[Alex]

"Patrick Turner" wrote in message
...
I repair, rewire, or seriously modify at least 5 AM radios per year.
I've done 3 this year and there are 3 waiting to be done, and its only
May.

I built a complete radio in 1999, 10kHz of AF bandwidth, variable
selectivity, linear IF amp, linear detector, linear AF amp with
EL34 in triode with 12AX7 and 12dB FB. Rola 12" Deluxe speaker from
1953
with a dome tweeter from 1972, because the 12" speaker is fairly flat
to
5kHz.

I have never heard any other AM sets that come close except
the one in the AM-FM tuner at my website, which I designed, and of
course the
Quad AM tuner all tubed, that isn't bad at all.

Could you please give a link to your website? Is there a schematic of
that
radio, mentioned above, which you designed? Is it TRF?
Regards,
Alex

Patrick Turner.

Try

http://www.turneraudio.com.au/am-fm-...x-decoder.html

Patrick Turner.
Interesting.
My comments:
1. I am not that fanatical to build a stereo-decoder around LC tanks which
require precise tuning.
2. It was funny to see a reactive tube for AFC instead of a tuning diode (or
any diode like 1N4148).
3. As far as the AM detector is concerned, I think it is not optimum.
Voltage on the detector appears to be about +35V. Forward bias current is
about 90uA. Differential impedance of the diode is about 250ohm (no signal),
220pF capacitor impedance is about 1.5K at 455KHz. Thus at no signal the CF
is loaded by about 1.5K. The CF, running at 1mA DC current would have about
0.5mA/V transconductance, therefore output impedance is about 2K. Thus the
gain of the CF is only 0.4 or so.

For the detector to begin detecting, obviously the AC current through the
diode shall exceed the DC component (90uA). It will require about 150mV on
the input to the detector or conversely 400mV on the grid of the CF.
Sensitivity of such a detector is rather poor (as you know, sensitivity of a
pure p-n diode is about 25mV.)

It is easy to improve the result 10 times -- just reduce the bias. Connect
R17 not to the ground, but to point "R" (+31V). After that the diode will be
biased only by the self bias of the tube, which is about 4V. Sensitivity of
the detector will increase 10-fold.

Regards,
Alex

[Alex]

"Patrick Turner" wrote in message
...


Alex wrote:

"Patrick Turner" wrote in message
...

I repair, rewire, or seriously modify at least 5 AM radios per year.
I've done 3 this year and there are 3 waiting to be done, and its only
May.

I built a complete radio in 1999, 10kHz of AF bandwidth, variable
selectivity, linear IF amp, linear detector, linear AF amp with
EL34 in triode with 12AX7 and 12dB FB. Rola 12" Deluxe speaker from
1953
with a dome tweeter from 1972, because the 12" speaker is fairly flat
to
5kHz.

I have never heard any other AM sets that come close except
the one in the AM-FM tuner at my website, which I designed, and of
course the
Quad AM tuner all tubed, that isn't bad at all.

Could you please give a link to your website? Is there a schematic of
that
radio, mentioned above, which you designed? Is it TRF?
Regards,
Alex

Patrick Turner.

The kitchen radio I mentioned from 1999 above
has a different circuit to one I just gave you at
http://www.turneraudio.com.au/am-fm-...x-decoder.html

I don't have the kitchen radio schematic at the website.

But what my kitchen radio does have is..

3 gang tuning cap.

Two RF coils with ferrite cores for high Q and two C gangs are coupled
so that there is no sideband cutting of AF
right across the AM band, which means each coil is tuned to a slightly
different F.

There is no ferrite antenna, just loose coupling from a wire antenna to
one input coil.

There is a variable µ twin triode used as a cascode RF amp stage and RC
coupled to the 6AN7 F converter
to make an IF signal.
There is AVC voltage applied only to the cascode input triodes.

The IF amp is 6BX6 with unbypassed Rk and no AVC applied, so its fairly
linear.
There is a cathode follower and diode detector cabable of detecting
about 10 times the actual voltage detected with very good linearity and
bandwidth.

Two standard IF trannies are used, but the first one after the F
converter
has one IF coil monted on a slide and a knob on the front of the radio
rotates to move the coil to coilo distance about 10mm.
This allows the radio to be tuned normally to a single
a single peak when tuning and then after closing the coils closer, the
IF BW is much increased to allow 9Khz of audio to be detected, but all
while
retaining very good skirt selectivity and rejection of
stations only 45kHz away even if they have 20 times the transmitting
power.

There are in fact 6 tuned circuits in all, so the attenutation
of signals away from the wanted stations is excellent,
yet what is wanted sounds very nearly as good as FM.

The 3rd gang of the cap is for the oscillator.

I paid very careful attention to tracking.

TRF sets are quite poor performers today because the stations are now
close together,
unlike in 1935, when the band wasn't crowded.

TRF usually used only 3 tuned circuits which needed careful tuning,
and they gave a peaked response, not a flat topped band band pass
character of a critically coupled IFT.

The Superhet with IFTs is definately The Best hi-fi AM tuner you can
have if its built right,
and 99% were atrocious, and all mainly conformed to the lowest common
denominator of utter crap.

When Radio was king before TV, there were very many makers all vying for
a market share.
They all agreed with each other to ensure the standards were lousy.

Patrick Turner.

May I ask you, Partick, which AM radio station has the least distortion? I
live in Sydney, and I wold say, BBC (around 600KHz) is the best. Others use
some elaborate compression and clipping which makes one's ears tired very
soon.

Do the stations INDEED transmit 18KHz of bandwidth? With only 9KHz grid?

On the stereo-decoder note, what is the phasing of 19KHz pilot with respect
to the 38KHz (suppressed) carrier: do the zero-crossings coinside (every
second one) or peaks coinside (every secon ones)?

I would guess, the peaks. That way it would have been easier to derive 38KHz
simply by a class C frequency multiplier.

By the way, in the Russian stereo system, there is no 19KHz pilot, but just
a 38KHz vestige with a proper phase. It gets boosted by a q-multiplied LC
tank and fed to a diode ring, similar to what you did..

Regards,
Alex

[Patrick Turner]

Iain Churches wrote:

"Phil Allison" wrote in message
...

Patrick T wrote:

I don't know how many tube amps you have designed and or built or
repaired count. Many tube amps will oscillate at HF when connected
to a capacitor load.


** Only a psychotic lunatic would deliberately connect a capacitor across
the output of a tube audio amp.

Quite wrong Phil. 0.22µF is a standard (albeit tough) test
for tube amp stability. You will find it mentioned in articles
by Tremaine and Crowhurst.

In order to gain unconditional stability ....


** An unnecessary, pedantic, pseudo technical wank.

Unconditional stability is more important now that is was
in the hey-day of tube amps when many speakers were 15
Ohms, and formed benevolent loads.

It matters not what the speaker ohms are.

15 ohms ain't necessarily easier to drive or
inclusive of less reactances than any other Z

For a given geometric layout of windings in any OPT,
there may be 2 lots of sec windings which give a match for 16 ohms
when windings are in series, and 4 ohms when in parallel.
The OPT leakage inductance is an imperfection present when implementing
a
load coupling idea such as an OPT.
This leakage is in effect in series with the perfect model of OPT
which has no winding resistance, leakage inductance or shunt
capacitances.

The change in the way the sec windings are arranged to suit either 4
ohms or 16 ohms
makes not the slightest difference to the amount of reflected effective
leakage in series with the primary 'perfect' winding, and as long as ALL
the sec windings are used in either series or parallel arrangements, and
as long as the
current density is the same for all wires in S windings, then the
winding losses
all stay constant along with shunt C when 'looking into' the primary
of the OPT.

A 16 ohm voice coil will have perhaps twice the turns of a 4 ohm voice
coil.
So it suits higher voltage and lower current.
But the amount of inductance is much increased, and an average
16 ohm speaker will have virtually no loading effect on an amp
at 50kHz, allowing output tube gain to go high, and thus allowing
oscillations.
A 0.2 uF has about 40 ohms reactance at 20kHz, but only 16 ohms at
50kHz.
The inductance value of leakage appearing looking into the 16 ohm Sec
winding of the OPT
is much higher than if the S winding effectively had 1/2 the turns as
would happen in the parallel connection for 4 ohms.

So its the OPT with the 4 ohm outlet that will be less affected by tests
with 0.22 uF,
unless of course the maker of the OPT has the 4 ohm outlet from a CT on
the 16 ohm winding,
just like so many lazy dumbass makers do these days and always have, ARC
included.

So, your statement I quote below is far far too simplistic and
and does not indicate any real truth of the amp-speaker interface
situtation either right now, or 50 years ago....
""Unconditional stability is more important now that is was
in the hey-day of tube amps when many speakers were 15
Ohms, and formed benevolent loads.""

Stability has ALWAYS been important, but makers knew 50 years ago as
some (but not all)
know now that unconditional stability AND excellent bandwidth at full
power costs several
beans and maybe a few extra walnuts towards the production costs.
Mainstream amp making has always been dominated by really craparse
designs, like Quad-II amps
and many Leak models. Its SO easy to build something better than these
sample
poorhouse thinking did. But it COSTS MORE, and the amp becomes heavier,
and it won't sell as well as the worser amp which people are
prepared to put up with. Heck, only BBC engineers had the slightest idea
what unconditional stability actually meant at all.

Phil says, about unconditional stability, that it is ""An unnecessary,
pedantic, pseudo technical wank.""

And in some situations he's perfectly correct, especially in a radio AF
output amp
where there is only going to be ONE speaker connected, the one inside
the
cabinet with the radio chassis.

But Phil isn't correct in other situations where any speaker ever made
might
find itself being hooked up to some amp which will oscillate
because of the way the the speaker fails to load the amp at HF due to
the
speaker inductance.
Or the speaker has some shunt C like most ESL which will cause a phase
shift
and cause the sine wave response to become peaked between say 8kHz and
50kHz.
The critical damping network should not only stop
oscillations when no load is present, but also stop oscillations when
ANY value of pure C is used as a TEST LOAD.
( Are you listening Phil? I never said you expect any speaker to ever be
a pure C.)

If you connect say 2uF across the output of many amps, ( and including
some SS )
there will be a peaked sine wave response at low levels where the music
is
listened to, maybe 1/20 of the clipping output power.
If the amp is tested at full PO of the 1kHz level into an R load,
and has 2 uF connected, then by 10kHz, the amp will have a pure 8 ohm
reactance
and the response will begin to sag, and by 20kHz, there will be only 4
ohms reactive loading
and the waves will be severely distorted due to over loading and vain
attempts
by the NFB network to overcome a declining RL value.
So NEVER test amps with pure C loads above about 1/4 of the maximum
clipping VOLTAGE
level of mid F performances into rated R loads.

So, with output levels low, you will see the peaked response caused by
the
2 uF cap loading. A typical peaking occurs at 32kHz, maybe the peak is
+6dB above the
reference voltage output at 1 kHz.
So despite the lowering of C load value as F rises, more output voltage
is produced across this load than at LF.

Some portion of the rise in response above 8kHz towards a peak may be
well within
the audio band, which is not wanted.
Hence the need to keep leakage L low, which means more interleaving,
thicker interwinding insulation layers, and this always means
less units per hour can be produced in a factory, and that what is made
has to be done by
the best and highest paid skilled workers.
((((( Bean Counters was a guy who carried a pistol during his factory
tours.
When he spoke to unionised workers about the skills and time they needed
to wind what the
engineer told them to, he shot the worker on the spot.
Then he shot the engineer, and used their bodies in the factory boilers
which kept the workplace just above 13C most winter days.
Back at Cumpunny Headquarters, he told the design team who shivered in
fear
to make the PO 20% higher and weight and size 20% lower,
and told the marketing guys to create a line of utter bull**** to
promote the product.
This is how profitable ventures are realized. Never give anyone what is
ideal.
Give the *******s just enough to keep 'em alive. No need to
tempt them to get fat.)))))

The phase shift caused by Csh makes the phase
relationship of Vin at V1 grid to VFB at the V1 cathode less phase
coherent,
so much so that the NFB becomes LESS effective than when no cap is
connected.
AND the leakage inductance and C load get together to make a series
resonant circuit.

So, Peter Walker and others deliberately made sure their ESL speakers
have SOME series R looking into the input of their speakers.

Its extremely unusual to find any speaker ever made that
presents itself as a pure C load to an amp.
ESL are the ones that have much C in their make up, but because there
is always a step up transformer, there is always some series input
resistance,
and possibly some shunt R as well.

The model I use to test amps for their suitability for use with SEL
is a 2uF + 1.5 ohm in series, plus 15 ohms in shunt across the R&C.

Many tube and other amps with a 2uF load give a very typical peaked
response at 32kHz, maybe +6dB
and maybe +3dB at 20kHz. But with an added series 1.5 ohms, the
peaked response is reduced to maybe 2dB max at 32 kHz, and
maybe only +0.5dB at 20kHz, which is quite acceptable.

The added 1.5 ohms is the DAMPING resistance needed in what is an
effective series resonant
LC circuit so that the Q of the resonance is much lowered.

The leakage L + C form a second order low pass filter.
For an unpeaked response from such a filter you MUST have some shunt R
across
either L or C or both, or some series R .
This is very basic LCR theory, and all speaker and amp makers need to be
very aware
of the need for critical damping of LCR networks if they wish to
see a flat response before or after a pole where the beginning of
attenuation has been planned to occur.

Some peaking is allowable with pure C loads.
It is a normal function of an LC filter with insufficient damping
But we do NOT NEED a perfectly flat response above 20hHz, so
damping brought by added R to the LC circuit does not need to be greater
than what is needed to stop the amp oscillating.
Some ringing will always occur in amps with a C load,
and its not necessary to prevent it all.

The F of the ringing is usually above the audio band and does not
matter.

Patrick Turner.



Iain

[Patrick Turner]

Robert Casey wrote:

Phil Allison wrote:




** Go shove your fat head down the dunny -


We used to do that to deserving people in college. Then flush that
toilet. We called it a "Ka-Boom"...

What a "dirty kaboom" is is left as an exercise for the student...

A more sneaky way to effectively direct heads of personel
and toilet bowls to be in a union of sorts
is to surrepticiously use the tooth brush of the personal to clean the
toilet.
He usually cleans his teeth next day without realizing the
hygiene deficit.

With luck, it gives him a case of the poopal ****es just like he's given
us mental ****es.
So it may have made it worth us sneaking into his bathroom at 3am
and with a small phial of our own crap in case he has cleaned his toilet
too well.

With luck, you will find out that our crap is as bad as his.



Patrick Turner.

[Alex]

"John Byrns" wrote in message
...
In article ,
"Alex" wrote:

Thanks, Partick.

Your reply is constructive and informative, unlike the other one.

My case is not a Hi-Fi amplifier, but a radio with a single-ended stage.
OPT
is quite lousy. It is designed for Rl=7.5K (for 6M5, EL80 or 6F6 tube at
250V). Its leakage inductance, referred to primary is as large as
165mH --
the windings are not interleaved.

There is an interesting variation on this capacitor thing that you will
see in some old radios. Rather than using a single capacitor which
slowly rolls off the high frequency response, two capacitors are used,
one across the primary, and another across the secondary of the output
transformer. The two capacitors, along with the leakage inductance of
the transformer, form a third order low pass filter. This provides a
sharp cutoff to attenuate noise energy above the desired audio band
while providing flatter frequency response below cutoff in the desired
audio band, than a single capacitor can.

I have never seen that, but it is an interesting idea. Might be useful on
SW. Requires a set of switches to throw relatively large caps in circuit. As
I understand, you need to turn the feedback off for this 3-rd order filter
to have effect. Turn the NFB off and the mechanical resonance of the
speaker will be undamped. So, there needs to be a special circuit, gradually
disabling the feedback with the frequency rising, but not affecting gain. It
can be done. And what if we place more LC pads between the secondary and the
speaker? Potentially can make the radio fit to receive SSB...


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

[Iain Churches[_2_]]

"Patrick Turner" wrote in message
...


Iain Churches wrote:

"Phil Allison" wrote in message
...

Patrick T wrote:

I don't know how many tube amps you have designed and or built or
repaired count. Many tube amps will oscillate at HF when connected
to a capacitor load.


** Only a psychotic lunatic would deliberately connect a capacitor
across
the output of a tube audio amp.

Quite wrong Phil. 0.22µF is a standard (albeit tough) test
for tube amp stability. You will find it mentioned in articles
by Tremaine and Crowhurst.

In order to gain unconditional stability ....


** An unnecessary, pedantic, pseudo technical wank.

Unconditional stability is more important now that is was
in the hey-day of tube amps when many speakers were 15
Ohms, and formed benevolent loads.

It matters not what the speaker ohms are.

15 ohms ain't necessarily easier to drive or
inclusive of less reactances than any other Z

My point was that most if not all conformed to
IEC/EN/BS EN 60268-5. (The ESL was the
exception - Russ Walker used to joke that Peter
his father was granted a dispensation by the Pope:-)

Specifications are sometimes less rigourous these days.

I have been at demos where amps that perform
perfectly well into Tannoy, Kef and B+W are
trembling on the threshold of instability when
presented with some other loudspeaker as a load.

Iain

[Phil Allison]

"Alex"

May I ask you, Partick, which AM radio station has the least distortion? I
live in Sydney, and I wold say, BBC (around 600KHz) is the best.


** The BBC ???????

Not here in bloody Aussie - maaaaaateee.

Surely you mean " ABC Radio National" on 576 kHz.



Others use
some elaborate compression and clipping which makes one's ears tired very
soon.

** Yaaaawwwnnnnn ...


Do the stations INDEED transmit 18KHz of bandwidth?


** No - but the recovered audio bandwidth derived from ABC Radio
National here in Sydney exceeds 10 kHz for sure.

I own a valve, Aussie made Hi-Fi AM tuner with 15 kHz ( -3dB) bandwidth
able to prove that anytime - the Audiosound AM100.



With only 9KHz grid?

** Not relevant in this country pal - with its huge distances between all
the major cities.

That one fact alone allows Australia to have the highest audio quality AM
radio in the world - or we did until about 20 years ago when AM
broadcasters all turned to brainless " talkback " programming.

Peeeeeuuukee....


On the stereo-decoder note, what is the phasing of 19KHz pilot with
respect
to the 38KHz (suppressed) carrier: do the zero-crossings coinside (every
second one) or peaks coinside (every secon ones)?

** A moot point - I would have thought.

Like " ... is the glass half empty or half full ?? "



By the way, in the Russian stereo system,


** So you are really called Alexei - eh ??

Are you another " fat ******* " wearing an ill fitting, shiny cheap
suit ??

Know any good jokes ??



....... Phil

[Phil Allison]

"Iain Cheurchus CRIMINAL LIAR "


I have been at demos where amps that perform
perfectly well into Tannoy, Kef and B+W are
trembling on the threshold of instability when
presented with some other loudspeaker as a load.


** ********.

Churches needs a bullet in the head.




.... Phil

[Jon Yaeger]

in article , Phil Allison at
wrote on 5/5/08 9:04 AM:


"Iain Cheurchus CRIMINAL LIAR "


I have been at demos where amps that perform
perfectly well into Tannoy, Kef and B+W are
trembling on the threshold of instability when
presented with some other loudspeaker as a load.


** ********.

Churches needs a bullet in the head.




... Phil




Phil,

And YOUR head needs some serious "shrinkage" . . . .

I'm inventing a new DSM category just for you, Phil. It is "scatasavant".
A smart guy with a manic potty mouth that's out of control - much like
Tourette's. You'll be famous yet . . .

[Phil Allison]

"Jon Yaeger = CRIMINAL GARAGE RODENT "


** Need lotsa rat bait.

So he will breed to death.




..... Phil

[Jon Yaeger]

in article , Phil Allison at
wrote on 5/5/08 9:55 AM:


"Jon Yaeger = CRIMINAL GARAGE RODENT "


** Need lotsa rat bait.

So he will breed to death.




.... Phil



To "breed to death" -- what a way to go . . . Thanks for sending me good
thoughts, Philthy!

[Patrick Turner]

Phil Allison wrote:

"Iain Churches MASSIVE ****ING IDIOT "


** Only a psychotic lunatic would deliberately connect a capacitor across
the output of a tube audio amp.

Quite wrong Phil.

** As if a know nothing, rote learning moron like Chercus would know.

0.22µF is a standard (albeit tough) test
for tube amp stability.

** It is a completely ABSURD test that does not represent any kind of
real loudspeaker.

In order to gain unconditional stability ....


** An unnecessary, pedantic, pseudo technical wank.


Unconditional stability is a "must"

** Utter ******** !!

**** off - you vile

CRIMINAL ****WIT !!

..... Phil

You are the only person with the view that capacitor load testing
of amplifiers is a useful tool to make amplifiers as stable as we may
want them.

Unconditional stability is found in very many tube and SS amplifiers.

We don't listen to square waves but we use them to test amplifiers.
And we use C loads to TEST them, and adjust them for
minimum square wave overshoot and minimum unpeaked sine wave response.

See my other recent post.

There ARE situations where complete stability with NFB isn't
necessary, for example in a radio AF amp where
the speaker used will always be the only speaker connected
and located within the chassis cabinet.

Patrick Turner.

[Patrick Turner]

John Byrns wrote:

In article ,
"Alex" wrote:

Thanks, Partick.

Your reply is constructive and informative, unlike the other one.

My case is not a Hi-Fi amplifier, but a radio with a single-ended stage. OPT
is quite lousy. It is designed for Rl=7.5K (for 6M5, EL80 or 6F6 tube at
250V). Its leakage inductance, referred to primary is as large as 165mH --
the windings are not interleaved.

There is an interesting variation on this capacitor thing that you will
see in some old radios. Rather than using a single capacitor which
slowly rolls off the high frequency response, two capacitors are used,
one across the primary, and another across the secondary of the output
transformer. The two capacitors, along with the leakage inductance of
the transformer, form a third order low pass filter. This provides a
sharp cutoff to attenuate noise energy above the desired audio band
while providing flatter frequency response below cutoff in the desired
audio band, than a single capacitor can.

Regards,

John Byrns

It was quite common to use caps from anode to the B+ so thus across a
driving triode's load
and the OPT primary, and indeed there is a 3rd order LPF.

But they'd never do this with loop FB connected.

The trouble with no FB in a radio AF amp is that the distortion
is so high that its magnitude competes with real HF content of the
signal.

If the AF bandwidth is a typical 3kHz, then what we hear as HF content
is
a combination of real HF of the source and all the artifacts of THD and
IMD
as surrogate HF content. Hence the coloration of the sound we hear.

Its acceptable for speach because what is important is the the content
of what is said
in news broadcasts and reports from ppl using mobile telephones,
and not the fidelity.

Patrick Turner.



--
Surf my web pages at, http://fmamradios.com/

[Patrick Turner]

Alex wrote:

"Patrick Turner" wrote in message
...
I repair, rewire, or seriously modify at least 5 AM radios per year.
I've done 3 this year and there are 3 waiting to be done, and its only
May.

I built a complete radio in 1999, 10kHz of AF bandwidth, variable
selectivity, linear IF amp, linear detector, linear AF amp with
EL34 in triode with 12AX7 and 12dB FB. Rola 12" Deluxe speaker from
1953
with a dome tweeter from 1972, because the 12" speaker is fairly flat
to
5kHz.

I have never heard any other AM sets that come close except
the one in the AM-FM tuner at my website, which I designed, and of
course the
Quad AM tuner all tubed, that isn't bad at all.

Could you please give a link to your website? Is there a schematic of
that
radio, mentioned above, which you designed? Is it TRF?
Regards,
Alex

Patrick Turner.

Try

http://www.turneraudio.com.au/am-fm-...x-decoder.html

Patrick Turner.
Interesting.
My comments:
1. I am not that fanatical to build a stereo-decoder around LC tanks which
require precise tuning.

But becoming an expert in servicing such things requires that you design
and build
such a thing as you would service. Its called doing an apprenticeship.

2. It was funny to see a reactive tube for AFC instead of a tuning diode (or
any diode like 1N4148).

no problem though; the tube works well...

3. As far as the AM detector is concerned, I think it is not optimum.
Voltage on the detector appears to be about +35V. Forward bias current is
about 90uA. Differential impedance of the diode is about 250ohm (no signal),
220pF capacitor impedance is about 1.5K at 455KHz. Thus at no signal the CF
is loaded by about 1.5K. The CF, running at 1mA DC current would have about
0.5mA/V transconductance, therefore output impedance is about 2K. Thus the
gain of the CF is only 0.4 or so.

I was able to get a very much larger linear output
from the CF detector than is required.

It works, try it. The CF idle dc current of 1mA is OK.
But the gm is much higher than 0.5mA/V.
As the tube turns on harder gm much increases.
It wouldn't work well to get a negative going voltage.
But OK for a positive going voltage.
THD is well under 1%, better than 5% as in most detectors.




For the detector to begin detecting, obviously the AC current through the
diode shall exceed the DC component (90uA). It will require about 150mV on
the input to the detector or conversely 400mV on the grid of the CF.
Sensitivity of such a detector is rather poor (as you know, sensitivity of a
pure p-n diode is about 25mV.)

Doesn't matter because the signal to be detected from the IFT is so
large
that one can get about -12Vdc of AFC voltage, more than needed.


It is easy to improve the result 10 times -- just reduce the bias. Connect
R17 not to the ground, but to point "R" (+31V). After that the diode will be
biased only by the self bias of the tube, which is about 4V. Sensitivity of
the detector will increase 10-fold.

The sensitivity you mention isn't needed or wanted.

The R16 and R17 network are 330k and 100k = 430k,
as as the cathode of CF is at about +33Vdc, there is a dc current
through the diode = 33/430 mA
or about 0.09mA, and this stays fairly constant with changes to AF
voltage in the 220pF.
I found having a virtual constant current discharge of this C was much
better than having
a large R taken to a voltage not much different to Ek.

I suggest you build this detector and give it a decent test using a
455kHz modulated test
signal. The main advanatge is that at high percentages of modulation
used these days the distortion
remains low until very close to 100%.

In many radios the detector suffers cut off distortion as % mod rises.

With very small signals, the detector suffers no diode threshold effects
because the
diode is 'on' at all times.

Patrick Turner.


Regards,
Alex

[John Byrns]

In article ,
"Alex" wrote:

"John Byrns" wrote in message
...
In article ,
"Alex" wrote:

Thanks, Partick.

Your reply is constructive and informative, unlike the other one.

My case is not a Hi-Fi amplifier, but a radio with a single-ended stage.
OPT
is quite lousy. It is designed for Rl=7.5K (for 6M5, EL80 or 6F6 tube at
250V). Its leakage inductance, referred to primary is as large as
165mH --
the windings are not interleaved.

There is an interesting variation on this capacitor thing that you will
see in some old radios. Rather than using a single capacitor which
slowly rolls off the high frequency response, two capacitors are used,
one across the primary, and another across the secondary of the output
transformer. The two capacitors, along with the leakage inductance of
the transformer, form a third order low pass filter. This provides a
sharp cutoff to attenuate noise energy above the desired audio band
while providing flatter frequency response below cutoff in the desired
audio band, than a single capacitor can.

I have never seen that, but it is an interesting idea. Might be useful on
SW. Requires a set of switches to throw relatively large caps in circuit.

The examples I have seen didn't use switches, the cutoff frequency was
fixed at 5 kHz, or whatever frequency it might have been that the
designer choose. The capacitor values, as well as the amount of leakage
inductance, are critical to obtaining a flat response below the cutoff
frequency. To provide several switch selectable filter cutoff
frequencies, all with the desired response shape, it would be necessary
to switch the leakage inductance as well as the capacitors! Patrick, do
you have any practical ideas of how one might provide several switch
selectable values leakage inductance in an output transformer? As far
as simply switching the capacitors out, I would have to check the math,
but I think that leads to a gradual roll off with the capacitors
switched out, starting below the cutoff frequency with the capacitors
switched into the circuit.

As
I understand, you need to turn the feedback off for this 3-rd order filter
to have effect.

Yes it is hard to imagine NFB being used around this sort of filter
circuit, the radios I have seen that used it did not have negative
feedback. Even if there weren't a stability problem, the NFB would tend
to negate the desired effect of the filter. NFB from the output tube
plate would seem possible though.

Turn the NFB off and the mechanical resonance of the
speaker will be undamped.

There may be some mechanical and acoustic damping of the speaker driver,
and certainly you could use power triodes to drive the speaker and
provide damping. IIRC the radios I have seen this circuit used in, did
mostly use triode output tubes. Also the vast majority of tube radios,
and small phono amplifiers, use pentode output tubes to drive the
speaker without NFB, so it is hardly an unheard of situation.

So, there needs to be a special circuit, gradually
disabling the feedback with the frequency rising, but not affecting gain. It
can be done.

If the need for NFB is felt, one could take the feedback from the
secondary at low frequencies, crossing over at higher frequencies to
feedback taken from the plate.

And what if we place more LC pads between the secondary and the
speaker? Potentially can make the radio fit to receive SSB...

Adding "more LC pads" sort of misses the whole point of this circuit
which is to take advantage of the already existing, read free, leakage
inductance of the OPT to create a third order LPF. Adding "more LC pads"
would mean actually paying real money for the additional inductors
required.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

[John Byrns]

In article ,
"Phil Allison" wrote:

"Alex"

May I ask you, Partick, which AM radio station has the least distortion? I
live in Sydney, and I wold say, BBC (around 600KHz) is the best.

** The BBC ???????

Not here in bloody Aussie - maaaaaateee.

Surely you mean " ABC Radio National" on 576 kHz.

Others use
some elaborate compression and clipping which makes one's ears tired very
soon.

** Yaaaawwwnnnnn ...

Do the stations INDEED transmit 18KHz of bandwidth?

** No - but the recovered audio bandwidth derived from ABC Radio
National here in Sydney exceeds 10 kHz for sure.

I own a valve, Aussie made Hi-Fi AM tuner with 15 kHz ( -3dB) bandwidth
able to prove that anytime - the Audiosound AM100.

With only 9KHz grid?

** Not relevant in this country pal - with its huge distances between all
the major cities.

That one fact alone allows Australia to have the highest audio quality AM
radio in the world - or we did until about 20 years ago when AM
broadcasters all turned to brainless " talkback " programming.

Peeeeeuuukee....

On the stereo-decoder note, what is the phasing of 19KHz pilot with
respect
to the 38KHz (suppressed) carrier: do the zero-crossings coinside (every
second one) or peaks coinside (every secon ones)?

** A moot point - I would have thought.

Like " ... is the glass half empty or half full ?? "

It's hardly a moot point, if you adjust it so the peaks coincide the
glass will always be totally empty and you will never get any stereo
separation. The ideal adjustment is where the zero crossings coincide,
although some error here can be corrected by a compensating adjustment
in the matrix, however the signal/noise ratio will suffer.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

[John Byrns]

In article ,
Patrick Turner wrote:

John Byrns wrote:

In article ,
"Alex" wrote:

Thanks, Partick.

Your reply is constructive and informative, unlike the other one.

My case is not a Hi-Fi amplifier, but a radio with a single-ended stage.
OPT
is quite lousy. It is designed for Rl=7.5K (for 6M5, EL80 or 6F6 tube at
250V). Its leakage inductance, referred to primary is as large as 165mH
--
the windings are not interleaved.

There is an interesting variation on this capacitor thing that you will
see in some old radios. Rather than using a single capacitor which
slowly rolls off the high frequency response, two capacitors are used,
one across the primary, and another across the secondary of the output
transformer. The two capacitors, along with the leakage inductance of
the transformer, form a third order low pass filter. This provides a
sharp cutoff to attenuate noise energy above the desired audio band
while providing flatter frequency response below cutoff in the desired
audio band, than a single capacitor can.

Regards,

John Byrns

It was quite common to use caps from anode to the B+ so thus across a
driving triode's load
and the OPT primary, and indeed there is a 3rd order LPF.

How do you get a 3rd order filter out of this, it looks like a 2nd order
filter to me?


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

[Patrick Turner]

Alex wrote:

"Patrick Turner" wrote in message
...


Alex wrote:

"Patrick Turner" wrote in message
...

I repair, rewire, or seriously modify at least 5 AM radios per year.
I've done 3 this year and there are 3 waiting to be done, and its only
May.

I built a complete radio in 1999, 10kHz of AF bandwidth, variable
selectivity, linear IF amp, linear detector, linear AF amp with
EL34 in triode with 12AX7 and 12dB FB. Rola 12" Deluxe speaker from
1953
with a dome tweeter from 1972, because the 12" speaker is fairly flat
to
5kHz.

I have never heard any other AM sets that come close except
the one in the AM-FM tuner at my website, which I designed, and of
course the
Quad AM tuner all tubed, that isn't bad at all.

Could you please give a link to your website? Is there a schematic of
that
radio, mentioned above, which you designed? Is it TRF?
Regards,
Alex

Patrick Turner.

The kitchen radio I mentioned from 1999 above
has a different circuit to one I just gave you at
http://www.turneraudio.com.au/am-fm-...x-decoder.html

I don't have the kitchen radio schematic at the website.

But what my kitchen radio does have is..

3 gang tuning cap.

Two RF coils with ferrite cores for high Q and two C gangs are coupled
so that there is no sideband cutting of AF
right across the AM band, which means each coil is tuned to a slightly
different F.

There is no ferrite antenna, just loose coupling from a wire antenna to
one input coil.

There is a variable µ twin triode used as a cascode RF amp stage and RC
coupled to the 6AN7 F converter
to make an IF signal.
There is AVC voltage applied only to the cascode input triodes.

The IF amp is 6BX6 with unbypassed Rk and no AVC applied, so its fairly
linear.
There is a cathode follower and diode detector cabable of detecting
about 10 times the actual voltage detected with very good linearity and
bandwidth.

Two standard IF trannies are used, but the first one after the F
converter
has one IF coil monted on a slide and a knob on the front of the radio
rotates to move the coil to coilo distance about 10mm.
This allows the radio to be tuned normally to a single
a single peak when tuning and then after closing the coils closer, the
IF BW is much increased to allow 9Khz of audio to be detected, but all
while
retaining very good skirt selectivity and rejection of
stations only 45kHz away even if they have 20 times the transmitting
power.

There are in fact 6 tuned circuits in all, so the attenutation
of signals away from the wanted stations is excellent,
yet what is wanted sounds very nearly as good as FM.

The 3rd gang of the cap is for the oscillator.

I paid very careful attention to tracking.

TRF sets are quite poor performers today because the stations are now
close together,
unlike in 1935, when the band wasn't crowded.

TRF usually used only 3 tuned circuits which needed careful tuning,
and they gave a peaked response, not a flat topped band band pass
character of a critically coupled IFT.

The Superhet with IFTs is definately The Best hi-fi AM tuner you can
have if its built right,
and 99% were atrocious, and all mainly conformed to the lowest common
denominator of utter crap.

When Radio was king before TV, there were very many makers all vying for
a market share.
They all agreed with each other to ensure the standards were lousy.

Patrick Turner.

May I ask you, Partick, which AM radio station has the least distortion? I
live in Sydney, and I wold say, BBC (around 600KHz) is the best.

The BBC does not broadcast at 600kHz into Sydney areas.

I think you mean 2FC, or 2BL. One is used for Radio National.
Networked RN is on FM elsewhere in Oz.

A Google search would tell you a lot more.

I RN at Canberra at 2CY, 846kHz, and music programmes sound very well
indeed.
Audiophile quality.
Never mind the compression and MP3 formats.
2CA at 1053 here does ancient pop music and adverts, so I never listen
to it
but the processing used on the old music source material makes it sound
better
than in 1970. The gear is simply better and the digital compressing
removes clutter in the audio and it sounds cleaner, but I don't wanna
hear
'House of the rising sun' another 400 times each month.


Others use
some elaborate compression and clipping which makes one's ears tired very
soon.

I find the opposite is true.

Its the intellectual content or lack of it that prevents me listening
or watching most broadcast media including TV and FM.

I don't have a working TV in my house because its a corrupting
influence.




Do the stations INDEED transmit 18KHz of bandwidth? With only 9KHz grid?

In the ACT, the closest any two stations are is 1008, racing info junk,
and 1053, old mouldy pop and advert junk.
Both these stations are only 5km away from me with 1053 at 5kW,
and 1008 at 300W.

Only 45kHz apart. A good AM set will pull in 1008 without hearing 1053
feintly in the background. Many AM sets fail the test miserably,
including SS junk.

A good set would allow 15kHz of AF modulation on the carrier, ie
sidebands extending 15kHz each side of the carrier.

There is another station on 1008, in Tassie, but see the list of
stations in Oz and their F.

AFAIK, where full AF BW up to 9kHz does not cause interference to other
stations, it is allowed.

Find out with Google.


On the stereo-decoder note, what is the phasing of 19KHz pilot with respect
to the 38KHz (suppressed) carrier: do the zero-crossings coinside (every
second one) or peaks coinside (every secon ones)?

I have never bothered to check.

The decoder just works.

It took lots of fiddling around though.

I would guess, the peaks. That way it would have been easier to derive 38KHz
simply by a class C frequency multiplier.

By the way, in the Russian stereo system, there is no 19KHz pilot, but just
a 38KHz vestige with a proper phase. It gets boosted by a q-multiplied LC
tank and fed to a diode ring, similar to what you did..

Yeah, you could have 38kHz, not totally suppressed, instead of the
19kHz which is 10% of the maximum AF signal,
so to prevent IMD between upper AF and 19kHz, the AF used to modulate
the
carrier must be cut off sharplyat 16kHz.

Maybe the russian system works better.

But of course the 38kHz is too low, and should be 50kHz, and then the
pilot could be at 25kHz, and that would make receivers easier.
But the GE-Zenith system allows for multiplexing and there can be
complete audio channels with 67kHz and 96kHz sub carriers.

Patrick Turner.




Regards,
Alex

[Iain Churches[_2_]]

"Jon Yaeger" wrote in message
...
in article , Phil Allison at
wrote on 5/5/08 9:55 AM:


"Jon Yaeger = CRIMINAL GARAGE RODENT "


** Need lotsa rat bait.

So he will breed to death.




.... Phil



To "breed to death" -- what a way to go . . . Thanks for sending me good
thoughts, Philthy!


Excellent:-)
That has to be the typo of the year.

[John Byrns]

In article ,
"Alex" wrote:

On the stereo-decoder note, what is the phasing of 19KHz pilot with respect
to the 38KHz (suppressed) carrier: do the zero-crossings coinside (every
second one) or peaks coinside (every secon ones)?

I would guess, the peaks. That way it would have been easier to derive 38KHz
simply by a class C frequency multiplier.

Your guess is incorrect, the 19 kHz pilot tone and the regenerated 38
kHz carrier must be phased so that the zero-crossings coincide, that is
the way the original system specification was written. If the decoder
is adjusted so that the peaks coincide it will be impossible to achieve
any stereo separation at all. If the phasing is such that the
zero-crossings do not coincide, but are close, it is possible to achieve
full separation by adjustment of the matrix separation control, but the
signal to noise ratio will be degraded.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

[Alex]

On May 6, 7:17*am, John Byrns wrote:
In article ,

*"Alex" wrote:
On the stereo-decoder note, what is the phasing of 19KHz pilot with respect
to the 38KHz (suppressed) carrier: do the zero-crossings coinside (every
second one) or peaks coinside (every secon ones)?

I would guess, the peaks. That way it would have been easier to derive 38KHz
simply by a class C frequency multiplier.

Your guess is incorrect, the 19 kHz pilot tone and the regenerated 38
kHz carrier must be phased so that the zero-crossings coincide, that is
the way the original system specification was written. *If the decoder
is adjusted so that the peaks coincide it will be impossible to achieve
any stereo separation at all. *If the phasing is such that the
zero-crossings do not coincide, but are close, it is possible to achieve
full separation by adjustment of the matrix separation control, but the
signal to noise ratio will be degraded.

Regards,

John Byrns

--
Surf my web pages at, *http://fmamradios.com/

I see, John.
Thanks for the info.

If it is so that the sero-crossings coincide then it better suits a
PLL-based multiplier. Probably the early designers had that method in
mind, not the harmonic frequency multiplier... In case of Patrick
Turner it indeed makes no difference, because he fiddles with at least
two LC tanks to get the phase right. In his circuit the phase
adjustment is so broad, that he can possibly swap left and right
channels if he wanted to.

Regards,
Alex

[Phil Allison]

"Patrick Turneroid Psychotic Freakoid "


You are the only person with the view that capacitor load testing
of amplifiers is a useful tool to make amplifiers as stable as we may
want them.

** ???????????

Wot drivel.


Unconditional stability is found in very many tube and SS amplifiers.


** That don't make it necessary.

You ****ing idiot.




....... Phil

[Alex]

"Patrick Turner" wrote in message
...


It works, try it. The CF idle dc current of 1mA is OK.
But the gm is much higher than 0.5mA/V.
As the tube turns on harder gm much increases.
It wouldn't work well to get a negative going voltage.
But OK for a positive going voltage.
THD is well under 1%, better than 5% as in most detectors.

Patrick, do you have a quality signal generator capable of nearly 100% AM
to test?
Otherwise, how could you measure the detector THD?

I guess such AM generator can be built on the basis of a DSP and a good
DAC... Possibly it might be my next weekend project.

For the detector to begin detecting, obviously the AC current through
the
diode shall exceed the DC component (90uA). It will require about 150mV
on
the input to the detector or conversely 400mV on the grid of the CF.
Sensitivity of such a detector is rather poor (as you know, sensitivity
of a
pure p-n diode is about 25mV.)

Doesn't matter because the signal to be detected from the IFT is so
large
that one can get about -12Vdc of AFC voltage, more than needed.

Yes, agree. With such a strong signal any detector will work, even the so
called cathode detector -- just remove the silicon diode, remove the 33K
pull-down resistor and use the sharp cut-off tube solely.


It is easy to improve the result 10 times -- just reduce the bias.
Connect
R17 not to the ground, but to point "R" (+31V). After that the diode
will be
biased only by the self bias of the tube, which is about 4V. Sensitivity
of
the detector will increase 10-fold.

The sensitivity you mention isn't needed or wanted.

Understand.

The R16 and R17 network are 330k and 100k = 430k,
as as the cathode of CF is at about +33Vdc, there is a dc current
through the diode = 33/430 mA
or about 0.09mA, and this stays fairly constant with changes to AF
voltage in the 220pF.
I found having a virtual constant current discharge of this C was much
better than having
a large R taken to a voltage not much different to Ek.

I suggest you build this detector and give it a decent test using a
455kHz modulated test
signal. The main advanatge is that at high percentages of modulation
used these days the distortion
remains low until very close to 100%.

My favourite detector is the emitter detector preceeded by a source
follower. Its sensitivity is 25mV. A great disadvantage though that it can
not stand a carrier more than 1.5...2Vm because of the reverse breakdown
voltage of the transistor. Linearity is perfect.

In many radios the detector suffers cut off distortion as % mod rises.

Agree. Some can not stand more than 60% of modulation, even at low audio
frequencies.

With very small signals, the detector suffers no diode threshold effects
because the
diode is 'on' at all times.

Yes, the diode is always ON, but it does not do good, contrary to a common
belief. Because of high diode bias, the detection (rectification) will begin
after the 455KHz AC component will exceed the DC component. Below that the
diode is virtually a resistor of about 250ohm. So your CF is loaded by a
250ohm in series with 220pF, representing about 1.6K impedance. From that
you need at least 150mV of the signal on the diode (more on the grid) for AC
exceed DC.

Regards,
Alex

Patrick Turner.


Regards,
Alex

[Phil Allison]

"Alex"

** So you are really called Alexei - eh ??

Are you another " fat ******* " wearing an ill fitting, shiny cheap
suit ??

Know any good jokes ??




....... Phil

[Phil Allison]

"Alex"


** So you are really called Alexei - eh ??

Are you another " fat ******* " wearing an ill fitting, shiny cheap
suit ??

Know any good jokes ??



....... Phil

[Phil Allison]

"Alex"


** So you are really called Alexei - eh ??

Are you another " fat ******* " wearing an ill fitting, shiny cheap
suit ??

Know any good jokes ??




....... Phil

[Patrick Turner]

Alex wrote:

"Patrick Turner" wrote in message
...


It works, try it. The CF idle dc current of 1mA is OK.
But the gm is much higher than 0.5mA/V.
As the tube turns on harder gm much increases.
It wouldn't work well to get a negative going voltage.
But OK for a positive going voltage.
THD is well under 1%, better than 5% as in most detectors.

Patrick, do you have a quality signal generator capable of nearly 100% AM
to test?
Otherwise, how could you measure the detector THD?

I have a Topward sig gene which produces good AM adjusrable to over 100%
mod.
Its SS.
My own home designed and built AM gene has all tubes and can do 100% +
mod, but its
not as linear.

To find out THD, the modulated envelope wave form of the carrier in the
gene
is compared with the detector output AF using a dual trace CRO.
Once lined up so the AF trace outlines the envelope shape the %
moulation can be changed
but there isn't any discernible departure between AM wave and the AF
recovered,
so I guess THD is or close to 1%.

I have compared the input AM waves to recovered audio in other detectors
that are not so linear.



I guess such AM generator can be built on the basis of a DSP and a good
DAC... Possibly it might be my next weekend project.

Why would you bother?

Just digitise the antenna signal and use digital processing to extract
the audio.
It may not sound as well as a CF and diode.
There are whole virtual radio chips available.

50 ways to leave your lover, and 51 ways to decode AM,
52 reasons why I didn't get laid last night.
Many different ways of doing lotsa things.



For the detector to begin detecting, obviously the AC current through
the
diode shall exceed the DC component (90uA). It will require about 150mV
on
the input to the detector or conversely 400mV on the grid of the CF.
Sensitivity of such a detector is rather poor (as you know, sensitivity
of a
pure p-n diode is about 25mV.)

Doesn't matter because the signal to be detected from the IFT is so
large
that one can get about -12Vdc of AFC voltage, more than needed.

Yes, agree. With such a strong signal any detector will work, even the so
called cathode detector -- just remove the silicon diode, remove the 33K
pull-down resistor and use the sharp cut-off tube solely.


It is easy to improve the result 10 times -- just reduce the bias.
Connect
R17 not to the ground, but to point "R" (+31V). After that the diode
will be
biased only by the self bias of the tube, which is about 4V. Sensitivity
of
the detector will increase 10-fold.

The sensitivity you mention isn't needed or wanted.

Understand.

The R16 and R17 network are 330k and 100k = 430k,
as as the cathode of CF is at about +33Vdc, there is a dc current
through the diode = 33/430 mA
or about 0.09mA, and this stays fairly constant with changes to AF
voltage in the 220pF.
I found having a virtual constant current discharge of this C was much
better than having
a large R taken to a voltage not much different to Ek.

I suggest you build this detector and give it a decent test using a
455kHz modulated test
signal. The main advanatge is that at high percentages of modulation
used these days the distortion
remains low until very close to 100%.

My favourite detector is the emitter detector preceeded by a source
follower. Its sensitivity is 25mV. A great disadvantage though that it can
not stand a carrier more than 1.5...2Vm because of the reverse breakdown
voltage of the transistor. Linearity is perfect.

In many radios the detector suffers cut off distortion as % mod rises.

Agree. Some can not stand more than 60% of modulation, even at low audio
frequencies.

With very small signals, the detector suffers no diode threshold effects
because the
diode is 'on' at all times.

Yes, the diode is always ON, but it does not do good, contrary to a common
belief. Because of high diode bias, the detection (rectification) will begin
after the 455KHz AC component will exceed the DC component. Below that the
diode is virtually a resistor of about 250ohm. So your CF is loaded by a
250ohm in series with 220pF, representing about 1.6K impedance. From that
you need at least 150mV of the signal on the diode (more on the grid) for AC
exceed DC.

My detector is a vast improvement on many others I have inspected in
normal
old radios.

Patrick Turner.

Regards,
Alex

Patrick Turner.


Regards,
Alex

[Patrick Turner]

Phil Allison wrote:

"Alex"

** So you are really called Alexei - eh ??

Are you another " fat ******* " wearing an ill fitting, shiny cheap
suit ??

Know any good jokes ??

I realize Alexei is your hero and mentor
and you don't mind that he's a "fat barstard".
Nor do I, and I enjoyed his sense of humour.

Somehow, Alexei's ideas about everything coincide with
Osar Wilde's ideas that life was too brief to be serious about it.

I could suggest you should adopt more of Alexei's attitude....

Probably, Alexei is still around somehwere and keeping a few smiles on
people's faces.

http://en.wikipedia.org/wiki/Alexei_Sayle

Patrick Turner.



...... Phil

[John Byrns]

In article
,
Alex wrote:

On May 6, 7:17*am, John Byrns wrote:
In article ,

*"Alex" wrote:
On the stereo-decoder note, what is the phasing of 19KHz pilot with
respect
to the 38KHz (suppressed) carrier: do the zero-crossings coinside (every
second one) or peaks coinside (every secon ones)?

I would guess, the peaks. That way it would have been easier to derive
38KHz
simply by a class C frequency multiplier.

Your guess is incorrect, the 19 kHz pilot tone and the regenerated 38
kHz carrier must be phased so that the zero-crossings coincide, that is
the way the original system specification was written. *If the decoder
is adjusted so that the peaks coincide it will be impossible to achieve
any stereo separation at all. *If the phasing is such that the
zero-crossings do not coincide, but are close, it is possible to achieve
full separation by adjustment of the matrix separation control, but the
signal to noise ratio will be degraded.

Regards,

John Byrns

--
Surf my web pages at, *http://fmamradios.com/

I see, John.
Thanks for the info.

If it is so that the sero-crossings coincide then it better suits a
PLL-based multiplier. Probably the early designers had that method in
mind, not the harmonic frequency multiplier...

I don't know, it could easily be that the early designers had the
harmonic frequency multiplier method in mind. Consider the following
argument. First IIRC there is a 90 degree phase shift between the
primary and secondary of a double tuned transformer, my whole argument
depends on that fact so please correct me if I have that point wrong.
Second a 90 degree phase shift at 19 kHz translates into a 180 degree
phase shift at 38 kHz, which simply results in a reversal of the left
and right channels, while a 90 degree phase shift at 38 kHz results in
no recovery of the stereo difference signal and hence no stereo
separation.

Now as long as the phase shift in the 19 kHz filters and amplifier is a
multiple of 90 degrees at 19 kHz the phase shift at 38 kHz will be a
multiple of 180 degrees at 38 kHz, whose only effect will be to reverse
the two channels. Let's assume that we have a single tuned 19 kHz
filter preceding the grid of a 19 kHz amplifier tube and a double tuned
transformer with a center tapped secondary winding connected to the
plate circuit of the 19 kHz amplifier tube, providing us with a multiple
of 90 degrees of phase shift at 19 kHz. If we ground the center tap of
the 19 kHz transformer secondary and connect a diode from each end of
the secondary to the grid of a 38 kHz amplifier tube we will have a
frequency doubler to generate the local 38 kHz carrier needed to
demodulate the stereo difference signal. This type of frequency doubler
will cause a 90 degree phase shift at 38 kHz because the peaks of the
resulting 38 kHz signal will be aligned with the peaks of the 19 kHz
pilot, rather than having the zero crossings aligned as they should be.
But we aren't done yet, the 38 kHz signal out of the diode frequency
doubler is distorted and contains harmonics of 38 kHz. If we connect
another double tuned transformer to the plate circuit of the 38 kHz
amplifier, it will remove the distortion on the 38 kHz waveform, and at
the same time introduce another 90 degree phase shift at 38 kHz
effectively canceling the effect of the 90 degree phase shift introduced
by the doubler circuit. The secondary of the 38 kHz transformer then
drives the stereo demodulator or switching diodes.

So it appears to me that the original designers of the pilot tone stereo
system had LC type decoders in mind, not PLL decoders.

Another factor might be that the original designers wanted to keep the
amount of 19 kHz leaking into both stereo channels the same. I haven't
done the math yet, but it isn't obvious to me that this condition would
accrue if the signal specified aligned peaks rather than aligned zero
crossings. I will have to check to see if this is actually the case.

In case of Patrick
Turner it indeed makes no difference, because he fiddles with at least
two LC tanks to get the phase right. In his circuit the phase
adjustment is so broad, that he can possibly swap left and right
channels if he wanted to.

I haven't looked at Patrick's stereo decoder in a while, I will have to
take a look and refresh my memory before commenting. Some of Patrick's
designs do tend to be a little Baroque.

I just took a very quick look at Patrick's tuner design as I am posting
this, and the first thing I noticed is that the Ratio Detector won't
work as drawn, but presumably it works just fine a built. The stereo
decoder design appears to be much more straight forward than his first
design, which was quite unusual and likely didn't have very good
performance.


Regards,

John Byrns

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