MXL 603 -> KM84 Replacement PCB

[joulupukki]

Matt will likely sell the transformer separately, if you email him.

Nope. I emailed today and he quickly said that they don't sell it outside of their own kits unfortunately.

 

[joulupukki]

I just put the MP TX into the DIY build and with quickly recording some acoustic guitar strumming:

- The MP mic still has higher output (I expected this)
- MP still has more upper frequency response (and sounds more clear)
- With the MP capsule and TX on the DIY build it's closer, but the MP build still has more upper frequency material in comparison

I'm hoping to visit a studio soon that has a pair of KM-84 mics and take the MP & DIY builds there to see how they all perform. That will be a lot more telling as to judge my particular DIY build.

Edit:
If it’s not the transformer or the capsule that’s giving the boost in upper frequencies & clarity, which component in the build would be the main contributor? The JFET? The 22uF electrolytic caps in the MP builds vs the 4.7uF tantalums in the DIY builds? C4 in MP is a 1uF and in this DIY build is a 4.7uF. The lack of the C2 cap in the MP build? A combination of all of the above? Something else?

 

[jp8]

I would put my bet on the removal of C2, which increased the gain, but also increases THD. Transformer distortion is mainly below 1kHz, THD increase of the JFET across the whole audio bandwidth. It's mainly 2nd order, tube like distortion, which in moderate amounts may sound benign and give a sense of clarity and detail if the fundamental frequency is in the mid range.

The other caps you mentioned will only affect gain and noise in the lowest octave(s) if they were chosen too small.

Would be awesome if you could share some of the shootouts with the KM84 and your DIY mics! Are you also going to make recordings of instruments other than guitar?

Jan

 

[joulupukki]

I would put my bet on the removal of C2, which increased the gain, but also increases THD. Transformer distortion is mainly below 1kHz, THD increase of the JFET across the whole audio bandwidth. It's mainly 2nd order, tube like distortion, which in moderate amounts may sound benign and give a sense of clarity and detail if the fundamental frequency is in the mid range.

You’re right about C2. I removed it in one of the DIY mics and now it’s definitely got more gain (a little more than the MP now at the same preamp level). It doesn’t seem to have adverse effects that I can tell. The THD didn’t really change relative to the new amount of gain. I was a little bit aggressive getting C2 out and one leg of it broke and cracked the tiny cap … so I won’t be putting it back in and will probably remove C2 from the other DIY mic as well.

Comparing the MP and the DIY mic with C2 removed, the higher frequencies are now a little closer to each other, but I’d still give the edge to the MP. It just has something a little extra that helps the definition.

The other caps you mentioned will only affect gain and noise in the lowest octave(s) if they were chosen too small.

Gotcha. That’s kind of what I figured after reading more about it this morning. My guess at this point is that it’s just a sum of all the parts (capsule, JFET, caps, and TX) that’s contributing to the subtle differences between the two mics. It could also be the vents on the mic bodies & capsules as well … in addition to the difference in space between the nose cone and capsule. Probably not enough to worry about.

Would be awesome if you could share some of the shootouts with the KM84 and your DIY mics! Are you also going to make recordings of instruments other than guitar?

I’m not 100% sure yet whether I’ll be able to get into the studio or potentially borrow/rent their real KM84. If I do, I know I’ll try guitar and/or whatever else they’ve got there … within reason. Is there any particular sound source you’d be interested in hearing the difference on?

 

[joulupukki]

After removing C2 from my DIY mics, they are a lot closer in sound to my MP SDC-84 mics. Here's a quick mix with no effects or EQ added, just volume automation and panning, trying to keep the MP and the DIY at the same exact volume. I recorded this with one of each mic at the same time as close together without touching as possible. Recorded through an Apollo Twin X with the Neve 1073 preamp emulation and then directly into Logic Pro. If anyone wants the individual tracks to compare them, just let me know.

What differences do you hear between them in a rough mix?

MP SDC-84:
View attachment MP-SDC-84-You-Are-My-Sunshine.mp3

DIY KM-84 (Central 2N3819 JFET, 3U cardioid capsule, and 3U GZT-84 transformer):
View attachment DIY-KM-84-You-Are-My-Sunshine.mp3

 

[jp8]

Nice song, nice playing!

I couldn't tell one from the other. But I have to admit, I have great difficulty hearing subtle differences. Especially in the top end, because my ears are perfect 9.5 kHz Low-Pass filters. So if anyone else hears differences, I would be the last one to deny that.

If there are any audible differences between the KM84 and your mics, I would expect them to hear in the lower and higher octaves, especially on loud instruments. So if you are curious about it yourself and have the opportunity, I would say: any bass instrument, toms, snares, overheads on drums, violin, etc. Like someone else already said, guitar and voice are mainly in the mid octaves and not very loud, so differences due to non-linear distortions will be less noticeable than in the extremes of the audio range.

The lower octaves, say below 100 Hz, could sound different because then the transformer might saturate and start to distort heavily. And they all have different transformers. The higher octaves, say above 6kHz, when played loud, will produce Difference Frequency Distortion and Intermodulation Distortion in the mid and lower octaves. Due to the masking effect, the THD of the higher octaves will hardly be noticeable, but the DFD/IMD tones will end up in the sensitive 1kHz-6kHz region and may cause harshness. I would expect this to be more noticeable with your MP and DIY mics without C2 and less so with the KM84. I would not consider the MP and DIY to be worse than the KM84 if that were indeed the case, but I would say they have different use cases.

Jan

 

[joulupukki]

Nice song, nice playing!

Thanks!

I couldn't tell one from the other. But I have to admit, I have great difficulty hearing subtle differences. Especially in the top end, because my ears are perfect 9.5 kHz Low-Pass filters. So if anyone else hears differences, I would be the last one to deny that.

Dang. My right ear can definitely hear higher frequencies than my left so I definitely partially understand that!

If there are any audible differences between the KM84 and your mics, I would expect them to hear in the lower and higher octaves, especially on loud instruments. So if you are curious about it yourself and have the opportunity, I would say: any bass instrument, toms, snares, overheads on drums, violin, etc. Like someone else already said, guitar and voice are mainly in the mid octaves and not very loud, so differences due to non-linear distortions will be less noticeable than in the extremes of the audio range.

The lower octaves, say below 100 Hz, could sound different because then the transformer might saturate and start to distort heavily. And they all have different transformers. The higher octaves, say above 6kHz, when played loud, will produce Difference Frequency Distortion and Intermodulation Distortion in the mid and lower octaves. Due to the masking effect, the THD of the higher octaves will hardly be noticeable, but the DFD/IMD tones will end up in the sensitive 1kHz-6kHz region and may cause harshness. I would expect this to be more noticeable with your MP and DIY mics without C2 and less so with the KM84. I would not consider the MP and DIY to be worse than the KM84 if that were indeed the case, but I would say they have different use cases.

Good points! Hopefully I'll get a chance to try out a real KM-84. In the meantime I'll have to go visit someone with a drum set and see how these to. With C2 missing they're definitely not going to be able to handle a high SPL. I know MP offers this same build optimized for snare drumps and has a higher-value C2 cap, a different transformer, and a different value for C4 as well.

I'm just happy I've got these Takstar KM-84 conversions working as well as the MP for the type of stuff I need them for. It's been a fun & rewarding project! It'd be great if I could repurpose the low cut hole in the Takstar CM-60 body to use as a pad switch. But, the upper part of the transformer is in the way for pulling that off.

 

[jp8]

Perhaps with a dedicated CM-60 board, the transformer will just fit between switch and XLR. There's 26 mm between them. Transformer measures 25mm... I could give that a try later, once I have finished my CM-63 boards. Two are ready, working on the third and last one.

Jan

 

[MicMaven]

After removing C2 from my DIY mics, they are a lot closer in sound to my MP SDC-84 mics.

My vote: The second one marked KM-84 sounds bigger, livelier, more musically exciting - not a lot, just enough to notice. The KM-84 has more percussive presence, i.e., it has more punch and sizzle, e.g., picks striking strings are more crisp and articulate and percussive. I like the overall tonal balance - the mandolin sounds right and the banjo is sufficiently far back to avoid the usual harsh percussive whack banjos have up close. If you were producing my record, I would have you use the KM-84. While both are VERY close in overall sound, the KM-84 JUST slightly more energetic. And that is just MY take ... James

 

[joulupukki]

Interesting observations. Thanks James!

I did just discover a fairly significant difference between my DIY KM84 and MP SDC-84 mics. The DIY mics are a lot more susceptible to RFI coming from my cell phone ... like at least 10 times more susceptible than the MP SDC-84s. I wonder if that's the diff between the 22uF electrolytics in the MP and the 4.7uF tantalums in the DIY? Or maybe not related to the component values or type at all ... like the mic body? This side of the equation is definitely won by the MP mics.

 

[jp8]

Electronics are always full of surprises... I would have expected both to have neglible RFI susceptibility. JFETs do not act as AM demodulators like BJT's do. This RFI susceptibility is typical for mics with a BJT output, like the Schoeps. Or even worse, single-ended BJT output designs like Behringer B-5. RFI susceptibility can be reduced significantly with a dedicated filter, like the CM-63 has. The CM-60 doesn't have this filter and is way more sensitive to cell phone interference.

Electrolytics are more like inductors at cell phone frequencies and do nothing to suppress RFI interference. But its parasitic inductance, together with other parasitic inductances and capacitances in the circuit, could develop resonant circuits which can raise or attenuate susceptibility at certain frequencies. I have RFI tested several mics between 500 and 1200 MHz and noticed that they differed in the frequency at which maximum sensitivity occurred, probably due to different circuit parasitics. But I did not test a KM84 style circuit. Perhaps I'll do that later. Your observations at least made me curious!

Question: they were connected to the same input on the same preamp or mixing console with the same XLR cable? It could also be the mic input which is AM demodulating the RFI signal. Using my RF jammer I observed different RFI sensitivities of different mixers, so I would not rule it out.

Jan

 

[joulupukki]

Electronics are always full of surprises... I would have expected both to have neglible RFI susceptibility. JFETs do not act as AM demodulators like BJT's do. This RFI susceptibility is typical for mics with a BJT output, like the Schoeps. Or even worse, single-ended BJT output designs like Behringer B-5. RFI susceptibility can be reduced significantly with a dedicated filter, like the CM-63 has. The CM-60 doesn't have this filter and is way more sensitive to cell phone interference.

Interesting. I never tested the CM-60 for this before I disassembled them for the project.

Electrolytics are more like inductors at cell phone frequencies and do nothing to suppress RFI interference. But its parasitic inductance, together with other parasitic inductances and capacitances in the circuit, could develop resonant circuits which can raise or attenuate susceptibility at certain frequencies. I have RFI tested several mics between 500 and 1200 MHz and noticed that they differed in the frequency at which maximum sensitivity occurred, probably due to different circuit parasitics. But I did not test a KM84 style circuit. Perhaps I'll do that later. Your observations at least made me curious!

Question: they were connected to the same input on the same preamp or mixing console with the same XLR cable? It could also be the mic input which is AM demodulating the RFI signal. Using my RF jammer I observed different RFI sensitivities of different mixers, so I would not rule it out.

Yep. All connected to the same UA Apollo Twin with the same XLR cables. I had one pf each of the MP and the Takstar KM84 conversion plugged in right next to each other and can watch their levels. When my cell phone gets close, it’s a disaster. Fortunately, if I move my cell phone about a meter away the interference goes away. I just need to be conscious about it when guys in the band come to record that they don’t bring their cell phones into the booth … and that mine is sufficiently far away.

One of the things I wondered is whether the body of the mic wasn’t grounded to XLR pin 1 but I used my continuity tester on my DMM and that all checks out as fine.

 

[jp8]

One of the things I wondered is whether the body of the mic wasn’t grounded to XLR pin 1 but I used my continuity tester on my DMM and that all checks out as fine.

XLR pin 1 should be connected to the body, indeed. If it's not, you'll get zero or little audio signal and loads of hum. The mic tube is the ground return path for the capsule, so it is essential XLR pin 1 and signal ground on the PCB have a low resistance path to the mic tube.

For RF, what matters is the impedance ("AC resistance") between pin 1 and the tube. At 800 MHz, a 5mm wire already has an impedance of ~25 Ohm. This is significantly higher than the DC resistance via the XLR screw, which I would estimate is in the 10...500 mOhm range. So every mm of wire counts. Maybe one mic has a much longer path from pin 1 to the mic tube than the other? I'm just guessing.

Takstar understood the importance of the short connection and in the CM-63, they have integrated a spring loaded grounding contact in the XLR. I wish I could find a source to buy these XLRs from. The best alternative I could find on Aliexpress has a metal tab which shorts the tube to pin 1 on two points.

Jan

 

[joulupukki]

XLR pin 1 should be connected to the body, indeed. If it's not, you'll get zero or little audio signal and loads of hum. The mic tube is the ground return path for the capsule, so it is essential XLR pin 1 and signal ground on the PCB have a low resistance path to the mic tube.

For RF, what matters is the impedance ("AC resistance") between pin 1 and the tube. At 800 MHz, a 5mm wire already has an impedance of ~25 Ohm. This is significantly higher than the DC resistance via the XLR screw, which I would estimate is in the 10...500 mOhm range. So every mm of wire counts. Maybe one mic has a much longer path from pin 1 to the mic tube than the other? I'm just guessing.

Takstar understood the importance of the short connection and in the CM-63, they have integrated a spring loaded grounding contact in the XLR. I wish I could find a source to buy these XLRs from. The best alternative I could find on Aliexpress has a metal tab which shorts the tube to pin 1 on two points.

Jan

Right. I thought of that later ... that the capsule wouldn't have had a ground if it wasn't grounded. Good call.

One thing I didn't do is trim the wire of the 3U GZT-84 transformer to make them as short as possible. I wonder if that could be the main difference? Also the MP (and in my mock PCB) the distance between the transformer secondary connections to the XLR pins is extremely short.

 

[joulupukki]

Interesting. I just did a quick test to see what would happen if I kept the transformer wires looping closely to the body of the transformer instead of flying above the circuit board (to get out of the way of fitting inside of the mic body). The RFI pegged the preamp well past the clipping point. I put them back how I had them and it's back to what I started with. I've also tried (without desoldering) twisting a couple of them and it's improved. So, there's probably some ideal way of routing these transformer wires and likely having them be shorter would help, though it makes me a little nervous so shorten them and have only one routing option.

Update:
Here’s the current configuration that seems to give the least amount of noise from my cell phone:

 

[jp8]

Missed your last post, because GroupDIY didn't notify me there were new posts in this thread, but I noticed you already started a new thread, specifically on this topic.

Anyway, I finished the KM-84 board for the CM-60. It just fits, if you clip the solder cups from the XLR pins. Will order the board together with some others I haven't finished yet. I have added a ferrite bead in the ground (L1, bottom left) to see if that will reduce RF sensitivity.





Jan

 

[MicMaven]

Takstar understood the importance of the short connection and in the CM-63, they have integrated a spring loaded grounding contact in the XLR.

SHURE also does this. See images of two types of SHURE XLR connectors used in SM and Beta series models.

Serving suggestion: I am a big fan of scraping the paint off internal points of interest to assure electrical continuity from stem to stern in microphone bodies. In addition to the other good suggestions raised above, you might also consider scraping a bit of paint of strategic points of contact so everything is clean and has continuous electrical conductivity.

Sidebar Comment -- Amateur radio operators employ various methods for reducing RFI/EMI in the shack, including adding additional shielding, working to avoid Pin-1 type ground problems (as JP8 mentions, and adding ferrite beads to susceptible lines as jp8 also says. In fact, many hams rely far more on ferrite beads than they shook, their shacks are festooned with them, instead of finding and eliminating the real source or problem. Just saying ...​

​

I sometimes use a pair of 22 nf VISHAY K223K15X7RF5UL2 capacitors for RF protection - Connect two 22nF capacitors
between Pins 2 and 3 of the XLR and Ground. This should be as close the XLR jack as possible for best EMI/RF suppression. Connect Pin 1 to the Ground Lug. I think this is consistent with others' recommendations.​

James /K8JHR

 

[joulupukki]

Good points @MicMaven. I think the reason those XLR connectors (with the big ground tab) work in those other mics is because the ground tab screw is accessible from the outside of the mic body. As you tighten the screw, the ground tab gets pressed up against the body. I think I would have to drill a new hole in my mic body to pull that one off. No doubt it’d make the electrical connection to the mic body be a lot stronger.

I’m definitely familiar with the ferrite beads in the shack. I just haven’t done anything like that on a smaller scale before inside of a microphone.

73/KB0YD

Also, rather than bore this build thread with RFI/cell interference chat, I started a separate thread about it: https://groupdiy.com/threads/is-there-a-way-to-reduce-cell-phone-interference-in-diy-km-84.86707/

 

[MicMaven]

. I think I would have to drill a new hole in my mic body to pull that one off. No doubt it’d make the electrical connection to the mic body be a lot stronger.

I’m definitely familiar with the ferrite beads in the shack. 73/KB0YD

Hmmm ... I do not think that is quite correct, old man. The set screw works by backing it out against the inside of the screw hole in the handle, and the ground pad merely rests up against the inside wall of the handle - notice the pad is located away from the set screw - making the two somewhat redundant. (Parenthetically, I avoid using ferrite beads as much as possible, preferring to locate the source of the problem and work it out from there. I would probably just move the cell phone to another room ... Different strokes, ya know.)

PS - Nice photo on QRZ.com and I am confident the backdrop is the Pulteney Bridge in Bath, UK, although my multiple appearances there likely preceded your visit by some years!

73 and good luck with the mic build. James/K8JHR

 

[joulupukki]

Ok, I guess I was just thinking it'd need to be a similar arrangement as how this old SM58 I've got ... where that set screw is accessible from the outside of the mic body. I ought to pick up some of those XLR connectors to try out. That flange would definitely make a stronger connection.

(correct on Bath)