MXL 603 -> KM84 Replacement PCB

[kingkorg]

I placed my UMIK-1 mic 20 cm from the tweeter in my FR measurement setup and adjusted the level of a 1 kHz sine wave until I measured 94.0 dB SPL in REW. I then replaced the UMIK-1 by the CM-60 (or any other DUT for that matter). I use a fixture to align the front face of the microphone with the centre of the speaker and to get exactly the same distance from the tweeter as the UMIK-1. I then read the output voltage of the mic from the RMS Voltmeter in my Audio Analyzer. This is the value listed in the first line of the table. Admittedly, the 19.3 mV is quite far off from the spec (12.6 mV). In most cases, what I measure is quite close to the specified value. E.g. the CM-63 was quite close, actually. With "close", I mean within +/-2 dB, which AFAIK is a normal tolerance of (budget?) microphones. This makes me believe that my method is OK. Or at least acceptable for DIY measurements.

At 0.1% THD, the output voltage measured 128 mV, which is 20*LOG(128/19.3) = 16.43 dB above 94 dB SPL. This amounts to 110.43 dB, which I rounded to 110 dB.

If you think my measurements or calculations are flawed, please let me know. I know you are far, far more experienced in measuring microphones than I am.

Jan

Nope, that's pretty much how i would do it. I was just curious if you had some other way to calculate it. It is possible, but makes my head hurt every time, so i just find it easier to derive using practical measurement. Thanks for the reply!

 

[joulupukki]

I don't know why I didn't put this graph in my previous post. Just hadn't thought to compare it yet, but this is pretty interesting. In terms of shape of the frequency response, the 3U and Takstar CM-60 capsules are pretty close. Hmm. This was using the Takstar CM-60 as the mic & circuit.

The 3U capsules I ordered have a 3mm long extension where the CM-60 capsules are only about 0.5mm. I've been trying to figure out how I could (temporarily) make some sort of extension so I could try out the CM-60 capsule in the MP SDC-84 body & circuit just to see how well it performs in that circuit. Any ideas?

CM-60 Mic w/ 3U Capsule Left, CM60 Capsule Right:


...and the 3U Capsule in the MP SDC-84 mic looks nearly identical to when it's in the Takstar.

Last night when I was comparing recordings of my acoustic guitar using the MP SDC-84 between the MP SDC capsule and the 3U capsule I couldn't tell the difference in sound (was using the free 4U+ Blind Test plugin for Logic). That makes me wonder if much difference could be heard if I could use the CM-60 capsule in the MP SDC-84 mic circuit? Maybe for mids-focused instruments like the acoustic guitar they'd be similar but on a drum set with a lot more high frequency info the difference would be more drastic?

 

[joulupukki]

It looks like I'll finally get my PCBs in the mail on Monday. In the meantime, I got anxious and tried my hand at PCB layout using EasyEDA. If, for whatever reason, the Graeme PCBs don't end up working with the CM-60 donor bodies, I'm thinking something like this might work. Basically the board is 82mm in length instead of 80mm and that'd allow me to slide on one of the XLR insert connectors. Or, if I found a MXL-style donor body, I could just use a saw and trim the board down by 2mm to make that also work. Most realistically, I'll figure out how to make the Graeme boards work, but still fun to experiment with possible ideas! I'm sure there are plenty of mistakes though. Nice thing is, 5 of them shipped from JLCPCB are just under $20 USD.




I did attempt to get the existing XLR connector off the CM-60 board. What a disaster. I ended up using pliers and just chopping it off the board. So one of the CM-60 boards is now a goner. I need to just order a pack of those XLR inserts for the 2nd CM-60 mic (which I'll do if the build in the first mic turns out really well).

 

[jp8]

Nice work!

Just wondering: C3, C6 an C5 are not interfering with the tube?

Jan

 

[kingkorg]

Last night when I was comparing recordings of my acoustic guitar using the MP SDC-84 between the MP SDC capsule and the 3U capsule I couldn't tell the difference in sound

Those MP capsules are made by 3U, or by same 3rd party manufacturer for both of these companies.

 

[Migs 31]

Nice work!

Just wondering: C3, C6 an C5 are not interfering with the tube?

Jan

Very good point.
@joulupukki hat's probably why, they are placed "dead centre", middle of Graeme's PCB.

M

 

[joulupukki]

Nice work!

Just wondering: C3, C6 an C5 are not interfering with the tube?

Jan

I dunno yet. Judging from his build guide, they don’t seem to be dead center and seem to be fine. They aren’t the taller electrolytic radials I’ve seen in other builds, but tantalum caps, which don’t seem to sit that tall. Here’s a shot from the build guide showing them not dead center:

Those MP capsules are made by 3U, or by same 3rd party manufacturer for both of these companies.

No idea really. If I was going just off of the captured frequency response of pink noise, I’m more inclined to think that the same 3rd party manufacturer makes the 3U and Takstar capsules and that the MP comes from a different manufacturer.

 

[Migs 31]

"I dunno yet. Judging from his build guide, they don’t seem to be dead center and seem to be fine. They aren’t the taller electrolytic radials I’ve seen in other builds..."

Ok, maybe I have a later (2020/2021) or earlier version of his PCB, than the one you have there...?
But on mine, they (C3-C5-C6) are definitely centered.
M

 

[kingkorg]

I dunno yet. Judging from his build guide, they don’t seem to be dead center and seem to be fine. They aren’t the taller electrolytic radials I’ve seen in other builds, but tantalum caps, which don’t seem to sit that tall. Here’s a shot from the build guide showing them not dead center:
View attachment 122190


No idea really. If I was going just off of the captured frequency response of pink noise, I’m more inclined to think that the same 3rd party manufacturer makes the 3U and Takstar capsules and that the MP comes from a different manufacturer.

It would be awesome if you could post images of all 3 capsules from the back, looking at termination screw and backplates

 

[joulupukki]

Ok, maybe I have a later (2020/2021) or earlier version of his PCB, than the one you have there...?
But on mine, they (C3-C5-C6) are definitely centered.
M

Ah, you're totally right. Looks like the earlier build is the one that is more towards the edge. I'll keep experimenting with a different layout.

It would be awesome if you could post images of all 3 capsules from the back, looking at termination screw and backplates

When I get home today I'll take a few pics hopefully to show the diffs.

 

[Chrisfromthepast]

While you are revising, take a knotch out of the corners of the xlr connected side. Trying to cut the phenolic later is tough.

This is a different circuit, and the layout is a bit cramped near the connector, but this is the idea.

 

[joulupukki]

What is the point of the notch. Will the whole thing not fit if the board is straight at the bottom?

 

[joulupukki]

Ok, looks like I also had the wrong size of WIMA caps on the design. This would fit everything a little better with the caps more towards the center. I guess the only trace running on the bottom side of the board would be the run that goes to the primary + of the transformer (TP+).

 

[Chrisfromthepast]

What is the point of the notch. Will the whole thing not fit if the board is straight at the bottom?

Notch it so that it will fit into the connector housing.

 

[joulupukki]

Notch it so that it will fit into the connector housing.
View attachment 122223

Ah ha! Of course. I was getting it confused in my head of how it works with the slide-in mount. Thx for clearing that up!

 

[joulupukki]

I wonder if something like this could work where it's more of a universal PCB that'd work in either a CM-60 donor or an MXL-style mic? In a Takstar CM-60, the overall board length is 84mm and the XLR insert is soldered directly onto the board (on the taper part). The CM-60 appears to use a 0.5mm pin length capsule. But, if you wanted to use a 3mm pin length capsule, you could saw off 2mm of the board (with a hack saw or whatever). Or, if you wanted to use it in an MXL-style mic where the board just screws on to the XLR housing, you could saw off 4mm of the board?

 

[joulupukki]

It would be awesome if you could post images of all 3 capsules from the back, looking at termination screw and backplates

From left to right: MP SDC, 3U SDC 3mm, Takstar CM-60:






Also of note, the 3U capsule's outside diameter is just a hair larger than both the MP and Takstar mic bodies:

 

[kingkorg]

From left to right: MP SDC, 3U SDC 3mm, Takstar CM-60:
View attachment 122232
View attachment 122234
View attachment 122235
View attachment 122237
View attachment 122236

Also of note, the 3U capsule's outside diameter is just a hair larger than both the MP and Takstar mic bodies:
View attachment 122233

Thanks mate!

 

[jp8]

I wonder if something like this could work where it's more of a universal PCB that'd work in either a CM-60 donor or an MXL-style mic? In a Takstar CM-60, the overall board length is 84mm and the XLR insert is soldered directly onto the board (on the taper part). The CM-60 appears to use a 0.5mm pin length capsule. But, if you wanted to use a 3mm pin length capsule, you could saw off 2mm of the board (with a hack saw or whatever). Or, if you wanted to use it in an MXL-style mic where the board just screws on to the XLR housing, you could saw off 4mm of the board?
View attachment 122226
View attachment 122227
View attachment 122228

The first idea would change the distance between capsule backplate and cone, if my underdtanding of your idea us right. That might change the FR. How, and to which extent, I don't know. @Kingorg: I guess you can elaborate on this.

The 2nd idea of sawing off part of the board should work. Just a pitty the XLR insert in the MXL body is rotated 180 degrees and does not line up with the board.

Jan

 

[joulupukki]

My boards from Graeme arrived today! I used one of the Takstar CM-60 mics and converted it to this DIY KM-84 circuit. Fortunately, both the 0.5mm CM60 capsule and 3U 3mm capsules work and so I was able to test both and compare them. The 3U capsule has nearly an identical frequency response as the MP SDC-84. The CM60 capsule seems to put out lower volume and still has a different frequency curve.

I did a test recording and cannot perceive the difference (when gain matched) between the MP SDC-84 and this DIY KM-84. But, there are definitely a couple of differences:

1. The MP SDC-84 (like they claim) is more sensitive and is about 4dB louder than the DIY KM-84 going through an Apollo Twin X, Neve 1073 preamp emulation with the input gain set to -50dB
2. If I add a gain plugin in post and crank it as loud as it can go and listen to the self noise, the MP SDC-84 has a lot more higher end noise than the DIY KM-84. I'd still like to record something like drums to see if I could perceive a difference there.

Because these Graeme boards were really built for the MXL screw-in mounted boards and the Takstars solder directly to the XLR insert, I had to improvise a little bit. It's not pretty, but it does work. I had to squeeze the board in just barely between the XLR posts on the insert and then I used left over components leads to connect the board's XLR pads to the XLR insert posts. Definitely not ideal, but it does work. From nose cone to XLR pins, the length is just a little bit shorter than the CM-60 board was which lets the 3mm 3U Capsule screw all the way on (whew).

I had to break out my file to get the 3U GZT-84 transformer to fit inside the board. I didn't have to file away enough to expose any inside copper layers, but it was at least 1mm in parts of it. Also, because the XLR pinouts are coming directly from the XLR insert, I swapped the transformer's secondary wires so that electrically it matches up to the schematic.

Just like has been discussed before, I had to shave away a little bit of the slot of the nose cone to allow the board to fit. The Takstar doesn't use any mounting screws for the nose cone and it's just held on by shoving the board into the slots. The wire from the capsule spring connection was just long enough to work.

Here are a few build pics (and trust me, the XLR connection doesn't look very pretty):






I've got it biased right now so the voltage on the drain is exactly at 10.0 V. At that voltage, the 2nd harmonic is exactly the same level as the 3rd harmonic. I'm not sure what is ideal as I don't fully understand that part yet. Should the 2nd harmonic be lower than the 3rd? Should it be higher? Should it be even? Completely clueless there. Haha.


Is the mic better than the Takstar CM-60? I hope so ... it certainly seems to mimic the MP SDC-84 pretty well! I've still gotta try out recording other sources ... and need to figure out what to do about covering that low cut hole in the Takstar body. Good news is that yeah, it's possible to make one of these boards work with a Takstar donor body.