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Best Amp for 0Hz-100Hz square & sawtooth signals driving coils for pulsed magnetic fields.

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I'm working on project to produced Pulsed ElectroMagnetic Fields (PEMF) at LOW frequencies (0-100Hz) using various wave functions. (sine, square, triangle, sawtooth)

The whole design is very similar to what an audio amplifier and speaker setup would be since speakers are essentially coils with low resistance/impedance.

The input signal would ideally come from a 3.5mm headphone jack or even better Bluetooth as I see some small pre built class D amplifiers include such.

I've considered using OpAmps, transistors, Mosfets  but seems pre-built class D amps gets me near 95% there so why build from scratch.

Any off the shelf class D amp would work except for one problem. The low cut of frequency of all pre-build amplifiers is often between 14Hz and 20Hz and understandably so since they are made for audible sounds.

Ideally, I need to account for super low frequencies, 1Hz or even lower, 0.5Hz, 0.1Hz etc.   

Power between 10 watts and 50 watts.

Though I can solder and have build projects before, Ideally, the less building the better. 

So, my question is this. 

Is there any off the shelf audio amplifiers that would also amplify signals even as low as 0-20Hz ?  Probably what would be considered a bad quality/cheap amp?

If not, are there any that could be easily modified by changing any input filers (RL) that would allow lower frequency signals to pass into the amplifier?

Something like this would be great if could amplify 0-20Hz:  https://www.amazon.com/Bluetooth-Receiver-Amplifier-Audio-Fosi/dp/B087FTV362/

Looking at the specs of the chip (TPA3116D2) it contains ...


...it is not clear to me if  a lower input frequency can be permitted by removing or changing external components

Would not be easy on this pre-built device since it does not use normal regular size capacitors and resistors that would be much easier to work with.

I'm open to suggestions. I may be going in a completely wrong direction. It seems there should be a simple answer but my project is not so common.

Requirements again:

  1. Amplify low power signal from 3.5mm jack or phones bluetooth.
  2. input signal 0-20Hz made up of various common waveforms
  3. Total Power 10-50watts
  4. Drive copper coils/speakers 4-8ohms
  5. Adjustable output power/volume knob
  6. pre-built or minimum modifications to a pre-built amp. Worse case will build it
  7. Fast and Easy is more important than cost at this point.

Thanks again for any suggestions or feedback.  I'm open to anything that may take me in an unforeseen direction.






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On 11/17/2020 at 8:19 PM, HarryA said:

Hi HarryA,

Thanks for the suggestion. It looks good but in the first paragraph of  the  datasheet makes me wonder sense is says...

"The LM3886 is a high-performance audio power amplifier capable of delivering 68W of continuous average power to a 4X load and 38W into 8X with 0.1% (THD a N) from 20 Hz –20 kHz."

20Hz?  I wonder if they just say that to appease savvy audio people or can the chip actually amplify DC & sub 20Hz frequencies.  I would think no caps on the input side would be important so the diagram you mention in there without caps make sense for sub 20Hz but I wonder if chips like this have some internal filter for filtering out super low frequencies.

Like you mention,  it says "Pure DC"  amp which would be 0Hz signal which would be great but not of this is clear.  Everywhere I look you'd have to make at least some assumptions.

I'm not 100% sure what terminology I should be searching for to be led to what I need.  Maybe DC Amplifier?.... but I still need some frequency response.  If my project was just amplifying square waves it would be easy.  I have some triggered input mosfets that pulse on/off for that.   The problem is really amplifying the sawtooth.   I've tried varying the duty cycle of the mosfet trigger pulses that with the right Capacitor should smooth them into a sawtooth but that has not been accurate.  At least it does not appear correct on the oscilloscope.  Maybe need bigger caps.

Any other suggestions on how to amplify DC to 20 Hz signals that are made up of higher frequency harmonics themselves.

Maybe the pre-build ebay item could work... but I'd need to remove any capacitors on the input side?

Thanks again


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The OPA548 op amp looks promising. It has up to 5 amperes output into 4 ohms or better. It can be bought in a complete amplifier see OPA348 on Ebay or Amazon.com just search for OPA548. The data sheet is here: https://www.ti.com/lit/ds/symlink/opa548.pdf

As the amp can be used in a dc pwoer supply makes it look useful. If you have an Amazon account you can ask about the complete amplifier low frequency range else I can.OPA548.png.92f920c80613534325a6a7b3398c8060.png



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Q: Can the module amplify square wave or pulse? What is the module bandwidth?

A: The module can amplify low-frequency square waves or pulses. Because it is an audio amplifier, the total bandwidth is not high. Signals within 100K can be amplified. The module is compatible with DC input.

This is the over priced one with best Q/A


The best price is 57.80$ Amazon.com  - actually it's 35.60$ plus 5$ s/h on Ebay

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