P331 shown with optional 1RU matching vent panels.
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 bring subtle life or organic size and 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.
(Ray LaMontagne, Bonnie Raitt, Aimee Mann, The Barr Brothers, Joe Henry)
Below is an email from Ryan after just 48 Hours with his new P331…
Here’s a note from the first client that gets to use the Whitestone. I told him I reprinted everything just because I needed to make a little change:
Damn, What did you change? These all sound like they are sitting perfect now! I can sign off on them, so good Ryan!
It’s a good start! Hopefully this project will be crossing Kim’s path soon.
Stampede Origin Studio
For any amplifier to be truly great you have to start with good clean power. The P331’s linear power supply is the most critical “component” for audio integrity next to the actual gain structure and circuit topologies within this amplifier. High voltage plate/anode tube power supplies start at around 320V DC and are filtered (up to 6 times) and smoothed with active regulators. It only uses film capacitors and real inductors (chokes) to remove ripple and noise from the high voltage power supplies.
(The War On Drugs, Kacy Musgraves, Arcade Fire, Bob Dylan, The Beatles)
“Seriously using it on more than half the work I’m doing. Scary good work tool. It has quickly become an essential tool, the most versatile box I’ve ever found to subtly colorize a track. Congrats to you and Dave!!!!”
The LOADING circuit is part of the fully differential (balanced) Class-A tube amplifier. It 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 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.
AUTO-PADDING (PRE, POST, OPEN) – One of the key elements of the P331’s LOADING circuit is its auto-padding feature. As gain is increased to the tubes, the signal is padded down in equal measure. It’s a cool feature and essential to the concept of the unit. To make it even cooler, we provide the ability to control where in the circuit the padding occurs (PRE or POST) or disable the auto-padding all together (OPEN). When in PRE, the auto-padding occurs to the signal before it hits the tube. In POST, the auto-padding occurs after the tubes. While in OPEN the auto-padding is disabled and your Loading knob allows you to crank it up while using your output Gain as a “master volume” of sorts similar to how a guitar amplifier with a master volume would function. It’s all about experimenting and letting your ears guide you.
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.
(Bonnie Raitt, Bettye LaVette, Belly, Aimee Mann, The Milk Carton Kids)
My favorite things about using the P331 is how fun and easy it is to dial up gorgeous enhancements to a wide variety of sources…and how it’s the type of gear that encourages you to trust your ears and your instincts.