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help understanding amplifier schematic

Discussion in 'General Electronics Chat' started by whiz115, Nov 9, 2008.

  1. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Triodes and pentodes sound different - and triodes also put out less power. Some people prefer the way a triode sounds, and accept the lower power.
     
  2. flat5

    flat5 Member

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    Using a DC voltmeter, black lead (ground) to chassis ground.
    Red (+) lead to pin 3 of the EL84, with power applied and the tube warmed up for a minute, no signal applied, measure and record the voltage.
     
    Last edited: Dec 7, 2008
  3. whiz115

    whiz115 Member

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    hi flat5 sorry for my late responce... what's that you wanted me to do? :rolleyes:

    i measure 9.5VDC
     
  4. dave

    Dave New Member

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  5. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    And what value cathode resistor was that?, you've changed it so much.
     
  6. whiz115

    whiz115 Member

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    220ohm.

    the EL84 don't turn red anymore.

    first time in my life i hear such ~5.5W the amp is really strong and i'm not kidding at all! it reminds me the TDA1557 as power output. (maybe my speakers are too sensitive i think >90dB).

    The capacitors at the el84 anodes helped alot, but noise still exists especially when i let the input floating. (shorting the inputs makes the noise less in that case)

    distortion? well.... if my mp3 player volume and my amp's volume are wide open then yes you don't listen anything other than distortion. but if i'm having my mp3 reasonably high there is NO distortion even if i'm having my amp's volume at maximum.

    quality of sound? not bad! but something is missing and i don't know what... my LM3875 does alot better with this song:

    YouTube - THE PRODIGY - Breathe

    old but still like it and usualy it's my favorite test for speakers and amplifiers.


    1. should i connect the screen grid directly to (+) to see what kind
    of impovements it might bring?

    2. what should i do with the remaining noise? especially when the input is floating? sould i decouple the screen? i was thinking to order a choke but it's going to be huge in size and heavy and i don't like it.
     
  7. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The amplifier cannot reduce distortion from the MP3 player that has its output clipping because its volume control is way too high.
    The amplifier will also clip if its input signal is too high.

    The output transformer reduces the level of low frequencies and high frequencies.
    The high output impedance of the output transformer stops it from damping resonances of a speaker.
    The bass is muddy not clear and tight.
    The high distortion of the tubes causes a form of compression so that the "punch" of some songs is missing.
     
  8. whiz115

    whiz115 Member

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    nevertheless it's fine...

    distortion is fine (acceptable even without nfb)

    output power is awesome (this is not 5.5W!!!! unless i have never heard true 5.5W, my 2.1 active PC speakers suppose they are 10W RMS) and it's half loud!

    sound quality is not bad i wasn't expecting miracles with 4-5 resistors and some capacitors i just was wondering what is that i don't like in the sound...


    NOISE is the problem! very little but still exists! and what's annoying is the noise of the amp when i have no source connected.

    if i fix all the anoying things and my circuit is as perfect asi wanted to be i'm planning to make it have low and high input so i can choose what i want to connect... Mp3s, guitars, sound cards, etc.
     
  9. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    That's because the 220 ohm resistor has reduced the anode current to 43mA, just under the maximum 48mA listed.

    Distorted sound is louder than clean sound, so valve amps sound louder than transistor ones of the same power - so this is to be expected.

    A high impedance open input will pickup mains hum, it's to be expected.

    I think you have a wildly imaginary idea of what 'no distortion' sounds like :D

    If you measured the distortion you would be amazed how high it is.
     
  10. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    PC speakers are tiny inefficient drivers, and wildy exaggerated power claims.

    Like I said previously, the high distortion levels of valve amps also make them sound FAR louder than their actual power rating.
     
    Last edited: Dec 19, 2008
  11. flat5

    flat5 Member

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    If you want less hum when no signal is applied, change the input network, the two components connected to pin7 ECC83.
    Change the 1M resistor to 50k or 100k, what you have. Even try 10k.
    Change the input cap from .01 to .1uf. Let us know what happens, please.
     
  12. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The 3" drivers in my pc speakers have the biggest magnets I have seen on little speakers. They are driven with 3.5W RMS each and are pretty loud with bass flat down to about 100Hz.
    The drivers look like this one:
     

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  13. flat5

    flat5 Member

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    Interesting speakers. What is the model number and manufacturer of the speaker system, please? Or is it home brew?
     
  14. whiz115

    whiz115 Member

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    ok nigel thanks for the explanations...

    @flat5 i'll try them and i'll let you know.

    something else now...
    i want to simulate low dumping factor so i can understand what's happening when an amp can't control well a speaker when resonating...

    i'm thinking to try a resistor in parallel with the speaker but i don't know
    what value to choose so i can have intense results.
     
  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Dumping?? Hee, hee.
    A tubes amplifier has a low damping factor. It is the same as having a resistor in series with the speaker. A speaker with poor damping sounds boomy due to the resonance that keeps it "ringing" long after the signal has stopped which also makes it sound "muddy".

    When everybody had a vacuum tubes amplifiers, speakers were made with reduced low frequency output so that the poor damping allowed them to resonate which boosted the bass. Those speakers lack low frequencies when driven from a solid-state amplifier that has an output impedance of 0.04 ohms or less.
     
  16. whiz115

    whiz115 Member

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    while tube amps like mine have output impedance 8ohm..right?

    ok...so what should i do to simulate low damping factor? i got a chip amp
    on my desk...should i connect a 4ohm/5w resistor from the output in series with the speaker?

    i'm curious to hear how it sounds with low damping and with "high".......
     
  17. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    For an 8 ohm speaker, yes.

    Yes, that will ruin the damping, and reduce the volume as well.
     
  18. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A tube amp without negative feedback probably has an 8 ohms output impedance at its 8 ohms tap on the output transformer. So with a modern amplifier that has a very low output impedance, connect an 8 ohms resistor in series with the 8 ohms speaker to simulate the speaker being driven from a tubes amp.
     
  19. bassnut

    bassnut New Member

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    re Hum. Where you ground all the 0vs on the chassis will make a huge difference, preferably at one point so you dont get earth loops (power supply current in the same wire as input ground).

    Also, I hope your not using guitar speakers, they are unsuitable for music. Guitar speakers have no-coil gaps at each end of the voice coil assembly so they dont burn out when over driven, thats why they can be driven hard and sound so loud, but they distort to hell (again, the kind of sound guitarists look for ;-).
     
  20. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    So you think a 5W amplifer is going to overdrive a guitar speaker? :p
     
  21. whiz115

    whiz115 Member

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    So...

    doing my tests using 4.7ohm,2.2ohm/5W resistors, 50 and 1000Hz tones and some songs, i've noticed that indeed there's a difference in the sound, the bass is less "aggressive" and the effect can't be undone be increasing the volume so i can overcome the power loss from the resistor.

    i didn't noticed any difference to the high frequencies... as low as you get the "effect" is more obvious.
     
    Last edited: Dec 20, 2008

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