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Help me diagnose a reverb driver circuit I built for belton "digilog" module

Discussion in 'Amp Tech Center' started by elipsey, Mar 12, 2014.

  1. elipsey

    elipsey TDPRI Member

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    Hi,

    I built a solid-state driver circuit for a Belton "digilog" reverb module and installed it in my home-brew amp. The pre-amp is built in the fashion of a Marshal, and the power amp is p-p 6V6s, with LTPI. Except for the messed up reverb, I think it sounds pretty good. :) The reverb driver circuit includes a 5VDC supply derived from the filament supply using a bridge rectifier and 7805 linear regulator, as well as an lm358 dual op-amp dip used to buffer the send and return signals.


    Power supply:
    [​IMG]


    Driver circuit, installed in the amp:
    [​IMG]


    A 512K reverb mixing resistor is installed after the first gain stages and before the cathode follower pair, and the reverb send and return are connected before and after this mixing resistor. The Belton reverb module seems to be working correctly; it's input and output pins look clean, and reverb-y, respectively, on my scope. However, the op-amp that buffers the wet return signal seems to be outputting a half-wave of it's input. There is gross distortion in the signal after the reverb mixer, which I believe results from the half-wave reverb return signal being superimposed on the original pre-amp signal at the mixing resistor. Of course, maybe there is something else going on that I don't understand... In any case, when I disconnect the reverb return, the signal after the mixer looks better, and the amp sounds normal again.

    Schematic from Belton docs
    [​IMG]

    Here's a link the Belton design documentation for the digilog, which I got this schematic from:
    https://www.smallbearelec.com/Projects/BTDR-1H.pdf R1 and C1 are 330k/5000pf for a 200hz high pass. R2 is 47k, expecting 6-8VAC at the input, which seems to be in the neighborhood of what it actually gets. R3 is omitted. The 5V supply measures 4.9VDC which is well withing spec for the reverb module, seems like it should be fine from the op-amps, and it's pretty much free of ripple. I tried disconnecting the resistor that connects the high-pass filter input to the non-inverting (+) input of the second op-amp (see schematic), but it didn't seem to help. Does this just mix the dry signal back into the output, or do something subtler like NFB, or something? I don't really understand why it's there.

    I double checked that layout of my little DIP circuit board pretty carefully for mistakes, solder bridges, &c, although I suppose there could still be a mistake there that I missed.

    Can anyone suggest what I might try next?

    Thanks in advance,
    Chris


    The input looks ok
    [​IMG]


    Half-wave output at return, why?
    [​IMG]


    Wet output looks nice and wiggley, but still half-wave
    [​IMG]
     
  2. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

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    Maybe try posting this over in the stop box area? I know its not a stomp box, but there are lots of OP amp guys over there in pedal land.

    Wondering if maybe there's a DC offset problem somewhere? Is it clamping like that when disconnected? Maybe an impedance mismatch with what you're trying to drive, and it is just requiring too much current from that little OP amp? Just a wild guess there.

    Sorry, I'm not much good with Operational amps...but I have an engineer working for me whom is. I can ask him.


    :)
     
  3. Cleeve

    Cleeve Tele-Holic

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    most op amps need both + and - power supply voltages. In stomp boxes op amps are run with the input at a midpoint between the 9 volts and ground, so the op amp can still deal with a signal that swings positive and negative relative to the 4.5 volts middle zero. I hope you get what I'm saying- check the op amp data sheet, you might need to derive a negative voltage from the filament AC as well as the positive you already have. If you have some batteries you can make a + and - voltage for the op amp and test. The schematic shows the inputs of the op amps referencing to ground, so they pretty much gotta have + and - supply voltages in order to work..

    Oh and you right about the second op amp feed back etc on the non inverting, it's rigged so with the control pot all the way down, there will still be dry signal.
     
  4. elipsey

    elipsey TDPRI Member

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    Thanks guys, you're right. It needs a dual ended power supply, or to be biased and ac coupled, but that's really just the start.

    I ended up posting this over at http://music-electronics-forum.com/t35974/
    It seems like those guys are more into this kind of stuff, although the stomp box forum probably would have been a good bet too...

    Anyway, it turned into a whole ball of wax. Check it out over there if you want.

    I would maybe lock this this thread if I knew how...
     
  5. CoolBlueGlow

    CoolBlueGlow Tele-Afflicted

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    Yep, Cleeve's on it. Wish I were sharper on Op Amps. I apologize, but I'm just not that good with them.

    Fortunatley, that's why we have a forum!

    :)
     
  6. BiggerJohn

    BiggerJohn Friend of Leo's

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    If you are running only a single supply, you have to make a half Vcc reference for the op amp inputs. You cant just ground that non inverting input like you did.
     
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