Revox MPX Filter Owners Manual

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Madeby: WILLI STUDER

Manufacturer of Magnetic Tape Recorders
Regensdorf 8105, Switzerland

World-Wide Distribution through :

R E V O X International,

Regensdorf 8105, Switzerland

a Division of ELA AG

REVOX - MULTIPLEX - FILTER

The REVOX Multiplex-Filter is a lowpass filter required for recording stereo
multiplex broadcasts on magnetic tape. It is to be connected between the FM
Tuner and the tape recorder and its presence is necessitated by the characteris-
tics inherent in the FM stereo multiplex system.

Under the sub-carrier modulation method, which is now the most widely adopted,
the two audio channels that make up the stereophonic signal are transmitted
via one FM transmitter. (Multiplex) For the sub-carrier, a frequency of 19 kc
is being used. (Pilot tone) This frequency is doubled to 38 kc in the receiver
and it substitutes the side-modulation carrier which is suppressed on the trans-
mitting end. The side-modulation covers a band from 23 kc to 53 kc and it is
being transmitted in addition to the regular audio band. In any magnetic sound
recording process a certain amount of high frequency emphasis is used in an
effort to improve the signal to noise ratio. This is particularly so at the slower
tape speeds, thereby making it possible that overmodulations of the magnetic
emulsion may occur from the sub-carriers 19 or 38 kc frequency. The interference
thus created becomes audible as an annoying whistling sound. Unlike the ef-
fects produced by occasional overmodulation with music, a continuous high
pitched interference will be caused by the pilot tone.

The 38 kc carrier produced in the decoder may also lead to some form of distur-
bance. As that frequency is being used as a trigger pulse, distortion compo-
nents are being generated, the first order product of which having a frequency
of 76 kc. In the REVOX tape recorder the erase and bias frequency oscillator
operates at 70 kc, thus a difference-tone of 6kc may be produced if too high
a 76 kc component is contained in the decoder's output. This interference will
also be present when stereo broadcasts are being recorded monophonically.

A multiplex filter should mainly suppress the frequencies of 19 kc and 38 kc,
while the audio band from 30 cycles to 15 kc should be allowed to pass without
appreciable attenuation. Such a filter is to be connected to the high impedance
outputs of broadcast receivers and tuners: and it follows that the design of such
a filter should match thOSe high impedances. However, various considerations
such as suitability for universal applications, excessive physical dimensions
due to the large inductive components required, as well as economic reasons,
make production of a passive network not feasible.

The REVOX multiplex filter contains two amplifier stages in each channel for
impedance transformation. In addition these amplifiers provide the gain required
for compensating the level losses caused by the LC components of the filter.
The actual filter network could thus be placed into a low impedance circuit,
while proper matching for the receiver's high impedance output is being main-
tained. This makes matching of the filter's input not critical, which is of par-
ticular advantage where the tuner's output impedance is not precisely known.
The REVOX multiplex-filter is equipped with two inputs per channel, whereby
the impedances are 47 kOhms with a sensitivity of 5 milli volts (overload point
200 m/volts) and l MegOhm with a sensitivity of 100 m/volts (overload point
5 volts). The amplifier section uses two silicon transistors with excellent noise
performance due to their extremely low collector emitter leakage (100 micro/
ampere). Heavy negative feedback helps to achieve the required low output
impedance for the following filter components. The filter itself presents a low
impedance source for the diode inputs 1 and II of the REVOX tape recorder, thus
matching is also not critical.

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The filters have been designed for a linear pass-band up to 14 kc with a sharp
cut-off thereafter, reaching 40 db attenuation at 19 kc. Attenuation of the sub-
carrier frequency of 38 kc is also better than 40 db. This response characteris-
tic assures interference-free recordings of stereo multiplex-broadcasts Without
deteriorating the audio signals.

The voltage required for operation of the filter is supplied via the remote control
socket when used in conjunction with a REVOX tape recorder. For other applica-
tions any 24 volt dc source capable of supplying 2 milli/amperes may be uti-
lised. Ripple of the supply voltage is not critical as a double section Ac-filter
is incorporated in the circuit.

INTER - EQUIPMENT CONNECTIONS

Inputs :

Depending on the output voltage of the receiver
or tuner, two shielded cables of suitable length
are to be soldered to the respective input ter-
minals .

Maximum permissible input voltage

2 00 m/volts
5 volts

5 m/volt input
100 m/volt input

Outputs :

Solder two shielded cables to the terminals di-

ode land diode ll respectively.

+25 Volt are available from pin 3 of the remote
E control socket on the REVOX. No wire is required

to the negative terminal as the ground return is
-_ + 25 V carried via the shields of the diode cables to avoid

a ground-loop. Where an auxiliary dc supply is
used, a two-conductor cable will of course be

The sensitivity of the diode inputs I and II on

the REVOX (3-36 should be adjusted with trim-
pots P5 and P7 so that, with record level con-
trols in the 12 o'clock position, a deflection of
0 W (100%) is reached on the respective V'U
meter during loud passages of music.

+ 25 V Diode 2

Diode l

{mm-q

A suitable cable for installation of the multiplex filter between tuner and REVOX
tape recorder can be found by cutting the REVOX type NAC inter-connecting
cable into two suitable lengths.

(NAC 220 6 feet, NAC 230 9feet, NAG 250 15 feet)

TECHNICAL SPECIFICATIONS

Sensitivity

Output level

Distortion

Signal to noise ratio
unweighted

Cross

talk rejection

Current requirement

Semi - conductors

5 mV (< 200 mV) at 47 kOhm
100 mV (< 5 volts) at l MOhm

25 mV to max. 1 Volt on 47 kOhm

< l %o for max. input levels of 200 mV

and 5 V respect.

> 55 db for input levels from 5 mV to 200 mV

> 67 db for input levels from 20 mV to 800 mV
this corresponds to the normal output levels
of broadcast receivers and tuners .

> 70 db

ZmA (25V)

4 s 1 1i 0 o n transistors BSY 72

REVOX - MULTIPLEX - FILTER
FREQUENCY RESPONSE CURVE

-3db/

-40 db

kHz kHz

SOHZ

1000 Hz 14 kHz