DVD-Software.info

[Patrick Turner]

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.

[Patrick Turner]

Bob wrote:

[Patrick Turner]

Bob wrote:

[Jon Yaeger]

in article 1131928841.537937.234860@g14g2000cwa.googlegroups.com, Bob at
wa2ise@ix.netcom.com wrote on 11/13/05 7:40 PM:

[Patrick Turner]

Jon Yaeger wrote:

[Bob]

Duh, now I see it "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". For some
reason I read it to say "If i switch off the FM receiver 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"

My bad....

[Bob]

"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 ).


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

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. "

Maybe the hum is in the BA1404 chip circuit? Try listening to it with
another good receiver and see if the hum is present or not. Hum is a
big problem with local homebuilt AM "micro" transmitters, usually it's
the RF signal paths being switched from one path to another thru
rectifier diodes in power supplies, causing undesired amplitude
modulation as seen by a receiver.. FM should be less sensitive to
this, but it may still "leak" thru and be heard at the levels you are
talking about. FM receivers do a good job of rejecting AM, but some
still can get thru. Like the 50db down you are getting. The fact
that you still have hum after shutting the receiver's power supply down
but before the filter caps discharge, and that you can't hear it on
radio stations, would make me think it's the BA1404 chip and or
residual AM from the rectifier diodes of the BA1404 chip power supply.
Try passing that power supply's DC output wires thru a ferrite ring
with a few turns bifilar wound to block off the RF from flowing down
those wires to the power supplie's rectifier diodes. That may
eliminate the undesired AM and the hum.