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Thread: Using Op-amps as reverb driver / recovery

  1. #1
    Member PositiveNegativeMan's Avatar
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    Using Op-amps as reverb driver / recovery

    Does anyone here have much experience of this? I want to incorporate a spring reverb to an amp I did a while ago. My intention when I first built it was to keep it as small as reasonably possible. Now I want to add things in!! It's barely got space for the reverb so adding a tube driver is out of the question. There are a few things I can tweak to get a little extra space. So OP AMP seem the obvious choice - but I have no experience working with them. I know that I could build a stand alone reverb, but I don't want to. Building the smallest amp I could get away with was an interesting project and was almost as much fun as it was frustrating!!
    Has anyone any experience?

  2. #2
    Old Timer tedmich's Avatar
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    op amp driven reverbs are very common, PV typically used a dual 4558 and there are kits available (google amp reverb kit)

    FYI the Belton BTDR-1H digital reverb module is tiny, cheap and VERY well reviewed, and an easy op amp circuit is available.

    Reviews here:Belton BTDR-1H Digital Reverb Module

    Belton DigiLog Reverb ?(neunaber technology - WET? Effects Pedals)?

    kit:General Guitar Gadgets - Reverb Complete Kit


    schematic:
    http://www.generalguitargadgets.com/pdf/ggg_verb_sc.pdf

  3. #3
    Senior Member Enzo's Avatar
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    If you go to the Accutronics web site, they have sample drive circuits.

    But I agree with tedmich, the PV circuit is simple and has been used for decades. Steal it.
    Education is what you're left with after you have forgotten what you have learned.

  4. #4
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    Another design that is probably worth a look is the op-amp reverb driver/recovery section that can be found in Laney amps, simple, effective and easy to tweak IMHO.
    JM2CW
    Cheers
    Bob
    Hoc unum scio: me nihil scire.

  5. #5
    Member PositiveNegativeMan's Avatar
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    Thanks!!

    Thank you for the suggestions, I will begin my studies / planning. I think when this is finished the amp should be rechristened as The TARDIS...

  6. #6
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    The thing that you need to keep in mind is that, if you are using garden-variety opamps, you need to use a reverb pan with higher input impedance vs. a tube-driven circuit. The typical Fender tube Class A SE reverb driver uses a transformer to deliver the drive signal into an 8 ohm pan input, much like any Class A SE guitar amp. If you hung a speaker there, instead of a pan, you'd have audio output (a neat way to test these circuits BTW). An opamp circuit uses a pan that has a MUCH higher input impedance, typically 150 ohms, because the opamps cannot deliver much current.

    I find that, to my ears, puny reverb drive current = puny reverb tone. If you want to go for the SS approach, you might want to enlist the help of small power amp IC's, e.g. TDA2614 (6W), LM380 (2W), LM384 (5W), etc. and drive a low impedance pan. It's really a SS version of the Fender circuit, but you don't need the transformer. The ability to deliver good drive current will improve the reverb tone tremendously.

    Recovery into opamps is easy, because current drive isn't a factor.

    If you are scratch-designing a circuit, keep in mind that you need to limit low-frequencies in the drive signal utilizing low-value coupling caps. Excessive lows will overdrive the springs and cause them to "boing". Not a pretty sound.
    John R. Frondelli
    dBm Pro Audio Services, New York, NY

    "Mediocre is the new 'Good' "

  7. #7
    Senior Member Enzo's Avatar
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    I agree with John about the wimpy drive. Look at any number of SS designs and you will see a little push pull transistor drive stage after the op amp. The suggested circuits at Accutronics feature this approach. The appeal of an approach like the Peavey is its simplicity, but you can get fancier easy enough.
    Education is what you're left with after you have forgotten what you have learned.

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    Yep, I totally agree, that's one of the reasons why I stated the Laney reverb drive / recovery section was well worth a look, as, to increase the driver's output current capabilities, they "bridged" the two sections of a dual op-amp.
    Cheers
    Bob
    Hoc unum scio: me nihil scire.

  9. #9
    Old Timer tedmich's Avatar
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    Quote Originally Posted by jrfrond View Post
    The thing that you need to keep in mind is that, if you are using garden-variety opamps, you need to use a reverb pan with higher input impedance vs. a tube-driven circuit. The typical Fender tube Class A SE reverb driver uses a transformer to deliver the drive signal into an 8 ohm pan input, much like any Class A SE guitar amp. If you hung a speaker there, instead of a pan, you'd have audio output (a neat way to test these circuits BTW). An opamp circuit uses a pan that has a MUCH higher input impedance, typically 150 ohms, because the opamps cannot deliver much current.

    I find that, to my ears, puny reverb drive current = puny reverb tone. If you want to go for the SS approach, you might want to enlist the help of small power amp IC's, e.g. TDA2614 (6W), LM380 (2W), LM384 (5W), etc. and drive a low impedance pan. It's really a SS version of the Fender circuit, but you don't need the transformer. The ability to deliver good drive current will improve the reverb tone tremendously.

    Recovery into opamps is easy, because current drive isn't a factor.
    Sounds like you and Rod Elliott think alike


    his design with a NE5532 and discrete transistor PP drive looks like it might sound great, like all the stuff I've bought from him has.
    Last edited by tedmich; 07-18-2009 at 07:22 AM.

  10. #10
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    Quote Originally Posted by tedmich View Post
    Sounds like you and Rod Elliott think alike
    It sure seems that way!
    John R. Frondelli
    dBm Pro Audio Services, New York, NY

    "Mediocre is the new 'Good' "

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    Mesa/Boogie did something similar in their Nomad line of amps, with a 5532 dual op-amp as the drive and recovery stages for the reverb.

  12. #12
    Old Timer tedmich's Avatar
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    NE5532 are highly respected as good (and relatively cheap) dual audio Op amps, along with the more expensive OPA2134. John, Enzo, Robert and Rod Elliott agree that the section of the circuit into the reverb pan needs to move a good amount of current as opposed to the wide voltage swing, low current of a typical Op amp circuit. This is why John suggested, and Rod designed, a circuit using a beefier audio amplifier front end, with the standard op amp for the signal return after the pan. The theory makes sense and I think I'll try Rod's Elliott Sound Product kit once I get some extra $. Or maybe just go with a Belton D-verb and a nice NE5532 or OPA2134 driving it.

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