Technology & Devices from the last Century...

The playback of the Naxatras III tapes has revealed a series of complications which, I am almost certain, are faced by all those who already possess or intend to acquire an R2R machine. Firstly, the overwhelming majority of machines available today are used (or heavily used, depending on their history), secondly the available data and tools (from user manuals and service/calibration manuals up to standard tapes and measurements standards) is very limited and, thirdly, the available know-how is decreasing at the moment, as people who have been trained formally to service this type of device are scarce. In practice, if you are not Jesus Agnew or someone with a similar level of engagement and experience in this area, it is difficult even to clarify whether the tape recorder is properly configured and calibrated for the intended purpose. To do this, you need access to standard tapes and at the moment there is only one manufacturer supplying them (with… rumors for a second one to appear in the near future) and, also, you need access to service manuals and methods and tools specific to the machine in hand (provided you know exactly which model it is, for what use and for what market). I leave out of the discussion the need for spare parts needed for standard maintenance, usually rubber and metal components which they will need to be replaced if the device is in use (and some of them, even if not in use). In any case, it is clear that this market segment needs to develop intensively to remedy these deficiencies if we are serious about boosting R2R machines back into the role of an analog reference source.

About the Tandberg TD20A SE

Having said that, the tape machine used to listen to Naxatras III tapes was relatively "new". It was a Tandberg TD20A SE from 1993, which was reviewed at that time for the Greek edition of Audio magazine, was bought new and had only minimum hours of operation. Those who had followed Tandberg (a highly respectable manufacturer from Norway) will recognize that the SE suffix was for machines that featured a custom Tandberg equalization curve, in addition to the standard IEC/CCIR curves. This was based on two circuits, the Dyneq, and the Actilinear, and it dramatically improved the signal to noise ratio, reaching up to 80dB(A) from the 69dB(A) of the regular TD20A. Obviously, the special equalization had to be used for both recording and playback, so it did not play a role in the project at hand. Also, observant readers should already have noticed, on the photos, the existence of "The Final Edition" inscription on the faceplate. This particular 1993 machine is one of the 60 manufactured in total by Tandberg at the time, on the occasion of celebrating their 60th anniversary (actually, it is the No.46). These were the last R2Rs made by the company.
The TD20A SE is a 1/4-inch tape machine, with half-track configuration (meaning it can record/playback two stereo tracks in only one direction on the tape), which can operate at 7.5 and 15ips with IEC/ CCIR equalization (as far as I know, there is, also, a NAB version). It is a single capstan-four motor design, using two motors to directly drive the supply and the take-up reels, one to drive the capstan/flywheel assembly, using a belt-based transmission and one for the transport control, through a digital control circuit. Historically, the machine was considered very good and was one of the audiophiles' favorites in the decades preceding the 2000s. The specifications given by the company are particularly satisfying, with a rotational accuracy of ±0.5%, W & F at 0.03%, a 20Hz-30kHz (±2dB) frequency response and a noise level between -69dB(A) and -80dB (A) depending on the equalization curve used (IEC or Dyneq/Actilinear). Distortion figures were quoted as 0.5% (with Tandberg's equalization and 320nWbm magnetic flux as a level reference) and 2% using the standard IEC equalization at 0dB recording level as a reference.

Some Measurements

Laboratory evaluation of a tape recorder today is not a simple matter. Methodology and specifications belong to past decades (past century, to be precise) and today's instruments offer possibilities and precision that were not available at the time. Briefly, one can evaluate a tape machine in three different ways, which are not, however, the subject of personal taste but should be applied under certain conditions and offer different information: First, in Playback mode, with a standard calibration tape, which has been recorded under controlled conditions by a tape recorder of established quality and confirmed correct (and sometimes traceable through a certification process) settings, a method which gives information on the playback quality of the DUT (and allowing to calibrate the respective mechanical and electronic components). Secondly, in Recording/Playback mode (assuming a 3-head machine), to evaluate the recording section (record/erase head geometry, bias oscillator, filters and level, equalization accuracy, etc). Such measurements are made only after the tape machine is calibrated in Playback mode and then, given a thoroughly calibrated recording section the playback section can be reassessed. This may seem like a vicious circle, but it makes some sense, as, in the absence of more sophisticated standard/calibration tapes, it is the only way to apply current measurement techniques based on sweeps and nested sweeps, for example. Thirdly, the "Source Monitor" mode, where all the electronic circuits and mechanical parts related to the machine/tape interface (bias circuits, equalization, tape path, and transport mechanics and the magnetic heads) are bypassed, usually evaluates the performance of the input and output stages. In fact, what is evaluated in this mode is the Monitor loop in a Tape/Monitor setup and is a way of confirming the neutrality of these circuits (and the relevant signal attenuators).
In this case we have implemented various versions of all these ways in the TD20A SE in an effort to explore both its capabilities and its condition (although it has a few dozen hours of operation only, but -in any case- it is a 26-year-old machine.) as well as the evaluation options that are available today.
Before the measurements and, in general, before we used the tape recorder, the only adjustment made was the azimuth of the playback head, using the Lissajous pattern method at the three frequencies available on the calibration tape.
The first parameter we evaluated was the frequency response. To measure in Playback mode, the calibration tape (a broadcast studio-recorded tape of unknown condition -to be honest) was used. This was a basic setup tool and it is a full track recording (i.e, a signal has been recorded throughout its useful width and not in two separate tracks -which is a requirement for measurements and calibration procedures that are common to all channels, such as azimuth). The tape complied with the standard requirements for measurements and signals for tape machines evaluation (often referenced in the relevant literature), including three frequencies (100Hz, 1kHz, and 10kHz) as well as an elementary sweep (20Hz-20kHz) at a level of -5dB.
The frequency response with this method indicates that the TD20A SE is close enough to Tandberg's specifications but has a somewhat higher low cutoff frequency and some noteworthy differences between the channels in the mid-high and top-high frequency range.
In Recording/Playback mode, the general picture does not change but it seems that there is a higher loss in the high-frequency range (keep in mind that the graph reaches up to 80kHz), indicating that some calibration/maintenance is required to the electronics and the mechanical parts associated with the recording.

Measuring the response in Monitor mode confirms that all the problems depicted in the two former measurements are not related to amplifiers and attenuators of the tape recorder but, moreover, to the tape-machine interface (which was more or less expected). Here the response comfortably goes up to at 60kHz for a -2dB attenuation, with the channel balance being better than 0.5dB.
Distortion measurements at Playback mode were performed using all three available frequencies of the standard tape. The machine performs better in the middle and upper bands (using the 1kHz and 10kHz signals) and appears to have low noise from the power supply (with the 50Hz component being below -70dBr, in reference to 0dBVU recording level.

A familiar intermodulation cluster appears, normally seen in most analog mechanical systems, and is due to the momentary changes in the rotation rate. However the relative components are at a low level.
The findings are most apparent in the 100Hz measurement (probably because the graph resolution helps). Here a modulation by a 10Hz component can be observed, which corresponds to a rotating part with a diameter of about 12mm. The only such TD20A component is a scrape idler that probably requires some maintenance or adjustment (or both…). Distortion values were rather reasonable. At 1kHz, the third harmonic (which is considered to be the most critical based on most texts about tape machine measurements) was close to -55dBr, using the 0dBU reading as a reference, which corresponds to about 0.18 %.

Distortion in Recording/Playback mode was much higher (0.56% at 1kHz, for the third harmonic), while, as expected, in Monitor mode, the whole spectrum of harmonics was below -80dBr, a value corresponding to 0.007%, with the noise from the power supply being unexpectedly low for an analog device, at -110dBr for the basic hum noise components (50/100 Hz) and their by-products.

Finally, we also attempted a THD+N versus frequency, versus level measurement. This is a nested sweep measurement that requires signals that are not included in a standard tape, so we could perform it only in Recording/Playback and Monitor modes. During the former, the performance was rather mediocre, with the relative curves being between 1% and 2% with clear upward trends in both high and low frequencies. However, it should be reminded that the recording section of the machine plays a crucial part here and we know that it is not at its best.
In Monitor mode, the performance returned to reasonable levels for contemporary high fidelity. We measured a 0.06% at the lowest recording level (-12dBr using the 0dBU reading as a reference) and a 0.04% for 0dBr.

In conclusion, the TD20A appeared to be reasonably functioning, despite its relatively old age and long periods of inactivity, and could provide relatively good (and rather enjoyable as it turned out) playback quality. It also seems that is feasible to restore its performance to the original specifications with some careful maintenance. Nevertheless, it is, also, obvious that the machine does not constitute, in its present state, a reference playback system to be used reliably in a recording quality review.

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