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#1
12-16-2004, 11:51 PM
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Improving the fidelity of AM reception audio
This is a set of mods to improve the fidelity of AM detectors commonly used in tube AM radios and the AM section of tube tuners.
We split off the AVC circuit from the audio detector. Because the filter cap in the AVC circuit will reverse bias the detector diode some. This will cause the high amplitude negative peaks of the audio (which causes very low RF valleys) to be clipped off. This rarely happened back in the old days, but with today's heavy audio processing by radio stations, it can be a significant source of distortion. Most AM radios use tubes like the 12AV6, which provide a pair of detector diodes. The second is usually not used and is tied to ground in most radios. Here we unground it and use it for the AVC circuit. A 100pF cap coupled to the plate of the last IF tube feeds the new diode. This also gives a benefit of keeping the gain down as we tune across a station, so we don't get blasted as much when slightly off tuned. A second mod is to reduce the usually 200pF audio RF filter cap. Here we used 50pF. This avoids "tangent" distortion; the clipping of negative going peaks of higher frequency audio. Contrary to popular belief, most AM stations broadcast a full 10KHz of audio, for an RF bandwidth of 20KHz. Sure, station frequencies are spaced every 10KHz, but not in the same town. Another mod is to reduce the heater voltage applied to the detector diode tube. Here in a series string AA5, we can do this by paralleling a 330 ohm resistor with the 12AV6 heater. This will cause the 12AV6 to see a heater voltage of 10V. This lower heat will produce less "contact potential" seen by the diodes. Less stray electrons boiled off the cathode. Less bias for the RF signal to overcome. Sure this also impacts the triode, but its self bias only drops from 1V to about 0.6V. Which is not serious. If this becomes a problem, one can "borrow" some negative bias off the 12BE6 local oscillator circuit by connecting a set of resistors totaling around 30 megs from grid 1 of the 12BE6 to the grid of the 12AV6. You probably won't need this. If, in a tuner, the detector is a solid state diode, replace it with a 6AL5 or equivalent sub-mini tube. And use both diodes, one for audio and the other for the AVC. And run the heater on reduced voltage of around 4 to 5 volts. Last edited by wa2ise; 12-16-2004 at 11:53 PM. 
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#2
12-17-2004, 02:09 AM
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That's about it, Bob, but this is a trick some of us want to do to our vintage SS receivers, if we are truly lucky enough to have ANY music stations left. Take the instance of my current GM Theftlock AM/FM cassette unit, which has a moderate response (yes, plain ugly FLAT with a little bass but better than some). Assuming I manage to deal with the code, I would love to figure out how to widen it. 1994 Pontiac Grand Prix if I knew the model after driving it only 8 days I'd run for office.
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#3
12-18-2004, 07:56 PM
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Quote:
http://pw2.netcom.com/~wa2ise/radios...tml#solidstate about halfway down. Only thing is that car radio sets are likely pretty crampt inside so fitting that new filter might be tough. A new wider band ceramic filter might work (*IF* you can find one that is the same frequency as the old one in your digitally tuned radio, less critical if the tuning is analog) if you can find some. You might be able to find a usable one from a junked FRS radio or cordless phone.
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#4
12-19-2004, 08:50 PM
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I will do some research to find which model it is and that might help both of us.
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#5
12-30-2004, 09:25 AM
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See that 4700 pF cap across the 470K grid resistor? If this was not a module with those resistors and caps, one could in essence remove the 4700 and replace it with a smaller version to get more high end audio through. Unless you feel daring enough to buy the components and remove the module altogether.
Tha 4700 pF cap is there to filter out whines and whistles of RF interference. Experiment with the value until you get the best balance. This is likely true also with the solid state variety. I thought that the 200 pF was a neutralizer to eliminate oscillation due to phase shift???? The separate diode idea is always an excellent choice. I have replaced the tube rectifiers with solid state (germanium) diodes and got excellent results in both fidelity and sensitivity. Why do you suggest changing a SS one with a tube? One can also detune the IF stages to get a wider audio response, since too high a Q narrows bandwidth if you so dare. But first mark the original setting with magic marker, pencil or nail polish, and don't go nuts with the screwdriver or you can break the lugs, and go overboard with turning them and forget where they were originally. One loses some sensitivity and selectivity, but those are the trade offs for sound quality. This is true for both technologies. BTW, 5KHz is the highest note that is considered musical anyway ("The Science of Music" & "The Psychology of Music". Good reading). Gabe
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#6
12-30-2004, 10:14 AM
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Yes, and I could probably synthesize a bland, monotonous 5 kHz tone myself...I'm reminded of the political catch phrase when I point out, "It's the harmonics, stupid!" (No disrespect meant at all)...Just as color needs complementary hues, sound needs harmonics.
Before we get into bandwidth hoohah, here it is... 535 LSB|----10 kHZ----||545 carrier||----10kHz----|555 USB (an example). It's been capable of that since at least the 1930's, and in the early 90s, with AM stereo, some were pushing it to around this: 529 LSB|----16 kHz----||545 carrier||----16 kHz----|561 USB Digital looks something like this (oversimplified): ~530 LSB|10 kHz IBOC-AM|*Analog 545-530||545 carrier||*Analog 545-550|10 kHz IBOC-AM|560 USB (an image, phase modulated or similar to FM, will appear at every 30 khz or three channels out upper AND lower sidebands, due to the nature of the IBOC signal, wiping out any weaker signals at that frequency). If you owned a GM car radio from the late 50s or early sixties built with tubes and especially that plate reverb, please do not tell me how low-fi AM is. You would know better. Last edited by asynchronousman; 12-30-2004 at 10:18 AM.
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#7
01-03-2005, 04:02 PM
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Right... but at that high a frequency, the item producing the note.. the highest string of the piano... may only be putting out one or two harmonics tops. That brings us up to 15kHz at best. How much real music is up there at 5kHz? Now for a note of 880 Hz, there is plenty of room for harmonics to 5kHz. That is a pretty high A (the 60th key from the left). The standard A that is used for tuning in a band is 440 Hz. Most music is between 50 and 2kHz, again needing really only 1-3 harmonics for very decent fidelity... since it is recreated in the ear anyway!
To hear it in AM radio, it is assumed that they actually transmit it to begin with. Assuming that they transmit within the bandwidth the FCC regulates (in the US for foreigners), if you have ever listened to a crystal radio through a decent amplifier, it sounds pretty darned good for being limited. But you know this.  "The stupid one gets offended". No offense taken. Gabe
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#8
01-08-2005, 11:27 PM
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Antennas can help too? with AM many homes are nosiy at RF
IMO&E PC's Modern TV's and anything else with a switch-mode PSU, will generate quiet a lot of RFI/EMI. which a meer intrenal antenna WILL pick up.
A random wire up highish outside, feeding into a 'low-noise' down lead - some fig 8. wire, or some 300 ohm TV ribbon, looped to stop rain entering yr home, and earthed to a real RF ground, will be a lot better than just the ferrite rod on the back. if you add a tapped coil and a vane cap you can tune the wire so it 'thinks' it is from 600 metres to 200 or so metres long. this alolowers the bandwidth of the antenna system and lowers nosie even more, it also increases gain. If the earth is accessible under the run of yr wire up in the air, you could run an in-ground radial back to earth of the same length as yr random wire. Even an exterior remotely tuned loop, using a varactor, is possible, you put the loop made from rainbow/ribbon inside some EV stable pipe in circle and yr done, maaate. Timbo
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#9
01-09-2005, 12:01 AM
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Quote:
I will assure you that ANY 10000 Hz signal, mono or stereo, will sound much better than Realplayer ever will without major effort (that's how a local college gets the local signal to the sister almost 200 miles away). I wonder if high speed streaming is the only way it will work. I'll never know, as I despise Real's "security" issues and poor quality enough to avoid it, and any alt player I've tried won't play Real whether I want it or not. That leaves out almost ALL of your nation's streaming radio stations I'm afraid, which I really love to hear to correlate to what Ian and Robert used to tell me. Oh well,,,
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#10
01-09-2005, 12:21 AM
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Quote:
Mother Nature can be a lot worse, though! I was undeneath a MAJOR storm cell for 15 miles once and the entire AM band SQUEALED and SCREECHED this ungodly, baleful and terrifying din. I though the radio had busted right then a there for 5 minutes until I switched to FM and it WORKED. Since it was my prize AM stereo Delco radio, I had thought it was ruined, but when I came back out of it, it returned to normal (major sigh of relief). You, being in OZ don't have as many stations and they have better co-channel separation too.
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