Proton at 670 brochure

Page 2

Reinhold Weiss visual
design, Larry Schotzs

r.f. circuitry, and Protons
production skills make
the AT-670 a winner.

AMPLITWE (OBI VERSUS FREQUENEV llil). FM TM iCIIGN; PRMDM "-670
1.0000 AP

2.00M
DJ

*2 t DDD

4.000 _
26 100 1k 10k 20k

Fig. 1-Frequency
response, FM section, of
left channel (solid curve)
and right channel (dashed
curve).

pleasing to look at but whose looks dont get in the way of
proper operation. As for the AT-670's circuit design. I need
reveal only that Protons special consultant for some years
now has been Larry Schotz. who has no rival when it comes
to r.f. design.

This combination of visual and electronic design talents.
aided and abetted by Proton's production facilities. has
produced a tuner that any FM radio lover and connoisseur
of elegantly styled audio components would be proud to
own. The only negative comment I have concerning this
tuner's performance has to do with its AM section. Frankly, I
cant believe that Schotz had anything to do with the design
of the AM circuitry; it must have been left over from a much
earlier model! In any event. most of us dont buy tuners of
this category to listen to AM. For that purpose. a table radio
usually does just fine.

The AT-670 lets you program 18 FM station frequencies
and nine AM frequencies. for a total of 27 presets. The FM
tuner section is equipped with the refined Schotz Noise
Reduction (SNR) system. which can be turned on and off as
required. Generally. it can be left on since. for strong sig-
nals. it does not impair reception quality in any way. The
tuner also has a Normal" (wide) i.f, mode and a Narrow"
mode. The latter is to be used only in areas where stations
crowd the dial and interfere with one another. since it does
degrade audio performance a bit.

Control Layout

At first glance. the black front panel looks as if it doesnt
have enough controls to operate the AT-670. All you see are
a tiny on/off button at the left. a Preset" up/down switch. an
AM/FM band switch near the center of the panel. and a
large rotary tuning knob at the extreme right. Only close
examination reveals a tiny button labelled Open/Close." A
push of this button. once power has been turned on. causes
an entire section of the panel to move smoothly forward,
disclosing additional required controls. (It reminded me of
the many secret revolving doors weve all seen in detective
and mystery movies.) The controls that come into view are a
switch that turns on the SNR circuitry. another for select-
ing automatic or manual tuning. a third that selects Mono"

or "Stereo" operation. and a fourth that selects the "Normal"
(wide) or Narrow i.f. mode of FM reception. Two tiny
pushbuttons labelled Memory" and Enter." also found
here. are used to set station presets. A softly lit display at
the center of the front panel indicates stereo reception,
tuning accuracy. and if. and tuning modes.

Mains Control" jacks on the rear panel allow control links
to other Proton 600 series components. This feature enables
you to designate one unit of a Proton 600 series system as
the master power controller and to turn all components on or
off by pressing the power button on the designated master
unit. In addition. major functions of this tuner. any other 600
series component. and even some Proton TV monitor/re-
ceivers can be controlled by Protons AH-681 system re-
mote. in addition to the usual output jacks at the rear of the
tuner. there is an output level control and provision for
mounting the pivoting AM antenna Ioopstick provided with
the AT-670. The antenna input is a standard 75~ohm coaxial
connector. but an impedance-matching transformer is sup-
plied for those who use BOO-ohm twin-lead cable from their
antennas. Additional slide switches on the rear panel select
7511.8 FM de-emphasis for use in the US. or 5011.8 de-
emphasis for use in some foreign countries. and 10-kHz
(U.S.) or 9-kHz (European) spacing between AM channels,

Measurements

As usual. I tested the FM tuner section first. Figure 1
shows the frequency response. Output levels from both
channels are virtually identical. and at 15 kHz response is
down only 1.0 dB for the left channel and 0.8 dB for the right.

Figure 2 plots the quieting characteristics of the FM tuner.
Using the "Normal." or wide. i.f. bandwidth. 50-dB quieting
in mono is reached with a signal input of slightly less than 10
dBf (one of the best figures I have ever measured for this
important specification). and 36 dBf of signal input pro-
duces 50-dB quieting in stereo. Repeating this test using
the Narrow if. mode results in only slightly poorer 50-dB
quieting sensitivity. 13 dBf in mono and 37.5 dBf in stereo.
Turning on SNR while in this if. mode. however. makes the
monophonic figure slightly worse (18 dBf) yet improves the
stereo figure to 32 dBf. At a strong signal level (65 dBf).
mono S/N is just short of 80 dB in Normal" mode while
stereo SIN is 73.5 dB. increasing to 75 dB at still stronger
input signal levels. These results. though excellent by any
standard. appear to fall slightly short of claims made by
Proton in their published specifications. This is only be-
cause Proton quotes A-weighted S/N figures. which would
inevitably be a bit higher than the figures I obtained. The
EIA/lEEE Tuner Measurement Standard. which i use. calls
for S/N measurements of FM tuners to be unweighted and to
be made over the bandwidth from 200 Hz to 15 kHz.

Figure 3 shows how mono and stereo THD + N vary with
signal strength. In the Normal" i.f. mode. THD + N for 65-
dBf signals is 0.08% in mono and 0.27% in stereo Switch-
ing to the "Narrow" mode reveals the penalty incurred by
using this mode to reduce interference from strong adja-
cent- or alternate-channel signals: THD + N increases to
0.46% in mono and to 1.8% in stereo.

l obtained close correlation between the results shown in
Fig. 3 and those shown in Fig. 4. The latter graph shows

52

AUDIO/MARCH 1992