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Guitar Reverb Pedal

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    Guitar Reverb Pedal G. Forrest Cook Aug 27, 1997 www.eklektix.com Rev 3: Aug 10, 1999 This circuit was inspired by a friend who wanted a reberb for his
    Message 1 of 1 , Jan 13, 2003
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      Guitar Reverb Pedal G. Forrest Cook Aug 27, 1997 www.eklektix.com

      Rev 3: Aug 10, 1999

      This circuit was inspired by a friend who wanted a reberb

      for his portable guitar amplifier. I originally tried using NE5532

      low noise op-amps for the buffer stages but they were too noisy

      for the low level circuits so I switched to discrete 2N3904

      transistors, they seem to be fairly quiet. This circuit has a

      nice feature not found in most reverbs, a drive control for the

      spring amp. The drive can be turned way up and a nice smooth

      distortion effect will come out of the spring. The circuit will

      run on a 9V battery but will probably run it down fairly quickly,

      it draws about 30ma idle and around 100ma with a full signal going

      through. I designed the circuit to work with a 12V gell-cell.

      A 9VDC "wall wart" transformer will also work nicely, if it hums

      you may need to add a 1000uF capacitor across the DC power lines.

      Be sure to measure the DC voltage from the wall wart, they seldom

      agree with the label and vary a lot with load current. The circuit

      should work ok up to 15VDC. I built the prototype circuit right

      into the reverb spring box for minimal size, it is a good idea

      to remove the spring before drilling the holes. All audio input

      and output lines should be wired with shielded coaxial wire, ground

      the shields at one or both ends, this may affect hum levels on the

      input line. All resistors are 1/4 watt.

      The rev 2 version of this circuit adds a buffer in front of the 386

      amplifier IC, this solves a problem where distortion generated in

      the 386 chip was feeding back from the 386 input terminal into the

      dry channel and making it to the output terminal. I also added

      better decoupling to the preamp stages with the 100 ohm/220 uF

      capacitor combination on VCC2.

      For the prototype's reberb spring unit I used an assembly from one of

      those '70s vintage Sansui stereo component reverb boxes, the ones

      with the blurry rainbow light show on the front. It works great

      but is perhaps a bit tinny sounding, I am only using a single spring.

      A company called AccuTronics makes reverb spring units, they

      have a web page at www.accutronics.com but it is mostly empty

      at this time (12/4/97). The circuit works with a spring

      input impedance of 8 ohms and an output impedance of 2.5K

      ohms, the AccuTronics 4AB3C1B 17" unit or the 8AB2A1B 9" units

      are good choices. My favorite of the two is the 4AB3C1B, it has

      a deeper resonance and the extra length helps to smooth out more

      of the distortion that is generated in the 386 amp.

      The 8AB2A1B unit still sounds very nice, both could be used with a

      switching arrangement.

      I tried using the circuit with the reverb spring from a friend's Fender

      Twin amp but there was some nasty sounding distortion on the reverb

      return amp, I have a hunch that it could be fixed with an impedance

      matching transformer but have not tried that yet. Other possible fixes

      might be to run a small DC bias through the reverb pickup coil or put

      a resistor across the reverb output to limit the signal.

      Accutronics reverbs are being sold here:

      http://www.stf-electronics.com/Page0014.Html

      and here for more $$:

      http://www.vibroworld.com/tech18.html

      I was able to get two tanks from stf electronics, they had the best

      prices.

      Here's some more data on reverb spring units (tanks):

      vibro world: http://www.vibroworld.com/tech18.html

      Accutronics Reverb ID# 4AB3C1B

      style------------------^

      input impedance---------^

      output impedance---------^

      delay time----------------^

      connector grounding--------^

      non-locking-----------------^

      mounting plane---------------^

      Part# primary user Len Springs Input/Output AC/DC US Price

      4AB3C1B Fender 17" 2 8/.8 ohms 2,250/200ohms $45.00

      4BB3C1B Acoustic SLM/Ampeg 17" 2 150/26 ohms 2,250/200 ohms $45.00

      4EB2C1B Peavey 17" 2 600/56 ohms 2,250/200 ohms $45.00

      4FB3D1B Music Man General 17" 2 1475/200 ohms 2,250/200 ohms $45.00

      9AB2C1B Fender Mesa Boogie 17" 3 10/.8 ohms 1,925/200 ohms $50.00

      9FB2A1C General 17" 3 1,925/200 ohms 2,575/200 ohms $50.00

      9FB2A1C General 17" 3 1925/200 ohms 2575/200 ohms $50.00

      8AB2A1B Mesa Boogie 9" 3 10/.8 ohms 2575/200 ohms $35.00

      8BB2A1B SLM 9" 3 190/26 ohms 2575/200 ohms $35.00

      8DB2C1B Marshall 9" 3 310/36 ohms 2575/200 ohms $35.00

      8EB2C1B Fender 9" 3 800/58 ohms 2575/200 ohms $35.00

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