I am in need of an amplifier to drive two 20 W 4 Ohm speakers. They are used more like solenoid pushers than for audio generation. I have been looking into designing my own, but before sinking a lot of time into it I'd like to know if it's a waste of time.

The signal being amplified will be between 0.1 Hz and 1Hz.

So to reiterate the question, are there any fundamental problems with devising my own circuit to drive these two speakers at these low frequencies?

  • \$\begingroup\$ do you need analog voltage control? why not just switch them on and off (with a mosfet for example)? (DC might still fry your speakers though) \$\endgroup\$
    – Wesley Lee
    Commented Aug 3, 2016 at 20:01
  • \$\begingroup\$ @WesleyLee Well that might prove difficult since the current setup uses an audio file as the driving frequency. The response needs to be quite accurate to the desired waveform. \$\endgroup\$
    – Nate San
    Commented Aug 3, 2016 at 20:16
  • \$\begingroup\$ Speaker systems have degrees of freedom that can maximize the peak currents beyond what you might expect. I've seen a worst case CTF of 6.6 reported in the literature. So while 20W into 4Ω suggests a peak current of 3.16A, a CTF of 6.6 might imply that a peak capability nearer 21A is needed. Sure, this peak current need only be delivered for less than a millisecond. But it could complicate the design of protection circuitry, if nothing else. \$\endgroup\$
    – jonk
    Commented Aug 3, 2016 at 22:31
  • \$\begingroup\$ You ought to have no problem getting an amplifier to work in this way; the tracking/focus of CD players wouldn't work without amplifiers and linear motors. \$\endgroup\$
    – Whit3rd
    Commented Aug 4, 2016 at 6:37

1 Answer 1


You should be able to do what you want. You need

  • A DC-coupled audio amplifier.
  • No capacitors in the signal path.
  • Dual-rail supply so the signal can swing positive and negative.

Your other option is to use a H-bridge and PWM the speaker. This is a lot easier to do cheaply than at was a few years ago.

  • \$\begingroup\$ Install a op-amp in the feedback path that is an integrator with a time constant longer than 10 seconds. Use a TL071 or similar op-amp with a 10M resistor and 10uF capacitor in the op-amps feedback loop. That should give a time constant of 16 seconds. It is to correct for DC drift that last longer than the time-constant. \$\endgroup\$
    – user105652
    Commented Aug 3, 2016 at 20:33
  • \$\begingroup\$ Do you know of such an amplifier that would work? I have searched and either the results are low power, or they are thousands. There seems to be no DC coupled in the mid range. \$\endgroup\$
    – Nate San
    Commented Aug 4, 2016 at 14:46

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