Fm transmitter fit for a stereo encoder

[Gideonstk]

Dear all,

I have built harrys' stereo encoder ( **broken link removed** ).
and i have tried it with my transmitter that i have built a few years ago from the esp site ( **broken link removed** ).

Well it does transmitt but there is no stereo seperation, my guess is that my transmitters' audio input is filtering the 23-53khz side frequencies therefore i get only the l+r with the pilot tone (the stereo led does light up).

Can anyone help me understand how i modify my transmitter to not filter these frequencies? it seems all most all the diy transmitters out there are not fit for working with stereo encoders without any modifications.

Much thanks, Gideon

 

[audioguru]

The value of R1 in the transmitter is 10k and combined with C10 causes high frequencies to drop above 16kHz. Change R1 to 470 ohms and add an opamp buffer to the output of the stereo encoder.

Get rid of that horrible LM358 opamp (6kHz max and has crossover distortion) and replace it with a wideband, low distortion and low noise TL072 dual opamp. Since another opamp is needed, use a TL074 quad opamp.

Harry's stereo encoder has pre-emphasis (treble boost) but doesn't have a filter to stop boosted high frequency audio from beating with the 19kHz and 38kHz. The cut in high frequencies of the LM358 is too gradual and is dependant on level. The beat will cause odd extra sounds to occur.

All the MP3 to FM transmitters use the BH1417 IC that has its RF frequency crystal controlled with a frequency synthesiser. It is stereo, has pre-emphasis and filters and works very well. Here are two schematics for it. The Micromitter has an RF attenuator on its output to reduce its range.
https://electronics-diy.com/schematics/BH1417_fm_transmitter.htm
https://electronics-diy.com/BH1417_PLL_Stereo_FM_Transmitter.php

 

[Gideonstk]

Bless you audioguru ,

i will change R1 to a 470ohm, and i will buy the tl072/tl074 on my next stop in the electronics shop. im not sure how to buffer the output of the encoder, i will try to figure it out from the opamps' manual.

i have been reading and considering the bh1417 micromitter project that has been mentioned many times in this forum for some time. i decided to go with harrys' encoder as a better way to learn the basics.
also because i have had quite a bad soldering experience with these surface mount chips before.

Thanks, Gideon