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| NOTE: The Millett MOSFET-MAX (based on the MAX V1.2 PCB) takes a departure in previous MAX/MiniMAX designs. The headphone delay-relay is entirely re-designed by Toole Audio. It is a power-on/power-off delay muting and DC offset protection circuit, but with a much smaller parts count than any before. Moreover, it presents a much faster response time in the actual relay switching. Many conventional relay-delay circuits still present a noticeable thump when switching the signal circuit into the amp. The Millett MOSFET-MAX relay is much faster and nothing but a small click will be heard in most headphones. Moreover, the entire relay circuit can be easily applied to existing older Millett MAXes (V1 PCB) and MiniMAXes with only the updated parts selection detailed here and on the MOSFET-MAX BOM (more on that later).
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The new MOSFET-MAX relay-delay 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 Hybrid, the MOSFET-MAX incorporates coupling caps (CA7-L/R) on the output to block DC. However, these caps take a number of seconds to fully charge, a prerequisite to their blocking ability. If your headphones are plugged in before the caps have charged (allowing them to block the DC), your headphones will be exposed to many volts of DC. In the case of the MAX/MiniMAX/MOSFET-MAX with V+ set at 27VDC, as much as 13.5VDC is on the headphone jack before the caps have charged. The relay circuit delays switching the headphone jack into the amplifier output until the caps have charge.
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| The product of the values of RM1 and CM3 determine the delay time. RM1 is sized at 2Mohm, this makes the resistor a bit more difficult to find in stock at some places or at a regular low priced for common V-D resistors, but you only need one of them and it's a small price to pay for the advantages of the new circuit. The reason this was done and perhaps the only negative consequence of the 24VDC circuit is that the CM3 capacitor now has to be rated at 35V. It has to see the full voltage of the 24VDC circuit plus some safety factor. This means that the CM3 capacitor is larger at all uf ratings. So, we arrived at the 2M resistor in order to keep the size down of CM3. This kept the footprint on the MAX V1.2 PCB at the same size and it also allows the easy retrofit to any MiniMAX with this parts selection.
As before, we are recommending that you size CM3 to 470uf. As mentioned above, the 24VDC nature of the circuit effectively cuts the delay in half. However, we chose to double the value of the RM1 resistor. This will still result in a cold start delay time of ~45 seconds, just as its been with the MiniMAX. Even a fast turn-off/turn-on should still result in a delay of ~20 seconds. This is what we recommend as a minimum. The board's pads can accept a 10mm dia. capacitor. 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. 3M resistors are also available, if you find that necessary. Note also that sone previous types of relay-delay circuits are configured with bi-polar caps. Those are not necessary for the MOSFET-MAX relay-delay circuit. Note that while the quality of the electrolytics in the delay circuit are not critical, they should be rated at the 105deg. C. rating. This is due to their proximity to the front plate and to the middle two heat sinks in the output buffer. Quite a bit of heat can develop in this section and the last thing you may want to fail is the delay capacitor. 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. If you retrofit a MiniMAX to this circuit, we highly recommend that you go back with the DIP socket (two DIP-8 sockets will work quite nicely). |
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Here is an example of using the Toole Audio 24VDC Relay-Delay in an existing MiniMAX:
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| Note that the CM1 and CM3 caps used in the MiniMAX are retained. This is not optimum, but you can easily see how the circuit works and the improvement in "thump" is noticeable. Likewise, the DM2 diode remains the 1N4148. This is OK, too - unless you want the very longest life of the relay.
The important changes are as follows:
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