Low-SPL Control Resolution

[K O'Connor]

Hi Guys

In the Precision Power Scale thread I discussed using a "build-out" resistor to improve the control resolution at the quiet end of the Power Scale sweep. This method can be applied to all previous Power Scale installations. Ideally, we want the "audible transition" of sound-to-silence to occur exactly at the end of the PS sweep. The value required depends on the supply voltage in the amp, the kit type, and how quiet you wish the minimum loudness to be.

Classic-PS and the SB-series both have a build-out resistor already. The main purpose of the inclusion of this resistance was to keep the mosfets 'on' at the full-CCW end of the Power Scale pot sweep. However, if the sound disappears before full-CCW, then adding a bit more resistance in series with the pot-0 lead will be effective.  In these Power Scale forms, it may only take a few kiloOhms, maybe up to 10k, to achieve this AT alignment with the pot sweep.

There were quite a few DC-PSK variations, and then the SF-series. The DC-PSK may require a build-out resistor up to 33k.

The Super Value SV-series followed, with many variations as I tried to use component values within the kit to eliminate dead spots of the pot sweep. Build-out resistor values of 10k to 33k are typical. Dead sweep at the CW or CCW pot ends shift with the supply voltage, requiring a tweak of two resistor values in the kit to optimise it for a given B+.

The Precision Power Scale Zh also needs a build-out resistor of 10k to 33k. I was too quick ordering these Zh has no loud-end dead spot but will have a quiet-end dead spot without the build-out R.

The Zp Precision Power Scale circuit has an on-board AT pot allowing the installer to set the audible transition EXACTLY. Zp has no dead spots and I believe it represents the ultimate performance in Power Scale kits.

The Precision Power Scale kits are still labeled as SVn but with a Zh or Zp suffix, as SVn-Zh and SVn-Zp. The SV2-Z forms incorporate VCK, since VCK is always needed in Power Scaled cathode-biased amplifiers. VCK is still available separately as it has other applications.

[K O'Connor]

Hi Guys

As I mentioned in the Precision Power Scale thread, the actual value of the pot matters with all of the "amplified pot" circuit forms EXCEPT for the newest Z variation - trurly The Ultimate in Precision Power Scaling control.

i was disappointed when testing out a Zh build that there was an excess of voltage drop at the highest output PS control position. It turned out that the pot I was using was off value by 15%. they are usually off value by 10% - a surprising consistency of quality - haha.

The problem arises from mixing a wide-tolerance 20% component like a potentiometer with tight-tolerance 1% resistors in a voltage divider. The voltage division becomes unpredictable in practical terms even though mathematically or in simulations you can see what happens when the pot is at either extreme of its nominal value range. This is not good enough. But, as reported, there was a hardware solution that removes the pot value from the gain equation for the circuit and the desired ideal performance with no dead spots is achieved.

The SV1-Z and SV2-Z product descriptions on our site (London Power, as of April 17, 2026) are for this Ultimate form of Precision Power Scale kit. It took a while to get there and I believe it should meet all expectations. It is amazing to me how well players have accepted the previous offerings, although loudness reduction has always been achieved despite occasional disparities of end-of-sweep resolution.

[makinrose]

Thanks for all the info on your development process. It really great to hear how even after all these years your PS technology continues to be improved.