This is a brief circuit examination of the 1176LN "slam mode", where all the pushbuttons are pressed to create an effect with the compressor that is different from pressing only one of the ratio buttons.

The ratio switces are made up of 4 DPST switches, each with an 'A' side and a 'B' side. The 'A' side is used to switch a tapped offset voltage between about Vbias and about -4V (depending on the bias voltage calibration point). The 'B' side is used to switch a tapped amount of signal from the output of the preamplifier stage into the GR amp/rectifier stage. About 80% of the signal is used at 20:1 and about 16% of the signal is used at 4:1.

When all 4 buttons are pressed, this has the effect of shorting out several of the taps. As can be seen in the circuits below, only the two end switches need to be pressed to get the same electrical result. What you end up with is a divider made up from the two end resistances.

In the case of the 'A' side, the resulting tap is closer to Vbias, but not the same as when just the 4:1 button is pressed, since there is a lower resistance to the -10V rail. It's more like a tap position that would correspond to a 10:1 button.

In the case of the 'B' side, the tap is slightly lower than mid way, approximately the position of a 10:1 divider point.

 

Slam mode ratio switches A

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Ratio switches A side in normal and slam mode condition.

 

Slam mode ratio switches B

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Ratio switches B side in normal and slam mode condition.

 

So what does this mean? It appears that the effect of pressing all the switches in creates tap points that correspond to a 10:1 ratio position, however with lower impedances as seen by the stage it feeds. It also appears that pressing just the 4:1 and 20:1 switches is electrically equivalent to pressing all 4 switches.