Input Impedance

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2000Z28M6

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I must say I'm new to mic preamps and from what I understand the input impedance adjusts how much the mic gets, but is there general settings that work better on a dynamic mic vs. a condenser?

For example if I'm putting a dynamic mic like a SM58 on a guitar cab, what kind of impedance works better?

Same for a small diaphragm condenser, is it strictly by ear and preference?

I've been playing with the knob but I cant seem to decide which one I like better, when set high on the 58 on the GTR cab it breaks up harsh and when I back off on the impedance it breaks up smoother, yet it doesn't seem to have as much edge and presence.

So just wondering if there is any advice from the pros :)

Thanks all.
 
For a condenser mic, put it on the highest setting. You don't get much tonal variation with lower values, just degradation in performance. There are probably exceptions, but there aren't many.

For dynamics, I think you've got it figured out, so set to taste.
 
Ok. Thanks makes sense.

I made a crude diagram to show the MPA's impedance dial, saves time knowing what the actual marks represent.


Dont laugh it's simple!

3310688370_8819500390_o.jpg
 
2000Z28M6 said:
Hey Found a really nice explanation.

http://www.microphone-data.com/pdfs/Mic impedance.pdf

Might not be entirely correct here....

Hope it helps someone else.
Click to expand...

No, I stand by what I wrote. The exception is for transformer-output condenser microphones, which most aren't. Transformerless condenser mics are the large majority, especially in the low end of the market. As such, they are nearly pure resistive sources, and don't change their frequency response according to the attached load. They just output a lower-level signal and experience distortion at lower SPLs.

Yes, you can change the frequency response of a transformer-output condenser mic to some degree with a variable input impedance control, but you are still loading down the active circuit that is on the other side of the mic's transformer, and thus potentially still degrading the mic's performance.

Now, some people might enjoy that distortion, but that's what it is, distortion.

Next, the impedance of many preamps, especially the IC preamps you will find on the lower end of the food chain, is really not that complex at all: again, nearly all resistive. So if you load down a resistive source impedance with a low impedance, mostly resistive load impedance, all you get is poorer performance from that circuit.

Let's take for example the picture on p.3 of that link--it's an ART Gold series preamp. It is a transformerless input pre with a discrete transistor first stage, and a resistive variable input impedance control. So the ART's control doesn't generate any tonal variation of its own; it is entirely dependent upon its load interacting with a complex source impedance of the mic. If the mic doesn't have a complex source impedance, then you really aren't accomplishing anything helpful by reducing the input impedance.

If you get to the point of using a U87 into a transformer-input pre looking for certain tonal variations, then you are advanced enough to ignore what I have written. Although I would encourage you to try to understand it first.
 
Well you just gave me a headache....:o

I hope your info is correct and not just throwing words out there.

Like I said I'm no pro nor do I have alot of experience with many pres, but i did notice a difference when lowering impedance.

Honestly the SM58 sounded smoother right around the 1080 mark than it did say around the 2040 area.

If the mic doesn't have a complex source impedance, then you really aren't accomplishing anything helpful by reducing the input impedance.
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And what starting impedance are you referring to here? 3k?

From that article it states 10:1 is the general starting point, is that not the case here?

I haven't played with the AT4040 but again I remember a sharper attack and thinner sound at the higher settings.

So I don't know if I still misunderstand what you are saying? I guess what i'm saying is that I didnt like the mic's sound at anywhere around the 3k mark.
 
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2000Z28M6 said:
Well you just gave me a headache....:o

I hope your info is correct and not just throwing words out there.
Click to expand...

Yeah, I just try to mislead people for fun :rolleyes:

I am actually saying the same thing as that article. In fact, my only real quibble with the article is it contradicts itself about pres "often" having complex input impedances, and then going on to say that many pres are IC input, and thus have a simple resistive impedance.

What I think they were trying to say is that a lot of high-end pres have a complex input impedance.

Note: neither the ART pre nor your AT4040 have transformers, so that entire section of the article does not apply to that chain. Resistive source, resistive load. Reread the article with that in mind.



Like I said I'm no pro nor do I have alot of experience with many pres, but i did notice a difference when lowering impedance.

Honestly the SM58 sounded smoother right around the 1080 mark than it did say around the 2040 area.

I haven't played with AT4040 the AT4040 but again I remember a sharper attack and thinner sound at the higher settings.
Click to expand...

OK, the SM58 has a complex source impedance (voice coil + output transformer), and I already agreed you should hear a difference there.

The AT4040, I would doubt there is a difference between 1K and 3K *if* you matched levels exactly after changing the input impedance. When you drop the input impedance, you will reduce the signal level, so you need to normalize them before you compare--it should be about 0.5dB difference.
 
Rite on, so basically you are saying STFU and set it @ 3k and know it's better than anywhere else?:o

Hey if anything I'm learning something here. thanks~!
 
2000Z28M6 said:
Rite on, so basically you are saying STFU and set it @ 3k and know it's better than anywhere else?:o

Hey if anything I'm learning something here. thanks~!
Click to expand...

No, that is not what I said. Reread my first post:

me said:
For a condenser mic, put it on the highest setting.

For dynamics . . . set to taste.
Click to expand...

I am done helping you now . . . good luck.
 
gosh dont get your feelings hurt, i was being sarcastic, i really do appreciate that you took the time to explain it to a noob like me.

So i'm aplogize if you didnt catch that. :) thanks again
 
As you have discovered some dynamics respond better to a lower load. I think I read somewhere that the optimum load on an SM57 was in the 500 ohm range which makes sense since the mic was designed in the era of the 600 ohm standard (almost every piece of vintage gear uses a 600 ohm input impedance whereas todays gear it is 10,000 ohms or higher!). Modern prosumer mixing consoles generally fall around 2k to split the difference between high and low mics. I can add that transformers themselves don't have an AC impedance (and sound is an ac , not a dc signal) but rather "reflect" resistance in ratio to their turns so circuits of differing impedances can be matched for better sound characteristics. You might think this is over your head but I bet you have used a direct box right? Same principle. You Bass or Guitar needs to see a very high load (a million ohms) to work properly but the mixer wants to see 2000 so inside the box is a transformer with the right number of turns (number of times a thin piece of wire is wound around a core)..hence the mixer sees 2000 and the bass sees 1,000,000 and everything functions to spec. As a rule condensors like to see a higher impedance. If you think that is funky the TRP preamp from AEA that was designed for ribbon mics has an input impedance of 18,000 ohms!

Cheers,
Ray
 
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