The IPT RigB, Rigid Metal Body for AT91 /AT3600 Cartridge Reviewed

Taking an Audio-Technica generator and upgrading the cartridge body is not a new concept. It was pioneered by Linn, who took AT’s generators and put them into exceptionally well-made metal bodies to make the Linn K9 and K18 cartridges. Naturally the compliance of AT’s cartridges, and thus the K series is perfectly suited to the medium-low mass arms that Linn sold to partner the LP12, thereby giving buyers a complete Linn front end. Though tough for me to admit as one of Linn’s toughest critics (to put it mildly), those metal bodies gave the cartridges a solidity which was obvious to the user as soon as they handled the cartridge on its own.

Whether black as in the K9 or red as in the K18 (a finish also used on the iconic Troika MC) the finish was anodised which gave it a hard and silky feel, exuding quality. Expensive styli completed the high-performance package. Stilton followed suit in the late ‘80s, taking MMs like the venerable A&R P77 and also some moving coils and ‘pimped’ them up with simple metal bodies to great effect such that even A&R (by then called ARCAM) had to follow suit and make their own body in-house.

Cartridge manufacturers often differentiate their ranges by the inclusion of a metal body as you move higher up the range. For instance, the £279 AT-VM740ML is Audio-Technica’s least expensive moving magnet to sport a metal body. While undoubtedly better than the polymer resin of the cheaper bodies, it’s still lacking essential rigidity between the cartridge body and the generator canister itself.

Simon and Gary of The Audio Files and Improved Performance Turntables (IPT) recently reached out to me to discuss a metal body of their own. The aptly named RigB (Rigid Body) is a metal body designed to fit the Audio-Technica AT91 generator. The same generator is used in the AT3600, a cartridge found on a vast number of OEM turntables, branded by Rega as the ‘Carbon’ and also available cheaply from the far east.

The body is precision CNC machined from a T6 alloy billet and black anodised for long, corrosion-free life. It replicates the geometry of the AT91’s plastic body with the canister held at the same position and angle and gripped by its flanks for the best possible interface between body and generator. The generator is held in place by a countersunk screw (included) and further secured by a few drops of Loctite 638 bonding material. A limited edition red body is also available in strictly limited quantities at a slight increase in price.

The body adds 1.9 grams in extra mass to what is already an especially light cartridge (2.5 grams as opposed to the original 0.6 gram body). The added mass is in fact seen as a positive thing for performance; particularly so for the lower compliance styli such as those of the AT3600 and Rega Carbon. The increase in mass drops the arm/cartridge resonance with most arms by about 0.6Hz which is insignificant.

I was supplied a pair of AT91 cartridges for A/B comparison tests and the RigB body itself. On first examining the body I was struck by the magnificence of its engineering. The current global pandemic has allowed IPT to utilise the time and resources of an engineering company normally devoted to working in the formula 1 sector. The curves around the mounts are beautiful, with the top of the body gently sloping toward the front as in the original. Tapped mounting holes take M2.5 screws directly, negating the need to fiddle with tiny nuts. They say that beauty is in the eye of the beholder but I think few could oppose the fact that the RigB is a real work of art.

Having spent some time admiring it I opened up a brand new AT91 and proceeded to remove the generator canister. It’s held in place by a single Philips screw at the top and a bonding adhesive. Once removed from the body it is secured into the RigB by the same arrangement using the mounting screw provided. When installed there’s no unwanted movement or play in the assembly.

IPT kindly sent a second AT91 so I could run an A/B test between the two. They also sent their new extended contact stylus, which is a nude-mounted line contact designed to show all that the generator is capable of. I give all styli a few hours of play before any serious listening.

Gary pointed out to me how Mechanically suitable the low compliance ATN3600 based extended contact styli will be for the (knife edge Type) standard Goldring Lenco Arm. “I’ve got one of these in restoration, and am very excited about the suitability of this synergistic combo.” While I am no Lenco expert having somehow managed to go over a decade in this hobby without getting my hands on one (working or for restoration) I can certainly say that the match looks ideal on paper.

We’ve covered both the AT91 and AT3600 in the past, though in both cases on cheaper budget turntables. The former was standard fitment on the Dual CS-435, and the latter were fitted to both the Dual MTR-40 and AT-LP60 and remains on the newer AT-LP60X. In neither case was I able to hear the true limits of the performance of either cartridge as in all cases the turntables were a limiting factor. Using my reference Technics SL-1200G however and a new high-end design of my own dubbed ‘Project Glacier’, I was able to hear what the AT91 is truly capable of.

The carts were run into the Musical Fidelity M6 Vinyl phono stage with the loading set to 47kΩ, 100PF. In my experience AT cartridges can become brittle and harsh at the top end with too much capacitance. 100PF coupled with the arm wiring at about 80PF in each case brings me close to the 200PF that remains my ideal maximum.

I started with an unmodified AT91. It’s character was consistent with what I remembered. A warm sound with lots of mid emphasis, wooly bass and a sightly muffled top end lacking in detail. Not an unpleasant listen, though not an especially refined or subtle one either. Though not a particularly exciting or lively listen, it does a great job of smoothing out less than stellar recordings and vinyl imperfections, especially on worn records and styrene 45s. Listening fatigue is virtually non-existent.

I then switched to the upgraded stylus and was, to put it bluntly, blown away. When Simon told me of their plan to produce a stylus upgrade for these cartridges I naturally assumed it would be a fairly standard .3 x .7 mil elliptical, narrowing to a .2 mil nude mounted elliptical at a push. When I found out it was to be a line contact tip I was surprised and couldn’t help but question whether the expense could be justified given the low-cost generator.

Suffice it to say that I brought the line contact stylus towards its first groove with some scepticism. Scepticism that quickly evaporated as the first few chords of the Eagle’s ‘Doolin-Dalton’ rang out. The highs, once muffled, splashy and sibilant, now crisp, clean and sparkly. Audiophile cliches, but true nonetheless.

The mids had relaxed somewhat, giving way to a sense of air that was either well masked or simply hadn’t existed previously. Bass was taught; and though it lacked the refinement you’d get with a better generator, it was still a vast improvement on the standard conical stylus.

Stereo image increased in all dimensions; again, perhaps not to the level of one of the pricier dual-magnet generators but still a marked improvement. What was once a fairly half-hearted, pleasant yet uninteresting listen was suddenly transformed into a highly capable cartridge.

I would even go as far as to say that the AT91 with its replacement stylus equals the VM95En with its nude-mounted elliptical stylus and dual-magnet generator, which perhaps just reinforces the obvious fact of the importance of the stylus in the playback chain. How many high-end cartridges could be improved with the addition of a better stylus tip?

I then swapped to the RigB body, fitted with an AT91 generator and the stock AT91 stylus. Again the sound was splashy at the top end, lacking in detail and with a significant boost in mid-range presence. Most notably different however was the bass, which was better defined and better controlled with greater impact than with the plastic body. There were less resonant overtones and the sound was over all cleaner, with less of the ‘wooly’ nature of the stock AT91 and a more refined and agile performance.

I then swapped out the styli. The difference with the line contact stylus in place was much the same as before, though now with the advantages of the RigB body. Bass notes were well handled with minimal bloom and blur. There remains a little mid presence but the boost is far less noticeable than with either the stock cartridge or stock stylus. Detail in the high-end is vastly improved and dimensionality improves a little further with the RigB body, a trait that the stock conical stylus wasn’t able to show. The AT91 generator certainly became the limiting factor in this setup, though with IPT having chosen a sensible line contact option rather than simply opting for the most extravagant stylus possible, the deficiencies in the generator can be excused and the cost of the stylus upgrade is more than justified by the improvement it brings.

I should state at this point that there are other styli available for the AT91 and AT3600 generators from LP Gear and many others. There is at least one Shibata, several line contacts, ellipticals (nude and bonded) and others besides. Some are vastly overpriced in my opinion given the limitations of the generator, while the IPT stylus falls at an acceptable price point where a package with a RigB body, a suitable generator and an upgrade stylus is still good value for money.

To summarise. The RigB is simply one of the best looking cartridges around. But it’s more than that. It’s a significant upgrade for a tremendously competent generator held back by its flimsy plastic body. I look forward to seeing what future RigB designs bring to their respective bodies. But for now at least, the AT91 variant leaps up into the leagues of the best budget cartridges, and in doing so surpasses a few mid-level models too. Highly recommended.