The stepped Attenuator is an alternative to the usual potentiometer controlling the volume. The difference is that the potentiometer uses a track of a resistive material like carbon or plastic, where a stepped attenuator uses a mechanical switch combined with traditional resistors. A metal film transistor should be less noisy than the resistive material in a potentiometer and hence the improvement sound quality. The other benefit is that the balance between the channels should be absolutely perfect. That’s the theory at least.
There are different types of stepped attenuators and they can be fitted with different types of resistors only limited by your financial situation. I have chosen a ladder type 100K logarithmic stepped attenuator fitted with traditional metal film resistors from Welwyn. I found in on ebay and it’s made by 8Audio, who I recommend for their good quality and price. This diagram shows the difference between a stepped and a serial attenuator volume control.
Here is a picture of the stepped attenuator installed in the Crack. It’s was pure luck that it could be fitted and only because there was a recessed middle on the attenuator making room for the Speedball PCB. I had to unmount the PCB to install the attenuator and I also had to make the shaft hole bigger (10 mm). It worked out well in the end though.
It’s not as smooth as a good potentiometer like the Alps Blue Velvet, it’s feels just like a switch, because it is a switch. The upgrade definitely made the sound even more clear and detailed than it was before and the balance is nothing short of perfect. I have to trouble with the steps or finding the right volume level. The only bad thing is the feel of a switch compared to a smooth potentiometer and I’m pretty sure I’m already used to that after a week or so. I recommend this upgrade and think it’s worth the price and effort, because of the increased clarity.
I also have to show you this. I got hold of one of the legendary G.E.C CV 2523 – one of the best tube for Bottlehead Crack, only rivaled by Western Electric 412a. Oh boy, I can’t recommend this enough. The clarity, the incredible sub bass, the sound stage and the smooth buttery, but extremely detailed high notes. The only downside to this tube, is the price. Unfortunately it’s not free. I got mine a little cheaper than usual, because it was a little skewed in both the glass and internals, but otherwise NOS and in perfect condition. The next project is to find a good Mullard, Siemens or Telefunken 12AU7. I’ll report back when that happens.