Does the surface mounted version of the MXL990 have the same circuit as the through-hole version?

[Resonite]

Hello, I've stumbled across an SMT version of the MXL990 microphone and was wondering if the PCB still used the same circuit as the older THT version.

 

[Khron]

Possibly / likely..? Posting some photos might help.

 

[Resonite]

I apologize for not sending any photos, attached are a couple. I can take more if you'd like.

 

[Resonite]

Possibly / likely..? Posting some photos might help.

Sorry, I didn't reply to this reply, rather, my own post. The reply above has pictures of the PCB and also attached is the reference circuit.

 

[RuudNL]

It looks like it is the same schematic.
Schoeps circuit, same resistor values.

 

[Drosselmeier]

your schmtc is incomplete There is a voltage converter on the board

 

[Khron]

The original 990 had a 16mm(?) electret capsule - have they switched to an externally-polarized one?

 

[Resonite]

The original 990 had a 16mm(?) electret capsule - have they switched to an externally-polarized one?

I'm not sure, although here's a picture of the capsule.

 

[Resonite]

View attachment 115254
your schmtc is incomplete There is a voltage converter on the board

I'm afraid that's the only schematic I could find.

 

[Resonite]

Thanks for all the information from everyone, I'll probably try to modify the circuit a little bit later tonight (EST).

 

[Drosselmeier]

The original 990 had a 16mm(?) electret capsule - have they switched to an externally-polarized one?

 

[Resonite]

Thanks for all the information from everyone, I'll probably try to modify the circuit a little bit later tonight (EST).

(I was thinking of replacing C3 and C4 and bridging them with a 10K resistor, although this circuit seems to be a little different)

 

[Khron]

View attachment 115257

Whoopsie...

and bridging them with a 10K resistor

What? Why?

 

[Resonite]

Whoopsie...



What? Why?

I'm going to add a low pass filter to the circuit (I also talked to a friend who's an electrical engineer, although he only confirmed it on the circuit that I have sent above). EDIT: I realize that I should increase the resistance value.
Reference: Mic EQ Pt.3

 

[Khron]

Then my issue is one of semantics. "Bridging them with a 10k resistor" sounded to me like you intend(ed) to just place a 10k resistor between the bases of the two PNP output transistors.

If you only want a low-pass effect, you could consider adding a 1-10nF capacitor in parallel with R7 (referring to the schematic in post #4 here). No need for cutting any traces, and capacitors in that value range can be found in NPO/C0G flavors (no voltage or temperature coefficient) - sticking with SMD packages should make it easy to just piggyback it on top of the resistor.

 

[ccaudle]

consider adding a 1-10nF capacitor in parallel with R7

Wouldn't that then leave the signal at R6 as just an unfiltered source follower output? It looks like you would get an unfiltered signal on the hot pin and a filtered signal on the return pin that way.

 

[Khron]

Wouldn't that then leave the signal at R6 as just an unfiltered source follower output? It looks like you would get an unfiltered signal on the hot pin and a filtered signal on the return pin that way.

Yeah, and..? That's only relevant in case you're planning on plugging the mic into some form of single-ended-input preamp.

Keep in mind, differential receivers amplify the difference between the "hot" and "cold" inputs.

 

[ccaudle]

Yeah, and..?

It would cut the slope of the LPF in half. That may not matter, or may even be preferred, just pointing it out as something to be aware of.

 

[Khron]

It would cut the slope of the LPF in half. That may not matter, or may even be preferred, just pointing it out as something to be aware of.

So it would (somehow) end up as 3dB/octave (as opposed to 6dB/oct, 1st-order filter slope)? How?

 

[Voyager10]

The through-hole MXL 990 also has a Schoeps style circuit - see pic below.

There's a schematic of the MXL 603 at RecordingHacks.com - http://recordinghacks.com/images/mic_extras/mxl/603S-circuitlayout.gif

Looks very similar, except there's no trimmer for adjusting the FET bias.