Op-Amp Questions, Marshall 3005

Discussion in 'The Workbench' started by xxx256, Mar 24, 2014.

  1. phiit

    phiit New Member

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    Hi, I just modded my 3005 combo with RC4558P opamp, while the hi input now sounds very very nice with this opamp I think it also seems to be slightly lower (or now to think of it, less "thumpy") in volume vs low input. The low input sounds about the the same before the opamp change (just as dirty and gritty) but hi input sounds way better just a 1/10 lower in volume.

    Am I imagining or did I f... something up :p From what I understand the hi input is supposed to be for hotter signal and thus being tamed with resistors? but both inputs are still before the opamp? :S
     
  2. BowerR64

    BowerR64 Well-Known Member

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    You should of soldered in an 8 pin socket that way you can buy a few different opamps and pop em in and out and try some different ones.

    I like the TL072 also and the NE5532
     
  3. phiit

    phiit New Member

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    No no! I mean the hi input's sound is just what I wanted but the lack of volume compared to low input is what puzzles me. If this is ok and the hi input has a lower volume with this opamp everything is as it should :) I just can't get it over my head why the difference is there unless the opamp is completely out of lowinput signal path?! It just doesn't make sense :p
     
  4. johan.b

    johan.b Well-Known Member

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    Do NOT mess with the bias on a solid state amp unless you have a tone generator a dummy load and know what you do. A tube amp is forgiving and you can mess a bit with it.a solid state amp will blow up if you set it too hot. Not forgiving. .There is only the modes of bias in solid state power amps. Cross over distortion mode, clean sound mode and magic smoke exit mode. ..
     
    Last edited: Jul 22, 2015
  5. phiit

    phiit New Member

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    I'm gonna update this, I just installed a opamp socket and tried another RC4558P chip and everything is just fine. No lack of volume or anything, just sweet sweet tone! :)
     
  6. Chris Juried

    Chris Juried New Member

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    Hi,

    Just a heads up regarding authentic and genuine Texas Instruments RC4558P ICs, which is really an awesome OpAmp. I have a sale, running now, here:

    Juried Engineering, LLC at Bonanza - ICs & Processors, Busine...

    and on ebay here:

    20 x Texas Instruments RC4558P Free Shipping New and Authentic USA Seller | eBay


    Best,

    Chris


     
  7. skunqesh

    skunqesh New Member

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    I know this is an old thread, but I recently picked up a Lead 12 5005 and am using it to learn a bit about SS amplifiers, etc.

    I noticed a similar problem as the one quoted above.
    Recently socketed the OP amp input so I could experiment - tried out a TL0272CP and noticed a similar effect on the Hi Gain channel.
    It's like the Hi/Low channels became reversed? is that even possible? Fwiw, the original chip works fine.

    thanks in advance for any advice!
     
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    4558
    is like the bottom of the op amp barrel. Low performance.

    Check the pin configuration and then you can try several different op amps.
    Make sure your pin configuration matches the original.

    And yes, definitely put in a socket instead of soldering the op amp to the board.
    Install the op amp correctly, paying attention to pin one location.

    I use : "HI-REL" sockets which are more expensive but more reliable also.
    [​IMG]
     
  9. jmp45

    jmp45 Well-Known Member

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    Necro post time. Wakjob, I have a trimpot bias. Is 470 ohm bias resistor a good replacement? Just curious if my amp has been modded and I should replace with a 470 ohm. TR3 BC212 is fried. I'm replacing this week with a 2N3906.

    [​IMG]
     
  10. jmp45

    jmp45 Well-Known Member

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    I measured the bias pot resistance in circuit @ 537 ohms. I'm just guessing that may be the actual resistance in circuit. I set it @ 470 ohms. Wondering if whomever modded this amp messed with the bias and fried TR3.
     
  11. chris84dc

    chris84dc New Member

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    Hi from Spain. I have the same problem, but I don't have a Lead 12, I have a Lead 20, so...I have the same problem with de noise. I change C4 and the problem is the same...Really Lead 20 schematic is some different that the Lead 12 , but I can´t read it because is handwritting
     

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  12. chris84dc

    chris84dc New Member

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    Maybe the fizz problem is the IC? Im going to put a socket for the IC and try TL072CP maybe is the solution...
     
  13. alpha al

    alpha al Active Member

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    Your amp uses a dual op-amp. All those you tried are dual, except LM380 and TL081.

    LM380 is a 2.5W power amp, commonly used as a headphone amp.

    TL081 is a single op-amp.

    That's why those failed to work. I'm surprised putting those in didn't cause damage.
     
  14. thetragichero

    thetragichero Well-Known Member

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    swapping op amps is great sport but little benefit (except for those selling on guitar electronics websites, especialy expensive burr brown stuff that doesn't belong in guitar gear). i would look to the components surrounding the op amp, and possibly earlier in the circuit (if you've got a noise problem metal film is a good idea). you got a fizz problem on a certain op amp gain stage? if it's non-inverting (quite likely), throw a small (pf... 110 220 470) across the feedback resistor
     

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