Усилители мощности моно — Limestone PP1

The PP1 is a mono valve power amplifier from Limestone, a Croatian maker operating under the company WiM-Systems d.o.o. Every PP1 is hand-assembled, and a stereo system needs two of them. Limestone describes its aim as combining vintage valve circuit practice with modern reliability, and the price starts at EUR 10,000 before VAT, with finishes and materials open to customer choice.

Key specs that matter

The output stage is a push-pull pair of KT88 valves running in class AB, rated at 45 W. Frequency response is 30 Hz to 30 kHz at the minus 3 dB points, total harmonic distortion stays below 1% at 1 kHz, and signal to noise reaches about 85 dB, equal to roughly 1 mV of residual noise. An input of 0.7 V RMS drives the amplifier to full output. The output transformer has separate 4 ohm and 8 ohm taps, brought out to gold-plated WBT binding posts. The chassis measures 25 cm wide, 25 cm high and 55 cm deep, and the weight is 18 kg.

Engineering and use context

The phase splitter and driver stage use Shuguang Black Treasure CV181-Z valves, and on request Limestone substitutes 6SN7 TAD or NOS Sylvania Bad Boy types. Since January 2025 the amplifiers leave the factory with Ray Tubes Select Series valves, and the Ray Tubes Reserve Series is offered as an upgrade. A precise bias control system keeps the KT88 pair at its working point, and bias is set with the help of a high-precision Simpson analogue meter. A microprocessor manages power, holding back the HT supply for a delay after power-up and draining the capacitors once the unit is turned off, and protection circuitry reacts if the bias voltage fails. The front panel carries a VU meter, and level is set by a motorised ALPS potentiometer. The body is made from 1.5 mm stainless steel with an open layout that leaves the valves exposed on top, and decorative Kerrock overlays are applied to the casework.

Pairing

With its moderate power, the PP1 suits loudspeakers of reasonable efficiency, and the choice between the two taps allows the secondary winding to be chosen according to the rated impedance of the speakers. The input sensitivity sits within the output range of common valve and solid state preamplifiers, and the built-in motorised volume control adds flexibility in how the gain structure is arranged. The long chassis needs a deep shelf or floor stand for each unit, and the open valve layout calls for free air above the chassis.

Alternatives

Limestone lists the PP1 as its only amplifier model, so the brand offers no other mono power amplifier at present.