Can you get high quality sound from an AM tuner? - Page 2

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Re: Can you get high quality sound from an AM tuner?


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That was the Philip Pik synchrodyne circuit.  It wasn't your typical kit
circuit (a lot more skill required to build and tune, and you had to
source all the parts yourself), but I agree with your point about the
slug tuning.  The demodulation didn't involve a standard "demodulator"
chip but a couple of CMOS chips (4016 quad switch, 4013 dual D flip
flop) and some comparators and op-amps.  The original papers are are
available on the hifi-am Yahoo group in the files section.  EA June 1975
and Proc. IREE, Sept 1975.

As for sound, it avoided the bandwidth problem PT referred to (bandwidth
determined by the audio bandwidth, with a TRF front end with 60kHz
bandwidth) and sounded very close to FM (although mono) in quality,
provided you listened to local ABC. The  AWA AM3 was sold in the US as
the McKay Dymek AM3, but it wasn't a patch on the Pik circuit for audio
quality.

DN

Re: Can you get high quality sound from an AM tuner?




Alan Rutlidge wrote:

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The problem with homodynes and synchrodynes is that their
selectivity depends on the profile of the audio filter following the
synchronous detector.

Stations which are 9kHz apart in Oz can interfere with each other because
sidebands of the modulation of one station can stretch out into the
bandwidth of another station, so you can get whistly monkey chatter
that a 9kHz notch filter cannot remove.
There needs to be good selectivity in the RF amp first stage but with no
attenuation of the modulation sidebands; this means a TRF input stage
with special tuning, and boy is thatv difficult to get right if you have never
built such a thing; a complete pita.

A superhet using tubes and IF trannies is a lot more fun although hard work to
build; the high IF voltage of about 10Vrms at 455khz is easily attainable
with a pentode and thus a maximum of 5Vrms of audio signal at 100% modulation
can be obtained
and there is negligible thd generated in the diode detector buffered by a
cathode follower.

But also a wide band ceramic IF filter could be used with solid state to get
the ideal passband of 20kHz at 445kHz but with very steep roll off each side of
the pass band.

There are other methods using 2 IF amp stages and having the IF at 2.5MHz,
so that its easier to get 20kHz bw after 6 tuned circuits in 3 IF transformers.
Winding the IFTs is easier as F goes higher, but again its a lotta effort and
pain
to teach oneself how to do it.

Its probably easier to digitise the antenna signal and use a PC to
count out the AM station you want.

The simplest tube set I have uses 6BE6 input mixer after a ferrite rod antenna,
6BA6 vari pentode IF amp at 455kHz, then a 1/2 12AU7 cathode follower buffer
to drive the diode
plus CR peak detector, followed by another 1/2 12AU7 cathode follower to
prevent loading the first RC filter so that you get very low thd in the diode
detector.

I get about 4Vrms average signal level and very clean from the set.
The 6BA6 runs with about -5 to -8 V of avc voltage which
makes the the amplification of the IF envelope fairly linear, just a bit of 2H.

I have also used an RF amp in another set which accepts all the avc,
so that the IF pentode is a 6BX6 sharp cut off pentode which is more linear
than the vari tube, and that sounds even better.

Audio bw can be increased by altering the distance between the
two IF windings on IFT No 1.

Unfortunately most IFTs you find in old valve sets have very high Q so that only

10kHz of IF bw is possible per IFT, so that after two such stages bw falls to
only
7kHz, thus giving 3.5kHz of audio.

But in some setys I have cut the former of the IF coils in the centre and
placed a cardboard tube sleeve over them to allow the coils to be slid slightly
closer to further flatten the response of the IFT.
Placing 100k across each IFT coil also reduces the Q somewhat and
broadens the bw but as some cost to IF amp gain.

Fiddling around with all these things are terribly prone to
oscillations at 455kHz which make the station tuning whistly
across all or part of the band.

Every old radio set you may find has had the layout carefully considered
and trialed and tweaked to get rid of stray parasitic oscillations, so if you
get an old set
to play with expect major amounts of time to get it working better.

Patrick Turner.






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Re: Can you get high quality sound from an AM tuner?


If you are handy with electronics, take a look at my page
http://pw2.netcom.com/~wa2ise/radios/feedback.html#solidstate
local AM can sound quite good.  In the USA most stations do broadcast
10KHz of audio, occupying 20KHz of RF spectrum.  Sure, AM stations here
are spaced 10KHz apart, but not in the same market.


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