When I googled I found a ‘stereo input’ FM transmitter. Most of the FM ‘bugs’ on the web and YouTube have a microphone. The circuit I linked to has left and right inputs, but mixes them to make a monophonic (not stereo) signal.
After looking at the schematic I noticed a mistake, so I thought it would be wise to let others know. I also explain some important things to know before building one of these. These issues can cause problems for the builder if he is not aware of them when putting it on the air.
The value of C3 is shown as .001 uF, but it should be .1 uF or more. This will prevent the transmitter from sounding tinny.
The value of C5 is shown as .01 uF but it should be .001 uF or 1000 pF (‘102’ ceramic disk). This also will stop C5 from affecting the quality of the sound.
Peter found another mistake. It says:
L4: 5 turns of 28 SWG wire on an intermediate-frequency transmitter (IFT) ferrite core
It should read:
L4: 5 turns of 28 SWG wire on an intermediate-frequency transformer (IFT) ferrite core
Thank you, Peter.
This L4 is used as an RF choke, to keep the radio frequencies from going back into the positive supply. It can be replaced with any RF choke, I think any value of 4.7 to 22 microhenrys should do the job. If you have a spare IFT, you could use it, but you have to take it apart and wind the wire around the core. I think 28 SWG is equal to 27 or 26 AWG, but you should be able to find a SWG to AWG to metric wire conversion table on the ‘net. The wire size is not critical.
Some important issues that might cause problems
This transmitter puts out enough power to make it a bit difficult to get it to operate properly. It may take some reading, but this is important to know.
Build this on a piece of PC board, not a breadboard. And keep all of the leads and wires short and unmovable. If they move, the frequency will change. Unstable wiring, especially the coil, will cause microphonics.
The 22 pF tuning cap VC1 sets the frequency. The schematic shows it as the only capacitor across the L1. The problem is that this makes it very difficult to tune to the right frequency because the capacitor will be very touchy. Just a slight adjustment will change the frequency a very large amount. Instead of this 22 pF adjustable, it should be replaced with a 15 pF silver mica or ceramic capacitor, of the type NP0 or C0G. Then connect a 1 to 5 pF adjustable capacitor in parallel (across) the 15 pF. This will be much less difficult to tune. Additional tuning can be done by squeezing or spreading the coil.
Also, it shows T1 as a BF494. This is hard to find, but can be replaced with a BC548, BC547, 2N3904, or other similar transistor.
It shows R4 and VR2 as the Power Control. The problem is that every time you change VR2, it will change the frequency. So then you have to go back and readjust VC1. It is best to replace both R4 and VR2 with a single resistor of 220 or 330 ohms. It could be higher for less power.
The antenna is tuned by VC2, another 22 pF adjustable capacitor. The problem is you need some way to tell when it is tuned to optimum output. You can’t measure the supply current, because it doesn’t ‘peak’ when the power out is optimized. What is needed is a field strength meter. This one is very simple, but could be even simpler. There is no need for the 47k variable potentiometer, it can be replaced by a 1k to 10k resistor and the DMM connected across it and the 470 pF capacitor.
This FM transmitter may not be legal in some countries because it may exceed the maximum power allowed. After building the first transistor’s circuit but before adding L2 and the output transistor, I would try putting it on the air. I have found that just the first transistor can usually be heard 50 to 100 yards or meters. If that’s good enough, then don’t build the last power output stage. Don’t add L2 and the second transistor’s circuit. Instead of 9 volts use 5 or 6 volts for the supply. And no antenna. The first transistor circuit should be strong enough to receive at 50 meters, with just the radiation from the coil.