audioguru Posted March 19, 2005 Report Share Posted March 19, 2005 Sorry, Sasi.I don't have the circuit of my old cable TV amplifier anymore. :'( Quote Link to comment Share on other sites More sharing options...
Kevin Weddle Posted March 19, 2005 Report Share Posted March 19, 2005 Hey Audioguru. How are you? What keeps Q2 and Q3 from creating too large a signal. Couldn't you use a parallel resistor to keep the gain down? We must know that the signal is constantly modulating a carrier and anytime the carrier is thrown into cutoff or saturation the meaning of the signal is lost. Although the square wave produced contains the rate of change, the higher amplitudes of the signal will not affect the Quote Link to comment Share on other sites More sharing options...
audioguru Posted March 19, 2005 Report Share Posted March 19, 2005 Hi Kevin,My FM transmitter circuit badly overmodulates when a loud sound or talker is too close to its microphone. I made its preamp gain high enough to pickup conversations about 2m away.I built the original circuit of a defective transmitter to see everything that was wrong with it and when its preamp was working (it was very affected by supply voltage changes and had much more gain and distortion) it would overmodulate like crazy!I think its overmodulation wouldn't be so bad if its RF oscillator produced only FM, since it would simply produce bad distortion in an FM radio, or cause interference to an adjacent RF frequency. Bad overmodulation does cause the AFC in my Walkman radio to work backwards and push the signal away. But since its RF oscillator produces AM in addition to the FM, overmodulation causes its RF carrier to stop momentarily whenever the RF oscillator is saturated and cutoff by overmodulation of its AM component.Adding a parallel resistor somewhere to reduce the gain would mess it up. If I add a resistor in parallel with the collector resistor of the preamp transistor then it would be cutoff and need rebiasing. Quote Link to comment Share on other sites More sharing options...
MP Posted March 19, 2005 Report Share Posted March 19, 2005 NE570. Not too complicated. ;DMP Quote Link to comment Share on other sites More sharing options...
audioguru Posted March 19, 2005 Report Share Posted March 19, 2005 Hi Mp,Thanks, I completely forgot about that one. I have an NE572 stereo one in my stock that I could try. Philips' "Compandor Cookbook" is full of applications for them:http://www.semiconductors.philips.com/acrobat/applicationnotes/AN176.pdf Quote Link to comment Share on other sites More sharing options...
MP Posted March 20, 2005 Report Share Posted March 20, 2005 You are certainly welcome. I have the old 1991 Philips Audio/Radio Handbook. Thanks for the link. MP Quote Link to comment Share on other sites More sharing options...
Guest Alun Posted March 20, 2005 Report Share Posted March 20, 2005 Hi Alun,The old CA3140 is too noisy for a high gain microphone preamp. There are lots of different low-noise audio opamps you could use.How about the LF351? then ? Quote Link to comment Share on other sites More sharing options...
audioguru Posted March 20, 2005 Report Share Posted March 20, 2005 Hi Alun,Happy Birthday soon!You are not very old like me so why are you selecting old parts?The LF351 hasn't been manufactured for about 7 years by National its inventor, but some shops might still have one. Its noise is lower than a 741 but not nearly as low as a TL071.Just rip apart an audio mixer, most have TL071 opamps inside.I don't know where you are, but don't use New Japan Radio's 1st version copy of the TL071. They tried to make their copy better than TI's original and ended up with it oscillating all the time. They had a big appology and warning about it on their website, but it's gone now. Quote Link to comment Share on other sites More sharing options...
MP Posted March 20, 2005 Report Share Posted March 20, 2005 Alun,I can see using an older op-amp if you have a bunch of stock you want to get rid of, but not as something to purchase.If you MUST use the LF351, I found about 28,000 of them around with my parts search. You should not have a problem getting one. But like audioguru has stated, go for the low noise op-amp in a new design. You will be much happier in the long run.MP Quote Link to comment Share on other sites More sharing options...
Guest Alun Posted March 20, 2005 Report Share Posted March 20, 2005 I can see using an older op-amp if you have a bunch of stock you want to get rid of, Quote Link to comment Share on other sites More sharing options...
audioguru Posted March 29, 2005 Report Share Posted March 29, 2005 Hi Guys,Here is a pic of my FM transmitter circuit built on a copy of Veroboard. It is Mod4, but using the circuit board of Mod3, so a few of its parts are very close together and I have resistor R6 and little ceramic cap C2 underneath. I have a 680 ohm resistor at the lower left as a current-limiting resistor for its LED. I don't have any space on this board for the 5.1V zener diode in series with the LED that shuts it off when the batt gets down to about 5.3V as a low-batt warning. Look at the tiny size of its trimcaps and electret mic. The mic has a thick rubber sleeve around it so is actually much smaller. The 1mm wire for my coils is so stiff that you can stand on them and they don't change. Quote Link to comment Share on other sites More sharing options...
Dazza Posted March 29, 2005 Report Share Posted March 29, 2005 Nice work audioguru :), I always find it helpful to have a pic of the project I want to construct. I'll get started on it once I have finished the two projects I am working on now ;), or at least finish putting them together then comeback later to get them working properly as I often do ;D. Quote Link to comment Share on other sites More sharing options...
Guest Alun Posted March 29, 2005 Report Share Posted March 29, 2005 Good job audioguru!It would be nice if some administrator could add your project to this site. When I get the time I will build it with some of the modifications Iv'e talked about.Could you tell me which is the least noisy pre-emphasis amplifier?The one I sugested with the LF351 - I have a few lying around, or the transistor one you sugested, I'm far too cheap to go out an buy a TL071! ;D Quote Link to comment Share on other sites More sharing options...
audioguru Posted March 30, 2005 Report Share Posted March 30, 2005 Thanks Dazza, ;DYours should also work fine.Thanks Alun, ;DOver here, a very popular TL071 costs less than a lousy old 741 or LF351. A TL072 dual costs even less! I always buy at least 10 and get a nice discount. Some experts like quieter audio opamps that cost an arm and a leg. This circuit is fine with just my transistor.I'm still working on my FM transmitter, trying little ferrite chokes throughout it. I had the RF oscillator direct-coupled from the preamp but the preamp's bias went haywire. I didn't think that the oscillator's base voltage would be dragged down so low I guess by the oscillations. It works best with a little choke replacing C3, but I didn't bother changing the schematic yet.I am also concerned with overmodulating it. A compressor/limiter would be too complicated for such a simple transmitter circuit. Because it has a lot of AM modulation, popping "P's" and other loud sounds cause its oscillator to stop for a moment. It will probably end up being modulated and tuned by a varicap for pure FM.Then (if it is ever finished) I'll post it as a project! ;D Quote Link to comment Share on other sites More sharing options...
trigger Posted March 30, 2005 Report Share Posted March 30, 2005 My veroboard sample will not as tidy as Audioguru does............ :PGood job Audioguru!!!I got to learn from you... ;D Quote Link to comment Share on other sites More sharing options...
audioguru Posted March 30, 2005 Report Share Posted March 30, 2005 Thanks Trigger,I learnt that keeping wires very short in VHF circuits is very important. ;D Quote Link to comment Share on other sites More sharing options...
Guest Alun Posted March 30, 2005 Report Share Posted March 30, 2005 Yes connecting a varactor diode in with the capacitcor will give pure FM with little AM. You sitll probably want to have a vairable capacitor in paralel as well.Can't you play around with the pre-amp biasing to clip or limit the outout to prevent over modulation or at least reduce it?Have you tried taking the output to the antenna via a smaller coil wound within the output tuned circuit? Quote Link to comment Share on other sites More sharing options...
audioguru Posted March 30, 2005 Report Share Posted March 30, 2005 Hi Alun,You have an excellent idea to solve my overmodulation problem! ;DLet the preamp transistor symmetrically clip at full modulation:1) Its existing max 5Vp-p output is way too high since full modulation is only about 20mVp-p. Use a decoupled voltage divider to feed its collector. Maybe only 1V.2) Use a voltage divider at its output to feed max 20mVp-p to the RF oscillator. The attenuation of 1V to 20mV is 50 times, that must be made-up by having 50 times more gain.OOps! The preamp transistor is already operating at max gain with pre-emphasis, more gain would require it to operate at nA. Also, without having the 100 times of headroom and negative feedback it has now, its distortion would be terrible.Mmmm, I have some opto-FETs that distort badly with a 20mV input, maybe they can be used as an audio limiter. Nope, they work backwards. With up to 20mV of input they are a linear resistor. Beyond that, their resistance increases. For a shunt limiter I need the resistance to decrease. I'll try them as the input resistor of a unity-gain inverting opamp. Then they should limit perfectly at 20mV output.See, I am talking about adding 3 ICs to this simple FM transmitter toy. Forget it. ;D Quote Link to comment Share on other sites More sharing options...
audioguru Posted March 30, 2005 Report Share Posted March 30, 2005 Hi Sasi,I don't know where to buy oriental transistors in Canada. Maybe in Hongcouver. Quote Link to comment Share on other sites More sharing options...
trigger Posted April 1, 2005 Report Share Posted April 1, 2005 nice pictures..... Sasi.So how are their performance then? Quote Link to comment Share on other sites More sharing options...
spleblem Posted April 1, 2005 Report Share Posted April 1, 2005 that is really greatim just wondering what are u are using alot of batteryies in series(if so what batterys AND HOW MANY) or do u have a mains power thought a transformer power packKIND REGARDSSPLEBLEM Quote Link to comment Share on other sites More sharing options...
Guest Alun Posted April 2, 2005 Report Share Posted April 2, 2005 Thanks Dazza, ;DYours should also work fine.Thanks Alun, ;DOver here, a very popular TL071 costs less than a lousy old 741 or LF351. A TL072 dual costs even less! I always buy at least 10 and get a nice discount. Some experts like quieter audio opamps that cost an arm and a leg. This circuit is fine with just my transistor.I'm still working on my FM transmitter, trying little ferrite chokes throughout it. I had the RF oscillator direct-coupled from the preamp but the preamp's bias went haywire. I didn't think that the oscillator's base voltage would be dragged down so low I guess by the oscillations. It works best with a little choke replacing C3, but I didn't bother changing the schematic yet.I am also concerned with overmodulating it. A compressor/limiter would be too complicated for such a simple transmitter circuit. Because it has a lot of AM modulation, popping "P's" and other loud sounds cause its oscillator to stop for a moment. It will probably end up being modulated and tuned by a varicap for pure FM.Then (if it is ever finished) I'll post it as a project! ;D I've manage to get my hands on a few TL081, are they suitable? Quote Link to comment Share on other sites More sharing options...
audioguru Posted April 2, 2005 Report Share Posted April 2, 2005 Hi Alun,A TL081 isn't spec'd for noise. It is the same as a TL071 but the quiet ones are labelled and sold as "TL071" and the lousy ones "TL081".Maybe a TL081 is made late on a Friday or early on a Monday, and whatever makes it noisy might affect its reliability.Try your TL081. Maybe its noise is at the borderline for quietness and you won't notice its hiss. Maybe all TL081's made recently are low noise. ;D Quote Link to comment Share on other sites More sharing options...
Guest Alun Posted April 11, 2005 Report Share Posted April 11, 2005 Hi audioguru,I've just attempted to build the oscillator section of the transmitter, it worked but the frequency was 200MHz with the capacitor set to it's highest value, the sinewave output was very smooth and clean too.I increased the diameter and number of turns on the inductor, and still no change in frequency.I replaced the inductor with a ready built 4.7uH ferrite inductor, and it made no indifference what so ever.The circuit is compact and less than 40mm square, and I ran it from 5V with a 100nF decoupling capacitor.I didn't have a plastic screwdriver handy to tune it so I just used a metal one, and I only took notice of the frequency when I wasn't touching the capacitor. I reduced the capacitors value and the frequency increased to about 250MHz then stopped oscillating, it was funny because once when I touched the capacitor the frequency increased to 438MHz, this was unexpected, because I thought more capacitance it would decrease frequency.The capacitor is a 39pf, it's quit a bit bigger than the one you used, do you think the capacitor has a parasitic inductance causing it to resonate at this frequency?Why else could this circuit be stubbornly oscilating at 200MHz? Quote Link to comment Share on other sites More sharing options...
audioguru Posted April 11, 2005 Report Share Posted April 11, 2005 Hi Alun,I don't know why your oscillator's frequency is so high. My FM transmitter can be tuned below the FM band, how far below I can't measure.You would expect your 39pF cap to work well at VHF due to its small value.Try another transistor. Maybe the one you are using has unusually low capacitance. Quote Link to comment Share on other sites More sharing options...
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