MANUFACTURER'S SPECIFICATIONS:
RECORD SECTION:
Input Impedances: Microphone, 1000 ohms, unbal.; line, 50,000 ohms, unbal.
Input Sensitivities: (after Dolby Level calibration) Mic, 0.5 mV for 0 VU; Line, 60 mV for 0 VU. MX Filter: 19and 38 kHz notch filters. Response with filter down less than 1 dB at 15 kHz. Outputs: (to tape recorder input) adjustable; maximum of 0.5 V at 1000 ohms output impedance.
PLAYBACK SECTION:
Input impedances (from tape recorder output): 50,000 ohms.
Input Sensitivity: Adjustable; minimum of 150 mV required for Dolby Level meter indication.
Bias Filter: 28-kHz low-pass to eliminate bias noise from output of recorder.
Outputs (after Dolby Level calibration): Maximum 1.0 V.
GENERAL:
Front panel: 12 7/8 x 4 7/8 in.
Overall Height with feet: 5 in. Chassis size: 11 3/8 X 4 1/8 X 7 1/2. Overall Depth with knobs: 8 3/4 in.
Weight: 10 lbs.
Price: $250.00 (slightly higher in some parts of the U.S.).
Audio buffs have been reading about the Dolby System for about two years, mainly relative to the professional A301 units which have gained such wide acceptance throughout recording studios all over the world. Now they can have a Dolby System for use in their own homes at a price well below the professional system, and yet one which will permit making recordings at 3 3/4 ips with practically the same quality they previously had at 7 1/2 ips, and without the increase in noise which previously had been the hallmark of the slower speeds.
While the "A" system separates the sound spectrum into four individual bands and processes them separately, the "B" system--as exemplified by the Advent Model 100--is much simpler, and processes the entire spectrum. It does this by increasing the high frequency content of the signal progressively as the level is lowered in the recording operation, and by providing a complementary decrease in high-frequency response in the playback operation. When properly adjusted, the complementary equalizations cancel out and result in a played-back signal which is identical with the original in frequency response, but with a reduction of tape hiss of the order of 10 dB. In addition, the Advent Model 100 provides input mixing facilities so a microphone signal may be added to a high-level signal from tuner, record player, or another tape recorder in any desired degree. Another advantage-and one which is particularly important to the cassette recordist-is that the continual changing of cables and plugs is no longer necessary because the record signal can be plugged into the recorder and the output from the recorder playback jacks can be plugged into the Advent and left permanently. The recordist can thus switch from record to playback without changing cables, and yet still have the facility for monitoring from source while recording on a machine which does not have a separate play head, and then after rewinding he can monitor from the tape by simply throwing a switch on the Advent.
Description
The Advent 100 has a multiplicity of controls on its front panel-nine controls, eight switches, two meters, three indicator lights, and a headphone jack, as seen above. Those black rectangles are switches-not convenience outlets.
All controls are clearly labeled. The rear panel accommodates a pair of phone jacks for microphone inputs, four pairs of phono jacks-high-level input, output to tape recorder, input from tape recorder, and output to amplifier. In addition, there are three holes for access to internal calibration adjustments, a meter test switch, a level-test point, two record calibration pots, and an unswitched convenience outlet.
Internally, the various sections are assembled on six circuit boards which plug onto connector pins in the chassis to facilitate servicing whenever necessary, with the power supply-regulated to supply 15 volts to the, circuits occupying the right end of the chassis.
The three indicator lights tell you when power is on, and when either channel is switched to "Dolby" operation. The layout is shown in Fig. 1. To get at all the controls and switches, both front and rear panels swing out, as shown in Fig. 2, making every component instantly accessible.
Fig. 1--Showing top view of the Advent Model 100 chassis.
Fig. 2--Showing Advent chassis with back and front swung out.
Fig. 3--Showing backside of Advent Model 100.
The Circuit
The microphone preamplifiers employ a single Motorola MC1303L to boost the level up to the equivalent of the high-level inputs, and both have separate controls to provide for mixing.
This IC is followed by a pair of bipolar transistors in each channel for additional gain, and these are followed by the multiplex filters, each having resonant circuits for 19- and 38-kHz suppression, both individually adjustable. All of this circuitry is on the first circuit board on the left as seen in Fig. 1.
The two record amplifier/processors occupy the next two circuit-board positions. Each of these contains six bipolar transistors, six diodes, and one FET. The circuit itself is reminiscent of early noise-reduction devices, except for the fact that the record amplifier does the reverse of what a noise-reduction circuit does. When the signal level is low, it boosts the high frequencies. The two playback amplifier/ processors are next, and these react much the same as the usual noise-reduction devices do-that is, they cut the high frequencies when the signal levels are low. Their complement of transistors and diodes is similar to the record boards with the addition of one more transistor.
The trick is, of course, to have the two processes track accurately, so the resultant is a flat signal. The final circuit board contains the output amplifiers and the meter amplifiers, each with two transistors per channel. In addition, this board has a calibration oscillator which provides a 400-Hz tone to adjust the various controls to the proper positions to ensure the complementary operation of the processing amplifiers, each of which has its own factory-set control for the "law" that the circuit will follow. Power supply regulation is accomplished by a single transistor with a Zener reference diode controlling the output voltage to the amplifiers.
Performance Measurements
Fig. 4--Showing recording curves.
Fig. 5--Showing playback curves.
How does one measure the performance of a device which has so many variables? We felt that to describe the Advent 100 properly, it would be necessary to provide some curves-not only as proof of how well it works, but to show how it works. Therefore, we first ran a series of curves, using a graphic recorder and the output from the CBS STR-100 record fed at the normal recording level, and at 10, 20, 30, and 40 dB below normal recording level. Then we made a second set of runs through the playback section with first the normal level signal, and then with signals 10, 20, 30, and 40 dB below the normal level. Then, of course, we repeated this performance through the second channel. The results are shown in Figs. 4 and 5. Note that at normal signal levels, the responses are essentially flat. If you plot the record curve for the -20 condition, and then plot the playback curve for the -20 condition upside down over the first, you will find that they are quite similar-in fact, almost identical. The fact that they are not identical is due to lack of calibration before starting the process, but the idea is to show how the response curve varies with signal level in each section, and how they are essentially complementary, as they should be. In actual use, after calibration, we again ran the sweep signal through the unit onto a tape, and then recorded the output from the recorded signal as it played back from the tape, and we found a line straight within ±2dB from 40 to 15,000 Hz. That is the proof of how well the system works.
Next, we measured distortion at a number of levels, and we found that THD was around 0.16% at normal signal levels, increasing to 0.28% at 30 dB below normal in the record amplifier. This is to be expected, upon inspection of the recording curve, in which the high frequencies-which contain the harmonics we were measuring-are increased. The reverse of this condition was observed in the playback amplifier, with normal-level signals showing a THD of 0.25%, and low-level signals indicating a reduction of THD to 0.12% all with a test frequency of 1000 Hz.
The measured signal-to-noise ratio on the playback amplifier was 70 dB re 1.0 volts output, and on the record amplifier it was 65 dB re 0.5 volts output these references being the maximum rated outputs for the two sections. No change in S/N was noted when the in/out switches were thrown, indicating that the processing circuitry made no difference in the noise output.
Operation
Setting up the Advent 100 is a fairly complicated process, but the accompanying instructions are clear and concise. In fact, instructions for setting the record level are provided for those machines which have separate record and play heads, and for those without separate heads and with or without separate record and playback level controls. This accommodates practically any machine. "Dolby Level" tapes are furnished with the unit-one on a cassette and one on a reel-so that either type of machine can be used with the Advent. To calibrate the unit, one puts the Dolby Level tape on the recorder and plays it, at the same time adjusting the PLAY CAL controls so the meters indicate the "Dolby Level." Then, without touching the PLAY CAL knobs on the Advent, one actuates the REC CAL TONE switch and adjusts the recording level controls on the tape recorder to the "0" level, then plays back the newly recorded tape to determine if the playback level reaches the "Dolby Level" mark on the meters. If not, one repeats the process, setting the record level control higher or lower as required, and plays it back again. This process is repeated until the playback levels coincide. After that, one sets the record level only with the controls on the Advent, and similarly sets the playback levels only with the controls on the Advent.
We would be inclined to suggest that the PLAY CAL controls be changed to screwdriver-adjust types to avoid the possibility of inadvertent changing after once being set for the correct playback "Dolby Level." If they are changed, the calibration procedure must be repeated.
Performance
Once the unit is correctly connected and calibrated, the only controls on the recorder that need be touched are those that serve to start and stop the tape and that change the operation from record to playback. This simplifies the recording operation considerably, as long as the recordist remembers this point. We first tried the Advent 100 with a Harman-Kardon CAD-4, a good cassette performer. After calibration, we recorded off the air from a good classical music station in stereo FM. On playback, the response was directly comparable to the original except for the tape hiss-it just wasn't there. Then we set the tuner to mono and recorded again with the "A" channel "Dolbyized" and the "B" channel normal. This would give a direct A-B comparison of the same material, instantly switchable from normal to noise-reduced operation.
The difference was astounding. In the "B" channel, we heard the usual hiss that we all expect from cassettes, while on the Dolbyized "A" channel, the hiss was practically inaudible. Advent claims a 10-dB reduction in noise, and that is about what we heard.
Next we hooked the Advent up to a ReVox A-77 and recalibrated, using the Dolby-Level tape on the 5-inch reel furnished. At 3 3/4 ips there was about the same reduction in hiss, but the response and freedom from noise were just about what we have learned to expect from the ReVox at 7 1/2. But when we tried the ReVox at 7 1/2, wow! That was a revelation. We have often maintained that every once in a while, every audio buff should find a place where he could hear a 15-ips tape, just to keep his reference as to just how good a fine original tape can sound. But the ReVox at 7 1/2 ips with the Advent 100 should give any 15-ips machine a good run for its money. More than that, what can anybody say?
-C.G. McP.
(Audio magazine, Jan 1971)
Also see:
SAE Model 5000 Impulse Noise Reduction System (Equip. Profile, June 1977)
SAE Mk IIICM Basic Amplifier (Jan. 1975)
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