A poor (wo)man's microphone measurement equipment

[OneRoomStudio]

So I built up and tested my circuit. It seems to work very well, with one exception - 60Hz line hum. Not surprising, but I can't figure out the best way to reduce/eliminate it. The good news is that initial measurements seem to look as expected (other than the 60Hz spike):

Simplified cardioid-only U87 circuit:


KM-84 style circuit:


This is the rig I built. It uses a Cinemag 10K:600 (with interwinding shield), and two 100pF variable capacitors. One cap is in series with the transformer output (as in my circuit above), and the other is only connected to the second set of probes to act as a "dummy" capsule for mics with dual diaphragm/backplate capsules. With the variable capacitors, I can dial in the capacitance to match the capsule I'm replacing.

 

[elskardio]

View attachment 125256

Would a simple attenuator work instead of the transformer?

 

[OneRoomStudio]

Would a simple attenuator work instead of the transformer?

Maybe. My goal with using the transformer was to block DC (my MOTU outputs are DC-coupled), isolate ground (capsules are not connected to external ground references), and unbalance the balanced output of the interface.

 

[kingkorg]

my MOTU outputs are DC-coupled

That's kind of weird, why is that? It would be messing with outboard gear. Also are you taking into the account THD produced by the transformer itself?

Also i presume as MOTU is professional piece of gear, the outputs are balanced, and the tip, and ring of output jacks are not coupled to ground. Even if they use impedance balanced outputs.

 

[Khron]

That's kind of weird, why is that?

At least some years ago, their clame to fame regarding that feature was being able to output control voltages / CV through the analog outs (for oscillators and whatever, i'm guessing).

 

[kingkorg]

At least some years ago, their clame to fame regarding that feature was being able to output control voltages / CV through the analog outs (for oscillators and whatever, i'm guessing).

Wow, had no idea. Wouldn't this be switchable?

 

[OneRoomStudio]

Wow, had no idea. Wouldn't this be switchable?

Nope.

 

[elskardio]

At least some years ago, their clame to fame regarding that feature was being able to output control voltages / CV through the analog outs (for oscillators and whatever, i'm guessing).

It's for Eurorack lovers I have the M4 and it's the same.

From a Soundonsound review:

"MOTU interfaces are favourites among the Eurorack community because they all tend to have DC coupled outputs. The M2 and M4 are no exception. The only other entry-level audio interface I've tested with this ability is the Native Instruments Komplete Audio 6 MkII which offers +/-2V. I measured the output of the M2 and M4 as +/- 3.6V, giving a very usable range of 7.2V."

 

[kingkorg]

Nope.

View attachment 126175

Wow.

 

[cyrano]

Some RME interfaces have DC-coupled outputs too. Headphones usually.

 

[OneRoomStudio]

Ok, upgraded my rig with shielded BNC probe cables instead of standard banana leads, and made BIG improvements. There's still a bit of 60Hz, but it's barely there.

Here's the simplified U87 circuit again:


The bump at the bottom is the noise floor, made up almost entirely of 60Hz, but much much lower level now. There's also a tiny blip at 180Hz...not sure what that is.

This is beginning to look like a useful mic circuit test rig!

 

[Khron]

There's also a tiny blip at 180Hz...not sure what that is.

3rd harmonic of 60Hz.

 

[OneRoomStudio]

3rd harmonic of 60Hz.

Yeah, just strange to not see the second at all.