So to see how much THD is introduced by the radio set one may use a
dual trace CRO to display both test
signal at the radio input and the detected AF and overlay the waves to
see how they compare. Using as much of the screen as possible, its not
difficult to see if there is less than 1% of additional THD added by
the radio set. All that takes time but its a very simple thing
Unfortunately it is also somewhat subjective.
and I
cannot understand why you would not have tried out all sorts of AM
detector ideas like I have to see what works best for you.
I already know that a well-designed diode detector works best for me; at least
since I regressed to tubes, in my IC days I would have answered differently.
With ICs you get a dc coupled amp and its possible to use an output
diode pointed at an RC circuit with the audio voltage and Vdc fed back
to the FB input port of the IC, and thus get what is supposed to make
the most linear detector imaginable because there is MUCH more NFB
being applied than when using a cathode follower like I do. The IC
input impedance at IF frequency needs to be high though. I have
circuits for doing all that but the CF I use is just fine.
You say that you ³have tried out all sorts of AM detector ideas², besides your
spin on the diode detector, and your ill fated attempts at building a syncrodyne
receiver, what have you tried?
I've tried a lot of variations on normal arrangements with IF coil
feeding an anode in a vari-mu pentode IF amp and following RC circuit.
The CF seems best to me.
I tried to build a version of the DG Tucker Synchrodyne circuit
published in Wireless World in about 1947. Maybe its now online some
place, and what prevented progess was the making of a suitable
balanced synchronous demodulator. Probably the best way is to use a
toroidal transformer with the core µ just right for range of RF
frequencies. But I never had time to explore how to make such a
tranny. The tucker circuit uses lots more tubes than a superhet and
needs much more practical expertise to get running without loud
whistles while tuning. It was a potentially excellent thing but it was
never to become popular because the superhet was king. Then tried
making a synchrodyne based on a self oscillating 6BE6, and there are
some simple circuits of those around, just RF input tube and 6BE6
needed only. That sort of worked a bit but monkey chatter and whistles
and controlling oscillations just right were easier said than done so
I abandoned the idea and proceeded with a good superhet. AGC is
applied to RF vari-mu cascode triode input amp and 6AN7mixer, then
6BX6 sharp cut off IF amp without AGC and using some unbypassed Rk for
local current FB. AF bandwidth is 10kHz, with minimum distance between
IF coils, but even with max distance AF BW is about 6kHz. Tone control
boosts and cuts F above 2kHz. 12AU7 CF detector used, and after
comparing the sound of my kitchen AM radio to countless others I have
repaired, my own design is very much better to listen to. I have a
Marantz AM/FM tuned hooked up to allow switching to FM in mono, and
when the same ABC news program is being broadcast at the same time on
local ABC AM stations and FM stations the AM radio I have sounds
clearer than the FM produced by the SS tuner. There is a subtle
difference, and a pleasant one. The AM from the Marantz is ****ing
awful.
I try not to give AM radio much attention in my life because I have
1,001 projects to complete for customers and several amp projects fr
myself in progress. But I get "trapped by AM radios" which ppl want
fixed. The majority are collectors items with outrageously poor
performance if they are restored to original condition, and because
ppl use compact flourescent lamps and 101 other things which create
noise, a ferrite rod antenna must be used and the MW RF input coild
junked. Then inputs for tuner and CD must be put in with a switch to
people are not stuck having to listen to the appalling programme
material such as talk back and commercial garbage laced with adds. The
speakers and amps need rebuilding to meet modern expectations. Thre
are some who might junk all the tubes and put in a board full of SS
parts but then that betrays tubes, and I don't do that trick.
What the
**** impedes your progress to your workshop and soldering iron?
Two things, first the fact that AM detectors are of academic interest only, as
there has been no AM broadcasting around this area, that needs anything better
than the crudest AM detector, for at least 15 years or so. *Second, I think I am
more interested in putting my soldering iron to work on the John Stewart
inspired ³improved 25L6² amplifier. *There are three binary design decisions to
be made before I purchase a chassis and begin punching it.
So it seems unlikely you'll ever listen to the benefits of my ideas.
Binary decisions eh. In a bind are you?
If you
have a broken leg, or are impaired or disabled in some way, then let's
hear about it.
Ah, you think you can drag me into an off topic discussion, it won't work, if
you want to explore that kind of discussion, please take it to email.
I guess you think some things are best kept out of the public gaze.
Its remarkable how many people are secretive about themselves, I'm one
who isn't. I have very little to feel ashamed of or embarrassed
about.
I googled * Telefunken AM rebroadcast receiver "Ballempfänger"
Nothing to be seen.
You weren't persistent enough, for a general description go he
http://www.radiomuseum.org/r/telefun...faenger_b.html
If you aren't a member, I'm not, you won't be able to see the schematic. *The
schematic is available from several other web sites though, you can find it in
two parts he
http://www.tsf-radio.org/schph.php?f...Ball-Empfanger....
and he
http://www.tsf-radio.org/schph.php?f...Ball-Empfanger....
While the design looks like the ³bean counters² were kept at a safe distance,
there still isn't a cathode follower in sight!
Those links open but the pictures won't open to reveal what you are
talking about.
I doubt I am missing anything.
AFAIK, not one single commercial example
exists of a radio with "infinite impedance detector" even though the
Selsted & Smith example is given in RDH4, page 1,495, Figs 27.56,
27.57
RDH4 does not have anything that works as well as what I invented for
myself 14 years ago.
In the US J.W. Miller offered both commercial AM tuners and Radios using
the
infinite impedance detector, I have one of their AM Tuners. *Altec Lansing
also
offered an AM-FM tuner that used the infinite impedance detector in the AM
section. *Sargent Rayment also offered a line of tuners and receivers that
used
the Selsted & Smith detector in the AM sections. *Ampex also built an AM-FM
tuner that used a perverted variant of the Selsted & Smith detector, I also
have
one of these.
I've never seen any of these things in Oz or any of their schematics.
Infinite Z detector isn't bad, but gives very low output.
That's an interesting comment as the output level depends on the input just as
with a diode detector. *My experiments with the circuit suggest that it does
exploit the characteristics of the low mu triodes to improve operation at low
carrier levels.
The infinite Z detector should work very well if a CF is used to
buffer the IFT output to produce a low Z drive which might then be
applied to a diode and then to resistance to then be able to extract
the average voltage across the resistance, and without the diode
having to charge a cap. Kinda like a half wave rectifier in a PSU
where the aim is to have the AF signal linear to average voltage. One
can use a choke and cap after the diode in the same way.
I've not needed to explore all that because I find the CF driving the
diode which charges the RC works just fine. The second CF can also be
rigged as a low gain AF amp with RC coupllng but R is a bias R of say
1M5 taken to a tap on Rk, so it appears as maybe 5M0 to the detector
circuit so cut off at high AM% is avoided.
The anode then can easily give gain = 4. it does mean that where one
has say 2Vrms of AF at the detector, you have 8Vrms at 12AU7 anode,
and that's getting high, tempting high THD, so I prefer having two CF,
with the second just to buffer the RC detector circuit and stop the
vol control loading. some Gain for the poweramp is easily created in a
tone control stage.
It's hard to beat a carefully designed diode detector though.
Depends. Most are distortion generators.
Those are not ³carefully² designed, it is easy to screw up a diode detector, it
is equally easy to build a good one but it costs a little gain. *This and the
output transformer are the areas where the work of ³bean counters² can be most
easily seen.
Indeed.
Most bean counters justify their employment by being able to reduce
the parts and labour needed to make something, and therefore
increasing shareholder profits and most often reducing the sound
quality and reliability in electronics produced by the company.
The opposite approach, of adding components that serve no useful function
is
just as bad, actually worse.
Each unto their own. everyone has a different way of doing things, and
commercial competition stifles anyone who dares to use one more nut,
bolt or resistor than the ****ing opposition brand. The result is that
they all have competitions to seewho can dumb **** down the most while
maintaining enough sales for the shareholders to dine well.
Hardly, commercial products today are overflowing with every imaginable feature,
each of which adds to the parts count. *Competition today seems to be based more
on features, with cost as a secondary issue, why make something more expensive
than it needs to be to provide the required features and performance?
I have a Marshall JCM2000 60W guitar amp pulled to bits on the bench.
It has an enormous lot of features and 5 times the number of R&C used
in 1960, and maybe 15 opamps and it is a complete PIA to work upon.
Nobody much gives a **** about the concerns I have, such as it should
be simple and easy. But some guitar amps are excellently configured
with ultra simplicity, real class A, and hardly any features. The
really good musos don't need features; they know how to entertain
without needing gear features to cover shortcomings.
If it makes you ill to use one more cathode follower than someone
somewhere said could be superflous, then don't. But I will. In my
vicinity there are no shareholders or bean counters, and my clients
get a good deal without having to pay for someone's Cadillac or
expensive lunches, neither of which contribute to sound quality.
An unneeded cathode follower doesn't contribute to sound quality either. *I
won't bother to ask who pays for your lunches and transportation, least it take
us off topic again, but I am reasonably sure they are accounted for in the price
of the amplifiers you build.
I make all payments around here and I hear the benefits of my circuits
as do my customers. I make very low wages with all I do because the
general public never pays artisans very much. I like my work, so I do
my work. Toilet cleaners get paid more. It worries me not, money ain't
everything.
Patrick Turner.