Help: Matching Dynamic Mic to Preamp

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kslight
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Re: Help: Matching Dynamic Mic to Preamp

Post by kslight » Fri Jan 18, 2019 8:16 am

According to shure, the SM7 is designed to plug into a preamp with 19-300ohms impedance. So I believe matching the impedance better should improve your results.

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ShinyBox
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Re: Help: Matching Dynamic Mic to Preamp

Post by ShinyBox » Fri Jan 18, 2019 9:11 am

Matching of impedance is not the way that designers/engineers think about signal flow any longer (since the 60's?)

Instead, a concept called bridging impedance is used. Here's the rough of it:

The source is designed with a low (ish) output impedance, the target is designed with a high (er/ish) input impedance.

For passive dynamic microphones (and ribbon microphones, see the plug, nudge nudge, wink wink, say no more), the best voltage transfer, as in best frequency response, will be obtained when the load is very high. As the input impedance of the microphone is lowered, frequency response will start to suffer in the low and high end.

So I don't think the issue is that the impedance is too high for the microphone. Something like the Cloudlifter will present a higher impedance to the microphone, and provide some additional gain, so might help (will at a minimum at least change) the situation.

You may see some mic pres labelled with switchable input impedance, and they will list something like 300/1200, etc. This is not the actual impedance presented to the microphone, as most of those input transformers are followed by a high input impedance tube, or amplifier. That high impedance is reflected back through the input transformer, so the impedance seen by the microphone is much higher (google reflected impedance).

Hope this helps

Jon

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losthighway
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Re: Help: Matching Dynamic Mic to Preamp

Post by losthighway » Fri Jan 18, 2019 1:49 pm

ShinyBox wrote:
Fri Jan 18, 2019 9:11 am
Instead, a concept called bridging impedance is used. Here's the rough of it:
......
So just to make sure I'm understanding the helpful takeaways from a carefully written, detailed response:

Raising impedance on the preamp (if adjustable, or using an intermediary device like the 'cloudlifter') is generally helpful for performance when working with large dynamics and ribbons?

It's interesting because I own a couple channels of Great River pres which have an impedance changing button which actually lowers the impedance significantly (from the default 1200 ohm to a mere 300 ohm) which the manufacturer recommends trying on ribbon mics. I don't always try it, but when I have it usually hasn't resulted in a preferable sound to my ears.

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ShinyBox
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Re: Help: Matching Dynamic Mic to Preamp

Post by ShinyBox » Fri Jan 18, 2019 3:17 pm

I believe that Dan (Kennedy, Great River Electronics) is changing to a different winding structure on the input transformer with the MP series pres, so in addition to the impedance changing, you are also getting a bit of a gain shift. Reflected impedance from the amplification stage after the input transformer would apply in this case.

On the Si pre that I manufacture, the input impedance is changed by switching a different resistor into the circuit, so the loading is changed without a gain shift. I can get away with this because it is a transformerless design.

Personally, I treat these as a "Yes/No" or "How does it make me feel?" button. You have the right source, with the right mic on it, as a last check, I'll hit the button, and it's either better or not. Some times less than ideal frequency response just works better.

But typically what most people hear with passive dynamics ( I believe there are some active ones out there, which wouldn't apply), and passive ribbon mics( there are active ones, the active circuitry sets the impedance the microphone sees to a set value), is that at higher impedance settings, you hear a lift in the high frequencies. If you really listen close, you can also hear some low frequency extension as well.

Regards

Jon

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Re: Help: Matching Dynamic Mic to Preamp

Post by standup » Fri Jan 18, 2019 7:58 pm

I have a pair of AEA RPQ mic pre’s here, obviously designed for ribbons. But I notice that any dynamic mic seems to sound different through the AEA’s — more open, deeper, more detailed. Looking at their page the input impedance is 63k Ohm. Interesting.

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