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 used 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, simplified circuit from the e12 is incorporated in the MiniMAX. 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, because their bias point sets the level of DC offset at the output.. As with the original
Millett, the MiniMAX incorporates coupling caps on the output to block DC. In fact, with the bypass caps on the primary electrolytic caps, the outputs on the MiniMAX are double protected. There is very little chance that cap failure will result in DC sent to the headphones. However, the output caps take several seconds to fully charge, a prerequisite to their blocking ability. The relay-delay circuit holds the headphone connections out of the circuit, until the capacitors have had sufficient time to charge and block the DC offset.
The actual time needed
before the capacitors charge can be highly variable, but most of us that have measured it usually see about 5-10 seconds
of DC offset spikes at the jack. Consequently, a delay time of 20 seconds was deemed to be safe (2X safety factor). However, depending on circumstances, this delay time may be cut in half. 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. Consequently, a large safety factor on the delay time is desired (4X, at least). Therefore, we are recommending delay times of 45 seconds or more. This amount of delay should not be objectionable. After all, tubes probably need a good half-hour (or more) of warm-up for the best sound. In a worst case scenario of quickly bumping the amp on and off, a standard delay circuit setting of 45 seconds will still result in 10-20 seconds of delay, more than enough to protect the headphones.
The RC 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. As with previous MAXes, we are recommending that you size CM3 to 470uf. This should result in a cold start delay time of ~45 seconds and a fast turn-off/turn-on 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. 2M and 3M resistors are also available, if you find that necessary. Note also that bipolar caps are are not necessary in the MiniMAX's relay-delay circuit .
As mentioned above, 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. I use Xicons (very inexpensive) with great success and those are specified in the BOM.
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 using the socket will allow for easy replacement and will prevent exposing the sensitive coils inside to too much heat when soldering.
There are several alternatives for the relay in the delay circuit. Here are the data sheets:
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:Monday, December 29, 2008 2:33:18 PM
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