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TLM102 output impedance / mic pre with transformer

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TLM102 output impedance / mic pre with transformer

Postby Guitarking » Thu Apr 08, 2021 2:17 pm

Hi everyone,

Once read that coupling the transformerless Neumann TLM102 with a mic pre WITH transformer wasn't a good idea. Didn't really understand why. I had to do with output and input impedance.
The TLM102 has an output impedance of 50 Ohms? The mic pre should be at least 5 times this number??
How does this work?

Is it good to use the TLM102 with my RME babyface inputs? Sounds good to me anyway.
And could I use the 102 with the GAP pre73 neve clone mic pre? And if so, is it best to put the pre73 at 300ohm or the normal (1200??)?

Thanks!
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Re: TLM102 output impedance / mic pre with transformer

Postby Hugh Robjohns » Thu Apr 08, 2021 3:16 pm

Guitarking wrote:Once read that coupling the transformerless Neumann TLM102 with a mic pre WITH transformer wasn't a good idea. Didn't really understand why.

I'm not surprised. It's nonsense!

I had to do with output and input impedance.

No, it's still nonsense!

The TLM102 has an output impedance of 50 Ohms? The mic pre should be at least 5 times this number?? How does this work?

The concept of voltage matching. The idea is that you want as much of the signal voltage produced by the mic to reach the mic preamp. To do that you need the lowest possible output impedance at the mic, and the highest possible impedance at the preamp -- but there are practical limits to both. So the ideal is a ratio of somewhere around 10 times, but anything above x5 will work fine.

For that reason, mic output impedances are usually between 30 and 200 Ohms (most being around a nominal 150 Ohms), and modern mic preamp inputs are typically somewhere between 1.2k and 5k Ohms. A few vintage preamps from the era of power-matching will have lower options under 600 Ohms, and a few designed for use with ribbons might go significantly higher (~20kOhms).

Is it good to use the TLM102 with my RME babyface inputs? Sounds good to me anyway.

Absolutely fine.

And could I use the 102 with the GAP pre73 neve clone mic pre? And if so, is it best to put the pre73 at 300ohm or the normal (1200??)?

Yes, it would work fine with either impedance, but the higher option is much better. The low setting can give a different tone option with dynamic mics. It shouldn't change the tone for mics with electronic outputs, but it could strain the output drivers unnecessarily and reduce headroom.
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Re: TLM102 output impedance / mic pre with transformer

Postby CS70 » Thu Apr 08, 2021 5:03 pm

To Hugh's excellent reply, I can add that in the last 6 years I've used the TLM102 with all kind of preamps, and it never failed me.. at least not for that reason. :lol:
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Re: TLM102 output impedance / mic pre with transformer

Postby Guitarking » Thu Apr 08, 2021 9:02 pm

Thank you Hugh (again) for your extensive answer.

CS70... thanks too! And when did the 102 fail you?

What would be the benefit of using a mic pre with transformer with the 102? Would it give more character?
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Re: TLM102 output impedance / mic pre with transformer

Postby CS70 » Thu Apr 08, 2021 9:14 pm

Guitarking wrote:CS70... thanks too! And when did the 102 fail you?

Let us just say that no mic can transform a bad singer into a good one! :lol:
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Re: TLM102 output impedance / mic pre with transformer

Postby Hugh Robjohns » Thu Apr 08, 2021 9:26 pm

Guitarking wrote:What would be the benefit of using a mic pre with transformer with the 102? Would it give more character?

Possibly, yes, depending on the transformer, and the signal level, and the nature of the source.
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