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I have a problem with LM13700 - it does not completely reduce the sound when the control current is zero. For testing, I put together the most basic circuit, where the control pin is connected to the ground. In my understanding, this should block the incoming audio signal. The simulation shows that the level drops down to picovolts, but my breadboard circuit is putting out 150-200mV Vpp and the incoming signal (250mV Vpp) is still clearly audible.

Could this be a problem with the chip? Faulty, fake? I tried two different ones - all the same, but I bought them from the same place. Or am I doing something wrong? Note that the circuit is powered by a single 9V supply.

Schematic Online Simulation

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Alex_G
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    According to the TI datasheet the minimum recommended supply voltage for single supply is 9.5V, so you're below that. You might try upping the voltage a bit and see if that makes a difference. – GodJihyo Dec 14 '22 at 18:00
  • Yes, I know about it. But I've seen some LM13700 circuits that use exactly 9 volts. Some circuits for guitar pedals for example. Although, I can try 12 volts to be sure. Thank you for suggestion. – Alex_G Dec 14 '22 at 18:38
  • Please show the exact circuit you implemented in hardware with particular attention to power rails, power rail splitting, decoupling capacitors and control lines. Also state what your power source is. – Andy aka Dec 14 '22 at 18:41
  • This is how it looks like - https://imgur.com/a/AiyTGxh I added resistor to ground at the output and tried to do some power supply filtering. – Alex_G Dec 14 '22 at 21:29
  • Also, my power supply is a universal power brick with selectable voltages, like 3, 6, 9, 12, etc. – Alex_G Dec 14 '22 at 21:37

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Should be able to get -30dB when compared with 1V which would be around 20mVrms with a 10uA bias. Could be the breadboard or the power supply or the source voltage.

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Source: LM13700 datasheet with mods

Ground the input and check the noise. Check the frequency content of the noise to see if it's amplifying a certain frequency. Check the power supply at the chip and ground to see if they are noisy.

Voltage Spike
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  • Thank you for the answer, I will try. Don't know actually how to measure frequency content, need to learn how to do it. – Alex_G Dec 14 '22 at 18:35
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    An oscilloscope with fft – Voltage Spike Dec 14 '22 at 18:48
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    @Alex_G If dealing with just audio frequencies, as it looks like you are, you could even do that in software using the microphone input on your computer. Specialized software exists, but also just recording it and playing it back in a music player with a spectrum analyzer visualizer would give the information you need. – Hearth Dec 14 '22 at 20:25
  • I keep forgetting that, I used that trick with a pezio electric motor to find out how far and fast it moved – Voltage Spike Dec 14 '22 at 21:15
  • Found it! It was hidden in one of the menus. I read that for frequency analysis I need to use the FFT function, and my oscilloscope should support it, only I couldn't find it. There is no information in the instructions on how to activate it. – Alex_G Dec 14 '22 at 21:23
  • @VoltageSpike here is an output without any incoming sound and 300 mkA Iabc current. https://imgur.com/a/6INJf1i I guess that it doesn't look very good. Unfortunately, I have nothing to compare with yet, but these jumps of 340 mV clearly do not improve the situation. I don't see any specific peaks on the FFT scale. – Alex_G Dec 14 '22 at 21:37