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Positive Feedback ISSUE 56
july/august 2011


pete riggle audio engineering

String Theory Tonearm

as reviewed by John Hoffman



string theory arm

Images courtesy of Pete Riggle





Sachiko Double Horn speakers with Fostex 208 Sigma drivers.

Electra Print Audio 300B Custom amplifier Electra Print Audio PVA pre-amplifier, Hagerman Coronet II phono stage with VCAP upgrades and a Hagerman Piccolo step up.

EAD T-1000 transport, Audio Magic Kukama DAC, Galibier Serac with graphite platter, Rega RB 251 with Incognito rewire and Pete Riggle Audio Engineering 12" String Theory Tone Arm. Accuphase AC3 cartridge.

Audio Magic Illusion 4D interconnects, speaker cable, and digital cable and Audio Magic Extreme series power cords. Zu Audio Mission speaker cables, Zu Audio Speaker wire, Zu Audio power cords.

Audio Magic Mini Reference power conditioner.


The final rays of sunlight leak through the picture window in my library. As shadows slowly advance to fill the room, the closing notes of a Louis Armstrong song, recorded over half a century ago, come to a close followed by the familiar thump of a stylus in the lead out groove. Perhaps there is time for one more album. Certainly there is room for a few more pieces of music in a day.

Currently I am focusing my attention on the newest arrival to my system, which is the Pete Riggle String Theory tonearm. Built from wood and brass, this arm has a striking visual appearance, yet there is a great deal of neatly executed engineering behind this pretty facade. I get up from my listening couch to cue up the arm, and put Satchmo back on the shelf. I select "First Time!" from Count Basie and Duke Ellington, for there are very few ways to spend an evening that are as pleasurable as the company of these two gentlemen. But I digress....

In Eastern Washington there is an enthusiastic group of audiophiles who gather several times a year for a small audio-fest. Members of the Bad BoyZ and Bad GirlZ of Eastern Washington include some notable folks such as Jeff Day of PFO, Stephaen Harrell of 6Moons, Kara Chaffee of DeHavilland Audio (token West-sider), the late Terry Cain of Cain and Cain, and Pete Riggle of Pete Riggle Audio Engineering. It was at a gathering a couple of years ago Pete first mentioned his interest in designing a tonearm, and then late last year the first batch of arms went into production. Early in the process, Pete decided to build the arm with a wood body and a suspended bearing assembly. When I got a look at a prototype, I was intrigued, and signed up for a review sample. The String Theory arm is available in lengths from 9 to 12 inches, with a factory direct price of $1600.

string theory arm

The foundation for the Pete Riggle tonearm is the VTAF, which is a device that allows for Vertical Tracking Angle on the Fly. Originally designed for the Rega series of arms, the VTAF has become an unqualified success in the analog world, and has been applied to a wide variety of tonearms. The device represents a degree of unconventional thinking in tonearm installation on a turntable, as it results in a mechanical uncoupling of the arm. With Rega arms this floating assembly requires proper tonearm cable dressing; with the String Theory arm, cable dressing is no longer a concern due to the fact that it does not exit the arm pillar. Incorporating the VTAF assembly into the initial design of the tonearm results in a greater realization of the benefits of the device. The threaded circular dial of the VTAF allows for precise VTA adjustment, with .025" of height adjustment per revolution of the brass dial. This device allows for the precise and repeatable VTA adjustments necessary to realize the full potential of a cartridge.

The body of the String Theory arm is cut from a 100 year old African Mahogany wood, and finished with fine layers of hand rubbed French polish. The mahogany was chosen for its excellent dampening characteristics; for the arm body proves to be resistant to energy that is developed by the turntable and cartridge. There are optional wood species available for the arm, which alters the mass and acoustic signature of the arm. Bloodwood or Ebony will have a higher mass, as well as a different set of dampening and resonance characteristics. On the other end of the mass spectrum, Western Red Cedar can be used to build an arm that is compatible with higher compliance cartridges, such as the Shure V15 series. In some respects, the String Theory arm is similar to a finely crafted instrument, where materials can be selected to produce a unique variation, with the presentation being altered to suit the needs of the owner.

string theory arm

The versatility of the mahogany can be further enhanced by alternative selections of the materials in the head shell assembly. Brushed brass or aluminum head shell components can be selected, with the lighter metal resulting in a two thirds reduction of mass. The highest percentage of tonearm effective mass is due to the cartridge head shell assembly, and brass and aluminum parts can be mixed or matched to adjust this parameter. The tonearm in my possession has aluminum head shell fittings, for I require compatibility with medium compliance cartridges. The overall mass of my 12 inch arm is 15.7 grams with a target resonance frequency in the 8Hz range. Brass fittings are available and intended for use with lower compliance cartridges. The Denon 103 series of cartridges are excellent candidates for use with the brass configured arm, as are many other older moving coil designs.

In the world of turntables there have been a few designs that suspend a tonearm wand from one or more strings in order to eliminate a conventional bearing assembly. While string bearing technology is unusual, there is a history of well executed string bearing tonearms, including the Well Tempered and Schroeder arms. The String Theory arm uses four finely braided Kevlar lines hung from a brass knob in a small square array. These strings are .12 of an inch in length, which minimizes any unwanted movement. The brass knob allows for an application of tension to the string assembly, which is how anti-skate is applied to the tonearm. The end result is a low friction bearing assembly that is uncomplicated in its design, but meets the criteria required for a high performance tonearm.

Located on the underside of the tonearm wand, directly opposite of the string bearing, is a small aluminum paddle assembly that extends into a cup used to hold dampening oil. Inside the cup is a small pin that is attached to a lever assembly that is placed under the cup. The pin-and-channel arrangement serves as a snubber, providing tonearm handling similar to that of a gimbal bearing arm. This device is ingenious; it allows for tonearm azimuth adjustment on the fly (AZOF). The String Theory arm is a uni-pivot design, so proper azimuth set up is critical. Although it can be a tedious process to go through on other tonearms, the snubber device allows the user has to move the lever in one direction and the pin will shift the paddle, thereby changing azimuth of the stylus. A small bubble level is located just behind the cartridge to gauge changes in position, while a mirror and listening tests can be used to find the final position. Once the arm desired azimuth has been found, lateral balance weights can be adjusted to move the paddle off the pin assembly. The snubber and pin arrangement is a smart piece of engineering, for azimuth on this arm is extremely easy to adjust.

The dampening cup located under the pivot point is a pathway for controlling any unwanted resonance nodes in the bearing assembly, and to a certain extent will optimize the cartridge and arm pairing. Pete supplies a syringe of high viscosity oil that can be added to the cup if the user finds it necessary. The degree of dampening applied to the tonearm is adjusted by the amount of fluid added to the cup. The amount of dampening force is increased when the additional fluid is added to the cup and extends up the sides of the paddle. But, the String Theory arm does not require any fluid in the cup; doing so offers an avenue for the owner to fine tune the cartridge and arm interface. There is plenty of leeway for experimentation with this option, and the type and amount of fluid used can be varied at the users discretion.

string theory arm

The counterweight for this arm is an under-hung variation of the Pete Riggle Counterweight for the Common Man (CCM). The under hung configuration provides a stability to the uni-pivot design. A series of small brass washers located between the counterweight stub and the wood arm body that can be increased or decreased in order to optimize the tonearm for cartridges of varying weight. The end stub for the arm is made of stainless steel, and the counterweight slides down the shaft and is locked down by a thumb screw. It is important to note that the location of the thumb screw will affect tonearm azimuth, and must be accounted for during the set up process.

The String Theory arm has a continuous six foot run of stranded copper 32 gauge tonearm wire with silver plated conductors and Teflon insulation. The head shell end is terminated with Cardas cartridge clips, which are extremely easy to use and fit snugly on the cartridge pins. A channel is cutout on the underside of the wood tonearm body, which is the pathway for the arm wiring. Shielding of the cable is provided by a thin walled brass tube in the arm body, and a foil and drain wire is used on the later half of the cable. The compliance loop portion of the wire is unshielded, as the stiffness of a shield would negatively affect the smooth operation of the bearing assembly.

Since anti skate cannot be set by a simple spring and dial assembly, the process of measuring the amount of force applied by the string bearing also required an innovative solution. With application of some critical thinking and engineering experience, the Pete Riggle Anti-skate Measurement Device (PRAM) was developed. Informally referred to as the "Skyhook", this is a small wooden stand that has two hanging lines placed in front of a scale. You may not have noticed this, but Pete Riggle does have an affinity for wood! It is his medium of choice for many projects. One line hangs straight down, and this is the reference point. The second line has a loop that attaches to the head shell lift, and the force applied by the string bearing will move this line away from the reference point. There is a linear scale behind these lines that measures the distance between the two, which corresponds to the amount of anti-skate force applied. One characteristic of longer tonearms is that less anti-skate force is required, and the parameters that Pete sets for his arms will have varying amounts of force that correspond to different arm lengths.

Pete does supply a set of measurement devices and tools for installation of the String Theory arm. Since I have an early version of the arm, the tools I received are clearly hand produced. Do not assume they are primitive or inadequate for they are certainly accurate devices. The mounting distance tool is once again made of wood and determines the placement of the VTAF bushing. Another template is the overhang gauge for the cartridge and the third one is a Baerwald alignment protractor. All the tools needed for setting the arm up are provided and the instruction manual for the installation and adjustment of the arm is reasonably uncomplicated. Bear in mind that this is not the easiest arm to set up, but it is not insanely difficult either. While it may be a tad more difficult to install and adjust than a Rega RB300, aside from the balancing of the uni-pivot. this arm aligns like any other pivoted one. When it comes time to make the azimuth adjustment, the true value of the snubber device becomes readily apparent.

string theory arm

I sent the VTAF bushing off to Thom Mackris at Galibier Audio to mount in a 1" aluminum arm board for the Serac. Upon its return, I then used the supplied tool to get the correct pivot to spindle mounting distance, which is accomplished by pivoting the arm board into the correct position relative to the spindle. An Accuphase AC3 moving coil was attached to the cartridge mounting plate, and the Cardas cartridge clips slipped on quite easily. There is a single nylon bolt that is fitted into the cartridge mounting plate, and it slips through a slot to the aluminum fitting that is attached to the arm wand. Installing the cartridge to the String Theory arm is actually a very simple process, and this single bolt arrangement makes cartridge alignment a simple task. The supplied protractors are adequate for this job, but are very basic items. The overhang protractor requires that I align it visually with the pivot point, and that task is a bit of a chore to get right. An accurate alignment can be achieved with a bit of practice, however I would prefer to have a visual marker for the center of the bearing assembly. Tracking weight is set by sliding the counterweight on the stainless steel stub, and measuring the result with a scale. The VTAF device sets the height of the arm pillar and allows for a wide range of adjustment. The next adjustment is azimuth, which the snubber device makes quite simple. First step is to move the snubber arm to the point where the small bubble level on the head shell shows it being level. I then have a small mirror that I use to get a closer look at the diamond and cantilever, and make another set of adjustments. When I am comfortable with final azimuth, I proceed to setting the side to side leveling weights. These are set to the point where the arm pin is no longer in contact with the sides of channel that is attached to the snubber lever. The final task is setting the anti-skate force, which is accomplished with the PRAM device. This instrument is delightfully simple to use and an accurate way of achieving proper anti-skate set up. Now the String Theory arm is ready to go, and I am ready to spin vinyl.

The defining characteristic of the String Theory arm is its combination of proper tonal color and its unflappable expression of micro dynamics that make up music. What this translates to is this: music has a robust and fleshed out tonal balance, yet the fine detail which makes it realistic is not lost in the pleasing texture of the music. Listening to the vocals on "Lullaby of Birdland" by Mel Torme [LuLu's Back In Town; Affinity AFF85] is certainly captivating, with a rich and layered texture that just draws me into the music. Torme's vocals are smooth and have a natural feel to them, and the flow of music is unhurried and idyllic in nature. The scat passages showcase Torme's formidable talents and the String Theory arm is instrumental in allowing my analog system the ability to access all the subtleties in this recording that makes it magical.

Certainly Mel Torme's vocals are legendary, yet the band accompanying him should never be overlooked. The String Theory arm does an extremely nice job of keeping the music together and never overtly inserting its character into the song. The opening bass passage is quick and lively with no hint of overhang. The brushes on the drum set have just the right swish to the sound and the cymbals have the appropriate metallic ring. The trumpet passages have a forceful attack, and have that distinctive "blat" that makes the instrument unique. The tonearm does not impart any significant tonal or timing inconsistencies in the music, and this in itself is no small feat.

string theory arm

Depending on the associated hardware used, the String Theory arm can be either a high or medium mass arm. The majority the arm effective mass is determined by the weight of the cartridge mounting assembly, which gives the owner a great deal of flexibility in adapting this tonearm to a wide array of cartridges. The primary function of arm mass is to control needle talk, which is an exciting force generated by a cartridge during playback. Basically, this event is where the cartridge generates energy while tracking the record and inputs it into the tonearm body. This energy then travels through the body, and is eventually returned to the cartridge, which results in smearing of the music. The 8Hz resonant frequency of the String Theory arm results in a greater immunity to needle talk, while a typical medium/low mass arm is tuned to the 10 to 12Hz range. The practical benefit of this design is improved transparency, image stability, and an overall coherency to the music. Listening to "Highway Patrolman" by Bruce Springsteen [Nebraska; Columbia OC38358] allows the strengths of the String Theory arm to be showcased. Springsteen's vocals have a natural and relaxed feel and his acoustic guitar has an easy flow throughout the song. The ambient cues in the recording venue are easy to discern, and are nicely balanced and consistent throughout the recording. In the background there is a harmonica and mandolin softly mixed in and the arm allows these instruments to come through with excellent clarity and definition. Performers and their instruments are solid in their image and well spaced throughout the sound stage. In terms of an overall presentation, the String Theory arm is an outstanding communicator of both content and context of music.

string theory arm

The dampening cup feature on the String Theory will add a degree of dynamic torsional resistance to the pivot assembly which can be altered by varying the amount of fluid in the cup. Mass and the inherent dampening characteristics of the wood tonearm body are two ways that the Riggle design addresses the issue of needle talk. The dampening cup can add another dimension of stability to the bearing assembly and provides alternate avenue to explore in terms of fine tuning overall performance.

The Accuphase AC3 cartridge is a relatively high compliance moving coil (18 x 10-6cm/Dyne), and in early listening sessions exhibited a small degree of mis-tracking in this higher mass arm. This set up issue was completely solved when I added a small portion of the supplied dampening fluid to the cup; the cartridge became a sure footed tracker in all types of music. Another benefit I experienced from the addition of the dampening fluid is improvement in the quality and quantity of the lower registers. Dynamic passages are now easily tracked, and bass notes are authoritative and precisely defined. "Rhythm of the Heat" by Peter Gabriel [Security; Geffen Records GHS2011] contains complex drum passages that have been programmed on a drum machine, and presents a challenging tracking test for many arm and cartridge pairings. The String Theory with the fluid option clearly defined the Bongo drum passages, which are lightening fast and widely varied in timbre. There are deep synthesized notes that are held for a longer time and these passages create a great deal of energy for the arm to deal with. Once again this arm/cartridge pairing has no issues with this task and the instrument has excellent weight and clearly defined texture. Once the correct amount of fluid for the cup was determined, I found the String Theory arm to be a tracking champion. While the default set up for the arm does not require fluid in the cup, in my application its use clearly resulted in improved performance in two major categories.

One point I should note is that String Theory arm I have for review is designed to be used with medium compliance cartridges and the Accuphase AC3 is not exactly that. For a period of time I had a Sound Smith modified Denon 103R on the table, and it performed flawlessly in the Riggle arm. The tracking abilities of the Denon were first rate in this arm and record noise was almost non-existent. While the AC3 is a superior cartridge in terms of sound quality, the mechanical characteristics are not in prefect alignment with this arm. Yet the fluid dampening feature effectively dealt with this issue and speaks loudly about the versatility of the arm.

What is there not to like about the String Theory arm? Well, I would say very little, especially considering the fact that I bought my review sample. The design of the arm is fundamentally sound and is a refinement of previously accepted tonearm technology; but with a few innovative twists. The azimuth snubber works like a charm and is responsible for an straight forward and painless set up procedure. The French polished Mahogany wood is certainly elegant in appearance, yet its internal dampening properties certainly contribute to the refined and genteel presentation this arm has. The string bearing is not a new concept, although the arrangement Pete utilizes allows for a simple and direct mechanism to apply anti-skate force. The PRAM device is certainly an innovative way to measure and adjust anti-skate force.

With all this forward thinking engineering built into this arm, there are a couple of oddities that a prospective buyer should be aware of. In essence, the String Theory arm is a craftsman type of product. Pete Riggle has an affinity for working with wood, brass, and adjustment knobs that can be set by hand. This arm has a hand crafted aura and it is an effort of a single man who reflects the skills of artisans and woodworkers from an earlier period. This arm is not a slick product that comes out of a machine shop designed with fantastical clearances and specifications. Which is not to say that the Riggle arm is primitive or unsophisticated, but rather that it is designed and manufactured with a philosophy that comes at the challenges of analog from a different perspective. For instance, there is a series of small brass washers located between the wood body and the tonearm stub. One could say that these washers represent a $3.25 worth of parts from the local hardware store, yet they offer a creative way to adjust the effective mass of the arm by allowing the counterweight to be kept in close proximity to the pivot point. There is one long wood screw that passes through the counterweight end stub and attaches it to the tail end of the wood tonearm. While this method would appear to be rudimentary at best, it achieves the goal of making a firm and secure bonding of these two pieces. In short, the String Theory arm is no more complicated or sophisticated than it needs to be, which is one reason why this arm can sell for $1600 and another arm will carry a price tag approaching $5000.

Well its another evening and the sunsets in Eastern Washington can be quite remarkable. From a picture window in my listening room I can see the hills and vineyards of Red Mountain reflecting the days last light. Once again the turntable is spinning, as I have been listening to a great deal of vinyl these days. The Riggle tonearm has taken permanent residence on my Galibier Serac and the only question now is what will be my final phono cartridge? Learning to Crawl by the Pretenders is on the table... and I have run though four records earlier this evening. My current analog rig sounds wonderful and has that ability to draw me into the music. A significant portion of this charm of analog in my system can be directly attributed to the String Theory arm. Beneath that beautiful French polished wood lies a remarkable bit of creative thinking in regards to the obstacles presented by analog playback and neatly executed engineering solutions. If you value a hand crafted product and the passion and integrity of a designer, than I would suggest you give the String Theory arm serious consideration. John Hoffman

String Theory Tonearm
Retail: $1600

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