Is this single transistor amplifier biasing the speaker?

Discussion in 'General Electronics Chat' started by jaydnul, Feb 4, 2016.

  1. jaydnul

    Thread Starter Member

    Apr 2, 2015
    I was watching this really cool video:

    and was just curious about the single transistor amplifier he made (19:10). Isn't that going to DC bias the speaker? If so, is that not a problem (clearly it's not because it works, but I thought any DC bias on speaker was to be avoided)?

  2. Lestraveled

    Well-Known Member

    May 19, 2014
    Yes. that is generally correct. But in a case like this, simple, cheap, low output, you can get away with it.
  3. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    The technical answer is: It Depends...

    Yes, a DC bias through a speaker coil usually is not a good thing. By shifting the audio centerpoint physically, it means the speaker has less movement range in one direction than in the other. For lower level signals you might not hear any difference. But for louder signals, the speaker might bottom out against the magnet structure, or strain against the springiness of the cone suspension in the other.

    Back in the tube days, a regulated power supply was expensive. A trick to filter power supply hum was to use the speaker coil as a filter choke.

  4. MrAl

    Distinguished Member

    Jun 17, 2014
    Proper output circuit and large series cap solves the DC current problem.
  5. crutschow


    Mar 14, 2008
    The coil they used for that was the speaker electromagnet coil, not the voice coil.
    Instead of having a permanent magnet as modern speakers have, those old speakers had an electromagnet and ran the B+ current from the power supply through the magnet coil to generate the magnetic field, and do double duty as a filter.
    The old Farnsworth radio my folks had on the farm where I grew up (model shown below) used such a scheme.
    (Note those knobs on either side of the dial. They were lever pots that moved up and down for bass and treble).