This is a cross post from Ramsey Electronics forums that I posted. I figured since the guys over there (God bless them) are only intending the FM transmitter kits as nothing more than a way to get stereo audio around the house and nothing more that I would include some info for us pirates or people who take part 15 to the next level to get quality audio out of that wonderful but crappy BA1404 chip..
I noticed the FM10 has a bit of a lowfi sound to it to be honest. It sounds great with most types of music but for modern rock and electro it's lacking a bit. Most high powered FM stations don't use much in the means of processing. They use the basics like the Orban units which are so popular. In all honesty in my opinion these processors don't do much besides clip protection and a bit of compression/expression via basic three band EQing. I am no smarty pants but I know what sounds good. The FM10 doesn't compare to commercial stations obviously but there is no reason the audio quality can't be made to sound better than direct in past a compressor. I suggest a basic 10 band EQ. A simple U shaped pattern is where to start. Well not so much the form of a U, but give the lows a bit of a boost. 30-80hz could use a boost in a down slope (30>80). Everything below 20 or 30 should be mostly blocked, so if your EQ can do it cut everything below that range. The FM10 and I assume the 25 seem to boost the 500-1k range. Slope these a bit to get more of a popular FM sound, or keep them mid range if you run a lot of talk radio on FM. For classical music it's probably best to keep these mid, for rock slope em. Beyond 5k is where it gets tricky in my opinion. I run the levels from 5k up on an up slope until about 10 (or 12)k. At that point I run my 16khz level way down to help cut any problems with my pilot signal. It seems if you up the 10 or so khz levels you will still get a hifi sound on most basic radios. The only place you notice the drop off from 12-14khz is on really good stereos. Besides if they are good sets you probably have a built in EQ on the stereo anyways .
So what kind of processing do you do on your ramsey transmitters? Do you just run them right into the box or do you do a bit of processing? Trying to get any of these kit transmitters to sound like the big boys is one heck of a job if you're a bit of an audiophile but even though everyone will tell you that it's not worth the trouble, trust me it is. I have a bypass button on my EQ/limiter/compressor and trust me a tiny bit of processing makes this kit sound like a pro station.
To add I must include some pirate info. FM10/25 puts out a shit ton of splatter and spurs. Though the original kits were intended as toys to be truthful, you can do some mods to really make it into an acceptible TX. First you must do the LPF mods on the audio, this will reduce <16khz so not to effect the pilot. Also it will reduce RF feedback once running an PA. Do the basic mods in the FAQ. No links, but just google FM10 FAQ and you will find the mods. The links change, so no point in posting them. Get a choke on that output transistor, replace the R8 470ohm resistor with a 1.5k resistor and run the FM10 at full 15-17 volts (my own mod I figured out) this will increase a bit of power on the output transistor and also make less heat on the 3 regulator diodes to less drift.
Other mods, clamp on a FM capacitor gang circuit to the OSC section. This will allow for fine adjust of freq. w/o using the stupid slug coil. Use NP0 CAPACITORS!. Replace anything in the OSC section with them. The higher grade, the better. Place a .1 cap from pin 5 (vcc in) to the ground block where ever it connects to the ic cap. This reduces a lot of spurs. Hell while you're at it do what I did and just put .1 caps (104) wherever power enters the chip and where RF doesn't run ramped or audio. Adding more caps reduces the ability of the board foil acting like inductors.
I have a ton of other tips including a good 17v supply schematic (only to use on the modified diode reg resistor mod) coming soon.
I plan to post pics of my totally remade FM10 along with its 2 watt amp.
Most people think VFOs on VHF is a joke. Well you got a point, but I flip the power to this hunk of mods and kit and get a pure signal out with <40db stereo seperation with little distortion. The VFO after a 1 min warmup is rock solid for up to a day. Not to bad if you ask me.
I will post pics soon of my frankenstein FM10 2-4 watt TX.
Another tip I forgot to add, make sure to cut the hell out of 20hz and below. The FM10/25 along with many other "kit" transmitters (maybe even the so called "pro" txes) for FM are bad at HPF above 20hz. Anything lower than that seems to screw up the stereo MPX in my experience via bass band harmonics along with deviation going crazy for slightly narrow BPFs. You can prove this to yourself by taking a basic BA1404 type kit, or even other kits that use other MPX chips, play audio into just one channel and listen to the opposite channel on a good stereo monitor (good radio will do as long as it's trust worthy) and you will hear deep bass distortions. Those are almost always low frequencies. Don't cut your low bands or else your stereo signal will lack the bass, but a good hpf on the audio feeds can do a bit. EQs usually don't have the ability to cut well at such low frequencies. I am lucky to have a HQ Yamaha 31 band EQ with a range setting allowing for double cutting. The only way around this is to get people who listen to not be audiophiles or cut down on bassy music. Not easy for stations like my own that play a lot of electro. Just make sure to drop it 20 and below. Give a boost at 40-60 to make up for it to the untrained ear. Most of all excuse all of the audio defects as the listeners fault lol.
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