DTS Audio Research & Technology SDD-3KP Pre-Amplifier



DTS Audio Research

The Korg SDD-3000 Digital Delay is an rackmount effects unit made famous by The Edge of U2.  For guitarists, this effects unit can be thought of as two effects in one.  The first is of course the fantastic-sounding analogue-sounding digital delay with its super-deep modulation.  The second is the preamplifier section, which when driving a Vox AC30 or similar amplifier can achieve really nice overdrive tones.  Depending on the setting, the preamp’s effect can range from subtle to extreme.  I tend to think it sounds best on the subtle side, as an “enhancement” of the Vox tone you already have.   Since there are a few nice modulation delays now today (Timefactor, etc.) that can sound similar to the SDD-3000′s delay effect, a useful effect would be one which replicated the preamp section in pedal form.  The schematic is easily traceable from the SDD-3000 Service Manual (see pages 7 and 9 of this .pdf).

It’s really not too complicated-   basically there is a switchable TL072  buffer, followed by two inverting amplifiers – a TL072 and the familiar 4558.  The gain control is done by the TL072, which allows one to drive the 4558 with a pretty big signal.  Neither the TL072 nor the JRC4558 are considered very good op amps in the HiFi world, but for guitar effects they are known to sound quite good…

As a U2 gear-junkie and frequent user of the SDD-3000 preamp,  I wanted to be able to duplicate the sound of my rack effects as closely as possible, which meant I wanted to be able to use this pedal in the very same way I use my SDD-3000 when it is in delay bypass mode.  Thus, I had two major requirements:

  1. to have the input and output attenuators (which I like to play with) just like on the actual unit
  2. to power the op amps with the +/- 15V supplied in the original effect in order to get the full headroom and to squeeze every last bit of tone out of them.

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