stage center reverb volume/noise

[jmasciswannabe]

Finished my stage center and I am glad to have a spring reverb, however, the volume gets louder though and with the increased volume I get noise. Don't know if this is the way it is supposed to go or if there might be a way to lower the volume output or at least get rid of the noise with the volume increase. Any ideas?

Thanks!!
Ian

[Mark Hammer]

The SCR doesn't really specify input or output impedance of the reverb pan, so I don't knoqw what it is optimized for.  My guess is that it expects a reasonably high impedance at both ends of the pan, though, since it doesn't show any anticipation of low impedances.

The implications of this for you are that signal transduction at the reverb pan input is not as efficient as it might be, which means that more gain recovery is needed at the other end.  That in turn will result in a poorer signal-to-noise ratio because the reverb signal isn't so robust.  You will note that the recovery stage uses an inverting stage with a 47k input resistance and 2.2meg feedback resistance.  That's a recovery gain of 47 or so.  Not huge, but if there is any hiss or hum to be acquired, that will certainly show it off.

If you have made the circuit with a PCB and 4136, or TL074, the following changes will be difficult to implement, but here goes....

Ideally, you want a reverb drive stage that can dump tons of current into a low impedance load, as is typical of reverb pans OR you want a way to make the existing circuit THINK that it is seeing a higher impedance load than it really is.  If you keep the existing circuit and stick a matching transformer, like the 1.5k:8R Radio Shack transformer so frequently used for DIY wahs, between the op-amp that goes "to reverb" and the reverb pan input, you should get more signal/drive into the reverb pan, more signal out the other side, and a better S/N ratio.

Alternatively, use either an NE5532, LM833, or a pair of NE5534's paralleled (see the "Dave Barber is a sick man" thread below) to provide more current drive at low impedances, in place of the op-amp section where you see the 470k feedback resistor and 220pf cap.  If the referred to thread makes no sense ask for directions here.  I can promise that it works if done right.

A touch of lowpass filtering on the recovery section is probably a good idea too.  Stick a 10-22pf cap in parallel with the 2.2meg feedback resistor for a high-end rolloff of about 7.2 - 3.3khz.  That rolloff is supplemented by a second lowpass filter around 3.3khz formed by the 4k7 resistor and .01uf cap after the reverb level pot  Finally, if the overall hiss is something you can't live with, a 330-470pf cap in parallel with the 33k feedback resistor in the output stage will also roll off more treble in the danger zone.

You may also want to consider dropping the 2.2meg resistor down a bit if you do manage to wring a more efficient passage through the reverb pan, since you won't need all that recovery gain.  A 1.5meg resistor may be sufficient.  You may find also that you can afford to drop the 470k resistor on the reverb drive stage down to 390k or maybe even less.