KM84 revisited

[jp8]

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!

 

[kingkorg]

 

[rock soderstrom]

They are described in detail on my website, which I have now launched.

Congratulations on your new website jp8! It looks great and I'm looking forward to reading it. Bravo!

 

[gyraf]

Darn, that website is a really good resource on all things KM84 - Thank you!

/Jakob E.

 

[jp8]

Thank you, Jakob! And to all who liked my post. I'm happy to hear you find it usefull! If you think anything is missing or I have it all wrong somewhere, just let me know. Ain't nothin' perfect on this world...

 

[pmfalcman]

@jp8 - FYI.: No issues on my phone. The site displays and works fine.

Motorola, so it runs practically pure Android.

 

[RuudNL]

As far as I see, the schematic of the Takstar CM-60 is not correct.
In the DC converter C4 should be connected to the junction of L1 and C5, not to the emitter of Q1.
Also I have a feeling that C6 (usually 22 or 27 pF) is connected between collector and emitter of Q1.
In general C5 is 6.8 or 7 pF. The position of R9 looks 'strange'. (Collector Q1 not decoupled.)

 

[jp8]

Thank you Ruud for your valuable comments! I assume you were referring to the schematics here on my website. I checked the circuit diagram, and you were partially right.

• C4 should indeed connect to the C5-L1 junction.
• C6 is connected as depicted! You'd indeed expect it to be wired as depicted in the CM-63 schematic, i.e. between Collector and Emitter, but it isn't. I removed the capacitor from the PCBA and measured its value using my Smart Tweezers. It measured 2.7 pF.
• I also measured the value of C5. It's value is 22pF.
• While checking the circuit, I also noticed another error: trimmer P2, which is actually not placed, does not shunt R8 but R9 instead. I assume R9 was added to reduce the output voltage, and with P2, the capsule bias voltage can be adjusted. There is NO decoupling capacitor on the R9/Q1/R10/D2/C3 node!

I have revised the schematic accordingly and published it on my website.