basic preamp circuit help

Discussion in 'The Projects Forum' started by Gonecat, Dec 8, 2009.

  1. Gonecat

    Thread Starter New Member

    Dec 8, 2009
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    Hi,

    I'm new here and for the most part suck at this stuff, but keep going for some reason. :(

    I'm trying to build this circuit for a pal (he needs to raise his 'woody' humbucking acoustic guitar pickup to line level and could use some tone controls, too):

    http://www.montagar.com/~patj/toneccts.gif

    i used a LM741 instead of the 709, which i read in the datasheet is a direct replacement. i wired it with 9v and 15v. Dbl checked the wiring. ...nada

    I am getting "-.25" volts at the output of the opamp.

    anybody know what might be going on or what to check next?
     
  2. mk1

    New Member

    Dec 8, 2009
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    Well I'm kinda new too.. but finding it kinda hard to see which input is inverting/non-inverting.. are you sure you have them the right way round? i take it its the top circuit?

    otherwise rly not sure! sry!
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    The opamp will not work with one input grounded. It must be biased properly at half the supply voltage or the opamp must use a dual-polarity supply which messes up the polarized coupling capacitors.

    I added biasing so the opamp can work.
    I recommend a low noise wide bandwidth audio opamp instead of the very old ones. A second opamp must be used to provide a 3M load for the guitar pickup and a low source impedance for this tone controls circuit.
     
  4. DickCappels

    Moderator

    Aug 21, 2008
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    The schematic is not clear, but the non-inverting input (+) should be grounded and the LM741 will need a negative power supply. There is no way it will swing close to its negative rail. -9 Volts on the negative power supply pin and + 9 volts on the positive power supply pin should make it work.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    Why add another power supply when a couple of resistors will bias the input of the opamp so it works?
     
  6. Gonecat

    Thread Starter New Member

    Dec 8, 2009
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    i am going to re-wire this tonight, thank you all for your input on this, and hats off Guru for the schematic, super cool of you...
    i'll update this thread with how it goes
     
  7. Gonecat

    Thread Starter New Member

    Dec 8, 2009
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    oh sweet, it works. :-D my morale is back up, thank you audioguru.
    i checked it out with my Les(s) Paul copy, sounded ok (though better with a booster infront).
    know any 'clean' boosts using the other half of this tl072 ?! :)
    cheers
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Gee, you were supposed to use the other half to buffer the input from the magnetic pickup!

    A tip - don't leave the inputs to an unused but powered opamp floating (disconnected); it will cause problems at some point. Connect the output of the opamp to the inverting (-) input. Connect the noninverting (+) input to ground, or to a convenient voltage source somewhere between +v and -v.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    I believe the attached is how he meant to say to connect it up.

    I changed a few values a bit so they would be values that you could actually buy.

    I increased the output resistor to 10k. AudioGuru had previously increased it from 600 Ohms to 2.2k, but if memory serves me right, line level audio is supposed to have a 10k impedance.

    I don't recall seeing 20uF caps in awhile. You can get them in 10uF or 22uF; either would work.

    I changed the output amp to be a voltage follower/buffer amp; basically non-inverting unity-gain (what goes in, goes out). C6 was used briefly, but removed, as were R8 thru R10.
    R1/R2 were selected to keep the input impedance at roughly 3.1 MEG, which is mighty close to the 3M that Audioguru had mentioned. R1/R2 are necessary so that the amp can operate with only one 9v battery.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    I would do it like this:
     
  11. Gonecat

    Thread Starter New Member

    Dec 8, 2009
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    bummer, i tried wiring it up like you said, Sarge, triple checked the wiring, cap poles, etc, but no luck. the guitar comes thru but seems uneffected (volume and tone stuff).

    any suggestions? did you try wiring this up and it worked for you?

    audioguru - i'm going to try this latest version of yours now...
     
  12. Gonecat

    Thread Starter New Member

    Dec 8, 2009
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    double bummer, tried this one, too, but same deal (i get guitar signal, but no effect).
    used ceramic instead of film for the .1uf, is that a prob?
    i get the feeling im messin something up, since neither of these last two schems have worked for me!
     
  13. Gonecat

    Thread Starter New Member

    Dec 8, 2009
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    Audioguru - the input section you just did a schematic for and posted - does that go with your original schematic's tone controls and power section or the one SgtWookie did? I am going to try a few things tonight but thought i'd ask...!
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    OK, wait a minute - did the tone controls work with the original schematic?

    It is very difficult to determine from the original schematic whether the amp was intended to be inverting or noninverting.

    It appears that Audioguru's orignal modification had a single TL07x amp as inverting, but with unity gain (what goes in, goes out inverted).

    I'd changed that to be noninverting, so the same polarity signal would go out as came in.
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    Well, it appears that my changing of the output amp from inverting to noninverting was a goof-up. :( My apologies, dude.

    Here's a corrected schematic with similar to Audioguru's input circuit.

    See the attached. It won't be that big of a change.

    1) Note that the - and + inputs to the 2nd opamp are different.
    2) Note the addition of R9, R12, and C6. R9 and C12 don't have to be exactly 47k; they could both be 10k to 100k, just as long as they are about equal in resistance. C6 can be any small capacitor between 10nF (0.01uF) to 10uF that you might have handy, but 0.1uF would be a pretty good value to use.
     
  16. Gonecat

    Thread Starter New Member

    Dec 8, 2009
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    killer, i'm tryin it. and i don't mind rewiring the whole thing if necessary!
     
  17. SgtWookie

    Expert

    Jul 17, 2007
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    Yeah, let's just take it one little step at a time.

    One thing I still don't like about it is that the output signal will be inverted from the input signal, which is why I thought it should be that way to begin with.

    But I don't want to change much at this point; let's just get you jammin' for the moment.
     
  18. Audioguru

    New Member

    Dec 20, 2007
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    Sgt Wookie,
    Your input opamp is an attenuator with a loss of 128 times and it couples half of the noise on the 9V battery to the input.
    My input opamp has no loss and it filters noise from the 9V battery so it is not fed to the input.

    Gonecat,
    The tone controls circuit is a standard Baxandall one that has been used for many years. The opamp in it must be inverting.
     
  19. Gonecat

    Thread Starter New Member

    Dec 8, 2009
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    Guess my post from yesterday didn't show up - i couldn't get any signal out of that one, Sgt, but when i replaced the 3M on the input with a 100k, i got signal. The bass control worked, but the treble didn't seem to do anything.
    Guru, i will try this latest one out tonight...
     
  20. Gonecat

    Thread Starter New Member

    Dec 8, 2009
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    ok, i tried the input section from AudioGuru's post #10 on this thread, along with the tone stack from post #18 and my 100uf cap popped like a firecracker, that was cool! yeah, smoke and all. i powered it with a 12v wallwart. am i wrong that this cap actually bridges from vcc to ground, even though it looks like it connects to those resistors? anyway, tried wiring "seperately" (and used a 9v) but still got no sound, but hey, nothing blew!

    earlier on, the tone stack portion was working fine, but you gents mentioned it needed to be buffered. is there no more straight-ahead way to keep the tone part that was working earlier and just add the buffer stage using the other half of the chip? or is that whats going on, and it requires changing the tone section?

    thanks a lot for the input
     
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