Soundgas Type 636P2 Stereo Preamplifier Sound On Sound Review

''Soundgas will be well known to British readers as one of the country’s leading specialists in vintage gear, especially old tape and disc echo units. Their USP is to offer comprehensively refurbished equipment, backed by a warranty and service options. They’re rarely the cheapest option, but when you’re spending thousands, this sort of peace of mind is invaluable.

Alongside the Space Echoes and Binsons, one of the mainstays of Soundgas’s vintage trading has been the Grampian 636 spring reverb. This owes its cult status partly to its use by dub mixers such as Lee ‘Scratch’ Perry, and partly to its unique saturation characteristics when overdriven — the 636 was reportedly a mainstay of Pete Townshend’s guitar sound. A design from the early days of solid‑state electronics, it uses germanium transistors to amplify the input signal and drive the reverb tank.

Germanium circuits frequently offered significantly worse noise and distortion performance than the valve designs they replaced, and were made obsolete in the ’60s when silicon transistors became widely available. Germanium semiconductors are also notoriously inconsistent, which is a serious problem when trying to match the sound across multiple channels or units. But they have character in spades, and hence there’s a thriving cottage industry today in audio devices that use germanium circuits. In the studio world, Chandler have an entire range of germanium‑based studio processors, and there are also plenty of guitar fuzzboxes and preamps based around germanium circuits.

Cooking On Gas

Over in Derbyshire, meanwhile, Soundgas struggled to find enough Grampian 636 units to meet demand, and many of those they did find required complete rebuilds. Hence the decision was eventually taken to develop a modern recreation. This has been produced in very limited numbers, largely dictated by the challenge of finding the correct reverb tanks, not to mention germanium transistors that were anywhere near in tolerance. The Soundgas Type 636 is also labour-intensive to make and consequently retails for almost £6000 on the rare occasions when units are available. As was already mentioned, however, the cult of the 636 revolves as much around the preamp circuit as its ability to add reverb to things, and so Soundgas have developed several further 636 products that omit the reverb tank. These include the pedal-format 636P, the 500-series 636-500, and the product under review here.

The Soundgas 636P2 is a 1U rackmounting two-channel preamp designed for use with microphones or instruments. Each channel has a phantom power switch and selectable input impedance, and Soundgas have also added Carnhill input and output transformers to balance the rear-panel input and output XLRs. Front-panel unbalanced jack sockets labelled ‘break in’ and ‘break out’ allow you to tap into the circuit after the input transformer and/or before the output transformer respectively. The 636P2 is very nicely made, with a sturdy metal chassis and faceplate. Apart from the aforementioned XLR and jack I/O, the only extra socket is an IEC connector that accepts a standard mains cable. You will need to specify your local mains voltage when ordering, as it does not use a universal PSU. Gain is controlled using vintage-style chickenhead knobs, and to the right of these you’ll find the 636P2’s only concession to onboard metering, in the shape of amber-coloured lamps labelled Overdrive. These don’t really provide useful information about signal levels, but they are an important part of the 636 circuit, the bulb serving as a crude limiter.

Unclean Machine

After this preamble, it shouldn’t come as a surprise to learn that the Soundgas 636 is not intended as a transparent, low-noise mic preamp for classical recording. I was particularly interested to try it because, as you can see from the photo, I have a couple of other germanium devices in my studio to compare it with. Chandler’s Germanium Preamp was reviewed by Hugh Robjohns back in November 2006 (www.soundonsound.com/ reviews/chandler-germanium) and remains a current product. It is an original design, not based directly on any vintage unit, and is transformer balanced on both input and output. The four-channel mixer below it, meanwhile, belongs to the very earliest generation of solid-state audio equipment, from around 1959-60, and features huge Gardner mic transformers on the inputs. It was made in the UK by Pye Electronics and saw duty at the BBC’s White City television studios, where it was employed as a ‘floor mixer’ to submix signals on their way to the main console. However, a direct comparison between these three units is hampered by their wildly different operating levels. No specs are provided for the Soundgas preamp, but even with the gain cranked, the hottest output you can get from it seems to be around +12dBu. By contrast, the Chandler preamp will happily kick out +34dBu — way more than any of my A-D converters can handle — and the Pye mixer is specified for “at least 90dB” end-to-end gain, which translates in practice as “hard to use unless you turn the master output way down”. The Chandler also has a unique approach to gain setting, wherein its Gain and Feedback controls interact in unpredictable fashion. 

Germanium Whip

When tested with microphones, the 636P2 and the Pye mixer sounded pretty similar. In both cases, it’s possible to get a relatively uncoloured sound by turning down the input gain, though the 636P2’s relatively low overall gain means you may then need to run its output into another preamp to achieve a satisfying line level. Turn up the gain, and both develop the same sort of crunchy saturation on peaks. The character is nearly identical to the ear, with the Pye perhaps sounding a touch ‘heavier’, but it’s clear from inspecting the waveforms that the clipping in the 636P2 is asymmetrical whereas that of the Pye is more or less symmetrical. The Chandler preamp, by contrast, is impossible to drive into obvious saturation without massively overloading the A-D converter that follows it. This sort of saturation on microphones has its place, but I personally wouldn’t require it often enough to justify the 636P2’s asking price, and it lacks enough clean gain to be useful as a general-purpose mic preamp. Where I think the unit comes into its own, though, is as a (nearly) line-level processor for synths and other electronic instruments. I was able to extract a surprisingly wide range of timbres from the same patch on my Korg MS20 simply by choosing different I/O paths. Going straight into and out of the XLRs delivers a warm, soft tone with a hint of growl when pushed, while the break-in jacks produce a more aggressive yet still sweet-sounding result. However, with unbalanced cables, a significant level of hum was audible unless I patched a DI box between synth and preamp. What was most surprising about recording synths through the 636P2 was the effect it had on modulation. The same degree of LFO pulsing, in the same patch, sounded much more obvious on the Soundgas preamp than on any of the others. I can only assume that this too is somehow due to the asymmetric clipping, which results in the positive-going waveform being trimmed while the negative-going side still has the headroom to modulate. Finally, the break-in jack can of course be used to plug in a guitar or bass, whereupon the 636P2 becomes the polar opposite of a clean DI. With humbuckers, in fact, it can be plain filthy, with a deliciously complex, chewy distortion that’s perfect for psychedelic or garage rock tones, and which can easily be further processed using a cabinet simulator if you want to bring it under control.
The Soundgas 636P2 won’t be your only preamp, but what it does is pretty much unique. Whereas some preamps aspire to offer ‘straight wire with gain’, this is more like a rusty coathanger. It’s neither clean nor quiet, but it adds instant character to anything you run through it. And although it can get pretty wild, there’s nothing accidental about the way it sounds — the two channels on the review unit were perfectly matched both in terms of level and tone. If you’re afflicted by sterile synths, dull drum machines or soulless samplers, the 636P2 will sort you out a treat.

Summary

Behind the 636P2’s innocent exterior lurks a seriously spicy preamp that excels for recording electronic instruments.’’

£ £2940 including VAT

W www.soundgas.com

Soundgas Type 636P2 - Twin Grampian 636 Mic Preamp

PUBLISHED APRIL 2025