Help: Matching Dynamic Mic to Preamp

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crandallwarren
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Help: Matching Dynamic Mic to Preamp

Post by crandallwarren » Fri Jan 18, 2019 5:38 am

Hey All,

I'm having an interesting issue and I have an idea of what might solve it and am hoping to maybe get some thoughts as to whether or not I'm on the right track.

I have two Metric Halo ULN-8's that I use for everything. I bought them for mobile stuff, but I'm now more studio based and I've never felt the need to get another outboard preamp.

I love SM7's and RE20's for vocals. I use these two mics with the ULN-8's all the time for spoken word stuff, but just recently I started using them for vocals again.

Here's the issue: I find that with both of these mics going into the ULN-8's, I often get this sound that almost sounds like soft tape saturation. It's like... fuzzy or buzzy in a way, but more frequency dependent than level dependent. Last week I did a session with a female vocalist and there were some moments where it sounded like we were clipping-- but, we weren't anywhere near clipping on the pre or converters or the daw chain.

So here's my question:
I used to own an Audient ASP880 and I remember being really impressed at how good the SM7 sounded through that pre with the variable impedance in "Lo". The input impedance of the ULN-8 is rated at 3k Ohms. Is it possible that this is maybe too high for the SM7 and RE20? Does anyone know if this would have a tendency to cause the sound I'm hearing in the recordings?

I'm wondering if investing in something like the Cloudlifter CL-Z might be a solution to this...


PS In terms of testing, I've confirmed this behavior with both mics, different ULN-8 models, cables, etc. So, it doesn't seem to be an anomaly, but something characteristic about the devices themselves.

THANKS!
Don't send a $1000 microphone to do a $100 microphone's job.

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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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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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.

crandallwarren
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Re: Help: Matching Dynamic Mic to Preamp

Post by crandallwarren » Fri Jan 18, 2019 9:45 pm

Hi Jon,

Thanks so much for your detailed response. I will definitely read more on reflected impedance. Yeah... I get the sense that this spec is sort've like power handling specs on power amps. It seems that not everyone is using the exact same language for the measurement. For example, according to the Cloudlifter's site, they recommend 1.5k Ohms as the optimal setting on the Cloudlifter CL-Z for driving an SM7 or RE20.

Additionally, it seems that the input impedance that the standard Cloudlifter CL-1 presents is also 3k Ohms... So, I wouldn't think that would be much of a change.

So, I guess the sense I'm getting is that, the issue I'm experiencing shouldn't really be one caused by this exactly and perhaps I need to look elsewhere? I guess I can take the plunge and experiment with different impedances and see if that resolves anything...

I'd be really curious if anyone else has had similar experiences with large diaphragm dynamics not getting along with certain pres.

Thanks again,

Collin
Don't send a $1000 microphone to do a $100 microphone's job.

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