Power Scale pot sweep

[K O'Connor]

Hi Guys

There is a compromise in the Power Scale pot sweep, depending on the specific Power Scale circuit, the supply voltage, and the amplifier.

With the older SB-style approach using the mil-spec pot, the original kit offering incorporated a linear pot. It is easy to see on a scope or meter the direct relationship between the pot setting and the output voltage. Due to the logarithmic nature of our hearing, a linear pot is not a good choice - unless you use the circuit for a bench supply - so, we changed to a log pot. The loudness change follows a log-squared curve but there ended up being better low-loudness resolution of control.

With the newer Power Scale kits, there is a compromise that causes a dead spot in the sweep at one end or the other, or at both ends. The dead spot at the top end means no voltage change occurs over that part of the sweep. This is not too critical for most players once they get used to the fact that you will not hear a loudness change until you get to the power level you are actually using - which is lower than they think. But... to the player this means the useful range of the pot is smaller.

A dead spot at the low end means the amp has gone completely quiet or off before the end of the pot sweep. How big this dead spot is is a bit more concerning from an ergonomic view. In the present SVn-D kits, R1,2,3 form a voltage divider with the Power Scale pot in parallel with R2. The divider values are chosen to give a 100x power reduction with the pot set to 12-o'clock, and to go to zero volts at full sweep. Most amps will be silent at some positive voltage out, so getting to a true zero is not necessary, more aesthetic. With R2=47k5 and R3=1k the output can go to zero, but changing to R2=43k2 or 44k2 and R3=2k21 gives  better sweep overall.

Have fun

[K O'Connor]

Hi Guys

The original form of the SV1-D is what I called SD-1, never released as such but I used it in my amps.

In that form, R2=56k2 and R3=6k81. This definitely does not go to zero volts out, but the amp sound would cut off right at the end of the pot sweep (values tuned by ear). When I decided to change the SV kits to this format for general release, the PCBs were labelled with these values, but later shipped with the newer values listed in post-1. Recent boards are labelled for the new values.

Note that a compromise with the top end dead spot is that if you try to eliminate it you may end up dropping voltage across the mosfets at the nominal full power setting of the Power Scale pot. Losing 10V or so in a 4-500V environment is no big deal BUT the mosfets will be running a lot warmer than they should at full power. Dissipation at full output should be essentially zero.