Low impedance class B amp

Discussion in 'The Projects Forum' started by BkraM, Jan 25, 2014.

  1. BkraM

    Thread Starter New Member

    Jan 19, 2014
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    Hello,

    I'm having some trouble with a class B amplifier. What I am trying to do is combine a class A (as pre amp) and a Class B amp (push pull)to drive a 8 ohm speaker. I know this will not produce the best quality sound but it's a learning curve.

    So far im still in the simulating fase, but i cant get it to work properly. The pre amp amplifies the Voltage signal properly when no load is attached to the output (+/-200 mV to +/-5 V). But when i connect the B class amplifier ( http://cnx.org/content/m32262/latest/ ) the output of the pre amp drops significantly.

    As i understand it the input impedance of the class b is approx hfe*Re. Incase of a 8 ohm speaker this is 800 ohm (hfe=100), this seems to be quite low. And is probably the cause of the drop in amplification, what am i doing wrong? here?

    Thanks,
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Yes, the input impedance of that class B stage is relatively low. You can increase that by going to Darlingtons or a Sziklai pairs for each of the output transistors.

    Such a simple class B amp will have a large amount of crossover distortion. That's why class AB is typically used which adds a slight amount of bias current to the output.
     
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  3. Jony130

    AAC Fanatic!

    Feb 17, 2009
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  4. BkraM

    Thread Starter New Member

    Jan 19, 2014
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    Thanks for the replies, I'll try them out.

    Would a class ab amplifier be better in this case? Not just for sound quality but would it better fit the required high impedance?
     
  5. crutschow

    Expert

    Mar 14, 2008
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    It would help the sound quality but not have a significant effect on the input impedance.
     
  6. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Try using a lower value of load resistor on the Class A driver. It will have to dissipate some power, but should work okay.

    Eric
     
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  7. BkraM

    Thread Starter New Member

    Jan 19, 2014
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    Jony130, I've build (sim) your amp (http://forum.allaboutcircuits.com/sh...914#post663914) works great!
    The role the C5 has in the circuit is very nice, took me some time to understand where the negative Voltage over the load came from.

    I'm wondering if i would add a volume control (pot meter) in this config would that be replacing R3? Any filtering (low high pass filters) would go in between T1 and T2?

    Thanks,
     
  8. BkraM

    Thread Starter New Member

    Jan 19, 2014
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    R3 doesnt seem to be a good spot voor gain control thinking about it, DC settings would go wild. Would add a variable resitor betwwen R4 and T1, outside the capactior loop. Is that the way to go?

    Build it on a breadboard, works great in practice too.

    THanks,
     
  9. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Hi, for the volume control simply add a pot at the input of the amplifier
     
  10. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Yes, C5 is charging via T4 transistor for the positive half.
    In this circuit:

    Vcc --->T4 collector-emitter --->C5 ---> speaker---->GND

    And discharging via T5 for the negative half.

    Positive C5 plate -->T5 emitter-collector--->speaker---> negative C5 plate

    why you want any filter between T1 and T2 ?
     
  11. BkraM

    Thread Starter New Member

    Jan 19, 2014
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    I was just wondering where in a amplifier filters would go for treble and bass?

    Added a potmeter above R4, does the trick. Will also try putting it in front of the amp.

    Thanks for your help!
     
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