The Technical Stuff
 
Since the TS series of pedals is covered in depth by R.G. Keen's The Technology of the Tube Screamer article, it's only really necessary to cover what changes were made to the original design.
The Changes
Changes were made in only a couple of areas. With the 'Boost' Control, the pedal can be set up such that it has minimal distortion for light playing with the drive control and the Boost can be used to set up a much more aggressive, overdriven tone. It also provides true bypassing. By changing just two capacitor values the low-frequency response is improved significantly.

I'll separte the changes as much as possible to help illustrated what change affects what characteristic. Most of which could be applied to any TS series pedal to help 'customize' it to the players individual wants or needs.

The Input Buffer
The 0.02uF capacitor on the input of the Buffer transistor causes some of the Low-Frequency Rolloff typical in the TS-9. In order to increase the 'Bottom End' , this capacitor can be increased. The first plot below shows the frequency response of the stock TS-9 circuit. If this capacitor is changed to a 0.1uF the low frequency response is increased. The trick is to not allow too much low of the low frequencies into the frontend of the clipper circuit to cause the pedal to start sounding mushy. The clipper section has it's own low-frequency rolloff components but before anything can be increased at that stage, it has to be allowed to get there to begin with. If you look closely at the simulation plots of the clipper section you can see the break point where the filtering of the clipper stage takes over from the filtering of the front end.

TS-9 Preamp frequency plot Screamer+ Preamp frequency plot
 
 
The Clipper 
The clipper adds some of it's own low-frequency rolloff in the the combination of the 4.7k and .047uF capacitor. By increasing the capacitor the low frequency response can be increased. To achieve the ability to adjust the drive down to a minimum amount of clipping, the 51k was reduced to a 22k. The combined resistance of the 22k and the Drive control determine how much gain the stage has before the diodes breakover. The 22k allows the drive to be adjusted such than the diodes will only break over on signals 100mv(peak) or greater at 1KHz. With the addition of a Boost control in series with the Drive control, it's not necessary to have full range adjustability in the Drive control. The value of the drive control was therefore reduced to 250k.  The values of filter resistor, filter capacitor, and series Drive resistor all interact in determining the overall frequency response and gain. 

The two plots below are of the clipper section. The plot on the left is for the stock TS-9 circuit. The plot on the right is with the 4.7k changed to a 3.3k and the .047uF changed to a .1uF. The combination of the two strikes a compromise between increased low-frequency response and gain-reduction.  


TS-9 Clipper response Screamer+ Clipper response
 
 
The Tone Control
The tone control was left intact as it exists in the TS-9. The response of the stage is determined primarily by the 0.22uF/220 ohm series combination connected to the wiper of the control. When the control is set to the center position it has a flat frequency response. The high-frequency rolloff is determined solely by the 1K and 0.22uF to ground on the input to the control. Depending on which side of center the control is turned, it provides either high-frequency boost or cut. The first plot below shows the tone control stage by itself. The second plot shows the response when the 1K/.22uF network is included.

The third plot shows what happens to the frequency response when a 100 ohm resistor is added in series to the first .22uF capacitor (after the 1K). This results in leveling off the midrange bump somewhat and allows more variance with adjustment of the tone control. It gives it a somewhat more full sound and less harsh.


Tone ControlTotal frequency responseNew Tone Section
The Drive Control
From a waveform point of view, it's possible to see what the different drive level settings do to the level of clipping. These similations show the unmodified TS-9 design first and the effect of the changes that were outlined above. The plots were made using a 600Hz waveform because this low enough were the differences can more readily be seen.
TS-9 Drive PlotScreamer+ Drive Plot

With the addition of the Boost control, the value of the drive control can be reduced to give a more fine adjustment. With a 22K series resistor replacing the 51K in the TS-9, the gain is even further reduced allowing for a more 'clean' signal than the TS-9.

The Boost Control

The Boost control allows an instantaneous increase in the amount of  'Drive' control. In addition, the 150pF capacitor provides some additonal high frequency rolloff removing some of the harshness resulting from the higher gain and imparting a slight tonal difference.

Screamer+ Frequency Plot - Boost on

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Since March 28, 2000.
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