The main idea here is that one slider controls the range, and the other controls the volume.
There are two versions: one can boost, the other cannot.

## Boost and Attenuate
```
// This effect Copyright (C) 2004 and later Cockos Incorporated
// License: LGPL - http://www.gnu.org/licenses/lgpl.html

desc: JDA Volume Adjustment
//tags: utility gain
//author: John Allsup based on "volume adjustment" by Cockos


slider1:12<0,60,0.1>Adjustment Range (dB)
slider2:0<-100,100,0.1>Adjustment Amount (%)
slider3:0<-50,50,0.1>Max Volume (dB)

in_pin:left input
in_pin:right input
out_pin:left output
out_pin:right output

@slider
  s1 = slider1;
  s2 = slider2;
  s = slider1*slider2/100.0; 
  adj1=2 ^ (s/6); 
  adj2=2 ^ (slider3/6);
  doseek < 0 ? doseek = 1;

@block
doseek > 0 ? (
  dadj=(adj1-adj1_s)/samplesblock;
):(
  dadj=0;
  adj1_s=adj1;
);
doseek = -1;

@sample
spl0=min(max(spl0*adj1_s,-adj2),adj2);
spl1=min(max(spl1*adj1_s,-adj2),adj2);
adj1_s+=dadj;

```

## Attenuate Only
```
// This effect Copyright (C) 2004 and later Cockos Incorporated
// License: LGPL - http://www.gnu.org/licenses/lgpl.html

desc: JDA Volume Adjustment Negative Only
//tags: utility gain
//author: John Allsup based on "volume adjustment" by Cockos


slider1:12<0,60,0.1>Adjustment Range (dB)
slider2:0<-100,0,0.1>Adjustment Amount (%)
slider3:0<-50,50,0.1>Max Volume (dB)

in_pin:left input
in_pin:right input
out_pin:left output
out_pin:right output

@slider
  s1 = slider1;
  s2 = slider2;
  s = slider1*slider2/100.0; 
  adj1=2 ^ (s/6); 
  adj2=2 ^ (slider3/6);
  doseek < 0 ? doseek = 1;

@block
doseek > 0 ? (
  dadj=(adj1-adj1_s)/samplesblock;
):(
  dadj=0;
  adj1_s=adj1;
);
doseek = -1;

@sample
spl0=min(max(spl0*adj1_s,-adj2),adj2);
spl1=min(max(spl1*adj1_s,-adj2),adj2);
adj1_s+=dadj;

```