Our goal for the Mark 4 was to duplicate the significant audible and measurable performance improvements we achieved in our upgrade of its sister amplifier - the Stereo 70. And we did - like the Stereo 70, it preserves the musicality of the original while dramatically improving the spectral performance to a near perfect balance that is neither warm nor analytical (just balanced). Another benefit of the highly linear all triode design is a significant improvement in imaging. Finally you will be very pleased with its substantially improved bass performance that is both deeper and much tighter (a major weakness of the original) along with a noticeable increase in dynamic contrasts. Not surprising, the circuit topology is virtually identical to our upgrade module for the Stereo -70 (highly linear all triode cascode amplifier direct coupled to a split load phase inverter) but incorporates slightly different compensation to to ensure perfect stability with the Mark 4.
While we were designing the pc board, we included a few optional features that many of you requested as well including a DC Balance potentiometer (to balance the DC bias in the output tubes) and an AC Balance potentiometer to perfectly balance the phase inverter stage. Installation is perfectly straightforward since the board is exactly the same size as the original including the exact labeling of the connectors. You simply de-solder the connections from the original board, drop in the upgrade board, and reconnect the wires to the same pads. So, within an hour you will be enjoying your Mark-4 like never before. Feel free to download our comprehensive instructions to see for yourself just how easy it is. You may purchase the board blank or fully assembled. You may also purchase the board and parts as a kit.
Blank Board Pair (PWB-4U) = $80
Blank Board Pair w/Parts (KIT-4U) = $122
Assembled Board Pair (ASM-4U) = $150
Driver Tube Option, 4 Premium 6922 Dual Triodes (OPT-V-4) = $64
DC Bal Adjust Option per Pair (OPT-DCB-4U) = Add $24
AC Balance Option per Pair (OPT-ACB-4U) = Add $24
Polypropylene Capacitor Option per Pair (OPT-PPC-4U) = Add $27