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4hv.org :: Forums :: General Science and Electronics
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Help with audio modulating

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Njubster
Mon Oct 20 2014, 06:51PM
Njubster Registered Member #6857 Joined: Sat Sept 22 2012, 08:25AM
Location: Srbija, Novi Sad
Posts: 40
I bought a lovely little pwm controller called SG3524N in a local shop, cause they had no TL494s, I followed the datasheet and I came up with attached circuit. It's working pretty fine for 12V, but I'm having trouble making it sing. I tried few things like applying audio signal to current limit comparator, or feeding audio signal to the base of external transistor, and then feeding the emitter current to the shutdown transistor base, but none of those worked.

I'm just a hobbyist, so I do have some huge gaps in knowledge, but it would be awesome if anyone could just give me an idea how could I achieve what I want :)

The schematic: Link2
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Sigurthr
Tue Oct 21 2014, 12:37AM
Sigurthr Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597
I've never used that chip but I am experienced with the SG3525, its brother. I capacitively couple line level audio into pin 6, the Rt timing resistor, to achieve audio modulation.

I was going to post a schematic I use for the 3525, but the pinouts are too dissimilar to be of much use to you, I think.
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Njubster
Tue Oct 21 2014, 03:10AM
Njubster Registered Member #6857 Joined: Sat Sept 22 2012, 08:25AM
Location: Srbija, Novi Sad
Posts: 40
Thanks for your reply. I saw some people using that technique with TL494, but I had no luck in making it sing like that.

Is the capacitor value critical?
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Wolfram
Tue Oct 21 2014, 05:57PM
Wolfram Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
Try something like this.
1413914268 33 FT166712 Flyback Audio
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Njubster
Tue Oct 21 2014, 06:08PM
Njubster Registered Member #6857 Joined: Sat Sept 22 2012, 08:25AM
Location: Srbija, Novi Sad
Posts: 40
Dude, I didn't even notice the mistake with voltage divider... Thank you... I will try it now :)
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Njubster
Fri Oct 24 2014, 09:52PM
Njubster Registered Member #6857 Joined: Sat Sept 22 2012, 08:25AM
Location: Srbija, Novi Sad
Posts: 40
Finally managed to get some audio output by connecting the audio out to Rt pin, but my phone starts to mess things up when it's connected, how could I fix this?
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Sigurthr
Sat Oct 25 2014, 01:43AM
Sigurthr Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597
Make sure you capacitively couple the audio in, don't DC couple it. (Use a series nonpolar 2-10uF cap, or polar with positive towards the Rt pin). DC coupling leads to all kinds of problems.
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Njubster
Sat Oct 25 2014, 06:41PM
Njubster Registered Member #6857 Joined: Sat Sept 22 2012, 08:25AM
Location: Srbija, Novi Sad
Posts: 40
Hey, thanks a lot, 2u2 electrolytic and it works :) Still needs a bit tweaking though.

Link2
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