Steven Swift wrote:
Hi Fi AM--
This was "almost" popular as a design project in the 60s. Even RCA in
their RC-19 Tube Manual have a circuit labelled "TRF AM Tuner-- for High-
Fidelity Local Broadcast Reception." (Circuit 19-8, p. 357)
Fidelity on an AM signal requires that most common circuits used in
radios be eliminated:
1: No AVC. This distorts the low frequency frequency response
2: No cathode bias bypass.
3: No diode detectors, unless the signal feeding them is greater than 10Vrms.
4: No AC coupling if diode detector is used (the "AC-loading" distortion
described in Terman, et al).
5: Speaker resonance 30Hz. Assumes the line out goes to a real "Hi Fi"
system.
All these "don't do" can be found in Terman, the Radiotron Designer's
Handbook and others.
Let me say a few words.
1. Many pages of RDH4 are devoted to AVC.
The time constant for AVC application is very long,
comprising of 1M and 0.047 uF, and measurement
of bass distortions resulting from well applied AVC is low
enough to be negligible.
2. Nothing wrong with cathode bias, especially nowdays when cheap
large value elcaps are plentiful, and we have better plastic caps.
RDH4 speds a lot of time on cathode bias.
3. Diode detectors are quite low distortion detectors even with
very low voltages of 100 mV if there is a constant current trickeled
through the crystal diode to keep them turned on with their
forward conducting voltage.
I gave details yesterday in another post of a detector which will change your
views about
diode detectors.
Diodes can be used with DC shunt feedback around an RF opamp,
and thd is negligible.
4. AC coupling is fine from an RC load fed by a diode.
The impedance fed by the audio + RF ripple voltage should be high,
like a cathode follower grid.
5. I have tried my radio with various speakers, and no trouble
making full amplitude signals at 20 Hz.
The LF pole is determined by the audio amp in the radio, but at the detector, the
pole is much lower.
On which pages are RDH4 and Terman "dont's" spelled out?
If you are willing to live with about 5-10% THD, then you can use more
common circuits.
True, but onje doesn't have to live with 5-10%.
linearize the IF amp and detector, and thd plummets.
However, there are dozens of "Hi Fi" AM circuits published
by the hobby magazines, tube vendors and kit makers. Have a look at them.
The RC-19 circuit uses a 6BA6 as an RF amp, followed by a 12AU7 used as a
detector and audio amplifier.
RDH4 has the circuit for the Selsted and Smith "infinite impedance "
detector, where a 12AU7 performs as credible detector, and as a diode,
but I think I'll stick with a germanium diode fet by a 12AU7 CF.
But does the RC-19 have enough tuned circuits to give over 70 dB rejection of
signals which are 50 kHz away from the wanted station at any place on the band?
The 6BA6 is a variable U tube, with a non linear
transfer curve.
If a large voltage is detected, there is quite a bit of distortion of the
detected
wave form.
Preferable is a 6AU6, a sharp cut off RF amp, with a more linear transfer curve,
although too much gain could be a problem, if so, use a lower Gm
pentode like a 6J7.
But having a 6AU6, or perhaps a 6BX6 is OK if biased with a cathode
resistance, and this R is left unbypassed to further reduce thd in the envelope
amplification.
With a load on the R amp typically of 25k, and Gm = 3 mA/V,
gain is 75, and if the Rk = 500 ohms, gain is 30, and thd reduced by 6 dB.
In a superhet, the voltage levels of the IF envelope coming from a mixer tube are
usually so low that the mixer don't maul the linearity of the IF signal,
even when the amplification is backed off with AVC.
Patrick Turner.
Good luck.
Steve.
--
Steven D. Swift, , http://www.novatech-instr.com
NOVATECH INSTRUMENTS, INC. P.O. Box 55997
206.301.8986, fax 206.363.4367 Seattle, Washington 98155 USA