joulupukki
Well-known member
Nice! That’s gonna be fun to build. That stash rivals mine and then some. I need to clean house and convince my band mates or someone to buy a couple from me. I don’t think I need so many! 
KM84 revisited
- Thread starter jp8
- Start date
Help Support GroupDIY Audio Forum:
Thanks for the heads up, @museric! Of course, I'm just as excited as @joulupukki and I hope you manage to build some great microphones. And that you also enjoy building them!
Did you already decide on which transformers to use?
Cheers
Jan
Did you already decide on which transformers to use?
Cheers
Jan
Great! @jp8 , you're tempting me to build a pair of these beauties too!If you have been following this and this thread of @joulupukki , you know that I have made two SMT variants of the KM84 circuit with several improvements compared to the classic KM84 circuit. For convenience, I have called these circuits KM84+ and KM84++. They are described in detail on my website, which I have now launched. You will find the links below. The KM84+/KM84+ article texts are not completely finished yet, but I plan to finish them during the Christmas holidays. I hope you'll enjoy reading the articles on my website and find them useful!
I will make KM84+/++ PCBs available for the Takstar CM-60 and CM-63 and Alctron T-02A on PCBWay, but at the moment of writing, I have only finished the building instructions for the KM84+, suitable for the CM-60 microphone.
Links to KM84 and KM84+/++ articles on my website:
KM84 circuit description: https://modimications.com/km84-style-charge-amplifier-circuit/
KM84+ and KM84++ description: https://modimications.com/km84-and-km84-head-amp-circuits/
Takstar CM-60 and CM-63 description and mods: https://modimications.com/takstar-cm-60-and-cm-63/
Available PCBs (at the moment only one, but you get an idea of what I'm working on: https://modimications.com/all-orderable-pcbs/
When more KM84+/++ PCB variants become available, I'll keep you informed via this thread.
Note: unfortunately, my website does not display correctly on mobile devices. I'll have to work on that.
Happy reading and Merry Xmas to all!
museric
Member
Yes, I did get the AliExpress transformers this time around (just a pair for the first build of the KM84+.) I think I’ll try the 3U transformers for the next pair, and grab those same time as the capsules. Yay!
Now - I’ve noticed that one component is out of stock vs. the rest of the parts purchase. I’m looking at alternatives to that Wima 1uF cap, and discovering that everyone seems to be out of the 5% tolerance for that size and value. The wait may be awhile according to both Mouser and Digikey - March or April. Ugh.
Get some other 1uF film cap. Wimas aren't sprinkled with magic fairy dust, and the transformers will have more loose tolerances than the film caps.
Keep a realistic perspective, is all I'm saying / advocating...
museric
Member
Good, yes, I was wondering about just getting another brand or a looser, and more available, tolerance. Thx!Get some other 1uF film cap. Wimas aren't sprinkled with magic fairy dust, and the transformers will have more loose tolerances than the film caps.
Keep a realistic perspective, is all I'm saying / advocating...
On the KM84+, you can either place a 1uF film capacitor (C4a), or electrolytic cap (C4). Whatever you like... Some people have very specific preferences regarding capacitors. I am not one of them, but I thought a red Wima capacitor looked nice on the black PCB. But I do believe that the value is important and that 1uF is the ideal value in my opinion. This is explained on my website.
The components that some people like to swap out and experiment with, are mostly through-hole technology parts, which many find easier to swap out than SMT parts. This is one of the reasons why I chose THT parts in some cases, instead of SMT to make the design more DIY-friendly. To this end, I also added a GND and Bias testpoint to facilitate bias adjustment and fault finding.
Jan
. But I do believe that the value is important and that 1uF is the ideal value in my opinion. This is explained on my website.The components that some people like to swap out and experiment with, are mostly through-hole technology parts, which many find easier to swap out than SMT parts. This is one of the reasons why I chose THT parts in some cases, instead of SMT to make the design more DIY-friendly. To this end, I also added a GND and Bias testpoint to facilitate bias adjustment and fault finding.
Jan
I received some pictures of a KM84++ PCBA with some assembly errors: R1, R2 and C1 were not placed in the right holes. Carefully watch the assembly drawings and the 3D PDF provided! I had to leave out some silkscreen printing in the High-Z area as that would possibly compromise the moisture immunity. So placement is not completely intuitive. There are also additional holes, allowing the use of different resistor types with different pitches. On the pictures I received, the resistors were placed upright in the closely spaced holes, instead of flat on the PCB. See picture below. The narrower 9mm pitch is for HVR82 series G-Ohm resistors from Aliexpress.
There were also issues with soldering C11. Most likely because a counterfeit part was used. The 101X18N102MV4E (aka EMDD101G102M1GV001E) is currently out of stock at Mouser, but is in stock at Digikey. If you're in doubt about replacement parts, either PM me or post your questions here.
Jan
There were also issues with soldering C11. Most likely because a counterfeit part was used. The 101X18N102MV4E (aka EMDD101G102M1GV001E) is currently out of stock at Mouser, but is in stock at Digikey. If you're in doubt about replacement parts, either PM me or post your questions here.
Jan
museric
Member
Oh good to know - earlier today I was just looking through the parts I've received so far (Digikey) and reviewing the board silkscreens for both the KM84+ and KM84++ and noticing C1 placement was not obvious from the PCB markings (I was only taking a cursory look during lunch, without the assembly document - thinking that it was something to be first verified with documentation - so this note is very timely.) I'll review what I have before I start assembly and let you know if I have more questions about this or other items. I hope to get a start on this in the next day or two, as I expect to have all parts by end of today.
Thank you Jan!
