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audio current (transconductance) amplifier

sarath azad
Prodigy 200 points
Other Parts Discussed in Thread: LM1875, LM3886

Hello,


Could you please provide some more information regarding audio current amplifiers especially for full range speakers (without crossover circuits)?

It seems the loud speakers are current consumers, rather then voltage. the sound from the speakers are proportional to the currents through them rather than voltage, and also they provide complex impedance to the driving amplifier rather than resistive load. So the voltage output may not be in line with the current output of the amplifier over the whole audio frequency range.

So the current (transconductance) amplifier are better for audio to driver loudspeakers rather than traditional voltage amplifier. Is it true? If its true? Are the amplifiers like LM1875 does voltage or current amplification ie will the output current be same even if the impedance of the speaker changes with frequency?

thanks and regards

S Sarath

  • 0
    TI__Guru**** 189401 points

    Hi, Sarath,

    All our amplifiers are voltage amps.

    -d2

  • 0 in reply to Don Dapkus
    Prodigy 200 points

    Hi Don,

    Thanks for the reply.

    Could you please tell if current amplifiers have advantage over voltage amplifier for loud speakers? 

    Or does the current amplification have problems over the voltage amplification, that we are not seeing them used in general?

    thanks and regards

    S Sarath 

  • 0 in reply to sarath azad
    Prodigy 225 points

    By definition, a transconductance amplier has an infinite output impedance.  If the loudspeaker could be equalised to have a flat load impedance over the desired frequency range, then a current-output amp has some theoretical advantages, and such an amplifier was proposed by Professor Malcolm Hawksford of Essex University a long time ago.

    In the real world, a moving coil speaker has a large rise in impedance at its resonant frequency, which will cause a large increase in output power to the speaker at this frequency if current drive is used.

    You might argue (and some people do) that this rise can be equalised away, but loudspeaker parameters change with time, and the equalisation is likely to require changing to allow for this.

    It is fairly easier to add current-derived voltage feedback to a normal chip power amplifier such as the LM3875, so you may want to experiment with this.  By suitable feedback networks, you can set the output impedance of the amplifier to anywhere between its normal, very low value all the way up to (almost) infinity.

  • 0
    TI__Intellectual 1240 points

    Hello Sarath-san,

    I'll interupt to discussion about Loud speaker current drive approach. Please take look at the file I attached. 4657.LM1875 Current Drivde Approach.xlsx This way is that I load is proportional to input voltage (Vin) . Even Loud speaker impedance change over frequecy, the current flow on Loud speaker is, Iload=Vin/R3. The output voltage is, Iload x (R3 + Loud SPK Impedance). The sensing resistor R3 should be pure resistive.  I hope this approach meet with your desire.  If you want higher current, you can take LM3875 or LM3886.

    Best regards

    T NAOKAWA

  • 0 in reply to Toyojiro Naokawa
    Prodigy 225 points

    Hello Naokawa-san.

    Yes, that is the sort of thing I was referring to in my post.  Your schematic show a pure current-drive to the loudspeaker, but it is possible to have a combination of both voltage-sensed and current-sensed feedback to set the output impedance to any desired value.

    Perhaps the original poster might like to download the free and easy-to-use TINA Spice simulator from the TI site and do a few virtual experiments?