In article ,
Patrick Turner wrote:
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.
HP had an interesting AM demodulator in their AM/FM modulation meter. The AM
detector used transistors, not ICs. A rough description as I remember it is as
follows. The transistors were configured in what was basically a darlington
configuration. Feedback was taken from the collector through a capacitor to a
pair of parallel diodes back to a summing junction at the base. The two diodes
were connected in opposite directions, with a pair of parallel connected complex
cognate networks in series with one diode at the end connected to the transistor
base. The modulation output was taken across the network that has the form of a
low pass filter IIRC.
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.
If you are willing to use an IC you can get a whole synchronous detector circuit
in a single IC in the form of one of the old CQUAM AM Stereo decoder IC chips.
There are instructions on the web that describe how to do this.
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?
Me thinks you misinterpreted what I meant by the term "Binary decisions".
Here are the three decisions I have to make, perhaps I should flip a coin and be
done with it.
1. The output stage will be of the distributed load type with 50% of the load in
the cathode circuit, and 50% in the plate circuit. The decision yet to be taken
is should I use the McIntosh output circuit, or an Acoustical style of output
circuit? This decision will have some impact on the layout of the power supply.
2. I must settle on the output tubes to be used, a QUAD of 25L6GTs in push-pull
parallel, or a pair of 7695s. Actually this is a ternary choice, the KT55 is
another possible choice for the output tubes, while these are cool tubes,
finding some would probably not be easy. The output tube choice has a major
impact on the chassis size, and layout, so I have to settle it before I purchase
a chassis and begin drilling and punching it.
3. The driver tube must also be specified, the binary choice was between the
6211, and the 12AU7, yesterday I made it a ternary choice by adding the 12AT7 to
the list. These tubes all use the same socket, so they can all be tried with
only a change of cathode resistor.
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.
I think you are just intimidated by the excellent German engineering in this
receiver, to eliminate any possible excuse of your not being able to open it, I
have posted a copy of the schematic on my web pages at this URL:
http://fmamradios.com/stuff/E1.pdf
--
Regards,
John Byrns
Surf my web pages at,
http://fmamradios.com/