OK, so I decided to publish my impedance converter circuit as well. As depicted here, it operates from 16V. It looks quite similar to Sergio's circuit, doesn't it? However, in this circuit, there's also AC feedback from the output to the JFET Gate, which makes this essentially a charge amplifier, like the KM84. The circuit, and some other circuit features not depicted here, resolve several of the KM84 shortcomings, so I named it KM84+++ (I also designed a KM84+ and KM84++ btw, which stay closer to the original KM84 design). Output can be a single-ended, transformerless impedance-balanced circuit, or through a 1:1 transformer. I designed a PCB for the Takstar CM-63 with the Lundahl LL1968 transformer. PCB is ready and waiting to be populated. If I'm satisfied with the results, I will make the PCB and complete schematics available through PCBWAY.
Vbe of T2 and R16 define the JFET bias current. R2 provides DC feedback, like in Sergio's circuit. I had ~3.5mA bias current available for this circuit, which must be shared between JFET and BJT. In the design depicted here, they have approximately the same current, but you can play with other current distributions, depending on the JFET, noise requirements, and output impedance requirements.
This circuit enables the use of more different JFET types, because it depends less on high gm values than e.g. a KM84 circuit does. Not having to adjust the bias may not be such a huge benefit for the average DIY, but I've read enough posts from people who do seem to experience troubles. Small trimmers are also not very reliable and take a lot of board space. "But what's that P1 doing on your PCB?", I hear you saying. Well, that's
optional trim pot to adjust the polarization voltage, so you can easily match the gain between two mics. It's not for JFET bias adjustment.
Another advantage of this circuit, or charge amplifiers in general, is that you can select the closed-loop circuit gain within a reasonable range by changing the value of C2. With Schoeps, it is more or less fixed and depends on the JFET chosen. Low-end roll-off is defined by R2 and C2 and to prevent noise, you want to increase R2 to higher values (e.g. 2, 3 or 5G) if C2 is decreased to obtain a higher gain. Due to the additional open-loop gain provided by the BJT, distortion is much lower compared to the KM84 and output drive capability has increased.
Enough for now. Once I have assembled the circuit, I will share the measured specifications (THD, Ein, etc.)
Jan
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