Whitestone Audio Instruments: P331 Tube Loading Amplifier

The P331 Tube Loading Amplifier is the debut product from Whitestone Audio Instruments. Dave Rosen investigated making a device for line level audio signals to enhance wife Kim Rosen’s [Tape Op #108] mastering rig. It was not intended for commercial sale, but beta testers implored the Rosens to release a production version, and that’s how I got my grubby hands on one. In years past, owners would have never shared this unit with any competing studios; I’m just saying... But this interest is not simply limited to mastering, as numerous mixing engineers have been buying the P331 for their setups. Even tracking engineers have been refusing to return their demo units, making this enhancement device desirable across the entire recording process.

Formally, the P331 is a Class A fully differential tube amplifier. Two tubes (dual-triode 6SN7GTs) are readily available should replacements be needed; The company offers factory replacement pairs for $79. Transformers are custom-built for Whitestone Audio Instruments, and the all-analog circuit is digitally controlled and supported by an onboard ROM chip. This means recalls are simple and accurate. There are additional features, including ways to adjust internal gain, tube loading, and more.

The P331 can shape a track through the following sections: Loading controls how hard the input signal hits the vacuum tubes, allowing users to alter where the audio sits along the tube’s response curve. User-selectable feed-back (Clean) or feed-forward (Bloom) modes give wide latitude to this process. To avoid the louder-is-better illusion, an auto-padding circuit attenuates the signal as load gain increases. The pad’s location is selectable among Pre (before the signal hits the tube), Post (after the tubes), or Open (which bypasses the pad entirely). Next is a parallel filter, called Lift. It can enhance highs, lows, or both (think smiley face). When engaged, the quasi-EQ’d signal is passed through the tubes and blended back in with the unaffected input signal at a set percentage.

A transformer section follows. Whitestone spent considerable time specifying, evaluating, and testing until they found the “Goldilocks” transformer for this design. The P331 is designed to run fully transformerless, so the transformers are here to serve as another tone shaping tool; plus they can be hard-wire bypassed (as can any section other than Loading), if not suited for the material. A solid-state output stage is the last stop. The fully balanced output amplifier controls a range of +/- 11 dB, switched in 1 dB increments. If 1 dB proves to be too coarse, a Resolution feature allows the user to back off from a given setting by either 0.5 dB or 0.25 dB. Talk about sweet.

At the AES debut of the P331, I grumbled at Kim Rosen. As a mastering engineer, I’ve spent years sorting out a hardware chain, and acquiring gear that provides similar tone-shaping powers as the P331. Don’t other engineers have similar journeys? Why would anyone want this device? But her answer stopped me in my tracks: “That’s precisely the point! This unit saves you all of that trouble.” And she’s right, of course. I admit that gear testing is usually fun, but I don’t have time to play Beaker (Dr. Bunsen Honeydew’s lab assistant on the Muppet Show) as much as I would prefer. I have deadlines and obligations. I guess now I’m just mad that this unit didn’t exist when I started.

It took a while to determine the best placement in my signal chain. I tried beginning, middle, and end. There are merits to all three locations. For my particular setup, I appreciated the impact of the tone shaping in the caboose position. Being able to fine-tune the output levels made the P331 an ideal intermediary between my chain and the converter. However, my chain is not your chain. I advise users to determine optimal location in their setup.

In use, the P331 is equal parts satiating and invisible. The results are true to the source, yet more potent, like a dash of audio salt. Stepped controls, paired with the capability to hard-wire bypass the sections, permitted me to close my eyes and audition without having to continually bypass at the router. Concentration is particularly important when using the P331. At first use, it’s not evident that the unit is “doing anything.” However, the improvements are obvious when you remove the P331 from the path. Listen for a few bars, then hit Bypass – you’ll feel like someone pulled the covers off you at 6 a.m. in January. Engage the circuit. Bypass. Engage again. Enjoy the evil smile moving across your face.

For a mixed-to-tape Americana song, I was concerned the P331 would be too much. However, the Lift function restored some of the missing top end. On an opera vocal, the Loading section allowed me to increase the program level up without compression. I found that Loading is not just “hitting the tube.” Each rotary switch position affects the tube’s internal gain. Paired with the padding, Loading forces the audio to different points on the tube’s response curve. This essentially allows us to control the tube’s operating point. In a traditional amp design, the operating point is fixed, which can constrain the device’s tonal personality. As implemented in the P331, Loading is fluid, much more nuanced, deliberate, and repeatable. Finally, on a pop record, the Duo Lift seemed perfect. However, there was some bite that didn’t sit quite right. I remembered that I could bypass the transformers (never necessary before this project), and it was magic.

Here are some points to keep in mind: Firmware updates must be done at the factory. A field-programmable circuit would have added appreciable cost, and Whitestone does not expect many (if any) firmware changes. It also runs reasonably hot (not Fairchild hot, but worth noting). Reserve a space above (and probably one below) in the rack. Quality has a price, and the P331 is no exception. Tight part tolerances, rotary switches for precise left/right channel matching, a custom high-voltage power supply, and a 1/4-inch thick aluminum faceplate all add up to a price tag targeted at working studios.

A principal mastering duty involves bringing out the best parts of a mix while respecting the hours (weeks/ months/years?) it took to get the track to your doorstep. We all love fun color-shaping devices, but most are too heavy-handed or inappropriate in mastering. Being able to shape sonics without overstepping bounds is a tricky proposition. It seems as if some hardware are butter knives while the Whitestone is a scalpel; such control is required in mastering. From loading your gear at the beginning of your chain to subtle tonal massaging as a capstone, the P331 is a well-thought, well-built, beautiful-sounding device.