The 500 Series LUNCHBOX: ISSUES AND THE SPECTRA SOLUTION



The use of the 500 series, “Lunch Box” format, has become an accepted solution to providing an array of professional audio options. With acceptance, has also come the understanding that the format has inherent limitations in performance, and is often considered a lower cost alternative to the “real thing”.

In many cases, the issue of the +/- 16 volt bi-polar power supply rail can constrain the performance of an audio circuit in terms of headroom, noise, and in many cases, and the increase of distortion of various types. For years we debated whether the performance of Spectra products would be diminished with the use of the reduced power supply voltage of the 500 format. If we undertook such a project, would we have to make the same excuse of, “almost as good”.


THE ISSUE WITH TRANSIENT PEAK OVERLOAD AND REDUCED HEADROOM

Transient peak headroom has been an issue with the professional audio industry from day one. In the tube era, while transient peak overload existed, the nasty audible effects of such overload did not. Tube circuits by the very design, do not have the transient speed of a typical solid state circuit, and overload, whether it is transient, or RMS type, is far less offensive to the ears. In fact, RMS overload in a tube circuit is often the goal of many. From the earliest introduction of solid state audio devices, the practice of allowing a minimum of 10dB of headroom became a requirement. Under this practice, the solid state audio circuit would hopefully remain stable, (and reproduce whatever audio signal appeared at the input). Unfortunately, audio transient peaks can exceed 20dB, especially with percussion instruments and many guitars. With those applications, another 10dB would have to be reduced from the input signal. As the signal is reduced, the noise level is increased. Additionally, the loss of dynamic range is dramatic. Most folks do not attenuate 20 dB, which in many cases, it is impractical. An audio signal providing peak to average levels of 10-20dB, will have far ranging effects on a conventional audio circuit. The most noticeable is the loss of extended harmonics of a given instrument. During peak transient overload, a conventional amplifier will have a specific recovery time. That amount of recovery time can be dictated by power supply stability, circuit type, (Class A versus Class AB), circuit bandwidth, and so on. Bottom line is that a snare does not sound like a snare, and a bass guitar fundamental is typically never heard accurately as the signal presented at the input of the amplifier, or mic/line preamplifier is lost, or altered.



ENTER THE 16 VOLT BIPOLAR POWER SUPPLY

As stated earlier, the 500 format uses a bipolar 16 volt DC supply. Most, if not all vintage, or non-vintage, discrete audio circuits employ power supply rails that exceed 16 volts, (typically not an issue with integrated circuits). Some are bipolar, some are not. Many circuits use 24 volts DC, and others, much higher DC voltages. The reduced voltage can, and will, affect the headroom, and thus the various characteristics of a conventional amplifier.


500 SERIES CHASSIS POWER SUPPLY CURRENT REQUIREMENTS

Another significant 500 format issue is power supply stability of the rack. There is no industry standard for 500 series module power supply consumption in terms of current. Some devices consume 500mA, others less than 100mA. Then there is in-rush current. The amount of current the device requires during initial turn-on. These numbers can be quite significant, sometimes many times higher than normal operating requirements, in terms of current requirement. Class A and tube circuits are generally the big current users with high in-rush characteristics, and operation requirements.

The important point is that the rack power supply rail remains stable, with no fluctuations in voltage/current provided to each respective module. Excessive power requirements from multiple modules in the rack, can and will, affect the audio performance of the rack as a system. There are more advanced 500 series racks that have addressed the supply issues with increased power supply current, isolated channel regulation, and so on. These units provide exceptional performance, but are considerably more money.


THE STX SERIES SOLUTION

For years, there was much discussion within our company, with regard to introducing a line of 500 series products that would provide the same sonic performance as our vintage consoles, 610 Complimiter models, as well as stand alone products like the M502. The statement, “almost as good as the original”, or “close” was unacceptable. The product performance had to exhibit the same high standards, irrespective of the power supply limitations of the 500 series chassis. The STX series of products meet the standards that are defined by our legacy designs, yet provides the user an affordable alternative that will provide years of reliable operation.


THE STX 100 MK2 AND STX 100D MIC/LINE PREAMPLIFIERS

The STX 100 MK2 provides stable, low distortion output levels up to +18dBm, irrespective of audio transient peaks. As a result, both models will provide full harmonic content, low distortion and noise, and best of all, reproduce the musical instrument or vocal content as it appears at the source. There is no guessing during setup. Performance is consistent and accurate.

THE STX 500 MULTI FREQUENCY GRAPHIC EQUALIZER

Originally introduced in 1967 as the Model 500 Microphone/Program Equalizer, was made famous by studios such as Stax, Ardent, Advision, A&M, and numerous others. The 500 series circuit was, and continues to be, unequaled in terms of phase accuracy, boost/cut range, and extremely low distortion. The unit provides +/-12dB of boost/cut, at eight selected frequencies, (four high and four low), via a two knob location design. The build quality is readily apparent with the use of Grayhill switches as per the original 500 design, as well as the use of American made high quality precision inductors not typically found in many 500 series products.

The STX 600 COMPLIMITER MICROPHONE PREAMPLIFIER

The STX 600 represents a unique solution to both ultra-fast limiting/compression and enough gain to operate as a preamplifier for microphone or line level applications. The input side of the STX 600 includes the same high quality transformer and famous 601 Complimiter module, (circa 1969). The input side of the STX 600 is identical to the input section of a C610 and V610. The two knob operation allows for quick setup as a peak limiter, peak limiter with mild compression, as well a mic/line operation level adjustment.




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