Troubleshooting Voltage Controlled - Low Pass Filter

[pradhan.rachit]

Hello experts,

I have been building a Voltage Controlled LPF for audio applications and have problems in troubleshooting the circuit issues.



In the above circuit, I've applied a 0.2Vpp/500Hz sine wave at the input port. I'm getting the signal at point A, but it fails to be present at point B. I tried to isolate the fault by removing each resistor next to it and noticed that the signal is present after R8 is removed from the circuit.
Also connecting the 20k resistor next to it causes the signal level to fall a bit.

Could someone help in explaining the working of the circuit? Also possible measure to be taken to rectify this problem.

Thanks!

PS : IC1 is LM13700

 

[chuckey]

R9 and R8 attenuate the signal by 82:1, turn up the gain on the scope!
Frank

 

[pradhan.rachit]

Aaah! I refrained from doing that :lol:
But, there is not signal at the output of the OTA at pin 9 either.

Any possible solutions? Thanks

 

[schmitt trigger]

Could it be that you have a defective component?

 

[pradhan.rachit]

I was able to find the fault. One of the transistors in the current source constructed from two BC559s was defective which caused the LM13700 to malfunction.

 

[chuckey]

Well done!
Frank

 

[pradhan.rachit]

Can someone explain how this circuit works? I was unable to find an explanation.

A video containing its response...
https://www.youtube.com/watch?v=axUHAwen-I4

 

[Audioguru]

You need to learn about the current-controlled transconductance amplifier to understand how the voltage-controlled lowpass filter works.
Many years ago the CA3080 was the first current-controlled transconductance amplifier, then later came the LM13600, then the LM13700. Look them up in Google and maybe you will find a description about how they work. Here is an applications note describing the CA3080: https://www.intersil.com/content/dam/Intersil/documents/an66/an6668.pdf