedance have on the signal?

2. How can I measure mic pre input impedance?


I hope all of that made sense. Sorry for the convoluted back story, but I wanted to give a little context for my situation and inquiry. I know input impedance can be used for creative effect and tone shaping, I am just trying to get my bearings for what I'm working with.

 

[joaquins]

LPF appears to be lower inductance on the transformer, so that would be your lower impedance set. Also same conlusion from higher level, since Tx ratio is greater, and secondary load will remain constant. The lower impedance the mic is loaded with lower resistance, so it may have more THD and less low end, but also less noise, some mics 'likes' lower impedance, like SM57 which response has the peaks damped by that. Also will give you less noise, as the voltage amplification on the transformer is higher, so for a low impedance mic, like some electronic output condensers may be good to get less noise, but also in this cases THD may be higher, even if low end doesn't change much, at least not in audio band. Is a trade off, as usual, use the one you like most for what you are doing, with low level sources on low impedance mic low impedance config seems better, for loud sources looking for a clean sound high impedance may be better as noise won't affect much since the source is louder and distortion may increase.

JS

 

[indiehouse Recording]

Cool, thanks for the info!

 

[abbey road d enfer]

Does anyone know anything about impedance matching? I know the rule is that the mic pre input impedance should be about 10 times greater than the output impedance of the mic.

Just to be little pedantic  , this is not matching. Matching is when the source impedance is the same as the load impedance. The 1:10 rule is called "bridging".

I was always under the impression that the higher the mic pre input impedance, the louder and brighter the signal will be. If that's true, it doesn't match what I found last night. What (I think) I found is that my 1200 ohms position is noticeably darker and quieter than my 300 ohms position.

Your "impression" is absolutely correct. The apparent discrepancy comes from the fact that the impedance modification in that case is obtained by switching the primaries in series instead of parallels, which reduces the overall gain by 6dB. So yes, the mic sees a better load, which improves its response, but you need to crank the gain a little.