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 Post subject: Improving a BA1404 IC FM Transmitter
PostPosted: May Fri 11, 2018 6:50 am 
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Joined: Nov Sun 18, 2007 8:26 pm
Posts: 317
Location: Sherman Oaks, Ca. 91411
I built this from a kit I ordered back in 1991 from "WJD Electronics" of Middleburg, P.A. ...probably long defunct now. It works and the range is actually OK - about 150 ft with a 1/4 wave wire antenna - though it really only puts out sufficient RF in the 100-105 mHz part of the FM band. The sound is also somewhat distorted and the modulation level is very low.

It was supposedly designed to connect to a standard CD or tape player but it clips easily unless the input level is very low. IC-1 is a 1458 audio chip. Although the paperwork supplied with the kit calls IC-2 a "WD 74S573", I believe it's actually a BA1404 chip with the original number painted over.

I realize that transmitters based on this IC have limited performance, but I was wondering if there is any way to improve the circuitry so it's at least usable. Yes, I know there are better FM transmitters out there ("Whole House", C.C. Crane, etc.), but I'm primarily interested in seeing if this one can be salvaged.

Sorry about the poor schematic quality - the original looks like a photocopy:


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 Post subject: Re: Improving a BA1404 IC FM Transmitter
PostPosted: May Fri 11, 2018 8:20 am 
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Joined: Sep Tue 30, 2014 6:08 am
Posts: 3360
Location: The Old Dominion VA 23518
G Forrest Cook had(has) an entire website devoted to the BA1404. I was an early enthusiast, hearing about the BA1404 from Don Lancaster's Hardware Hacker column in Radio Electronics/Electronics Now magazine in the late 80s.

I bought and built the Ramsey FM10 to solve a problem, but was disappointed in it's drift over time. Some of the G Forrest Cook mods solved my problems. I haven't used my FM10 in a while - too busy playing with some Panaxis stuff that was a bit better, but also unused now.

June, 1992 edition of Radio Electronics has the "ultimate" BA1404 circuit in Lancaster's column, on page 72 - it's almost verbatim, the Pioneer CD-FM-1 CD-to-FM modulator.

G Forrest Cook's website:

transmitters.tripod.com/links.htm - only the Google cached site is up for me.

I do have some of his stuff mirrored from 2003-ish too. I can send them to an email address (PDF and maybe some other stuff ZIP'd) if you'd like. I also have a bunch of Panaxis and other non-BA1404 FM transmitter stuff, not sure what is still available on the web...


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 Post subject: Re: Improving a BA1404 IC FM Transmitter
PostPosted: May Fri 11, 2018 10:25 pm 
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Joined: Nov Sun 18, 2007 8:26 pm
Posts: 317
Location: Sherman Oaks, Ca. 91411
Thanks for responding.

I have seen the info in that pdf before. The Ramsey FM10 used a transistor output stage and a different power supply, so most of the mods suggested would not really be applicable here, unfortunately. The modulator described in the Radio Electronics article is basically just using the BA1404 as an input/stereo generator stage, so I'm afraid there's not much I can use from that, either.

As mentioned, I'm primarily looking for a way to clean up the distortion and get a bit more modulation. Maybe some changes to the input stage/IC-1 circuit would accomplish that?


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 Post subject: Re: Improving a BA1404 IC FM Transmitter
PostPosted: May Sat 12, 2018 7:58 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2223
Location: Saskatoon
box13 wrote:
Maybe some changes to the input stage/IC-1 circuit would accomplish that?


That could be part of the problem. 1458 dual op amps are probably borderline operating at that supply voltage. An LM358 dual op amp would be a drop in replacement and would be more suitable for the low supply voltage.


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 Post subject: Re: Improving a BA1404 IC FM Transmitter
PostPosted: May Sun 13, 2018 3:18 am 
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Joined: Dec Tue 29, 2015 8:23 am
Posts: 203
Location: Australia
Like Findm-Keepm I also spent a few years playing with the BA1404. Cook's website is excellent (& is still up for me), but a bit short on practical implementation of his (very good) points. Looking at the schematic of yours, my own thoughts are:

  • A double-sided board with solid ground plane is almost essential for stability & to minimise coupling between sections (which results in noise & distortion). If your board isn't double-sided, I'd be very tempted to make one or use dead-bug/manhattan construction.
  • As Bob says the LM1458 is a weak spot - although older datasheets characterise them down to 3V or 6V overall (depending on manufacturer), in my experience distortion rapidly rises at you go below 12V (i.e. +-6V). The bandwidth of the LM1458 is also terrible - sub-10kHz or so.
  • That at least has the advantage of keeping out high frequencies near or above the stereo pilot. If you swap it for something better as Bob suggests, it'd pay to add decent low-pass filtering with a cutoff at 15~16kHz to the audio section.
  • Looking at your circuit there seems to be drafting or design error - there should be a 100k-200k resistor at the pilot signal output on pin 13. Pilot signal level should be set to produce 10% deviation (i.e. 7.5kHz) so, without that resistor, it's possible the pilot signal is overdriving the modulator & leading to distortion &/or low modulation of the audio (depending on how the modulator behaves). It might also cause issues at the receiver end too. Have a look at the BA1404 datasheet for further info/inspiration.
  • The output from pin 13 is also a square wave rather than a sine wave. Cook mentions the problems with that in a few places, but I don't think he provides an example solution. It'll get some filtering from the rest of the coupling network across pins 12/13/14, but a properly designed 19kHz low-pass filter might be a worthwhile mod for future experimentation.


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