John Doty wrote:

Telamon wrote:

Congratulations for the continuation of one of the most retarded
threads I've yet read. Never heard of biasing a diode or being unable
to understand doing so is pretty pathetic. Electronics does not get any
simpler than this.

John understands this stuff extremely well: I've argued detector issues
with him on rec.antiques.radio+phono in the past. John obviously doesn't
believe Patrick really understands what he's advocating.

I am not a university trained electronics engineer with a sound
backgound of mathematical ability.

But one doesn't need to understand a simple AM radio at the post graduate
level, complete with all the maths, to be able to build a set that works
better than nearly all the old junk I occasionally get to service or repair.

John has ofrten tested my ideas expressed here on the group, and I have never
minded that,
bearing in mind that one quick explanation to *all* readers may not be enough
to
provoke them into forming a clear mental picture of how they could
build a radio from scratch if they had to.


Biased diode envelope detectors are *not* simple: I've used them for
x-ray spectroscopy with scintillation counters, and they are tricky
beasts. A biased diode is far from an ideal switch: its dynamic
resistance varies with instantaneous signal level, making the circuit
bandwidth vary rapidly. The mathematics of this are rather difficult.

But your applications in X-ray spectroscopy and scintilation counting,
whatever that involves, may be somewhat different to an RF diode detector
circuit
where afaik diode detection is one heck of a simple concept, with very much
in common with a diode power rectifier.
The crystal diode detector works fine as a switch at 10.7 MHz in an FM
receiver discriminator circuit,
and 455 kHz is a doddle.

The math don't have to be known.

As I pointed out in another recent post, a comparison between the
AM envelope shape and the recovered audio can and should be made using
a dual trace oscilloscope, at low and high levels of signals, and at high and
low levels
of AM%, and at 20 Hz, 1 kHz, and 20 kHz of audio F modulation.
If the detector is then seen to be low thd with all tests, and no tube or
diode
is anywhere near an overloaded or over rated condition, then
you have the circuit working propoerly without having had to use
university standard maths.



I also think an emphasis on detector distortion under idealized test
conditions misses the real issues. The most annoying distortion on AM
signals doesn't come from the detector. Multipath, steep IF skirts, and
AGC all distort the modulation envelope. Perfect reproduction of such
distorted envelopes yields bad sound. I suspect that the great sound of
the old tube diode detectors actually results from their poor
reproduction of these sorts of envelope distortion (but this is a
difficult hypothesis to test).

I have built many tube amps and a few radios, and the better they measure,
the better they sound.
A wide bandwidth for the IF allows less critical tuning, so the distortion
effects
of an IFT or the front end being slightly off tune are negligible.



In any case, my 1934 Stromberg-Carlson 58-T, with its weak AGC, poor
skirt selectivity, and a classic diode detector has the best sound of
any of my AM radios, both to my ears and my wife's. One receiver it
beats is my Drake R-8, which uses a very low distortion (active full
wave rectifier) envelope detector. Of course, the R-8 is a much better
DX machine, but that's a different issue.

I have serviced dozens of old radios which came the the workshop much loved,
but some sounded quite dreadful for a variety of reasons in the
RF, mixer, IF detector and audio amps, not to mention the speaker.
I sometimes just repair these horrors, rather than modify them.

One spectaculary displeasing radio I was given is a 1957 Radiola with 7 bands
including the BCB,
and it included a tuning cap with 3 gangs, because an RF stage was included.
It was the deluxe radio model from that company, and even had a PP audio amp
with two 6V6 with FB, and a better than average 12" speaker.
It has proper active tone controls.
The tuner section gave a lousy 2 kHz of audio BW.
Turning up the treble boost did nothing; there is no treble to boost.
It was said that such radios sounded "mellow", and I dislike them intensly.
I have completely revised the circuit to my own, and now it sounds far
better,
with about 8 kHz of AF bw.
The SW performance isn't too good, and the alignments and tracking
are all very inaccurate, but I rarely if ever listen to SW.
One of these days I will build a cabinet for the Radiola chassis I have,
and build a speaker box.

Unlike the big Radiola, most old AM radios were designed to be cheap to make,

with a minimum of parts, and thus were slightly too simple for my way of
thinking,
or my ears.

When one extends the audio BW and reduces the thd from 5% to 1%,
then a good AM set sounds almost as good as thre same material being
broadcast on FM.

My kitchen radio has a full range speaker in a 60 litre reflexed box separate
from
the cabinet for the radio tuner and SET audio amp with NFB using one EL34.
I don't like the sound of a 6V6 with no FB powering a speaker with 6 kHz of
unflat bandwidth,
with the speaker mounted in a resonant cabinet with an open back and resonant
sides.

I don't like listening to distance AM stations, because the vast majority
transmit
programme material almost identical to what is available locally since
networked stations
have universally spread around the country, and the distance listening can
never be
without noises.

Patrick Turner.