MXL D.R.K. Condenser Mic as Donor, Alice, Royer??

[doorunrun]

Apologies for the terrible artwork, but here's what I think is going on on the daughterboard:
Pins are numbered 1-12, anticlockwise from top right, like a regular IC.
I'd guess, therefore:
5 - input from capsule
4 - noninverting amp dc blocking capacitor
6 - circuit ground
3,8 - to output coupling caps
7 - mid supply decoupling capacitor
9 - main supply decoupling capacitor
10 - battery input?
1,12 - from output coupling caps
2,11 - to XLR

By George...@Voyager10, I think you've got it! Excellent work! Thanks for taking the time to suss it out!

I didn't have any luck by reinstalling the '2134. The board(s) drew a lot of current and the phantom supply voltage at the XLR connection dropped to a very low value.

I see what you mean about the 10Ks. I'll revise my order...

Thanks so much for the help!

 

[doorunrun]

I maybe putting my foot in it.
I started comparing the two schematics and thought I might rebuild the daughter board as a OPA Alice.
I may have some SMD resistors laying around...

 

[doorunrun]

There's probably no point in a fancier op-amp unless you're inclined to change the 10K resistors as well.

About my earlier post...Doh! That's just what you said.

 

[doorunrun]

A photo of the daughter-board (DB) with the '103' (10K) SMD resistors removed. I rotated it and added pin numbering to help make things a little clearer:


To help get my head around the signal flow between main-board and daughter-board I printed @Voyager10 's terrific drawing out and drew in the cap to XLR connections and discovered something interesting.

Audio flow from Op Amp outputs to XLR
Non-Inverting Output: Pin 3 of DB --> Cap --> Pin 1 of DB --> Pin 2 of DB --> XLR Pin 3
Inverting Output : Pin 8 of DB --> Cap --> Pin 12 of DB --> Pin 11 of DB --> XLR Pin 2

Well this is backward. I think the reason for this is the way the capsule is made with the electrical contact on the rear side. Since I'll be using a capsule with connection on its front the easy answer is to just reverse XLR connections.

I have a good supply of 0603 MD resistors that will fit. I'll go with OPA1656 (or OPA1642) and install 2.2K resistors. I'm just wondering if the other resistor values should remain at their current values or more in line with the Alice circuit.

My guess is Marshall implemented it this way to avoid conflicts with copying someone else's design or to get the mic operational with a 9V battery.

Thanks!

 

[Voyager10]

Regarding the output polarity - it depends on where the polarization voltage gets applied, relative to the amplifier/impedance converter input.

There are two common arrangements for a simple, 2-terminal capsule. Option A is for one terminal to be grounded and the other pulled up to the polarization voltage via a big (1G) resistor. For option B, one terminal is connected directly to the polarization voltage and the other grounded via the big resistor. In both cases, the 'big resistor' end is coupled to the amplifier.

In both cases, an increase in air pressure moves the diaphragm closer to the backplate, the capacitance of the capsule increases, and the voltage on the capsule (momentarily) decreases. (i.e. the magnitude of the voltage decreases, irrespective of which terminal is positive with respect to the other).

In option A, decreasing capsule voltage sends the amplifier input negative. For option B, the amplifier input goes positive (you get "polarization voltage minus capsule voltage"). So option A will need some inverting arrangement to keep to the 'positive pressure = positive voltage' convention.

 

[doorunrun]

Ugh!
That daughter-board is very thin ceramic, and the metallic pads/traces are VERY easily burned off. I now know. I tried installing the 2,2K resistors and it wasn't a pretty sight (site).
Not holding out much hope for repair. At least the I/O pins are spaced at 0.1" here may be some possibilities to salvage something out of it.

 

[Voyager10]

Mmm. Bad luck, that sucks. It might be easier to build something on perfboard at this stage, with a SOIC adapter for the op-amp.

 

[doorunrun]

Mmm. Bad luck, that sucks. It might be easier to build something on perfboard at this stage, with a SOIC adapter for the op-amp.

Yes, that's were I'm headed. Thanks!

 

[doorunrun]

Here's what I've come up with so far:


The idea here is to fit the new daughter-board below the mic's main-board and fit it between the mic's mounting rails.
The header/socket connector is known as DuPont. I'm mounting only one of them and plan to use wire jumpers on the other side. The reason for this is the layout of the new break-out board with its SOIC pins won't allow it. It also allows a short direct connection to the IC's input pin. I hope to keep the layout about the same as the original.

Fingers crossed!

 

[doorunrun]

I drew @Voyager10's schematic in KiCad and laid out the board to mimic the original daughter-board. I wanted it as a guide when laying out parts and wiring the new break-out board. I'm hoping to use 1206 SMD resistors. Caps and diodes will probably we through-hole.


Edit: the KiCak schematic might help...

 

[Khron]

I drew @Voyager10's schematic in KiCad and laid out the board to mimic the original daughter-board. I wanted it as a guide when laying out parts and wiring the new break-out board. I'm hoping to use 1206 SMD resistors. Caps and diodes will probably we through-hole.
View attachment 126909 View attachment 126910

Pin spacing's probably double the original's (~5mm / 0.2" versus 2.54mm / 0.1"). But that's only if it's intended to match the layout of the "main carrier board"..?

 

[doorunrun]

But that's only if it's intended to match the layout of the "main carrier board"..?

Right. The break-out board I chose and the way they brought out the SOIC pins made fitting it into the existing layout more difficult. Well see how well it progresses.
On the new daughter-board the IC and components are facing down toward the main board. I'm going to try wiring on its other side with fine gauge wire and overlapping paths won't be, I hope, a problem.

 

[doorunrun]

I got the replacement daughter-board wired up and installed...


OK, it's working but output is low, I had to bring up gain pot on my Mackie 1202 mixer to get a decent level; even that was still a little low.

I'm not thrilled with how the capsule connects to the new board. I think what Marshall was trying to do was to lower external phantom voltage to match what you'd get with the 9-volt battery. I didn't take too many voltage readings, but did note polarizing voltage was around 40V.

I went back to using the MXL/Schoeps boards in this mic. I will do more bench testing and maybe come up with something better.

 

[doorunrun]

I've replaced the "122" SMD resistors (not 221 as previously thought) with 51Rs thinking this is closer to what the OPA Alice boards use. Also, thinking I need to relocate a 1G resistor to the daughter board to improve the capsule's connection routing. And, maybe replace the inverter board's zener with a 10 or 12V one.

BUT...after a good night's rest I think I might use this mic for another Royer Mod build. I've got all of the parts left over from the other one and a bunch of 5840s. I can share the power supply between the two for now. I might even be able to modify or replace the inverter board to give the mic pattern switch capability. Hmm...

I've already got two other OPA Alice mic builds, why do I need a third?

There I go thinkin' again.

 

[doorunrun]

Got the mic RoyerMod modded.


It's essentially the same layout as what I used for RoyerModing a MXL2001, see: "Royer MXL2001 cont'd"

After my earlier experience wiring up the daughter board, I used wire-wrap wire for short "pad-to-pad" connections. I used 28 and 30ga. teflon insulated hook-up wire sourced from Amazon for the longer runs. It's nice not having to worry about insulation shrinking or melting when soldering.

Hopefully transformer placement isn't going to interact with the 5840 tube. There's not a lot vertical space in that mic. I'm leaving room up near the capsule in case I can fit a pattern switching board in there. ATM, I'm not sure what that entails.

Initial testing of the mic looks and sounds good.

Have a great weekend!