I am working on a noise cancellation project and I am having problems with my design once I breadboard it and connect my 8ohm load.

Design Notes: I am not applying/mixing any kind of signal into the circuit, like audio that will go to headphones. I am using a microphone and preamp and then an inverting op amp with a high enough output current spec to get our desired output power. We are using a bone conductor/audio exciter as our output, not a traditional speaker.
We are tasked with cancelling out Snow machine exhaust noise, which is about 200-600hz.

Everything simulates perfect. I'm sure I'm missing stuff to help get advice, so please ask questions if needed! Thank you.

If I remove the 8ohm load, it'll work if I dial up the resistance of the load with a pot up to at least 250ohms.

Inverting Amp

  • \$\begingroup\$ It looks like the amp will only provide 2.8 v p-p with a 4 V supply. Peak current for an 8 Ohm load would be around 175 mA and an RMS power of about 123 mW at the speaker. How much power do you need ...you didn't specify it. You also don't seem to have any power supply capacitance , so may be being impacted by your battery internal resistance. \$\endgroup\$ – Jack Creasey Jun 28 '18 at 23:19
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    \$\begingroup\$ " I am having problems with my design " OK, I'll bite. What problems are you having? You have failed to state them. You show two waveforms. What are they? Where are they measured? What does " it'll work" mean, anyway? \$\endgroup\$ – WhatRoughBeast Jun 29 '18 at 1:28
  • \$\begingroup\$ You need a power amp chip that can drive a few amps but more important good specs on signal range and frequency with noise amplitude at given frequency range then suppression ratio before power amp with actual load impedance together as a list of design specs!! 1st !! \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 29 '18 at 2:47
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    \$\begingroup\$ Jack Creasey - I was just planning on outputting up to the 123mW to the speaker that the amp can drive. I am limited by the project design to a 4V battery. I am using a rail to rail op-amp, so I can't really get anymore power to it. This op amp is rated to supply over 300mW. TLV4112. Also, what do you mean by the power supply capacitance? Do you mean a coupling capacitor after the supply, but before R4? I didn't put one there because I need that signal to stay on Vcc/2 so that the DC bias on both inverting and non-inverting pins are equal. Or are you referring to something else? \$\endgroup\$ – Brady Nelson Jun 29 '18 at 2:51
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    \$\begingroup\$ I don't understand what the problem is, from the question. Note that if you have written additional details in the comments, they should be edited into the question. I see that you have a lot of numbers but it's too messy to get a proper overview. \$\endgroup\$ – pipe Jun 29 '18 at 13:56

If you're looking to deliver 123mW to the speaker, that's about 2.8VPP. In your scope capture, you're showing about 4VPP. If you cut your input or gain until your output is 2.8VPP, the part might be theoretically capable of driving it.

The first page highlights on the spec for this part indicates that it has rail-to-rail output and can drive 300mA. But--and this is a big but--it doesn't say it will do both at the same time. On top of page 5 are some tables showing what the maximum and minimum output voltages are under different supply voltages, output currents, and temperature conditions. It doesn't give you exactly 4V or 125mA, so some interpolation is necessary...best to just try it, but with an output under 2.8VPP.

  • \$\begingroup\$ Thank you for that advice! I didn't know anything about that but it totally makes sense looking at the graphs. As I look at the High-Output voltage vs High-Output graph, the Voltage on the left is Vdd voltage right or is it Vpp of the output? That makes it look like I "should" just barely be able to get around 150mW with about 2.8Volts. If I am reading it right? \$\endgroup\$ – Brady Nelson Jun 30 '18 at 5:49

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