stage center reverb tips

[vacuumdust]

HI ALL...I can't get the search function to work...I just ordered parts for the stage center reverb and was just hoping for some tips from those with success stories...I'm really concerned about keeping the noise down, which can be a problem with a lot of reverb units.  I've got the right tank...and from what I remember that's half the battle.  THANKS

[Mark Hammer]

Standard tricks for keeping noise down in just about ANY processing path is to use:

1) Companding (complementary compression going in and expansion coming out)
2) Pre-emphasis/de-emphasis (treble boost going in and complementary treble cut coming out)

Both of these are regularly used with flangers, choruses and analog delays.  Anderton has a "budget noise reduction" project in the yellow projects book, I think.  Taken from a Guitar Player article (which I have somewhere), it essentially boosts treble at the input and cuts by the ame amount at the output.

Many higher quality reverbs use limiters on the input so as to keep the springs from being splashed around by transient peaks.  So, one option is to implement a sort of dedicated Orange Squeezer as part of the input circuitry to the SCR.  Note that this will only serve to prevent misbehaviour by the springs, and will not do anything with respect to hiss.

Certainly one of the challenges in any spring reverb is to achieve the most efficient drive of the springs that one can.  The advantage it provides is that less gain needs to be applied in the recovery stage.  Obviously the bandwidth of the springs is not sufficient to actually transmit any hiss, so all the hiss is actually coming from the recovery stage.  If you can get a hotter signal at the input of the recovery stage, then less gain means less hiss.  If some treble boost going into the springs means that you can afford to cut some treble at the recovery stage, then once again you eliminate some of the hiss at the output.

So, take a look at the SCR schematic here: http://www.generalguitargadgets.com/diagrams/stage_center_reverb_sc.gif
The gain of the driver stage (IC1b) is 21.4 by my calculations.  The gain of the recovery stage (IC1d) is 46.8 by those same calculations.
IC1b has a low-end rolloff at around 360hz (set by R4 & C4), and a high-end rolloff around 1540hz (set by R7 & C1).  Although there is neither low-end or treble cut in the recovery stage, you will note that R2/C2 provides for a 6db/oct treble-cut rolloff starting just under 3.4khz.

Consider the following possible mods/changes:
1) You can provide some pre-emphasis (just the way Boss does it in their pedals) by placing a second RC combo in parallel with R4/C4.  For instance, if you placed a 15k resistor in series with a .01uf cap, and put that in parallel with R4/C4, you would get a gain of 31.3 for content above around 1060hz.  So, just a modest goosing of the mids and highs.  Is that enough drive gain?  Maybe, maybe not.  If you replaced R7 with a 220k resistor in series with a 500k trimpot, yu could achieve somewhat to somewhat more drive gain than stock.  Alternatively, you might simply plunk in a 560k or 620k resistor in the R7 position if you feel it can handle a little more gain.  Just note that increasing R7 will result in a lower rolloff produce by R7/C1, so you may have to change both values.  With R7=560k and C1=220pf the rolloff is around 1290hz ( a bit lower than stock), but with the extra treble boost from the parallel input components that drop in rolloff should not be noticeable.

2) With a hotter and brighter drive signal, you may be able to drop the value of R9 to, say, a pair of 1M resistors in series or a 1.8M or even 1.5M value.

3) Since you don't need the extra top end, given the limited bandwidth of springs, you can afford to take out some of the high end at the recovery stage itself.  With a 2M2 resistor, consider placing a 22pf cap in parallel with the feedback resistor for a rolloff as just under 3.3khz.  A second pole of lowpass filtering at the same point as the R2/C2 rolloff will yield steeper cut of the unnecessary treble, and less hiss working its way into the mix.

Just as a sidenote, the schematic shows R7 labelled as the "Dwell" control.  Strictly speaking, the Dwell control on Fender reverbs was generally a drive amount control, determining how hard the springs were being pushed, rather than how much wet signal was being mixed back in with dry, as it is in the schematic.

[petemoore]

  I used a MAX 1044 for the power supply, built and tested that before applying it.