The imaging of stereo mic arrays and loudspeaker monitoring is not as accurate as it should be — and the SHUpHLER MkIV aims to address that.

The SHUpHLER ['shuffler'] is manufactured by small British company Phaedrus Audio, and I reviewed the original model back in SOS July 2017 (https://sosm.ag/phaedrus-audio-shuphler). The product has continued to evolve in important ways over the last decade, and I thought the latest Mark IV model had sufficiently improved, that this device warranted a second look. I’ll concentrate mainly on the changes and updates here, so if you’d like more information on the background to the shuffling processes please refer to my original review (which is free to view).

Same... But Different!

As you’d expect, the MkIV performs essentially the same job as the original — it provides a variety of shuffling configurations optimised for different stereo recording and listening scenarios. Importantly, though, two of its shuffling functions are completely new, and the audio connectivity has been enhanced too.

Like the original, the SHUpHLER MkIV is housed in a deep extruded aluminium case. It measures 108 x 46 x 225mm (WHD), but instead of the hexagonal anodised metal front panel of the earlier design, the new model employs thick fibreglass PCB end panels. The original anodised metal panels required additional processing to ensure they were grounding effectively against the case. Phaedrus subsequently discovered that this new double copper-clad PCB solution is a lot better in that respect. As the panels have relatively small dimensions and are quite thick, they are easily robust enough to support the control knob and I/O connectors, and I had no issues with interference despite heavy provocation!

Originally, the pro version of the SHUpHLER used a valve gain stage, but the latest model is entirely solid‑state, using a handful of Texas OPA2320 precision low‑noise, low‑power, CMOS op‑amps. It’s designed to operate on a single‑sided 5V power supply, a benefit of which is that it allows the SHUpHLER mkIV to be powered directly from any standard USB supply, via a USB‑C socket on the rear panel (a USB‑C to USB‑A cable is included). As the unit only draws 100mA of current, it will work with all USB 1.1, 2.0 or 3.x outlets, but ideally a good quality dedicated USB power supply should be used, to realise the unit’s impressively low noise‑floor specifications of ‑100dBu (A‑weighted).

The MkIV SHUpHLER is available in two versions: a professional transformer‑coupled balanced I/O model, and an unbalanced consumer version. Both have four quarter‑inch jack sockets for line‑level connections in and out. Being directly USB powered, the 5V power rail restricts the maximum internal signal level to just +7dBu or +5dBV, and so that’s also the maximum I/O level for the unbalanced version. The pro model’s balancing transformers not only provide galvanic isolation to avoid ground loops; they also step down/up the I/O levels with a maximum of +18dBu (giving a potential dynamic range of 118dB).

The unbalanced transformerless model’s THD measurement is better than 0.001% at all frequencies, but the transformers in the pro model degrade that somewhat; I measured THD at 0.05% with a 0dBu input level. Naturally, the THD figure is also frequency dependent and, at 40Hz, it reaches 1% rising to 3% with a +16dBu input level. I think that’s to be expected in a transformer‑coupled device, and a fair compromise given the USB powering and flexibility of connection formats and I/O levels. And who doesn’t enjoy a little transformer warmth?

I measured the frequency response in the no‑processing mode to be within 1dB between 40Hz and 20kHz, and the roll‑off below 40Hz is primarily there because the Blumlein shuffling mode has the potential to expose a lot of LF rumble.

Shuffling Modes

As with the original model, five different shuffling modes are provided, selected with the only operational control: a six‑way rotary switch on the front panel. All shuffling processes equalise the Mid and Sides components of the stereo signal in specific ways, adjusting the stereo width at different frequencies to compensate for the inherent errors of stereo mic arrays and loudspeaker listening.

The first switch position disengages all processing circuits, but the signal path is still routed through the transformers and op‑amps. The next clockwise position is labelled Elliptical, and this replaces the sum‑and‑difference matrix of the original SHUpHLER. This Elliptical mode progressively reduces the stereo width as the frequency reduces, such that the lowest frequencies are reproduced in dual‑mono. Elliptical filters are commonly employed in vinyl mastering to avoid anti‑phase LF signals from reducing the groove depth and causing tracking problems. But it’s included here because it’s also very useful when listening over headphones, creating a far more natural listening experience if the source features hard‑panned sounds.

I’ll group the second and fourth shuffler modes, Cardioid and Blumlein, together because both are intended primarily for correcting issues with different kinds of stereo mic array. They both widen the stereo image at low frequencies (the opposite of the Elliptical mode) and the Blumlein mode is the stronger of the two. That one is designed for near‑spaced omni microphones, such as in binaural or Jecklin Disc setups. In essence, this shuffling mode converts small low‑frequency phase‑shifts between the two channels into amplitude differences, thereby enhancing the stereo imaging when heard over stereo loudspeakers. The Cardioid mode is designed to restore a sense of spaciousness in coincident mic arrays, while also increasing the forward focus (and reducing some of the rearward pickup).

Stereosonic and WaRP, the third and fifth modes, are related shuffling processes designed to enhance the imaging accuracy of stereo loudspeakers. The Stereosonic mode was developed by EMI’s boffins back in the 1950s to do two things. First, it reduces the slight blurring of the stereo image when listening over stereo loudspeakers — something that’s caused by the stage width at high frequencies being inherently wider than at lower frequencies. Its secondary function was to compensate for the HF beaming effect of the large‑diaphragm microphones that were popular at the time, which also exaggerated the HF stage width (particularly in EMI’s classical recordings).

The new WaRP mode replaces the previous BOF (Bride of Francinstein) process, which itself was a ‘corrected’ version of Stereosonic shuffling with less HF narrowing, and better suited to more modern recordings and pan‑pot stereo material. WaRP is a further development of BOF. It incorporates academic research from the mid‑1980s, which revealed that while conventional stereo speakers reconstruct accurate audio wavefronts below about 1kHz for virtual sound sources between the speakers, at high frequencies the required wavefronts effectively become sub‑sampled — the result being unwanted spatial ‘alias’ sound sources that confuse and blur the stereo image in ways not previously appreciated.

Richard Brice, who designed the SHUpHLER, developed a method to correct these unwanted spatial aliases called Francinstein T‑Sym, using linear‑phase filters. This solution is available in PSPatial Audio’s Mac‑only Stereo Lab software, and the WaRP process is essentially an analogue simplification of that digital algorithm. Comparing the Stereosonic and WaRP modes, the former reduces the stereo width above 2kHz by about 4dB, with a gentle transition over nearly four octaves from 200Hz up. In contrast, the WaRP process applies slightly less HF narrowing and the transition occurs more sharply over an octave around 1kHz. This might strike you as an insignificant difference, but in practice it actually makes a big improvement, greatly increasing the sense of realism and solidity of the stereo image with most stereo material.

Impressions

Phaedrus Audio’s SHUpHLER MkIV addresses stereo imaging issues that no other analogue hardware processor tackles in such a versatile, effective or user‑friendly way. Although the five shuffling modes were designed with specific applications in mind, the user manual includes a useful table listing how each mode might enhance (or not) a wide range of different situations, too. I used the SHUpHLER MkIV with a wide variety of stereo mic arrays, and was always able to find a mode that elevated the sense of spaciousness and/or enhanced the stereo imaging precision, without exception.

The new Elliptical mode is a real bonus for headphone listening, making a lot of wide stereo material far more natural and pleasant.

While I didn’t have the opportunity to compare the new WaRP process with the old BOF, it is noticeably more accurate than the Stereosonic mode at improving the realism and focus of most modern commercial recordings auditioned over stereo loudspeakers. I found the Stereosonic mode was marginally better for historical recordings (especially EMI recordings from the ’50s and ’60s, funnily enough!). I was concerned initially that I’d miss the sum‑and‑difference mode, but actually the new Elliptical mode is a real bonus for headphone listening, making a lot of wide stereo material far more natural and pleasant.

Having used the SHUpHLER MkIV in post‑producing several recording projects over recent weeks, with it hooked into my stereo monitoring setup the rest of the time, I’d be very reluctant to part with it. It’s highly recommended for anyone working with simple stereo mic arrays, or for optimising the stereo imaging of a monitoring system.