John Byrns wrote:

In article , Robert Casey
wrote:

Just posted a schematic of the Miller TRF receiver, with the "secret"
inductance values filled
in. a.b.p.radio

Hi Robert, that's certainly a cute little radio you have there. An
interesting point is that the separate "negative mutual coupling"
inductance, the one with the "secret" value, isn't even necessary and the
part can often be eliminated from the circuit. All you need to do is wind
L1 and L2 like a typical double tuned IF transformer, and if the coupling
coefficient is correctly chosen to yield the required value of mutual
inductance, and if the two windings are phased correctly to make the
mutual inductance "negative", then the separate coil like you used isn't
necessary, although you must retain the capacitor in the common lead of
"L1" And "L2", since that is part of the "secret". This scheme will work
in a circuit like the Miller "High Fidelity" Crystal Tuner where L1 and L2
are just single winding coils, there would obviously be problems applying
the idea to your circuit because of the extra winding you put on L1,
making it into a transformer by itself.

I think there is a reason why the CT choke is used for coupling the earthy
ends of two Ls in a pair of LC circuits.
I tried to use the mutual coupling solely via a 0.1 uF cap,
or about that value, and the reponse doesn't always come out nice and flat and
symetical
each side of the centre F.

Its not as easy as ypou think to get this sort of circuit to work like the
text books say.



I listened to the WABC "jpg" you posted, and the tuner certainly has a
good bandwidth, although I wonder how much pre emphasis WABC might have
been using and how well your receiver matches it, I will have to listen to
it again to see how correct the de emphasis seems to me, there was also
some background noise at several points, I will have to listen again to
see if it was part of the audio at points, or if it was interference of
some sort. The thing I didn't like about it was that it had pretty
horrible levels of distortion, and while this could be WABC's fault, I
have found that it is typical of these so called "High Fidelity" crystal
receivers. I have a couple of J.W. Miller "High Fidelity" crystal tuners,
and they have the same distorted sound. I think this is because the
crystal detectors produce really horrendous distortion, unless you have a
big enough antenna to get the audio output level up to at least the 2
volts RMS that Patrick recommends.

The type of detector used does affect the audio quality.

A germaniun diode detector should never be driven straight off
a tuned secondary IF circuit.
It should be driven by a low impedance source, ie a buffer stage such as a
cathode follower,
which is a 12AU7, 12AT7, or a trioded 6AU6 or some such.
The CF grid is direct connected to the active end of the last IF or RF tuned
circuit.
The earthy end of this LC is connected to a +30volt source from a resistance
divider from the B+,
and bypassed with 100 uF to 0V.

In my radio, the CF is 1/2 a 12AU7 and has a 15 k to 0V.

The diode anode is connected to the tube's cathode, and the diode's cathode
feeds
a 270 pF cap with 1M in parallel to 0V.

Then a 0.022 uF couples the audio output of the 270 pF to a series 100k and
screened lead
a 500k log pot to 0V.
This is the volume control.

The wiper of the volume pot feeds the grid of the other 1/2 of the 12AU7
via a screened lead.
This tube is set up as a normal plate loaded gain stage with 3k3 for Rk,
bypassed with 0.01 uF.
The DC carrying R = 47k, and B+ = 235v, but could be more.
The screened leads and miller C of the 12AU7 gain stage act as a second C to
filter the 455 kHz ripple voltage from the recovered audio signal from the
detector.

The 12AU7 gain stage plate signal is cap coupled via 0.033 uF to a
120k in series with one 1/2 of a dual gang 100k + 100k linear pot,
which is in series with the other 1/2 of the pot which forms
a resistance total of 320k to 0V.

The centre join of each 1/2 of the two pot tracks is the output to the power
amp.

There is a 180 pF compensation cap between the earthy side of the
0.033 uF and the join of the two tracks of the pot.

Each wiper of each pot section has a 0.001 uF cap taken to the join of
the two tracks of the pot.

The action is that the wipers each move up and down the pots in the same
directio
at the same time, so when at the top, the top track is bypassed with 0.001 uF,
and the botton track
has its 0.001 uF shunted, so the treble is boosted.

when both wipers are swung downm the top 0.001 uF is shunted,
and there is a 0.001 uF from output to 0V, thus the treble is cut.

Try this out you guys, you'll really like the low thd detection and tone
control!

Because a nearly constant current flows through the detector diode, its always
turned on,
so the forward conduction curve of the diode does not affect the signal
detected, even when the signal
is really low level.

For AVC, I have a 25 pF taken directly off the anode of the IF amp
to an IN914 with its cathode at 0V.
A negative going voltage is developed at the anode of the diode,
and its passed via 1M to the AVC control of the RF input tube.



I would try for another 10 dB or so of

audio output above that level before being completely happy myself. But
no way do these simple crystal sets sound "High Fidelity" to me because of
all the nonlinear distortion. The distortion probably does help give them
their bright sound though, sort of a psycho acoustical trick if you will,
but it does wear on one.

Crystal sets need quite some refinement to be hifi.

Using a cathode follower detector as I have outlined would work well to keep
the diode distortion out of the audio signal.

The system I use removes the loading effect of the diode and RC
filter from the tuned circuit. Even when a tiny RF or IF signal is present
at the CF grid, the ripple voltage on the 270 pF won't vary much
compared to when there is a huge RF or IF signal present.
If say the RF/IF signal is 40v peak to peak, then the
direct voltage level across the 270 pF rises from its 35v at no signal to
55 volts, no problem for the CF tube.
You can easily get a very nice 10vrms of audio signal from a
large % modulated signal.


Patrick Turner.





Regards,

John Byrns

Surf my web pages at, http://users.rcn.com/jbyrns/