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The Millett Hybrid MOSFET-MAXed - Output Stage

JFET-Input MOSFET Diamond Buffer

NOTE - Differences from the BJT Diamond Buffer (DB) version and updates from the JFET-mod:

  • 2sj74 and 2sk170 JFETs for QB2L/R and QB3L/R, respectively
  • RB12L/R trimmers are 1K ohm
  • RB1L/R is jumpered out
  • QB1L/R JFETs are 2N5486
  • RB8L/R, RB9L/R are populated
    (these are jumpered out on the BJTs)
  • RB4L/R, RB5L/R, RB6L/R,
    RB7L/R, RB8L/R, & RB9L/R are ALL 100 ohm
  • Heat sinks should be 1-1/2" tall minimum
    (1" may be used at 90ma bias w/special mods)
  • 220pF caps on QB8L/R

  • NOTE: You must decide which output stage to use - either BJT Diamond Buffer or MOSFET Diamond Buffer. You cannot build both. Please refer to the original Millett MAX website for info on the BJT Diamond Buffer.

    The MOSFET-MAX website and this page are based on the JFET-mod for the MOSFET diamond buffers. Details of the JFET-mod may be found in the MAX history section.

    When using the dual JFET-input for the MOSFET diamond buffer, there is no adjustment capability between QB2 and QB3 within each channel. Because of the relative high currents and the fact that the JFETs are providing dual-signal input per channel, it is recommended that the input-JFETs be matched in current within 0.5ma. Also, due to the potential of a stored-charge and/or static damage, the MOSFETs should be tested. Beezar.com or Amb's Audio Shop are convenient sources and are referenced in the MOSFET-MAX Bill of Materials.

    Manufacturer specification sheets for the MOSFETs, the complementary small transistor pairs, and the input JFET are here:
    2SJ74BL IRFZ24N IRF9Z34N  
    2SK170BL 2N5087/2N5088 2N5486  


    (click for larger pic)

    In the key plan above, note the positions of the dual-input JFETs. These JFETs are the primary difference in the JFET-mod MOSFET diamond buffer. QB2 and QB3 serve as the primary signal input to the diamond buffer. In the JFET-mod MOSFET diamond buffer, the 2SJ74BL and 2SK170BL are used in place of a 2N5087 for QB2 and in place of a 2N5088 for QB3, respectively. The only other difference is that they are reversed from the silkscreen for QB2 and QB3. All output MOSFETs are on the left as one faces the front of the board. The current JFET, 2N5486 is analogous to the PN4392 used in the BJT MAX. All other transistors are the same as in the BJT MAX - 2N5087 and 2N5088. Further simplifying things, 100 ohm resistors are used throughout the buffer except at RB1 (jumpered out), RB2 and RB3 (1K). Of course, the output power resistors are still 2.2 ohm as well.

    Due to different charge capacities between N-channel and P-channel MOSFETs, a compensation capacitor should be applied to the outer two pins of the N-channel MOSFETs. These capacitors and their locations are pictured in the key plan above and in the photo and description below.

    From Ti Kan (Amb) on the MOSFETs:
    "For each channel, if you use 100Ω resistors for RB8 and RB9, then you should also install the 220pF capacitor CB8. The N-channel MOSFET's gate capacitance is less than that of the P-channel, and this capacitor equalizes them. An alternative to this approach would be to omit CB8, but change the values of RB8 and RB9 to 121Ω and 75Ω, respectively. This will assure symmetrical slew rates for positive going and negative going square wave pulses."
    (referenced from Amb's M3 amplifier, but part labels - in bold - have been altered to agree with the MOSFET-MAX's part numbers.)

    file last changed:Sunday, February 28, 2010 12:02:38 PM
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