The Mk4 version is not intended to improve performance compared to the Mk3. The idea was to change the layout and earthing arrangement to make construction easier, and to do this I decided to include two channels on one standard size board (6" x 4") with a single star earth included on the board. While trying to work out a new layout some small changes and simplifications were made.
The bias chain no longer has zeners and has less smoothing, but the LEDs have low impedances, and supply rejection should still be good. Only a single feedback 200k is used, but now taken from a potential divider across the output capacitor. A single 4700uF is used instead of a parallel pair of 2200uF, just to save space on the board. The 0.1uF plus 6R8 Zobel has been omitted, this really does nothing useful, the 1R plus 0.1uF now added at the output keep the total load resistive at high frequencies with an inductive external load. The 470R across the 0.1uF is there for the output capacitor to charge through when no load is connected, and it is connected here rather than directly across the output just because it simplifies the board layout. I thought of leaving out the mosfet gate protection zeners, but still have no certainty how far the internal zeners can be relied upon. Crosstalk and supply rejection need testing before finalising the design. The anti-thump circuit had to go, but the new feedback arrangement could possibly reduce that problem.
GAIN AND PHASE SHIFT.
The gain and phase responses produced by a simulation are about the same as for the Mk3 version, again there is an almost perfect linear phase response from 1kHz to 20kHz, having the same effect in that frequency range as a constant time delay of 3.3usec, so having practically no effect on wave shapes. The -1dB frequency range is about 12Hz to 30kHz.
To avoid the mosfet mounting bracket used in earlier versions the mosfets are connected near the edge of the board so that they can be fixed directly to a heatsink. They could be soldered onto the board, but then there is some difficulty in ensuring they all line up correctly with the heatsink surface, and so the layout has been changed to allow the option of using terminal blocks as shown in the next photos. The blocks should be soldered in place before the 300R gate resistors so that these can be routed correctly to avoid obstructing the blocks. The inputs can also be taken to a terminal block if required.
The coil, shown next, is air-cored, and is made using 18-gauge enamelled copper wire (1.2mm dia.) which can be made by winding it on a 1cm dia former, for example an AAA battery as shown in the photo. There are 13 turns, giving an overall length about 17mm. The other photo shows how the gate protection zeners are fitted. To save space only two holes are provided and the zeners have two ends soldered together off the board. The connected ends are the same polarity, in the photo the cathodes (indicated by a dark band) are connected, but it works just the same if anodes are connected together instead.
SUPPLY REJECTION AND CROSSTALK.
Part 1. Supply Frequency Rejection.
Part 2. Modifying the Supply.
Part 3. Crosstalk.