GE-7 EQ Gain and 18 Volts

Posted: March 5, 2016 in DIY, Effects, Ideas To Be Developed
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GE7signalflow

Basics of the Signal Path

The input to the Boss GE-7 Graphic EQ has a buffer which is active at all times.  There is a slight emphasis around 2 kHz to help the guitar sound stand out.  Immediately after the buffer is the amplifier for the level control.  It’s a differential type with only one source voltage split with the level’s sliding potentiometer.  This slider will either boost or cut the whole signal when it is moved from the zero line.  From here, the signal is split; one path goes to the EQ section and one part goes to the positive input of another differential amp.  The EQ section feeds the negative input of the differential amp.

The math for a differential amp is Vout= A(V+in – V-in).  By adjusting the EQ sliders from the zero line, a boost or cut is calculated for the frequency band.  Since differential amps reject voltages which are the same, this enables the boost and cuts to amplify efficiently by only affecting the parts of the signal which are different from the source at the V+ input.

If a person uses the level control to boost the whole signal and then boosts or cuts with the EQ sliders, the results from any changes from the zero line are exponential differences between the dry, bypassed, signal and the result of the two differential amps.

The point is: if a person wishes to have a subtle effect, a choice should be made between using the using the EQ section dramatically with little or no level change, or to use subtle EQ changes with a greater level boost.  If subtlety is not a factor, very resonant sounds can be achieved by selectively boosting both EQ and level.  The only concern would be whether a person wants to drive the second op amp into overdrive or not and staying below that threshold if necessary.

The last part of the GE-7 circuit is a buffer for the wet signal.  A passive filter rolls off frequencies below 107 Hz and above 1.07 kHz.  This helps with noise and booming bass frequencies, while also helping the guitar’s focus remain on its target frequencies.

Raise The Roof

While there are mods available for changing the pedal to 18 volt operation, voltage doubling y-cables are available to feed two 9 volt power supplies to one pedal, an single 18 volt adapter can be used, or a multi-pedal power supply with an 18 volt output can be used.  The op amps used will not be damaged by increasing the voltage, as long as they aren’t fed by more than 30 volts (24 volts to be safe).

Increasing the voltage supply reduces compression and distortion by increasing the headroom available to the input signal.  When a 9 volt supply is used, the largest a signal can be before it distorts is just less than 4 volts.  Doubling the supply voltage also doubles the headroom (to a little more than 7.5 volts) and allows for a cleaner, more open, tone when the signal is being manipulated.  It doesn’t amplify the signal by a greater amount, because that is set by the ratio between components and is independent of the power supply.  It’s purely for improvement to the tone and gives some latitude for extreme settings on the pedal.

Example/Tangent

Any example of increasing headroom to reduce distortion and compression is the 18V Mod to EMG pickups.  The difference is quite radical and improves the sound by making it less squashed and sterile.  I have a guitar with the EMG Het Set.  I popped out the 81/85 set and installed the Het Set.  I didn’t change the 18V Mod.  It sounded awesome.  I recently switched it back to 9V and I’m not impressed; The dynamic range is smaller.  Even though EMG says the 18V mod isn’t necessary for them, I found the sound to be improvement over 9V.  Likewise, the GE-7 works fine at 9V, but 18V will lend a more open sound.

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