The Millett Hybrid Maxed - Tweaks
Millett 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 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 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 4-7 seconds
of DC offset spikes at the jack. Consequently, the general consensus during design of
the MAX was that at least 10 seconds of delay or greater was needed to be safe. A 10 second delay
should result with most headphones and the parts selection from the MAX
BOM. With the values on the MAX BOM, absolute delay should be around 30 seconds. However, since we're talking about protecting valuable headphones, a big safety factor is desired. Plus, if you happen to turn off the amp, then turn it back on before the caps have had ample time to discharge, the delay will be cut in half. For this reason, we recommend that you do not go below the minimum total values for RM2 and CM3.
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. The current selection for CM3 is sized
at 330uF. However, the board 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 larger capacitors should not be
an issue. Note also that the actual e12 is configured with bi-polar caps. Those are not necessary for the MAX.
AMB lists several relay alternatives for the e12. Here are the data sheets for the ones that are still available:
(listed on the MAX
Transistor Data Sheets:|
|QM1: BD139||QM2: 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.
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.|