Scott 99-A integrated mono amplifier
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Taken from 'Radio & Television News', February 1954

THE new Scott 99-A transcription amplifier appears, at first glance, to be a "front end" or equalizer-preamp, yet actually it is a complete 10-watt amplifier, power supply, and equalizer-preamp—all packaged in a single cabinet measuring 13¼" x 3¾” x 9¾”".

Since heat dissipation as well as the size was important, engineering studies made at the time this amplifier was being designed showed that a 10-watt power output would be most feasible. Higher power outputs would add considerably and undesirably to the size and would also dissipate too much heat to be radiated comfortably from a sheet metal cover. The 10-watt output proved satisfactory for reasons to be discussed shortly.

Beam power tetrode output stages were chosen for several reasons. The greater efficiency of the beam power tetrode meant less power lost in heat and more developed in sound output. This efficiency also allowed use of fewer driving stages which further decreased over-all heat and bulk. Considering all factors, use of beam power tetrodes meant a reduction in generated heat by almost two-to-one. The final system does not overheat; in fact, the metal cover becomes only moderately warm to the touch. The heat generated is conducted through, and radiated from, the plane surfaces of the all-aluminium chassis. Scientifically designed slots in the case, located directly above the output tubes, aid heat dissipation by the convection of heated air rising through these slots.

The reliable 6V6 output tubes were chosen because they produced the 10-watt output with less heat and less distortion than any other tubes tested. With carefully engineered inverse feedback, it is possible to achieve extremely low levels of distortion. At 10 watts, harmonic distortion is less than 0.8%. At full rated output the first order difference tone intermodulation distortion is less than 0.3%. This latter type of distortion is closely allied to that harsh, discordant growling most noticeable and offensive to the human ear. This distortion component measurement then represents the best criterion for determining over-all quality of a power output system.

Also engineered into the output system were the self-balancing phase inverter circuits which have been standard on H. H. Scott amplifiers for six years. These circuits automatically balance the output tubes, reducing distortion and making unnecessary any readjustments after replacement of output tubes in use.

Another factor of output systems receiving careful attention was the often overlooked requirement of clean, symmetrical clipping. On sudden peaks of music and sound, any amplifier will clip to some extent. Some amplifiers may indicate extremely low distortion levels up to their rated undistorted output, and yet above these ratings the clipping may not be clean and symmetrical. Ragged clipping causes extraneous damped transient effects, which sound very harsh and discordant, to be introduced. This was avoided in the 99 by careful engineering to insure that any clipping would be both clean and symmetrical. As long as this requirement is met, the human ear is unable to detect substantial amounts of clipping, and the over-all result is an output system which, though rated at 10 watts, gives performance audibly equal to much higher formal power ratings.

That a 10-watt power amplifier with associated power supply is incorporated into so compact an enclosure while still including sensitive low-level preamplifying and equalizing circuits seemed to make the hum problem important. Since D.C. heater operation was not feasible because of space considerations, an all-aluminium chassis was used to minimize circulating and ground currents which, in a steel chassis, would contribute to hum pickup. Positive bias on the heaters, together with an adjustable centre-tapped, hum-bucking potentiometer, reduced hum still further, as did careful engineering of parts placement (Fig. 2) and shielding of all low level circuits. This attention to detail brought over-all hum to the unusually low level of better than 80 db below full output, a level comparable to that generally achieved only by D.C. on the heaters. See Fig. 4 for the schematic of the amplifier.

The over-all frequency response is flat from 20 cps to 30,000 cps. Above 30,000 cps the response is rolled off in order to prevent possible ultrasonic oscillation. Such oscillation is often caused by unavoidable stray coupling between external input and output connections when amplifiers have unnecessarily extended frequency response. While such oscillation is, in itself, inaudible to the ear, it overloads the amplifier, causing noise and distortion in the audible range. Another feature, usually found only in higher priced amplifiers, is the sharp-cut-off rumble filter which prevents sub-audible overload below 20 cps. Unless the low-frequency response is limited sharply below the lowest audible notes, sub-audible signals may be generated by eccentricity of the record centre hole and by turntable rumble. These sub-audible signals often are sufficiently large to cause amplifiers to operate in almost continuous states of overload. Acoustical feedback from the loudspeaker to the pickup cartridge is also decreased considerably by eliminating sub-audible, but high power, low-frequency signals.

For record equalization, separate three-position bass turnover and treble roll-off controls are provided. These separate controls are patterned after those first announced in the H. H. Scott 212-A amplifier which was catalogued in 1950 but only few made because of Korea and parts shortages. While two controls must be adjusted for equalization, this type of control is inherently a simpler and less costly means of equalization than the somewhat simpler-to-operate single control giving the complete equalization curve.

In order to give maximum equalization flexibility, a study was made of those recording curves in widest use. A very widely used equalization curve for LP records is the original one introduced by Columbia. This curve uses more bass and treble pre-emphasis in recording than most and therefore requires less bass and treble compensation in playback. This curve was included in the 99 equalization.

On any professional instrument, such as the 99, it is necessary to make provision for older and foreign records, so the curve marked "EUR" in Fig. 3 was added to the equalization. This curve has a turnover of approximately 250 cycles and a flat high-frequency response.

The AES and new NARTB curves are also widely used. These curves are very similar except in extremely low frequency response. Since each curve is specified to an accuracy of ±0 2 db, each curve falls within the tolerance limits of the other. It is therefore possible to draw a single curve falling within these tolerances over practically the entire range of equalization. This combined curve was provided in the equalization, and it is entirely adequate for playing recordings made for either the AES or NARTB curve. (The RCA "New Orthophonic" curve is the same as the new NARTB). Since both turnover and roll-off equalizer controls have three positions, a total of nine record compensation curves are possible. For example, a combination of the Columbia turnover with the AES roll-off provides an almost exact match for London records. Similarly, the "EUR" low-frequency equalization used with the LP high-frequency response provides a perfect match for old 78 rpm Columbia shellac records. Table 1 gives a chart of equalization settings for many records. Further compensation for conditions such as pickup response, speaker response, speaker enclosure characteristics, room acoustics, and personal preferences may be obtained easily with the wide-range treble and bass tone controls.

These tone controls are continuously variable throughout their range and each provides either wide-range boost or attenuation of the response. The maximum treble boost or cut is ± 20 db, while the maximum bass range of the control is ± 15 db. These specifications are in addition to the progressive boost provided by the automatic loudness control.

Complete and continuous adjustability of the loudness compensation itself is an interesting feature. The automatic loudness control, of course, compensates for the human ear's insensitivity to extremes of treble and bass at low listening levels. This is done by progressively boosting the treble and bass response as the volume control is turned down. To maintain satisfactory tonal balance, treble loudness compensation is necessary as well as compensation for bass tones, if a "boomy" effect is to be avoided at low listening levels.

It should be pointed out that the loudness compensation provided in the 99 is based on extended listening tests by juries of listeners rather than upon blind application of the Fletcher-Munson curves. Using a variety of program material, the listeners were allowed to adjust calibrated variable condensers until the music sounded best to each listener. These adjustments were made at many different volume levels. The tests, over a period of months, were statistically correlated so that subsequent analysis provided loudness compensation curves with contours and degrees of compensation most pleasing to the human ear. This was particularly important because the attempts, in some loudness controls, to provide extreme Fletcher-Munson compensation had almost caused loudness compensation to fall into disrepute through the invalid application of the Fletcher-Munson effect.

For broadcast and recording applications, of course, little or no loudness compensation may be required or desired. The loudness compensation may be continuously adjusted or entirely removed by proper settings of an input level control in conjunction with the loudness control. This input level control also permits compensation for the different outputs of various commercial pickups. For example, to remove loudness compensation entirely, the loudness control may be turned to a high value thereby removing compensation from a circuit. Volume is adjusted by the level control which functions as a simple volume control.

An input selector switch is also provided. This allows ready selection of "Tuner," "Phono," "TV," "Tape," or other channels. Separate input jacks are provided on each input circuit. In addition to the usual constant-velocity "Phono" input for magnetic pickup cartridges, a constant-amplitude input is provided for FM pickups. The constant-amplitude input is obtained with a differentiating network which matches the constant-amplitude pick-up to the normal constant-velocity response of the preamplifier circuits.

On the underside of the chassis, input jacks are provided for connection of an accessory "Dynaural" noise suppressor, such as the Type 111-B. The "Dynaural" noise suppressor virtually eliminates high-frequency record scratch and low frequency turntable rumble, without eliminating music components which are audible to the human ear. The patented "Dynaural" noise suppressor should not be confused with fixed filters, since these latter eliminate audible musical frequencies under all conditions where undesirable noise and rumble are eliminated. The dynamic rumble suppression feature is particularly important with wide-range systems which include extended bass response speakers. All but the very best professional turntables are susceptible to some rumble, and sometimes rumble is actually recorded on records. Under such conditions, dynamic rumble suppression can increase listening pleasure without loss of any low bass musical tones. The inputs for the noise suppressor may also be used for dubbing records on tape while continuing to monitor through the normal sound system.

Speaker output terminals of 4, 8, 16, and 500 ohms are provided, allowing use of virtually any speaker or professional line-impedance conditions. A separate A.C. power outlet is provided on the rear of the chassis for powering accessory components such as turntable, tuner, etc. The power supply operates on 105 to 125 volts, 50-60 cycles. A "Slo-Blo" fuse provides circuit protection but prevents fuse burnout as a result of normal A.C. line transients.

It is believed that this new amplifier will be a significant contribution to professional music reproduction since the 99 exceeds all FCC requirements for FM broadcasting station performance. While its circuits are similar to those of previous H. H. Scott amplifiers, the over-all electronic and mechanical design is completely new and of rather remarkable simplicity. In design, construction, appearance, and performance, the 99 meets, in every respect, the amplifier standards established by its manufacturer.