Whitestone Audio Instruments P331 Tube Loading Amplifier


Tube-driven stereo audio enhancer

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Meet the P331 Tube Loading Amplifier

The Whitestone Audio Instruments P331 Tube Loading Amplifier was developed to bring professional tracking, mix and mastering engineers versatile, elegant, precise and repeatable audio circuit variation. There’s nothing like running a signal through a well-designed piece of analog gear, at “unity” gain without any “processing” applied at all. Simply exploiting the sound of some of these analog input and output gain circuits is often-times just the thing to bring subtle life or organic size and depth to recorded audio. The P331 is designed to enhance any audio signal…your individual tracks, on your mix-buss or in your mastering chain.


  • LOADING: The LOADING circuit is part of the fully differential (balanced) Class-A tube amplifier which utilizes 6sn7 dual triode vacuum tubes originally introduced in 1939. These tubes are still in production and can be purchased new without hassle. The LOADING section was designed to coerce an audio signal to hit the tube at different points on its response curve. As the gain increases to the tube, the signal is automatically padded down in equal measure. This allows precise and repeatable loads to be applied to the tubes. It also helps to ensure minimal level gain or loss, allowing you to audition the enhancement the tubes are imparting without drastic changes in level.
  • LIFT: The LIFT circuit is a gentle parallel filter that enhances the highs, lows or both the highs and lows of the signal. Audio passes through the LIFT circuit before the LOADING circuit. With any of the LIFT filters engaged, the audio is passed unscathed through the fully balanced P331 circuitry and blended with harmonic content of the LIFT and LOADING circuits working in concert.
  • XFORMER: The P331 is designed to run fully transformerless in its default state. This provides the purest, cleanest and most unaffected audio path through the unit. However, in keeping with the P331’s versatility, the operator may switch in output transformers and select two optional settings to load them in different ways. This provides additional options for enhancing the subtle textures of recorded audio.
  • OUTPUT GAIN: To faithfully reproduce the sound of the audio passing through the tubes and other circuits in the P331 implements a low distortion, fully balanced solid state amplifier for the output gain section. The output gain circuit offers 11dB of boost or cut in 1dB increments. However, for even finer control, the “Resolution” switch allows the operator to “zoom in” 0.25dB or 0.5dB.

Contact your Westlake Pro Sales Consultant TODAY to hear what the Whitestone P331 Tube Loading Amplifier can do for your studio!

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