Re: OPT equalising circuit

Alex wrote:
> 
> "Patrick Turner" <info@[EMAIL PROTECTED]
> wrote in message
> news:481B4967.C86278B8@[EMAIL PROTECTED]
> 
> > I repair, rewire, or seriously modify at least 5 AM radios per year.
> > I've done 3 this year and there are 3 waiting to be done, and its only
> > May.
> >
> > I built a complete radio in 1999, 10kHz of AF bandwidth, variable
> > selectivity, linear IF amp, linear detector, linear AF amp with
> > EL34 in triode with 12AX7 and 12dB FB. Rola 12" Deluxe speaker from
1953
> > with a dome tweeter from 1972, because the 12" speaker is fairly flat
to
> > 5kHz.
> >
> > I have never heard any other AM sets that come close except
> > the one in the AM-FM tuner at my website, which I designed, and of
> > course the
> > Quad AM tuner all tubed, that isn't bad at all.
> 
> Could you please give a link to your website? Is there a schematic of
that
> radio, mentioned above, which you designed? Is it TRF?
> Regards,
> Alex
> 
> > Patrick Turner.

The kitchen radio I mentioned from 1999 above 
has a different circuit to one I just gave you at 
http://www.turneraudio.com.au/am-fm-radio-tuner-multiplex-decoder.html

I don't have the kitchen radio schematic at the website.

But what my kitchen radio does have is..

3 gang tuning cap.

Two RF coils with ferrite cores for high Q and two C gangs are coupled 
so that there is no sideband cutting of AF 
right across the AM band, which means each coil is tuned to a slightly
different F.

There is no ferrite antenna, just loose coupling from a wire antenna to
one input coil.

There is a variable µ twin triode used as a cascode RF amp stage and RC
coupled to the 6AN7 F converter
to make an IF signal.
There is AVC voltage applied only to the cascode input triodes.

The IF amp is 6BX6 with unbypassed Rk and no AVC applied, so its fairly
linear.
There is a cathode follower and diode detector cabable of detecting 
about 10 times the actual voltage detected with very good linearity and
bandwidth.

Two standard IF trannies are used, but the first one after the F
converter 
has one IF coil monted on a slide and a knob on the front of the radio 
rotates to move the coil to coilo distance about 10mm.
This allows the radio to be tuned normally to a single
a single peak when tuning and then after closing the coils closer, the 
IF BW is much increased to allow 9Khz of audio to be detected, but all
while 
retaining very good skirt selectivity and rejection of 
stations only 45kHz away even if they have 20 times the transmitting
power.

There are in fact 6 tuned circuits in all, so the attenutation 
of signals away from the wanted stations is excellent,
yet what is wanted sounds very nearly as good as FM.

The 3rd gang of the cap is for the oscillator.

I paid very careful attention to tracking.

TRF sets are quite poor performers today because the stations are now
close together,
unlike in 1935, when the band wasn't crowded.

TRF usually used only 3 tuned circuits which needed careful tuning,
and they gave a peaked response, not a flat topped band band pass
character of a critically coupled IFT.

The Superhet with IFTs is definately The Best hi-fi AM tuner you can
have if its built right,
and 99% were atrocious, and all mainly conformed to the lowest common
denominator of utter crap.

When Radio was king before TV, there were very many makers all vying for
a market share.
They all agreed with each other to ensure the standards were lousy.

Patrick Turner.