tube stereo decoder with sample introduction Darius FM tuner circuit and images description of the EC900 EF95 input and mixing part A few words about the ratio detector tube stereo decoder implementation with example The VHF FM IF with EF89 2x 6BN6 and EF91 Low Phase Noise - push-pull input section with ECC91/6J6 
A stereo decoder with tubes is of course a "must" for a tube FM project. Basically, the stereo decoder
divided into two groups.
Group 1: Most common are the switching or multiplex decoder. In these, the output signal from FM demodulator to a demultiplexer
is given. The output of the VHF FM demodulator is described as
stereo multiplex signal .
With a clock frequency of 38KHz this multiplex signal (short: MPX) signal at the right time then switched to the left and right channels.
Group 2: Much less frequently (
RT50) to non-existent, the mixing decoder as shown in the blog featured. They work purely analog as known by the superheterodyne ago. A filter at the entrance filters out the carrier-
stereo difference signal from the MPX signal from the FM demodulator. In a
product detector is now demodulated by using the 38KHz carrier frequency difference signal. In a (resistance) matrix is \u200b\u200bobtained by superimposing the low-difference signal with the composite signal then the left and right channels. For such tasks analog tubes are of course ideal.
As an introduction to this technique is shown here
1990 model ideal.
For all I know you have the 38KHz stereo decoder with the 19KHz pilot tone gain. The easiest way to a Pilottonverdoppler to PLL systems
Johnson divider.
In multiplex decoders (Group 1) is the easiest demultiplexer aus Schalt- Dioden. Nahezu alle diskreten
Multiplexdecoder arbeiten so. Wird bei einem solchen Stereodecoder die Schaltfrequenz mit einer (Pilottonverdoppler) Röhre erzeugt, dann geht das in Hifi Kreisen schon als Röhren Stereodecoder durch. (
Link ,
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Link )
Beim Mischdecoder (2.Gruppe) wird mit einen Tiefpass das Summensignal also das mono Signal, abgetrennt. Dann gelangt es zur Matrix. Diese Stufen sind in Teil 1 des Schaltbildes unten zu sehen. Der Tiefpass besteht aus L1 C1 R2. (C2 ist ein Koppelkondensator. Das Gitter der folgenden Verstärkerstufe liegt nicht auf Massepotential.) R3 und R4 bilden mit C3 die
Deemphasis für the sum signal. The de-emphasis is the 1990 model that is before the matrix. The de-emphasis of the difference signal occurs before the mixer, that is carrier-. More on that later. With VR1, the level of the sum signal can be adjusted.
The 2nd Shows the circuit part Pilottonaufbereitung and 38KHz carrier recovery. The MPX signal passes through a high pass from C8 L2. About R14 passes it to a 19KHz resonant circuit L3 C9 VC1 of the 19KHz pilot tone filter out. R14 determines the goodness of this circle. Through the coupling capacitor C10 reaches the pilot tone on the grille of the ECC83. The bias generated by the starting current. It turns to eine von der Pilottonamplitude abhängige Gittervorspannung ein, die die Triode in der Verstärkung regelt. Amplitudenschwankungen des Pilottones werden so einigermassen ausgeglichen. Um die Rückwirkung der Gitter- Anodenkapazität zu kompensieren, ist der Neutralisationskondensator Cn angebracht. Die Katodynstufe und der push push Verdoppler wurden nachgerüstet und sind ein Tribut an das nicht kompatible Radio Daten System RDS. (Statt BAT... kann man auch eine Röhre verwenden.) Diese Art der Verdopplung hat gegenüber der sonnst üblichen Aussiebung der Oberwelle den Vorteil, dass keine 19KHz Reste mehr im 38KHz Träger enthalten sind. So wird vermieden, dass die dritte Oberwelle des Pilottones das RDS Signal in den Hörbereich mixed. To the anode of the Triodensystems ECH81 the 38KHz oscillator is coupled via C14 loose. The loose coupling of the parties of R14 and C14 not only allows the parties to adjust to maximum probes, including the phase of the 38KHz carrier to carrier-frequency difference signal to synchronize properly. At R25, the carrier is the, for a multiplicative mixture with the ECH81, of required amplitude. By clamping on g3 the ECH result is a negative rectified voltage from the Magic fan EM71 appears.
Now that the 38KHz carrier is recovered can, down to the product detector circuit part 3, the LR and RL signal for the matrix can be obtained. The resonant circuit of C18 L5 C19 is tuned to 38KHz and filters out the carrier frequency difference signal. R27 determines the goodness of the circle. The quality circle is set so that the demodulated signal, the de-emphasis is required. Advantage of the carrier-emphasis, is the noise that are distant from the front of the mixer 38KHz are strongly attenuated. This minimizes unwanted mixing products. About C20 trägerfequente the difference signal goes to the de-emphasis on the g1 Mischhexode. Your bias is generated within the model 1990 from the starting current. This causes a more extreme stereo effects without increasing total of the base width.
With this basic knowledge, which provides, together with the circuit described above explanation is the functioning of all tube stereo decoder of this type clearly. In another blog I am based on that, explain that stereo decoder.
Glossary
Stereo Decoder: He decodes the signal from FM demodulator, the left and right channels.
carrier frequency technology : transmission of several additional "news" with the help of sub-carriers in a "news" channel. Here: In addition to the sum signal (mono = L + R), the signal difference left minus right channel (LR) to a 38KHz subcarrier in amplitude modulation with suppressed carrier (DSB). When you hear the signal from directly, you can only see the sum signal (L + R) was. The carrier-difference signal is involved in the ultrasonic range.
stereo multiplex signal : between the left channel (L) and the right channel (R) is connected with a Überhörfrequenz from 38KHz to and fro. Listening to this multiplex signal from directly, you can only see the sum signal (L + R) was. You hear so mono because the switching occurs so quickly that the ear does not differentiate the switching process can. A demultiplexer operates to the synchronous multiplexer, can again be divided into left and right channels. When multiplexing from the left and right channel is created the same spectrum as the mixing of the differential signal to a subcarrier in DSB. Therefore, the transmitter and the receiver side, both methods are used.
Johnson divider : A flip-flop frequency divider which supplies at its outputs by 90 degrees phase shifted signals. Thus, phase comparison, a can be generated with the reference frequency (here, the pilot tone) in phase signal lying in a PLL system with XOR. (For example, the stereo decoder TA7342P and other IC's)
22/10/2008
The english translation under construction.
A valve based stereo decoder is a must in a valve VHF FM tuner project. There are two kinds of stereo multiplex decoders one is time division and the other is frequency division. Time division multiplex is most commonly used. This blog explains a valve based frequency division multiplex decoder.
In the time division topology the output signal of the FM demodulator (stereo multiplex signal, MPX) goes to a 1-to-2 demultiplexer . A clock frequency of 38KHz is generated in the decoder and switches the MPX signal to produce the left and right audio signal.
The frequency division topology is seldom seen stereo decoders. They are analogue and work in the way like a superhet receiver does. A filter at the input filters the subcarrier modulated left minus right signal (DSB L-R ) from the MPX signal. This signal is input to a product detector . This product detector mixes the DSB L-R subcarrier with the 38KHz carrier frequency generated in the decoder. L-R is output of the detector. In a (resistor) matrix the mono signal L+R is superimposed with the L-R signal to get left and right audio output. For analogue tasks like this valves are predestined.
The Model 1990 shown in this blog is an excellent introduction in this topology. As far as I know all kinds of stereo decoders get their 38KHz carrier from the 19KHz pilot sound. 38KHz is generated easily by doubling the pilot sound up to more complex systems using a Johnson divider.
In time division stereo decoders the demultiplexer is most easily made of switching diodes. Most of the discrete time division decoders work this way. If the clock frequency is generated by a valve some hifi enthusiasts are calling it a valve stereo decoder. ( Link , Link , Link )
In frequency division stereo decoders a low pass filter separates the mono signal (L+R) from the MPX signal and is input to a stereo matrix. These stages are shown in part one of the schematic below. The low pass consists of L1 C1 R2. C2 is a coupling cap, note that the grid of the ECC82 is not at ground potential. R3 R4 and C3 are making the de-emphasis for the (L+R) signal. VR1 makes the L-R level adjustable. In the Model 1990 the L+R de-emphasis is at the input of the matrix. The de-emphasis for the L-R signal is made at the sub carrier frequency at the input of the L-R mixer. Later more about this. The cathodyne stage makes L-R and L+R. These components are input to the stereo matrix.
continued...
21.Mai 2008
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