DIY Modifications
Feature Substitutions and Omissions
• To omit the DC jack, short the pads of J5 between the symbol "_/_" on the printable Silkscreen Layer. See image at right.
• An STX-3100-3C audio jack can be used in place of the STX-3100-9C jack for part J2. The auto on/off feature must be bypassed by shorting all unused pins of J2.
• The internal bass boost switch can be substituted for other DPDT switches or a 50kΩ potentiometer. Connection diagrams and details are given below.
• To disable bass boost, short the CB_L and CB_R terminals, independently.
• Diode D2 is reserved for reverse and over-voltage protection and is not required. To omit D2, leave its spot empty (do NOT short the pins).
Optional Modifications
Feature Substitutions and Omissions
• To omit the DC jack, short the pads of J5 between the symbol "_/_" on the printable Silkscreen Layer. See image at right.• An STX-3100-3C audio jack can be used in place of the STX-3100-9C jack for part J2. The auto on/off feature must be bypassed by shorting all unused pins of J2.
• The internal bass boost switch can be substituted for other DPDT switches or a 50kΩ potentiometer. Connection diagrams and details are given below.
• To disable bass boost, short the CB_L and CB_R terminals, independently.
• Diode D2 is reserved for reverse and over-voltage protection and is not required. To omit D2, leave its spot empty (do NOT short the pins).
Optional Modifications
DC Coupling:
Replace C2_L and C2_R with 330 ohm resistors. Capacitors C2 serve to AC couple the audio signal to the opamp. AC coupling eliminates potentially dangerous DC offset from the source, at the expense of audio quality. Additionally, this modification improves channel balance and noise at low volumes. Be sure to measure source DC offset before using a DC coupled amplifier! DC offset at either channel should not exceed 20mV with bass boost on.
Resistors:
Use 1/8W resistors instead of 1/4W. Physically smaller resistors will reduce lead length, for improved performance.
Capacitors:
Try WIMA capacitors for C2_L and C2_R, and any of the more expensive varieties for C1. See Bill of Materials.
Bass Boost Control Knob:
Omit the bass boost switch and both RB resistors (leave them out!). Use four short 22 gauge wires to connect pins A, B, C, and D to a 50kΩ potentiometer. This results in adjustable bass boost via a control knob. Terminal connections for Alps/Vishay and Panasonic 50kΩ potentiometers are given in the diagrams below. If bass boost is found to be too strong, install 51k resistors in RB_L and RB_R to reduce the intensity by a factor of two.
Replace C2_L and C2_R with 330 ohm resistors. Capacitors C2 serve to AC couple the audio signal to the opamp. AC coupling eliminates potentially dangerous DC offset from the source, at the expense of audio quality. Additionally, this modification improves channel balance and noise at low volumes. Be sure to measure source DC offset before using a DC coupled amplifier! DC offset at either channel should not exceed 20mV with bass boost on.
Resistors:
Use 1/8W resistors instead of 1/4W. Physically smaller resistors will reduce lead length, for improved performance.
Capacitors:
Try WIMA capacitors for C2_L and C2_R, and any of the more expensive varieties for C1. See Bill of Materials.
Bass Boost Control Knob:
Omit the bass boost switch and both RB resistors (leave them out!). Use four short 22 gauge wires to connect pins A, B, C, and D to a 50kΩ potentiometer. This results in adjustable bass boost via a control knob. Terminal connections for Alps/Vishay and Panasonic 50kΩ potentiometers are given in the diagrams below. If bass boost is found to be too strong, install 51k resistors in RB_L and RB_R to reduce the intensity by a factor of two.
Tip: Try to minimize wire length. Unnecessarily long wires will add inductance and capacitance to the feedback loop, potentially causing unwanted noise or oscillation.
