Whitestone Audio Instruments is the brainchild of Dave Rosen and his multi-Grammy® nominated mastering engineer wife Kim Rosen. Much of the way Kim works is exploiting the sound of the input and output gain stages of some of her favorite equalizers and compressors with actual processing bypassed. Just the subtle enhancement some of these analog gain circuits provide often times is just the thing to add a bit of “magic”. The goal for the P331 was to capture this kind of enhancement potential in a line amplifier that delivers the natural clarity of a well-designed tube circuit. At the same time, implementing fully switched controls that introduce harmonics and other carefully implemented, and repeatable, non-linearities to subtly enhance the signal without harming the recording’s original intent. 


The Whitestone Audio 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 with processing bypassed is often-times just the thing to help bring subtle life or enhanced depth to recorded audio. That’s what the fully balanced P331 is all about; giving you control of how an audio signal passes from input to output and how it can be enhanced throughout its journey.

In 1939, just a few weeks after the start of WWII in Europe, RCA Radiotron introduced the 6SN7 octal (8-pin) double triode tube to the American market. This was the birth of the true common ancestor of all modern double triodes used in high fidelity amplifiers. The 6SN7 is still in production after over 80 years, for good reason.

6SN7 vacuum tubes of the 1940s vintage were chosen for this amplifier due to their lower distortion capabilities that exceed that of the standard and ubiquitous 12A*7 variety. The 12A*7 varieties were incarnated due to commercial needs such that they were “better, cheaper, lighter” at the expense of embodying more distortion. With the advent of feedback, tube amplifiers could be mass produced with these less expensive 12A*7 style tubes using more feedback to reduce distortion.

While feedback reduces distortion, it also lowers overall amplifier gain. It might have been considered heresy to reduce the gain through feedback (and reduce distortion) in earlier tube designs since minimal gain was still extremely expensive prior to the 1950s. Thusly, earlier tube designs were lower distortion on purpose; especially for their critical role in modulation based communication systems that had more stringent requirements than the average 1950s guitar amplifier tube.

Notwithstanding, ultimate circuit topology can bring out more second harmonic characteristics than usual with any tube design. Hence the impetus for the variable circuit topologies within the P331 Tube Loading Amplifier to allow the user to do so as necessary or not at all.