An Open-Reel Tape Primer

I never in my wildest imagination thought that in the year 2022 I’d be writing a primer on open-reel tape. Or that The Absolute Sound would be reviewing a newly designed and built reel-to-reel deck aimed at audiophiles—the Metaxas & Sins Tourbillon T-RX explored by Jonathan Valin in this issue. Yet open-reel tape is making a strong comeback, with many new and rebuilt machines available in the market, along with an astounding number of highly desirable music titles from top artists. As Jonathan explains in his review, tape is the ne plus ultra of high-end audio.

In this brief tutorial, I’ll give you some background on tape machines and how they work, how to use and maintain an open-reel deck, and magnetic-tape fundamentals.

Let’s start with tape formats. Two-channel consumer tapes are usually provided on ¼” tape. If you’re of a certain age, you may remember from tape’s commercial zenith that there are two formats of ¼” tape, called “quarter track” and “half track.” The quarter-track format has four audio channels, two of which (one left/right stereo pair) are played at a time in one tape direction. At the end of the tape, the tape reel is flipped over and played in the opposite direction to access the other stereo pair of channels. The quarter-track format doubled a tape’s playing time at the expense of fidelity; the narrower tape tracks reduced the signal-to-noise ratio and dynamic range. By contrast, the sonically superior half-track format has just one stereo pair of tracks, and the tape is always played in the same direction. The modern renaissance in tape is based on the half-track format. Fig. 1 shows the track configurations of half-track and quarter-track tapes. (The “full-track” format uses the entire tape width for a single monaural channel.) Incidentally, some very-high-end professional machines record two channels on ½” tape for even greater fidelity. 

A second spec you need to know is tape speed. The standard for consumer releases today is 15 inches per seconds (ips). In the 1960s, pre-recorded open-reel tapes were usually recorded and played back at 7½ips, or even 3¾ips, to extend playing time. Faster tape speed results in superior sound quality, with more extended treble response, wider dynamic range, and lower noise. Professional machines can run at 30ips, which offers the best sound quality, but burns through an expensive reel of tape quite quickly. Today’s consumer machines usually offer 7½ips and 15ips playback speeds.

Incidentally, the Compact Cassette format is based on ⅛” tape, is quarter-track (you flip the tape over at the end), and runs at a speed of 1⅞ips. 

Given these specs, it’s surprising that cassette doesn’t sound worse than it does. In 1976, Sony introduced the Elcaset, an attempt to bring the performance of open-reel tape to the convenience of a cartridge. The Elcaset used ¼” tape and a tape speed of 3¾ips in a cartridge twice the size of a Compact Cassette. Despite the greatly improved performance, Elcaset was a commercial failure—it was introduced just as the Compact Cassette was making great strides in performance with improved tape formulations, Dolby noise reduction, and three-head decks.

The slower tape speeds of the earlier generation of pre-recorded open-reel tape allowed an entire album to be stored on a 7″ tape reel. Consumer machines of the day were largely limited to 7″ reels, with professional machines designed around 10.5″ reels. The high-end consumer decks offered today all accommodate 10.5″ reels. The playing time for a given reel size is determined by the tape thickness as well as the speed. A 7″ reel can hold from 1200 up to 2400 feet of tape, resulting in playing times of approximately 32 minutes and 64 minutes, respectively, at 7.5ips. Thinner tape is more fragile, and has significantly more “print-through” (explained later). 

A factor related to reel size is the type of “hub” in the deck—the mechanism for securing the reel to the machine. Machines that accept 7″ reels feature a consumer-grade “trident” hub, a thin post about the size of a turntable spindle with three blades over which the tape reel slides. The three spring-loaded blades then expand to hold the reel in place against the turntable. The professional-grade mechanism is the “NAB hub,” a large spindle that more securely clamps the reel to the tape machine’s turntables. You can see the differences between hubs in Fig. 2. Hub adapters are available that allow you to secure NAB reels to trident hubs. 

To recap, the first generation of consumer open-reel tape machines were based on 7″ reels, ran at a tape speed of 7.5ips or 3¾ips, were recorded in the quarter-track format, and used trident hubs for attaching the reel to the tape machine’s turntable. Today’s consumer machines are much more like professional decks, with the ability to accept 10.5″ reels with NAB hubs and to run at 15ips; they also play tapes back in the half-track format.

Now let’s look at the parts of a tape deck. You can see in Fig. 3 the main components of supply reel, take-up reel, heads, capstan and pinch roller, tension arms, and idlers. A tape that has been previously rewound is placed on the machine’s left-hand (supply) turntable, and the tape threaded through the tension arms, idlers, and capstan/pinch roller. During playback, the tape is slowly wound onto the take-up reel on the machine’s right-hand side, passing across the heads on the way. 

The tape is pulled across the heads by the capstan and pinch roller. The capstan is driven by a motor beneath the deck plate. Upon playback (or record), the rubber pinch roller moves into position to press against the spinning capstan, pulling the tape across the heads from the supply reel to the take-up reel. Some tape machines have two capstan/pinch roller pairs, one on either side of the heads. These “iso-loop” transports do a better job at isolating the stretch of tape passing over the heads than machines with a single capstan/pinch-roller. At the start of this article you’ll find a circular photo of the tape path through the head block of the Metaxas & Sins Tourbillon in which you can see the dual capstan/pinch rollers. (The capstans are inside the small round cutouts at the apex of the triangular red supports, and the pinch rollers are the black rings just below the capstans.)

The tension arms’ job is to maintain constant tension on the tape. The tension arms’ positions are sensed, with their positions used to regulate the torque applied to the take-up reel. Specifically, the tension arm is connected to the wiper of a potentiometer, with the voltage across one leg of the potentiometer indicative of the arm’s position. This voltage then controls the amount of torque applied to take-up reel motor through a servo system, thus maintaining constant tape tension through the entire tape path.

The playback head (also called the “repro” head, for “reproduce”) is where the magnetic signal imprinted on the tape is converted to an electrical signal. Some modern consumer decks have only a playback head; they lack a record and erase head on the assumption that the machine will be used for playback only. If you don’t plan on recording, a playback-only machine has the advantage of positioning the playback head in the optimal middle position in the head block, as well as much less circuitry. The head is made from stacked laminations the height of the tape’s track width. Coils of wire are wound around the laminations (Fig. 5). A gap in the laminations at the front of the head where the tape contacts the head is key to how the head works; a voltage is induced in the coils of wire that is proportional to the rate of change of the magnetic flux density (the signal recorded on the tape) across the gap. A tape head operates on the same principle as a moving-iron cartridge in that both rely on a varying magnetic reluctance to induce a voltage across coils. The playback head’s tiny voltage output is amplified by a built-in or external head amplifier, similar to a phono preamplifier.

There’s one more important thing to know about tape heads before we leave the subject: azimuth. This term refers to the perpendicularity of the head to the tape. If the head is aligned precisely at 90° with respect to the tape, we say that the head has perfect azimuth alignment. If the head is tilted slightly in either direction, the head has an azimuth error. 

It’s vital that the azimuth be set correctly, as even a slight error will result in a loss of high frequencies. I’ll share with you a story that vividly illustrates the effect of azimuth error on treble response. When I was a college student I had quite an ambitious audio system in my 1969 Toyota Corolla. The rear deck had been chiseled out, replaced with MDF to accommodate two 10″ woofers. The door panels housed 4″ midrange drivers and dome tweeters. In the trunk was a power amplifier and custom crossovers designed by my colleague at the stereo store where I worked, Albert Von Schweikert. The source was a high-end, line-level-output cassette player that I bought used. After installing the deck I immediately realized that it had severe azimuth error—the sound was dull and rolled off. Before I could get inside the device and fix it, I used a folded business card kept in the ashtray (remember those?) just below the tape deck. After starting a tape playing, I would push the folded business card under one side of the cassette, changing the tape’s angle in relation to the head to realize correct azimuth alignment (although in an unconventional way). I would adjust the card while listening until I heard the maximum treble response. 

Fortunately, correctly adjusting the azimuth in an open-reel deck is quite simple to do (and much more precise than my crude fix): The head sits on a spring-loaded platform adjustable with an Allen bolt on one side of the head that you turn to adjust the azimuth. You need, however, a dual-trace oscilloscope and a calibration tape, such as one from Magnetic Reference Laboratories, to correctly set the azimuth. Today’s high-end machines will have their azimuth correctly set, so you shouldn’t need to adjust it. 

Before we leave the subject of azimuth, here’s another anecdote. It’s common for tape machines to have very slight azimuth errors. If an album is mixed to different tape machines, or to the same machine after the azimuth has been adjusted, the master tape used to cut the LP will include tracks recorded with slightly varying azimuth. Note that if the record head had a +1 degree azimuth error, a +1 degree azimuth error in the playback head will result in perfect overall azimuth and no high-frequency loss. What matters is that the record and playback heads have the same orientation to the tape, even if that orientation isn’t perfectly 90°. At the MoFi mastering studio, the engineers study the mastertape to determine the perfect azimuth setting for each track, and if necessary, adjust the playback head’s azimuth on the fly as the tape plays and the master lacquer is cut. That’s dedication to the art. 

Just as in the LP system, magnetic tape uses an equalization curve on recording and a complimentary curve on playback. In fact, the playback amplification and equalization circuitry are so similar between a phonostage and a tape machine’s head-amp that a phonostage can be easily modified for tape playback. There are two primary equalization curves in use today, IEC and NAB. IEC is predominant in Europe, and NAB in North America. These curves differ slightly, so that if you play back a tape recorded with the NAB curve with IEC equalization, you’ll hear a slightly rolled-off treble and not as much low bass. Conversely, playing an IEC tape with NAB playback equalization will result in a slight bass and treble boost. Some machines, but not all, offer selectable playback equalization. Tapes offered for sale will indicate whether they were recorded with the IEC or NAB equalization curve. The circuitry for the IEC curve is simpler, one reason why IEC is considered better-sounding than NAB. 

Tape Handling and Deck Maintenance

The first thing to know about handling tapes is that tapes should be stored “tails-out.” This means that the end of the recording is at the outer edge of the reel. A tape that is stored tails-out is placed on the take-up (right) side of the machine and rewound before playback. For storage, the bit of tape that dangles from the reel should be secured to the reel flange with a short piece of low-adhesive “hold-down” tape specially made for this purpose. Blue tape indicates the tape is stored tails-out; red tape means the tape is stored heads-out. Why should tape be stored tails-out rather than heads-out? A phenomenon called “print-through” causes the magnetic field on one layer of tape to partially magnetize the layer it is pressed against in the tightly wound pack around the reel. In severe cases, you can hear print-through as a “ghost” of a signal, particularly if there’s a quiet passage where the print-through occurs. If the tape is stored heads-out, the print-through occurs before the recorded signal, making it much more audible. Tails-out storage doesn’t reduce print-through, but the print-through is more easily masked when it occurs after, rather than before, the wanted signal.

Tapes are best stored in a constant-temperature and low-humidity environment. Keep them away from strong magnetic fields, including loudspeakers with their powerful magnets. It’s a good idea to put leader tape at the head of the tape. This non-magnetic plastic tape allows you to handle the tape for threading through the transport without having to handle the tape itself. Leader tape can also be inserted between tracks to make it easier to find the starts of songs. You’ll need a splicing block, razor blade, and splicing tape specially made for this purpose. A good source for these products is spliceit.com. 

All the parts of the tape path that come in contact with the tape must be cleaned on a regular basis—as frequently as every hour of play, depending on the tape formulation. Some of the oxide on the tape “sheds,” leaving a brown deposit on the idlers, heads, capstan, and pinch roller. A Q-tip dipped in alcohol will do the trick, but you should not use alcohol on the rubber pinch roller, which will cause it to dry out. Keep the Q-tips and cleaning solution right next to the machine and get in the habit of giving the tape path and heads a quick clean on a regular basis.

Tape heads and metal parts in the tape path build up a residual magnetism that must be removed with a tape-head demagnetizer, also called a “de-gausser.” Residual magnetism on the tape heads and metal parts will gradually erase high frequencies from the tape. You must use extreme caution when bringing a demagnetizer near a tape machine. The demagnetizer must be turned on and turned off well away (several feet) from the machine or the sudden powerful turn-on or turn-off pulse can permanently magnetize any metal parts near it. After being turned on at a distance from the machine, the demagnetizer is moved very, very slowly over the metal idlers, tape guides, and heads, and then moved away from the machine very slowly before being turned off. The idea is to gradually randomize the magnetic domains until the net result is no magnetization. 

How Tape Works

Tape is made from a polyester material onto which a magnetic material is attached via a binder. The magnetic material is usually iron oxide—rust. Each discrete particle of iron oxide is called a “domain” and can be magnetized by an applied magnetic force, in our case from the record head. Each domain has either a magnetic north-south or a south-north orientation. When the domains are randomly magnetized, the tape has no signal and is said to be demagnetized. When many domains have the same magnetic orientation, the tape is magnetized. An audio signal encoded magnetically has a varying pattern of magnetic orientation within the domains that changes polarity in a precise analog of the audio signal.

Magnetic tape exhibits a significant non-linearity in the transfer of magnetic flux from the record head to the tape. As a very small and gradually increasing signal level is applied to the tape by the record head, the tape remains unmagnetized until the signal level reaches a certain threshold where the tape begins to become magnetized in a linear fashion. In this linear region, the magnetic flux remaining on the tape (“retentivity”) is directly proportional to the applied magnetic force. Keep increasing the signal level, however, and the tape becomes non-linear again—an increase in applied magnetic force results in little or no additional magnetism on the tape. This condition, in which all the domains are magnetized in the same magnetic orientation, is called “saturation.” Saturation is accompanied by compression of the signal’s dynamic range along with an increase in harmonic distortion, primarily the second and third harmonic. It’s a gradual process that is relatively benign sonically, and fairly common. In contrast, an overdriven digital-audio encoder produces a horrible “crunching” sound as soon as the encoder runs out of bits and the tops of the waveforms are flattened. Setting the recording levels for analog and digital recorders thus require very different approaches. It’s interesting to note that there are plug-ins for digital workstations that mimic, with DSP, the warmth and gentle compression of analog-tape saturation, which many artists and engineers find euphonic and musical.

Saturation can’t be overcome, but the non-linearity at very low signal levels can. The solution is “bias,” a very high-frequency AC signal that is mixed with the audio signal and recorded on tape. In practice, bias is a very high-frequency sinewave (typically 100–150kHz) added at ten times the audio signal’s amplitude. In addition to lowering noise and distortion, bias greatly expands the amplitude range over which magnetic tape is linear. It is bias that allows very low signal levels to be recorded on tape, even signals that are beneath the tape’s noise floor. It’s fair to say that magnetic tape recording wouldn’t exist as we know it without bias. Bias is adjusted for the specific tape formulation before recording; you don’t need to worry about it on playback.

Tape machines and magnetic-tape recording are worthy of a book-length treatment. I’ve hit just the essentials in this primer, and hope that they help you understand tape machines and encourage you to take the next step to the ultimate in music playback in the home.