Is this single transistor amplifier biasing the speaker?

Thread Starter

jaydnul

Joined Apr 2, 2015
152
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)?

Thanks
 

AnalogKid

Joined Aug 1, 2013
8,534
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.

ak
 

crutschow

Joined Mar 14, 2008
25,250
..................
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.
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).

 
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