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The Millett Hybrid MOSFET-MAXed - Tweaks

Millett MOSFET-MAX e12 Delay

NOTE: The e12 is a power-on/power-off delay muting and DC offset protection circuit designed by Ti Kan (AMB). The complete e12 is an excellent DIY project and incorporates many safety features not included in the MAX. Please take the opportunity to learn more about the e12 at AMB's Audio DIY site: AMB Audio Laboratories e12

A partial circuit from the e12 is incorporated in the MOSFET-MAX. The schematic is shown above. This circuit forms a protective delay sequence that prevents signal power from reaching the headphone jack until the Millett tube circuit has had a chance to warm up and charge the capacitors. Tubes freely pass DC. As with the original Millett, the MOSFET-MAX incorporates coupling caps on the output to block DC. These caps take a second or two to fully charge, a prerequisite to their blocking ability. However, the caps can only block steady-state DC, whereas the tubes may continue to spike and discharge potentially damaging voltages for several seconds.

The e12 relay holds the headphone jack out of the circuit until things settle down. The actual time needed before the tubes settle down can be highly variable, but most of us that have measured it normally see about 5-10 seconds of DC offset spikes at the jack. Depending on some circumstances, this time may go longer. More important, if the amp is turned off and turned back on before the CM3 cap has had time to completely discharge, the delay will be much shorter - perhaps half the time or less.. Consequently, a large delay time safety factor is desired in order to protect headphones. Delay times of 45 seconds or more should not be objectionable. After all, tubes probably need a good half-hour (or more) of warm-up for the best sound.

The product of the values of RM2 and CM3 determine the delay time. RM2 is sized at 1Mohm, the largest commonly available value for many leaded resistors. So, increasing the delay time should probably be done by increasing the size of CM3. Beginning with the MOSFET-MAX, we are recommending that you size CM3 to 470uf. This should result in a cold start delay time of ~45 seconds. More important, even a fast turn-off/turn-on should still result in a delay of ~20 seconds.. The board's pads can accept a 10mm dia. capacitor if necessary. So, whatever you can fit in there will work - a high-quality cap is not strictly necessary. Moreover, many lower grade caps are smaller in size than the typical UPW or FM caps most of us use, so finding significantly higher-rated capacitors should not be an issue. 2M and 3M resistors are also available, if you find that necessary. Note also that the actual e12 is configured with bi-polar caps. Those are not necessary for the MOSFET-MAX.

Note that the quality of the electrolytics in the delay circuit are not critical. Any commercial quality electrolytic should suffice. They should be rated at the 105deg. C. rating, however, due to the proximity of the delay circuit to the middle two heat sinks in the output buffer.

In addition, a DIP-16 socket is a nice touch. The relay will plug directly into one and allow easy replacement. You will have to snip some of the legs on the socket where there are no pads on the board, but as with opamps, it will prevent exposing the sensitive coils inside the relay to too much heat when soldering.

AMB lists several relay alternatives for the e12. Here are the data sheets for the ones that are still available:
(listed on the MOSFET-MAX BOM)
Fujitsu-Takamisawa RA-12W-K Fujitsu-Takamisawa RY-12W-K
Transistor Data Sheets:
QM1: BD139QM2: MPSA14
Alternates for the BD139 include any medium-power, BJT NPN transistor. All the equivalents for the NPN output buffer transistors apply (BD135, BD137, MJE243, 2SC3421/22, etc.).
The MPSA14 is a Darlington transistor with an extremely high collector current - 1A or over. Accepted alternates include: KSP13, KSP14, KSP25, KSP26, KSP27, MPSA13, MPSA27, MPSA28, MPSA29, 2N6426, 2N6427, 2N7052, 2N7053, BC372 or BC373. One of the 2N5087/88's WILL NOT WORK.

Some have had some difficulty with orienting QM1, the BD139. The case style can make it difficult to determine the correct orientation. The label should face forward on the MAX board, with the case molding line to the rear.

file last changed:Sunday, September 7, 2008 7:00:00 AM
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