Building adapter to connect dynamic handheld mic to 3.5mm TRS Lav mic input

Question

I want to use a pair of Rode Wireless GO IIs as transmitters for a pair of switched dynamic wireless mics (both low end SM58S clones without transformers Behringer SL85S) I want to use them for live sound applications, and the built in mics are too feedback prone compared to cartoid dynamic mics.

The Rode transmitter units have TRS mono inputs with plug-on power. When I hook the mics up directly, with XLR cold to TRS Sleeve and XLR hot to TRS tip, the levels are really quiet and the mic switch causes a horrible loud sound. Note that my XLR mics leave ground floating since they do not have transformers. I tried using a signal transformer (Neutrik 1:3:10 I tired the 1 : 10, the 1 : 3 as well as the reverse of each, 10 : 1 and 3 : 1)to boost the voltage level (or maybe to match the impedance, but I would be lying if I said I really understood impedance). The transformers helped with he loud pop when switching the mic, but the levels are still much lower than ideal, and it seems the transformers did not help at all.

Is it possible to use passive transformers to boost the signal level? My intuition is that the dynamic mics produce enough power but that there is some matching issue. I would say that my mic signals are 16db too low. I hope to avoid a pre-amp, as it would be bulky, hard to build, and difficult to power portably.

Answer 1

Problem one is that the Røde transmitters use a 1/8" TRS input with plugin power as far as I understand, and they don't bother mentioning their pin assignment. It's typical for TRS mono mics these days to connect TR to the microphone and use S for shield. That will work both with the old Soundblaster convention of providing plugin power separately on the ring as well as with stereo inputs already containing plugin power. Regarding the socket, bets are mostly off, but since Røde offers numerous stereo microphones, T and R might both be inputs with plugin power, either connected together (which would not work with microphones using a TS plug) or added in digital or analog in the transmitter.

Either way, you are not likely to get by without plugin power. That is different from the AKG wireless plugs (mini-XLR) where plugin power is on a separate pin and can just be left off.

For the safety of your microphones and for not biasing any prospective transformer you are going to use, you'll need a blocking capacitor removing the plugin power. Plugin power inputs may have an impedance of a few kOhm, so if you want to get going from about 20Hz, your capacitor would want to be something like 1/(2 pi 2kOhm 20Hz), namely 4uF. Note that this makes 20Hz the transition frequency (already +3dB loss, no problem for voice or even double bass or bass guitar but watch out for pipe organs and bass drums) and 2kOhm is a wild guess. The value can be larger: that's not critical but also likely unnecessary. You might want to use a Tantalum type since they have better behavior for higher frequencies than standard electrolytic capacitors and are quite smaller than non-electrolytic capacitors with similar capacity.

After blocking plugin power from the input, you should get reasonable results using the 1:10 transformer for boosting your microphone signal. Without the blocking capacitor, the transformer gets biased with DC current, and small transformers like that get saturated pretty easily, meaning the unblocked DC current would cause signal weakening and distortion.