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Post by indiehouse on Aug 14, 2014 8:51:57 GMT -6
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.
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Post by svart on Aug 14, 2014 9:25:21 GMT -6
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 convoluded 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. The magenta statement above is TRUE, to a degree, if the system is MATCHED, which mics are not. The frequency response is not part of the impedance equation, but of reactance. While the change in load might not even affect the mic if it's output current is high enough, then change in winding ratio of the input transformer might drastically change the sound or level. The 10:1 ratio thing is about maximum transferral of voltage, but only because there is no real matched source to load. If that was the case, the mic would have something like 600R output to a 600R input transformer.. But that's for another time. impedance is the opposition to the flow of Alternating Current(AC). Resistance is the opposition to all currents (but mostly applied to DC). lets look at the math.. F= 1/(2piRC) is the equation for an RC filter. This is the LPF that is formed with the series resistance of the cable and the parasitic capacitance to the shield. F = 1/(2pi(SQRT(L*C)) is the equation for an LC filter. This is the LPF that is formed by the inductance of the transformer, it's parasitic capacitance and the parasitic capacitance to the cable shield. For an RL filter, the equation is F= R/(2piL) This creates a HPF situation with the series resistance of the cable and the inductance of the transformer. We would need to know more about the output of the mic to determine a mathematical answer. However, a transformer is a complex device that has so many variables, it's also hard to say what is happening without using spectrum analysis. A pure loading situation would bring your level down as the current drive ability of the source (mic) is being limited. And then, you have reactance which is a complex amalgamation of R, L and C components of the system. So, while you might be seeing the knee of the high pass filter moving downwards as your inductance goes up (due to adding more windings in series in your attempt to increase the impedance), you might also create ringing or overshoot in the transformer itself, which adds the false sense of "clarity". Take note that Neve type preamps had SNUBBER(zobel) networks on their input and output transformers to keep ringing and overshoot to a minimum. If you are changing impedance without changing these network values, then all bets are OFF and who knows what will happen. The other option is that you simply don't have it hooked up correctly, or that it's designed incorrectly. it IS DIY after all.. (EDIT) Looking at the schematic for the 10468 input transformer, it's a step UP design, so what might be happening is that when you go from 1K2:4k8 to 300:4k8, you are changing the step up ratio and getting +6db of boost as well as a change in your primary impedance. This will change the whole dynamic of the system, and probably give you the changes you hear, which are introduced due to the change in winding ratio, not the change in impedance.IF the mic output impedance was higher, or more closely matched to the input transformer, then you'd get exactly what you thought you'd see. |
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Post by jimwilliams on Aug 14, 2014 9:55:18 GMT -6
Use an ohm meter if you don't have better test bench gear. The setting with the lower measurement is the 300 ohm position.
I can easily hear the 300/1200 switch on my Gap 73 even with an SM58. 300 ohms sounds darker at 15k hz.
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Post by indiehouse on Aug 14, 2014 13:09:27 GMT -6
Use an ohm meter if you don't have better test bench gear. The setting with the lower measurement is the 300 ohm position. I can easily hear the 300/1200 switch on my Gap 73 even with an SM58. 300 ohms sounds darker at 15k hz. I'll check it out some more tonight. I've measured with my DMM, and I get something like 12ohm on the "up" and 46 ohms on the "down" position. I measured when I had the CV4 plugged in last night, and got 7 ohms on the "up", and 12 ohms on the "down". Strange. |
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Post by svart on Aug 14, 2014 13:23:44 GMT -6
Say what? You can't reliably use an ohm meter to measure electrical impedance. Measuring DC resistance is barely useful as different windings can have different wire gauges, but this might not apply here, we don't know. Plugging in the mic also affects this due to also measuring the resistance of the output transformer of your mic. As I said in my post, what you hear is most likely from the +6db boost you get from changing transformer ratios and the change in reactances due to the change in input transformer ratios, and probably a change in the zobel response due to the change in ratio. It has nothing to do with the change in impedance because you are not using matched impedance, you are using a voltage transfer only, not a power transfer. That's why the input impedance needs to be 10 times higher, so that you have the least amount of current transfer and therefor no voltage drop. BTW, here's a primer with a calculator to show level difference between impedances. hyperphysics.phy-astr.gsu.edu/hbase/audio/imped.html#c4 |
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Post by indiehouse on Aug 15, 2014 7:42:50 GMT -6
Cool stuff. Trying to wrap my head around it, but this is good to know. Thanks!
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