Yes, it can be used no problem without it, it's just that the circuit will have less gain. Issue with this cap could explain loss of gain, either if it's leaky, or if it's shorted.
Getting 6-8 fewer dB out of a new DIY KM84 clone in a stereo matched pair
- Thread starter joulupukki
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Voyager10
Well-known member
If there's no appreciable AC signal at the source terminal, C3 is doing its job.
joulupukki
Well-known member
Alright, well, I've swapped out C3 (22uF 50V) with one I pillaged from a switching power supply that died. I also moved C1 from Mic A to Mic B. Still the same behavior with Mic B. It seems like all signs point to a bad FET. Is this a correct assumption at this point?
I personally haven't encountered two fets properly biased at similar operating voltages have this much diference in level. Maybe someone has different opinion, my gut feeling says it has to be something else, but i might be wrong. No way around it, if you suspect fets you need to swap those as well.
Voyager10
Well-known member
Well, unless the FET is a bad 'un, the only remaining option is that the drain is being too heavily loaded.
- Check R4 is 47K ?
- What is the level if the transformer primary is disconnected? It should increase - is it now the same on both mics?
I'm reluctant to assume the FET is bad, as it seems like Micparts grade them individually before shipping.
Last guess: are the source and drain connected the right way round? FETs can work 'upside down' but with different parameters.
- Check R4 is 47K ?
- What is the level if the transformer primary is disconnected? It should increase - is it now the same on both mics?
I'm reluctant to assume the FET is bad, as it seems like Micparts grade them individually before shipping.
Last guess: are the source and drain connected the right way round? FETs can work 'upside down' but with different parameters.
joulupukki
Well-known member
R4 is 46.9K on both mics. All resistors test nearly the same on both mics.
Unfortunately I've packaged them up and am about to drop them off with UPS to be sent back to MP so I can't test this.
They do grade them, especially for this matched pair. However, it could be that somehow (even though I thought I went quickly) I damaged the JFET so that it's not working properly in Mic B.
They're connected the same way in Mic A and Mic B so I'm guessing that's not it ... and also something MP would have spotted before sending them to me.
If it's not the JFET, MP will be able to fix whatever it is and at the end of the day (or in a week or so) I should hopefully have a great matched pair of mics. I definitely feel defeated, but not wanting to drag this on for too long if it wasn't the JFET and becomes beyond what I'm capable of diagnosing. What I need is a good stereo pair of mics to record an album with.
Thank you all very much for chiming in and helping me check for the obvious things! Hopefully I'll hear back about what it was and if I do, I'll report that here for reference to help potentially someone in the future.
If it doesn't work out for whatever reason, you can't go wrong with these. All of these are matched, very tight production tolerances. Every household should have a pair.R4 is 46.9K on both mics. All resistors test nearly the same on both mics.
Unfortunately I've packaged them up and am about to drop them off with UPS to be sent back to MP so I can't test this.
They do grade them, especially for this matched pair. However, it could be that somehow (even though I thought I went quickly) I damaged the JFET so that it's not working properly in Mic B.
They're connected the same way in Mic A and Mic B so I'm guessing that's not it ... and also something MP would have spotted before sending them to me.
If it's not the JFET, MP will be able to fix whatever it is and at the end of the day (or in a week or so) I should hopefully have a great matched pair of mics. I definitely feel defeated, but not wanting to drag this on for too long if it wasn't the JFET and becomes beyond what I'm capable of diagnosing. What I need is a good stereo pair of mics to record an album with.
Thank you all very much for chiming in and helping me check for the obvious things! Hopefully I'll hear back about what it was and if I do, I'll report that here for reference to help potentially someone in the future.
https://www.scandinavianphoto.no/line-audio/cm4-condenser-microphone-1053650?gad_source=1
Voyager10
Well-known member
I think returning to MP is about all you can do, it was looking less and less likely that it was a construction fault. It would be great it you can post anything you find out about the diagnosis.
joulupukki
Well-known member
I’ll for sure report back when I hear from MP about what it was. Hopefully they’ll let me know what the issue(s) is.
I do have a stereo matched pair of sE Electronics sE5 … and they work fine enough. But I can tell I will like these MP 84s better. They have less of a proximity effect and also don’t have too much high end like these sE5s do.
I do have a stereo matched pair of sE Electronics sE5 … and they work fine enough. But I can tell I will like these MP 84s better. They have less of a proximity effect and also don’t have too much high end like these sE5s do.
joulupukki
Well-known member
Speaking of DIY KM84-like builds, I see references to a couple of KM84 project kits on this forum site, but nothing recently. Did the group buys on parts for those stop? Just curious.
I can only attest the quality of these mics. I have the older CM3 and also Neumann KM84. Not sounding exactly the same but definitely on the same level.
The cm4 is a microphone everyone should own at least one of.
Have you thoroughly cleaned the circuit boards? Spray with IPA and brush off multiple times (well away from the capsules!!). Leakage would be a killer on these boards. Careful visual inspection of the boards with a powerful eyepiece is highly recommended too. Double check component markings while you are at it.
Attenuation switch - if you have one fitted - take it out of the circuit.
Check XLR wiring and assemblies.
Swap cables and input channels. Obvious but just a reminder to assume nothing.
Since you have two ‘identical’ items and you have swapped capsules, try swapping PCBs, then components.
The balance of probabilities is the components are good and the fault is in the assembly or elsewhere.
Don’t give up. Just my 2cents.
Attenuation switch - if you have one fitted - take it out of the circuit.
Check XLR wiring and assemblies.
Swap cables and input channels. Obvious but just a reminder to assume nothing.
Since you have two ‘identical’ items and you have swapped capsules, try swapping PCBs, then components.
The balance of probabilities is the components are good and the fault is in the assembly or elsewhere.
Don’t give up. Just my 2cents.
Ive had a pair of CM4's on my wanted list for ages , amazing value for money ,
and way ahead of the usual budget SDC mics in terms of flatness ,directivity .
Do Line Audio still do the multi patern mics , I seem to remember they were discontinued .
and way ahead of the usual budget SDC mics in terms of flatness ,directivity .
Do Line Audio still do the multi patern mics , I seem to remember they were discontinued .
Jfets aren't matched. Maybe not even the same Jfet in each mic. Change to a pair of matched Jfets. A J310 is a hi gain, fairly low noise fet. I don't know your circuit but that jumps out at me.
Chrisfromthepast
Well-known member
- Joined
- Mar 5, 2018
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https://groupdiy.com/threads/mxl-603-km84-replacement-pcb.72711/page-8
There is a PCB (probably still available) that worked for me. After you buy a capsule and transformer from 3u audio, and source a donor body, the rest of the BOM is available from mouser. Not exactly a kit, but you can definitely do it!
Yes!
I second the motion and recommend that option.
I got them PCB's from Graeme.
Used the parts I had available and both mics worked perfectly from the start.
Sorry to hear that you are having problems with your mics.
M
drtechno
Well-known member
Looking at the schematics, R3 should be adjusted to match gain.
Thank you all very much for chiming in and helping me check for the obvious things! Hopefully I'll hear back about what it was and if I do, I'll report that here for reference to help potentially someone in the future.
Also, if you use a single turn potentiometer, make sure it goes to the other pin. Some will have resistance at the bottom of the wiper differently from each other. This should be a multi-turn potentiometer anyways.
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joulupukki
Well-known member
I guess I could have experimented with different values of resistors for R3. In this case, the JFET was pre-biased with the kit and in this particular case it was a 4.22K resistor. Since I already sent it off for MP to evaluate, I can't try that now.Looking at the schematics, R3 should be adjusted to match gain.
Also, if you use a single turn potentiometer, make sure it goes to the other pin. Some will have resistance at the bottom of the wiper differently from each other. This should be a multi-turn potentiometer anyways.
For fun I was putting together sourcing all the components to build a DIY from a donor mic body/etc. and for a pair of these it was still gonna be between $500 - $600. Crazy how it all adds up. That was if I were to use a 3U transformer and capsule and Dale CMF55 resistors & WIMA film caps from Mouser ... and would still need to source or build a PCB.
Has anyone ever posted gerber files needed to print a PCB? Seems like one would be able to use a place like JLCPCB.com to inexpensively print a few prototype PCBs and be off to the races.
drtechno
Well-known member
R3 is supposed to be a trimmer pot instead of a resistor. Usually its a 10 turn bourns potentiometer.
This is a simple circuit. I wouldn't bother picking up a pre-made board unless its only a couple of bucks different from a proto board.
