TLM102 output impedance / mic pre with transformer

[Guitarking]

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!

[Hugh Robjohns]

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.

[CS70]

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.

[Guitarking]

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?

[CS70]

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!

[Hugh Robjohns]

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.

[NEWTON IN ORBIT]

Hi, I recently purchased a Daking Mic Pre One. I have a friend of over 20 years that was involved in the design, or redesign of every Jensen transformer in their catalog. At least until they were purchased by Radial Engineering.

The TLM line of mics, have a 50 ohm output z. This is pretty low, not as low as older Schoeps mics, but low. He insists that the Jensens need what was nick named a "Schoeps Switch" to work properly with mics that have this low of an output impedance. He has tested mics with these low output impedances into these transformers for many decades. Since the 1970's.

The TLM mics and low Z Schoeps will have an altered frequency response, and possibly ringing and overshoot when coupled with these transformers. Since Cinemag transformers are very similar, this will be the case with some or most of theirs too.

A -20 db pad, will often fix this, depending on where in the circuit the pad is, and a Switchcraft or Neutrik barrel could be made up with the series resistors in line, but then you must use an external phantom supply because the resistors will affect the current reaching the mic from the xlr jack from the internal +48 supply.

There is a white paper on how to make this "Schoeps Switch" circuit on the Jensen website.

So, the TLM mic's tone will be affected feeding some transformer coupled mic preamps, whether you hear the difference or not, is another story. All that matters is whether you like the sound or not.

I have some mic preamps this gentleman built for me, and while they do not have a Schoeps switch, the pad does the same thing, and I can tell you that not only does the level drop, but the mic's frequency response and timbre change for the better with the pad in on a TLM103, and TLM127. A shame about the level drop, but at 70 db of gain, it's not a huge deal.

Best Regards,
John

[Hugh Robjohns]

The 'schoeps switch' you mention basically inserts 68 Ohm series resistors into the hot and cold lines as they enter the transformer primary (where they won't affect phantom current or voltage). The idea is to increase the apparent source impedance of a transformerless mic from ~50 Ohms to ~150 Ohms.

And your friend is quite right in that 'correcting' the source impedance when using low-z transformerless mics will avoid certain performance issues with the preamp input transformer such as frequency response variations and transient ringing. Although a lower source impedance can also improve THD performance, so it's actually quite a nuanced subject and the actual results are highly dependent on the complete circuit design.

In general, the varying transformer loading characteristics create tonal and transient changes which may or may not be deemed musically beneficial. But, as far as the OP was concerned four years ago, there is no risk of damage to mic or preamp, which was their primary concern, I think.

The 'not a good idea' concern relates simply to
the idea you've shared, that plugging a transformerless mic into a transformer-input preamp may not extract the best performance each unit is technically capable of, due to degraded transformer behaviour.

But... the specifics will depend on the complete design of the whole preamp, as I say.

And, at the end of the day, transformerless mics like the TLMs were designed to achieve a higher level of performance when used with modern transformerless preamps.

[NEWTON IN ORBIT]

Hey thanks for the response. I was only commenting because there can be some validity to the OP's question regarding change of tone with output Z with some transformers.

I appreciate the very kind response.
I think for 50 ohms, a resistor value of 49.9 r is what Steve Hogan suggested.
I also think Steve would be happy to join in and discuss this kind of stuff if you'd welcome him on your forum.

Thanks for being calm and cool (and informative) in your reply, something that is very rare on most internet audio forums in 2025. It's much appreciated!
Best Regards,
John

[Hugh Robjohns]

I also think Steve would be happy to join in and discuss this kind of stuff if you'd welcome him on your forum.

Everyone one is most welcome!

Thanks for being calm and cool (and informative) in your reply, something that is very rare on most internet audio forums in 2025. It's much appreciated!

We hear that a lot...