Re: Multiplex decoder nearly rewired.


Hi Patrick,

It only took you 9 vacuum triodes to do what QUAD did with 3 PNP
transistors? I have a stereo radio from 1962 that uses only the two
triodes of a 12AX7 to implement the multiplex decoder, and also uses a
"diode ring demodulator". Also take a look at the stereo decoder used in
the H.H.Scott 345 receiver, which has a tube decoder circuit virtually
identical to the QUAD decoder topology right down to the "diode ring
demodulator", and it uses only a single pentode and a single triode in the
decoder proper, with another triode used simply to control a neon stereo
indicator light based on the output of the 19 kHz "rectifier"/doubler
circuit. The pentode and the two triodes are all contained in a single
6N11 compactron tube. The schematic is available on the web.

I have four questions about your changes to the QUAD circuit.

1.) When will we have a schematic?

2.) At several points in the description of your circuit you refer to a
"19 kHz Notch filter", this is a feature that the original QUAD design
didn't include. What topology did you use to implement the "19 kHz notch
filter"?

3.) The original QUAD design included a low pass filter with a 55 kHz
cutoff at the input to the demodulator "ring", does your circuit include
this low pass filter, or did you leave it out? This filter doesn't just
eliminate potential SCA beats, but is important to minimize the hiss and
noise on the stereo audio outputs even when SCA sub carriers aren't being
used. Without low pass filtering, filtering noise at the output of the
ratio detector around 114 kHz will be translated down to the audio range,
degrading the stereo signal to noise ratio more than need be. This noise
can be the result of weak signals, or it can be due to interference from
distant adjacent channel signals, especially in areas that use 100 kHz
channel spacing in the FM band.

4.) It isn't clear how you are compensating for the fact that the fact
that the L-R sub carrier signal is effectively transmitted at a lower
level than is the main channel L+R signal? QUAD used an auxiliary matrix
to perform this function, feeding inverted L+R audio into the outputs of
the decoder to cancel some of the L+R signal. There are at least two
other was to perform this function, are you following the QUAD lead, or is
the "compensation" capacitor in the cathode circuit of the input amplifier
a crude solution to this problem.?

Did the folks that "say diode ring demodulators are inherently noisy" give
any explanation for this claim? Diode rings are frequently used as low
noise RF mixers, so this claim seems counter intuitive. What kind of
demodulators did they suggest might have lower noise than a diode ring?

QUAD was not the only manufacturer to use this sort of diode ring
demodulator. This type of "diode ring" was the standard topology almost
universally used in discrete component stereo decoders, both tube and
transistor. Even Scott switched to this design in their later tube
decoders as mentioned above, and I would assume they carried it through to
their discrete component transistor designs too.

You mentioned that the QUAD decoder circuit uses four additional diodes to
switch between the monophonic and stereo reception modes. I can see
circuitry that looks like it automatically controls the diodes in the
stereo audio path to interrupt that path when a 19 kHz stereo pilot tone
is not being received. I also see the two diodes in the monophonic audio
path, but I can't see how they are automatically controlled? It looks to
me like you have to manually turn off one of the QUAD II power amplifiers
to get a monophonic output from the decoder when a stereo signal is not
being broadcast. Can you explain what I am missing here?


Regards,

John Byrns


In article <43778450.3155F14B@xxxxxxxxxxxxxxxxxx>, Patrick Turner
<info@xxxxxxxxxxxxxxxxxx> wrote:

> Some of you were following my progress with re-designing & rewiring
> of the stereo multitplex decoder I have within an ancient
> Trio receiver.
>
> I finally settled on a design using all tubes that resembles the
> method of operation of the first Quad add on MPX unit with 3 transistors
>
> except that I have all triodes.
>
> After the ratio detector, there is a V1 6DJ8 amp which amplifies the
> signal
> from the RD by about 5 times, then buffers its anode output with a
> direct coupled CF, V2.
> From this V2CF, a feed to a pair of parallel 19kHz tuned circuits
> filters out the 19kHz and V3 triode amplifies it and
> powers a tuned 19kHz tranny which has a CT secondary and two diodes
> then convert the 19kHz into its full wave rectified form which is mainly
> 38kHz.
> This signal is used to synchronize a 38kHz oscillator with 2 triodes
> very like the type
> Scott used but with a 6CG7, V4/5.
> The oscillator is a PP type, and the double tuned 38kHz tranny
> has two CT windings, pri for the anodes, sec for the connection via its
> CT to the
> composite signal from V2CF, minus the 19kHz pilot tone.
> A ring diode demodulator almost identical to the design used by Quad is
> then employed
> with 4 x 27k R and 4 x 1N9148 diodes. A pair 10k R off the two diode
> junction
> outputs charges a pair of 900pF caps, one for each channel.
> These RC values gave the right amount of de-emphasis.
> The signals from each channel are then fed to a first order RC filter to
>
> help remove switching noise, then fed to CF buffers, 12AU7, V6/7,
> which drive LC and RC 3rd order filters to further remove any switching
> artifacts.
>
> The L&R signals are taken to switch contacts to select stereo or mono.
>
> The mono signal is derived by de-emphasising the signal from the
> V2CF after the 19kHz notch filter with a simple RC filter, then to a
> switch.
>
> Another pair of CF buffers, 12AU7, V8/9 are used to buffer the signals
> recovered
> before being allowed out of the box to a preamp or other amp.
>
> I found that with the loudest radio station playing, I was getting
> a maximumm of 8vrms of audio output voltage.
>
> Stereo separation is determined by careful setting of the tuned circuits
>
> and correct compensation to ensure the gains at audio and the subcarrier
> signals
> were identical after the CF, V2, which is loaded with the effects of the
>
> notch filter.
> I was able to get over 30dB of separation from 20Hz to 8 kHz, abive
> which the
> sep dropped to about 20dB by 13 kHz, good enough.
> I suspect the separation drifts due to the caps I have used across the
> tuning coils, and I have to buy a brace of temperature stable caps.
> A few degrees of 38khz carrier phase shift relative to the subcarrier
> double side band
> signal can reduce sep from over 30db to 15dB, which is a lot.
> The compensation is achieved by just the right sized cap across the
> cathode R of the V1 amp stage, which has 22k as the anode R
> and 3.9k as the cathode R, so hence its approx gain of 5.
>
> I may switch to using 12AU7 instead of the 6DJ8, since there is
> probably too much gain.
>
> Noise was strange problem after all the bugs were ironed out.
>
> I have an FM stereo miniature transmitter based on the BA1404 chip
> running
> off a 1.40 V supply, ( which is the right voltage btw ).
>
> I think the standard emphasis circuit which I have used as per the app
> schematic
> manages to inject some noise, hiss, into the modulation of the supressed
> carrier signa
> so that when switching from stereo to mono, the hiss with stereo is a
> lot greater.
> There is also some hum on both stereo and mono, 100Hz, and I don't know
> where that's coming from, but it is in the signal from the ratio
> detector,
> so it isn't from the MPX decoder. Rails are clean as can be.
>
> If i switch off the FM signal gene and allow it to "run on" from the
> store of energy
> in the caps in its PS then there is no cessation of the hum, which is of
> concern
> because its 30mV at the tuner output. Hiss remains constant in stereo.
> but at least that's only a few mV.
> The max AF signals are up to 8Vrms, so the SNR is barely -50dB,
> or about as good as vinyl.
>
> Ah, HUM, the Royal Pain in the Arse when you cannot
> find any reason for it to exist.
>
> I can't hear any hum in the background of other stations, but then
> that's difficult
> to hear with a signal going on.
>
>
> But its sure sounds well compared to an Audio Reflex tuner I have which
> uses all chips,
> and which has a better
> SNR ratio; more like about -60dB.
>
> Later this week i will remove the SE 6BQ5 audio amps in this unit since
> I never ever use
> them for listening; they are not good enough for realistic levels in my
> lounge.
>
> Then i will tweak the PS further, and provide an all DC supply for
> each of the 13 tubes that will be left on the chassis.
> 2 x 6AQ8 for the 100MHz input/oscillatormixer/ AFC stage,
> 2 x 6BA6 for the 10.7MHz IF,
> 1 x 6AU6 for the limiter,
> and all the 5 tubes tubes above in the MPX unit.
> Then the AM section of the tuner which runs separately
> from the FM part has 6BE6, 6BA6 for mixer and IF,
> then 6AU6 in triode for detector, so quite a lot of tubes all up.
>
> Some folks say diode ring demodulators are inherently noisy.
> hence the need to keep the signal level high.
> But the 38khz carrier level is about 20Vrms, with a maximum
> of 8Vrms of imposed modulation, surely that's a high enough sig level.
> It'd have to be a lot higher than the system Quad used 44 years ago
> since their 3 transistor
> unit used only a 15V supply, thus limiting voltage swings to about
> 1/3 of what i am using.
> But would the type of diodes make a difference?
>
> Quad achieved a lot with just one transistor used for the synchronized
> 38khz
> oscillator.
> When there was no 19khz pilot tone as there sometimes isn't with a mono
> transmission,
> the bias voltages developed by 19khz amplification and stereo operation
> ceased to
> be generated so DC biased diodes were used automatically
> switch from stereo to mono, so the mono signal
> was taken direct from the buffered output of the ratio detector.
> DC biased diodes and fets are used routinely now to switch all sorts of
> things;
> they are considered a sonic pest by many,
> Quad was the only one to incorporate such a neat trick as feature at
> that time.
> Not really needed of course, but had the yanks done the same thing
> they'd
> have used 3 extra tubes and a lot more gear.
> Quad's methods were very ingenious.
>
> Patrick Turner.


Surf my web pages at, http://users.rcn.com/jbyrns/
.

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