speaker and crossover

Discussion in 'General Electronics Chat' started by apassionata, Feb 9, 2008.

  1. apassionata

    Thread Starter New Member

    Feb 9, 2008
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    hi. my friend mistakenly connected the (+) and (-) inputs of his speaker to the amplifier's 4ohm and 8ohm (both +) outputs when trying a new speaker cable.

    his amplifier is a 300b autobias integrated tube amplifier, and his speakers are 97db with two drivers with a passive crossover.

    he listened to the speakers with this wrong connections for more than 20 hours in high listening levels.

    even though he noticed that the sound was a little different, he though that it was because of the new cables.

    my questions are below:

    1) how can the speaker make sound connected to both (+) poles?

    2) do you think he damaged something in the amplifier, speaker or the crossover with this connection?

    3) if something was damaged because of this wrong connections with 20 hours of listening, would there be any signs?

    thanks.
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    Amplifiers made for home stereos for the last 40 years do not have outputs for different impedance speakers. Old amplifiers with vacuum tubes had output transformers that did.

    No harm was caused by the wrong connection.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    1) The output transformers' secondary has a center-tapped 8 Ohm winding. Taking output from both +4 Ohms and +8 Ohms would be equivalent to taking output between the common terminal and +4 Ohms. If his speaker was rated for 4 Ohms, it would probably sound OK; if 8 Ohms, not so good.

    2) It's hard to tell if anything is damaged with what information you've provided thus far.

    3) If something were damaged, it would likely show up as badly distorted output (damaged output transistor) , poor high frequency response (damaged crossover network, blown tweeter) or poor low frequency response (damaged crossover network or blown woofer)

    Try connecting the speakers properly, and see how it sounds.

    If the speakers' impedance doesn't match either of the amplifier's outputs, it just plain won't sound good. In that case, either the speakers would need to be replaced, or use an impedance matching transformer.
     
  4. apassionata

    Thread Starter New Member

    Feb 9, 2008
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    thanks for the answers. however, what i do not understand is:

    how can the speaker sound when + and - inputs of the speakers are connected to + and + outputs of the amplifiers? - pole output is not necessary for output?

    i do not mean that he connected the 8 ohm speaker to 4 ohm.

    he mistakenly connected the (-) input of the speaker to the (+) 4 ohm pole of the amplifier, and the (+) input of the speaker to again (+) output. so the (-) input of the amplifier was empty.

    any harm do you think?
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    The output transformer has a winding with a tap for 4 ohms. The tap is a center-tap;
    The impedance is good for a 4 ohm speaker from the tap that is marked "4 ohms" to ground, or from the 4 ohms tap to the 8 ohms terminal.
     
  6. apassionata

    Thread Starter New Member

    Feb 9, 2008
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    He noticed that after this mistake, although there is no noticable distortion in the sound in general, now he thinks that the bass does not go as deep as before. Do you think it is just psychological or if something had been damaged, can it be the reason for less deep bass?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    After listening to his stereo blasting at high volume for 20 hours straight, it's very likely that he damaged his hearing!

    Yes, it is possible that he damaged something in the amplifier, since he was basically running the amplifier wide open with only 1/2 of the load it was designed for. I'm surprised that he didn't notice the distortion and turn it down.

    There are a number of different amplifier classifications, and within those classifications are a great many different designs. It would be practically impossible to troubleshoot such a device from long distance without having so much as a schematic to look at.

    However, you could try swapping speakers from one channel to the other, see if there is any discernable difference. Try swapping the signal inputs to the amp, see if there is a difference.

    Check to see if there are any bypass buttons pressed on the amplifier. My sound system has a bypass switch for the CD player that completely bypasses the equalizer and bass/treble controls.

    Build a signal generator, and put some low-level sinewaves through the amplifier. Look at the output using an oscilloscope across the speaker connections to see if it's distorted (don't operate the amp without a load connected)
     
  8. legac

    Well-Known Member

    May 4, 2005
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    300B autobiased would produce 8watts RMS. The normal audible level of a 97 dB loudspeaker would not be more than 3 watts RMS. Your friend did not use 300B to play Rap music- I believe. Mistaken reversial of connection would neither harm the loudspealer nor the amplifier.The moving coil of the driver has operated within safe limit. It is easy to hear the sound of crash if the driver was damaged. FYI: some amplifiers have equiped with a switch at the rear side, which permits the audience to reverse the phase of the loudspeaker to fit with the resonance of the listening room.
    Cheers
     
  9. apassionata

    Thread Starter New Member

    Feb 9, 2008
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    Thank you for the answer. So basically +/+ connection instead of -/+ does not harm the speaker or the amplifier. Right?

    I ask this as the mistake was not -/+ connection instead of +/-.

    The mistake was connecting both inputs of the speaker to + outputs.
     
  10. legac

    Well-Known Member

    May 4, 2005
    54
    0
    I understand that the amplifier has 4 connection points for 2 loudspeakers labelled [+/-] and [+/-]. As you have mentioned one loudspeaker was connected to [+/+] the other [-/-] was not loaded?
    If a loudspeaker was connected to [+/+] then the output signal was generated
    by two output xformers connected in serie. The output impedance was -in principle- 4 time higher. Because the 300Bs are biased at class A the connection [+/+] would not draw excessive current. However it caused certain distortions but it should be OK after fixing the right connection. Your friend should not be worry.
     
  11. Lembit

    New Member

    Nov 13, 2007
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    your speakers were connected simply in series between the stereo channels and you were listening to difference signal A-B (or B-A). It certainly did not cause any damage to amp nor to the speakers, because the output power in such case is a lot less than maximum possible.
    Only there was no STEREO sound in that case whatsoever and your friend does not need stereo amp at all, if he was not able to realize in 20 hours, that there was no stereo.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    896
    When the program was the same on both channels (mono) then the amplifier had no load. That means the output transformer developed extremely high voltages to the output tubes and maybe damaged them or damaged the insulation of the output transformers.

    The bass frequencies would have been cancelled. Your friend should have noticed.

    Hee, hee. The amplifier is only 8W per channel??? Peanuts power.
     
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