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07-28-2026, 12:03 PM
Hi Guys
Oops! Missing parts are never good. Sorry.
For reference, the latest boards are yellow with black silk-screening (the only kits to be these board/screen colours). Along the edge of the board at the rear between QN3 and M2 is a label "SV2-Zpb". SV1 is labelled similarly as "SV1-Zpb". (Here, "pb" has nothing to do with lead)
QP4 is correctly listed as BC560C on both the schematic and the board, and as listed in the assembly procedure. This has been the same for all of the SV2-Z variations.
For best performance QP4 should be high-gain, so a BC560C is specified, with gain range "C" and any of the BC55xC family will suffice. Gain range "B" might be okay, but "A" will be problematic at lower voltages.Voltage is not critical as the device only sees the Vbe of QP5.
LTspice suggests that the ZTX560 will work okay. You have to install it backwards to the silk-screen outline for the QP4 position.
QP4 and QP5 form a feedback-controlled current source, with QP4 providing the feedback and control and QP5 as the pass element. In general, the QP4 position needs a high-Hfe device and since I use the BC550C/560C pairs in audio circuits, I already have them and they are ideal for this application. Many other devices will work here but tend to need more current through them and/or they tend to steal away too much current from the pass element.
For example, in typical audio applications, you could use 2N3906 for QP4 because the current is generally higher to improve slew rate. This device will work much better than an MPSA92, say, which has lower gain and other parameters (early voltage rating) that make it a compromise for such a function. The same applies to the input side of a current mirror.
Speaking of current-mirrors, QN3 and QN4 form a current mirror, but the currents are relatively high so a general purpose NPN like 2N3904 is fine for QN4, which is the input side.
In the VCK portion of SV2-Zpb, the voltage capability matches the Power Scale portion at 700V of input. The VCK portion is clamped so the input-output never exceeds about 250V.
Oops! Missing parts are never good. Sorry.
For reference, the latest boards are yellow with black silk-screening (the only kits to be these board/screen colours). Along the edge of the board at the rear between QN3 and M2 is a label "SV2-Zpb". SV1 is labelled similarly as "SV1-Zpb". (Here, "pb" has nothing to do with lead)
QP4 is correctly listed as BC560C on both the schematic and the board, and as listed in the assembly procedure. This has been the same for all of the SV2-Z variations.
For best performance QP4 should be high-gain, so a BC560C is specified, with gain range "C" and any of the BC55xC family will suffice. Gain range "B" might be okay, but "A" will be problematic at lower voltages.Voltage is not critical as the device only sees the Vbe of QP5.
LTspice suggests that the ZTX560 will work okay. You have to install it backwards to the silk-screen outline for the QP4 position.
QP4 and QP5 form a feedback-controlled current source, with QP4 providing the feedback and control and QP5 as the pass element. In general, the QP4 position needs a high-Hfe device and since I use the BC550C/560C pairs in audio circuits, I already have them and they are ideal for this application. Many other devices will work here but tend to need more current through them and/or they tend to steal away too much current from the pass element.
For example, in typical audio applications, you could use 2N3906 for QP4 because the current is generally higher to improve slew rate. This device will work much better than an MPSA92, say, which has lower gain and other parameters (early voltage rating) that make it a compromise for such a function. The same applies to the input side of a current mirror.
Speaking of current-mirrors, QN3 and QN4 form a current mirror, but the currents are relatively high so a general purpose NPN like 2N3904 is fine for QN4, which is the input side.
In the VCK portion of SV2-Zpb, the voltage capability matches the Power Scale portion at 700V of input. The VCK portion is clamped so the input-output never exceeds about 250V.
