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The Dynaco AF-6 is the AM/ FM version of the very popular FM-5 tuner which we reviewed back in October, 1972. It has the same simple styling with gold-finished, diecast panel-in fact, from a distance it looks very much like the FM-5! The tuning scales are different, but if you were looking for an extra AM switch you wouldn't find it. Although the same kind of rocker switches are used, they perform different functions.
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FM: IHF Sensitivity: 1.75 0V.
Input required for 50 dB S/N: 5.0 µV.
Ultimate S/N: 65 dB.
AM Suppression: 58 dB.
Stereo Separation: 50 Hz, 10 kHz 30 dB, 1 kHz 40 dB.
Capture Ratio: 1.5 dB.
Frequency Response: 20 Hz to 15 kHz ±1 dB. THD: 0.5%; stereo, 0.9% or less.
Output @100% Modulation: 2 volts max.
AM: Sensitivity: 5012V with ext. input.
THD: Less than 2%.
Selectivity: 20 dB @10 kHz, 55 dB @20 kHz.
Dimensions: 13 1/2 in. by 12 in. by 4 1/4 in. H.
Price: $240.00, kit; $350.00, assembled.
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Thus, the one at the center is the AM/FM switch but the left-hand switch now has a dual function. When the tuner is on AM it controls the audio response in 3 steps-narrow, medium and wide. On FM, the first position is mono, the second stereo and the third brings in a filter. This not only rolls off the high frequencies (4 dB at 10 kHz) but also introduces a blend which has the effect of reducing noise on weak transmissions. The switch on the right controls the muting and Dynatune circuits--about which more later.
At the rear are two pairs of output sockets-one to feed the amplifier via the volume control (the knob on the extreme left)
and the other pair are intended for use with a tape recorder as they provide a variable output. Also on the back panel is a fuse, and an a.c. outlet socket, screw terminals for AM and FM antennas and the AM loopstick itself. This is mounted on a swivel bracket so it can be adjusted-space permitting.
How It Went Together
Basically, the AF-6 consists of three pre-wired circuit boards, a complete front-end and an assortment of small components for the front and rear panels plus a power transformer. Thus, all the hard work has been done and it is really quite easy to put the pieces together. You do need a little patience and some skill with a soldering iron, but that's all. I would estimate the assembly time to be in the region of five hours although it could be done in less if you can avoid interruptions! Although the front-end and i.f. strips are factory aligned, most engineers will realize that small variations in stray capacities and so on will affect alignment so our kit was checked as soon as it was finished and then measured again after realignment with a signal generator. As you will see from the figures, the differences were insignificant and the specifications were met or exceeded-all without laying a screwdriver on the trimmers! Circuit Description
The FM circuit is very similar to that of the FM-5; the r.f. amplifier is a dual-gate MOSFET with another MOSFET as a mixer and the oscillator is a bi-polar type. The i.f. section uses two 4-pole ceramic bandpass filters with four IC amplifiers.
Detector is a ratio type and it is coupled to an emitter-follower to a phase-correction network, and a 67-kHz notch SCA filter, and an FET which controls the muting circuit. The IC multiplex circuit is a cross-coupled multiplier demodulator which provides additional 67 kHz rejection. A low-pass filter with dual 19-kHz and 38-kHz filters is followed by the de-emphasis network, the volume control, and an audio amplifier stage. Output impedance is 1000 ohms, so long connecting cables can be used without undue losses. The FM meter is connected after the first ceramic filter and limiter stage and also after the third IC amplifier limiter, so ample deflection is obtained from weak signals.
The FM muting circuit is controlled by a combined logic circuit which is fed by the detector output. It senses the d.c. shift and switches off the audio signal when the variation from center exceeds 80 kHz. It is also activated by a second signal from a 150 kHz high-pass filter. Any interstation noise is amplified and will operate the muting circuit.
Now to the Dynatune circuit. This is really a kind of amplified automatic frequency control and here is how it works: The detector's d.c. output is fed back to the front-end through an amplifier limiter. This signal controls the oscillator frequency, zero d.c. being the optimum tuning point. A tuning position one side of the station would give a positive d.c. voltage and turning the tuning capacitor the other side would produce a negative d.c. voltage. The voltage feedback circuit is a servo loop and obviously it must be switched off when tuning, otherwise it would tend to lock, or hang on to one signal all the time. Thus, when the d.c. level at the detector reaches a certain level by the action of moving the tuning knob, the muting circuit switches off the servo loop. When the logic circuit senses a lack of inter station noise, the servo loop and audio output is switched on again. Dynaco suggests that the MUTE position be used for tuning and then switching on the DYNATUNE when the station is tuned in. When the TUNE light is illuminated, you can switch over and the DYNATUNE will pull the station in to its correct tuning position. The TUNE indicator light is activated from the output of the logic circuit-which also operates the STEREO light with the addition of the 19-kHz carrier signal.
The ferrite loopstick is tuned by the first of the three AM tuning capacitor sections, the second and third tuning the r.f. and oscillator stages. The i.f. bandpass characteristics are defined by a 12-section LC filter after the first i.f. amplifier.
Then follow two more stages of amplification, the diode detector, and a transistor low-pass filter network which includes a 10-kHz notch to reduce adjacent channel interference. Signals for the AGC amplifier and meter stages are provided by a dual-diode detector. Power supply consists of a full-wave rectifier with transistor and zener regulation. Two more transistors are used in a delay circuit to avoid switching thumps.
Figure 2 shows the FM performance characteristics. IHF sensitivity was 2.2 µV before alignment-excellent by any standards. Using a signal generator, this figure was reduced to 1.75 MV-very near the theoretical minimum. At 5 µV input, quieting was over 55 dB and this increased to over 70 dB above 30 µV. Harmonic distortion was 0.3% on mono and 0.5% for stereo. At 10 kHz these figures were 0.8% and 1.2% respectively.
Stereo mute threshold was exactly 4µV, so weak transmissions would still be heard. Stereo separation, shown in Fig. 3 was around 40 dB at mid-band and 30 dB at 8 kHz and 40 Hz. Before alignment, these figures were only 2 dB less, with a maximum of 3.4 dB less at 15 kHz! Capture ratio was 1.2 dB. On the AM side, sensitivity came out at 41 microvolts (direct input) with THD below 2% at any input up to 100,000 pV. Bandwidth was down 6 dB at 6 kHz in the wide position, 12 dB in the medium position, and 17 dB in the narrow position. The notch filter attenuated the response at 10 kHz by 29 dB. I.f. rejection was slightly better than the specifications at 64 dB.
After assembly, the tuner was connected to a Sony 2000F preamp and Phase Linear amplifier with pair of AR-LST speakers. FM antenna was a simple three-element array (rotary) some 30 feet from the ground; location, about 35 miles north of Boston. More than 35 stations were received at good signal strength-and that muting circuit worked like a charm.
There was no distortion or "side skirt" noises, the station was either there right on the button or not heard at all. On the AM side, all the Boston stations were received with ease using the loopstick but stations further afield--in New York, for instance -came in better with an outside antenna. Quality was variable as few AM stations put out a truly clean signal in this area. In the wide-band position, overall sound was sometimes surprisingly good-not comparable with FM, but eminently listenable for all that.
All-in-all, the AF-6 is a top quality tuner capable of giving results as good or better than tuners costing a great deal more probably twice as much. It can be bought assembled at $350.00, but kit builders will save more money. And of course, the FM-5 is available for somewhat less, if you don't want the AM facility.
(Source: Audio magazine, Sept. 1974; George W. Tillett)
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