Why current-drive?

What is a current-drive amplifier?

A current-drive amplifier is a power amplifier with a high output impedance. It works as a voltage controlled current source or transconductance power amplifier: voltage in, current out.

  • When the output impedance of the amplifier is much lower than the impedance of the loudspeaker it is a voltage-drive amplifier. Most amplifiers are voltage-drive.
  • When the output impedance of the amplifier is about the same as the impedance of the loudspeaker it is a power-drive amplifier. A valve amplifier is often a power-drive amplifier. 
  • When the output impedance of the amplifier is much higher than the impedance of the loudspeaker it is a current-drive amplifier. There are almost no current-drive power amplifiers available.

Current-drive compensates for power compression

Playing music, especially loud music, heats up the voice coil

  • Loudspeaker drivers are very inefficient.
  • The most efficient drivers are about 10 % efficient, the most drivers are between 0.1 and 1 % efficient.
  • So, 90 to 99.9 % is turned into heat!
  • The temperature and the resonance of the voice coil rises.
  • Professional drivers, with very good heat radiation, can withstand a lot of heat and can have a power compression of 5 to 6 dB.
  • With 6 dB power compression the resistance of the voice coil is raised to 4 times its original value and a voltage-drive amplifier delivers only 25 % of its power.
  • With 3 dB power compression the resistance of the voice coil is raised to 2 times its original value and a voltage-drive amplifier delivers only 50 % of its power.
  • This is more common with normal sound levels at home and normal drivers.
  • So, the sound level is compressed.

The sound also changes

  • The impedance curve of the driver is not flat.
  • So, the relative damping is dependent on frequency.
  • This means that the middle frequencies of the driver, where the impedance is lower, the compression is the most.
  • So, if you turn up the volume (or the music gets louder), you boost the low and high frequencies of every driver more (actually the inverse of a loudness correction).

And with passive crossovers the sound changes even more…

  • Passive crossovers with compensation for changes is voice coil resistance are rare, if they even exist.
  • A raise in resistance lowers the cutoff frequencies of the to the driver connected crossover filters.
  • And a tweeter heats up faster than the midrange, and the midrange faster than the woofer.

The loudspeaker driver is an electric motor

  • And, as you know from high school an electric motor is driven by current.
  • So, when you drive a loudspeaker driver with current you leave any audible effects behind.
  • Actually there is some “power expansion”.
  • The current is controlled. So the output voltage of the current-drive amplifier is raised, with a heated voice coil and thus a higher driver impedance. The amplifier has to produce more voltage to maintain the output current.
  • As the impedance curve is raised, relatively it is flattened a bit. So the power demand in the middle frequencies, where the demand was the lowest, is raised the most.

Current-drive lowers the distortion in the loudspeaker

The impedance curve of a driver is a mess

  • The impedance curve determines the relation between voltage and current.
  • The impedance curve is highly non-linear.
  • The impedance curve is dependent on frequency.
    • Has a peak at the resonance frequency.
    • Has a rising response at high frequencies.
    • Has bump and peaks when there are resonances or vibrations.
  • The impedance curve is dependent on the level of the signal.
    • Mostly the impedance curve is measured at 2,83 Volts.
    • When you measure at another voltage level the impedance curve is different.
  • The impedance curve is dependent on temperature.
    • If you play loud the voice coil heats up and the impedance curve changes.
  • The loudspeaker driver works also a microphone.
    • That is the cause of the peak in the impedance curve at the resonance frequency and at other resonances.
    • Every sound has impact on the impedance curve. Sounds and vibrations of the driver itself. But also other sounds in the loudspeaker enclosure from other drivers and ports. And also sounds from outside the box.
    • To give an impression, at resonance the microphone signal is often more than 90 % of the input voltage. This gives the electrical damping.
  • The driver has a coil, a voice coil.
    • Coils are problematic. Coils with ferromagnetic materials are even more problematic. In the better loudspeakers coils with ferromagnetic are not used in the crossover filter, because they distort the signal.
    • But a driver has always a voice coil with ferromagnetic materials. The voice coil itself distort the signal because the impedance of the voice coil is non-linear (hysteresis, saturation).
  • So, the impedance curve is highly non-linear.
    • What does that mean?
    • If you apply a voltage to the driver, then the current through the driver has distortion. This is known as current distortion.
    • Also the other way around. If you apply a current to the driver, then the voltage over the driver has distortion.
    • So, what to use? Drive the driver with voltage or current?
    • I think you guessed it already.

The loudspeaker driver is an electric motor

  • And as you know from high school current drives an electric motor.
  • So, when you drive a loudspeaker driver with current you leave a lot of distortion behind.

Current-drive flattens and extends the frequency range of the drivers

  • With current-drive you get a nice HP type frequency response
    • A resonance and above that a nice and flat frequency response in both amplitude and phase.
    • Only at high frequencies when breakup appears and the dispersion narrows the on-axis response rises (with voltage-drive the narrowing dispersion compensates for the rolloff above the frequency with minimum impedance).
  • And current-drive extends the usable frequency range
    • With voltage-drive you need a closed box (rising the Q) or bass reflex (an extra resonance) to boost the bass.
    • If you dampen the resonance and if you dampen the high frequencies when the dispersion narrows you have a wider frequency range with current-drive.
      • The low frequencies are limited by the displacement of the drivers.
      • The high frequencies are limited by the dispersion of the driver, and depth, diameter and breakup of the cone of the driver are the determing factors.

Current-drive gives more headroom

  • Most musical energy is in the mid frequencies and less in the lows and highs.
  • Every driver has its lowest impedance in the middle of its frequency range, and a rising impedance at the low end and at the high end.
  • With voltage-drive the highest power demand is middle frequencies of each driver.
    • With a certain voltage, the highest current is where the impedance is the lowest.
  • With current-drive the lowest power demand is middle frequencies of each driver.
    • With a certain current, the lowest voltage is where the impedance is the lowest.
    • The highest power demand is at the edges of the frequency ranges of each driver.
      • For the most part these edges are cut off by crossovers, only the edges of the total frequency range need more power, but there is far less musical energy.
  • So, with music the power demand with current-drive is much lower and gives more headroom for dynamics.