The sub-out feature of the GK1 was a consequence of an unusual attenuation topology. Let me explain according to my understanding of this important component.
Level controls, in potentiometer (three terminal) form, ideally require two parameters for best sonics. One is a low source impedance to drive the pot, otherwise attenuation is highly variable with the source chosen; and the second is an extremely high target impedance for the circuit block being driven from the wiper.
The problem is the wiper. At the point where it slides on the carbon/plastic track, there is spring pressure, and this means that the wiper contacts the track, distorting it physically at a number of microscopic points, setting up a small number of alternate paths into the circular resistive substrate. These multiple paths will be pretty much at the same voltage if no current is flowing, but if a current is flowing into the wiper, then they will all differ in their voltage drops and the output will tend to jarble the sonics. The only way to avoid this problem is to draw no current through the wiper, and this mandates a very high target impedance.
Thinking on this, I felt that since we could not control the source impedance driving the preamp (digital and tube sources are different, as you'd expect), and since the input impedance into the GK1 was around 45K, fixed by the input stage topology, it might be better to place the volume control between the SS and tube sections. The Zout of the SS sections is around 30R, nice and low, and the Zin of the tube stage, with its bootstrapped, self biased cathode follower, is 4.3M (calculated), extremely high. This would mean that the SS section operates at full line level, but the tube operates only at the listening level set by the volume control.
This 'enclosed' environment thus defines the ideal point for a level control. Source and target impedances are optimally controlled, and almost no current flows from the wiper into the tube grid.
However, if we connect a sub amp to the wiper, then it must deliver a small current into the amp input via the wiper contact, blurring the waveforms as described above. This is the problem, and it flattens the sonics for the tube.
The only way around this is to insert a buffer, an opamp of good quality is fine, between the wiper output (sub-out) and the sub woofer amp. Andy has done this to good effect. This prevents the sonic effect on the tube output, as Gerado has noted, by restoring the wiper current to negligible levels.
It's worth noting, too, that the gain of the tube is around 0.92, that is, 1Vrms in will yield 0.92Vrms out, and if we take the sub-out at the 1Vrms point you can see that the bass frequencies will enjoy a small lift. It's very low, however, around 0.72dB, and I doubt most would hear it, (1.5dB is acknowledged to be around the finest resolution of the human ear), but it's nice to have it......
Hope this is helpful,
Cheers,
Hugh
