开云体育

So here is the full non-CORE DPA 406x teardown.

Scott Helmke already kindly posted a couple of photos of the capsule here, I am reproducing them below for full context (and because my own teardown wasn't as clean). The outside shows a 3-pin SMD device (turns out it’s a PNP), a 33K resistor and a RF blocking cap.





I went further and tore down a defective one (“thin tinny sound”) I scored on eBay (it had dirt and even a droplet of blue glue on the membrane, the presence boost cap was glued on with the same blue glue and maybe this was done carelessly).

The outer device from Scott’s photo is a PNP transistor, code “3F”, likely some flavour of BC857. You can't see the "3" in his photo, but the rest of the marking matches my unit.





The JFET is deeper inside the device, potted to the “lid” behind the back plate of the electret. Its marking has been removed (laser). The gate is connected to the back plate of the capsule using a bondwire.



The “toy” component tester identifies it as a JFET, but the values for |Vgsoff| and Idss are wrong. I have measured |Vgsoff| = 1.77V and Idss = 14.8mA.



This is not the little league JFET with 100-300uA Idss typically found in electret lavalier mics. It doesn’t have integrated gate diodes either. Because my DPA 4080 CORE U(I) curve also has a “knee” at about 50uA, I believe in both cases the JFET is selected such that it will bias at 50uA with the 33K resistor placed externally. Versus a low Idss JFET, this configuration offers higher transconductance even at a lower drain current because the device itself has a much higher Idss (see below for why).



Since the JFET doesn’t have integrated gate bias diodes, the potting compound also holds a back-contact chip resistor (this is very likely the high value resistor). “Back-contact” meaning one of the resistor’s terminals is on the back, and the top one is supposed to be connected with a bondwire. You can see where this back-contact pad was soldered to the case, making the electrical contact to ground. The resistor looks like laser trimmed thick film.





Another component in the potting compound remains unidentified (and I cracked it while removing the potting compound). Could be a ferrite bead on the gate (Schoeps has one). If it’s a capacitor, I have no idea what it would be doing on the gate side, so I don’t think it’s a cap.



To summarize, this is it, plus the mystery component which I left out:



As I eventually suspected above, it’s the same topology from the Audio-Technica , except that one depicts JFET with integrated gate bias diodes so it has to be a low Idss device. The topology keeps the JFET’s VDS constant at ~0.6V (within the variation of the PNP’s VBE), which bootstraps both Cgs and Cgd, so the capsule sees a very low input capacitance (which is a big advantage for tiny capsules, DPA’s is probably around 5-6pF). The output is a PNP emitter follower, which means it has very low output impedance (tens of ohms) compared to a JFET source follower.

The LSK189B model in LTSpice has just the right VGSoff and Idss to bias correctly in the DPA topology (I’m not saying the JFET actually is a LSK189, but that is a good model to simulate with, if someone wants to try out the topology in LTSpice), while BC857B is a likely candidate for the PNP.