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DIY Modifications - v2.03
Feature Substitutions and Omissions
Gain Adjustment
Optional Modifications
Feature Substitutions and Omissions
• To omit the DC jack, short the pads of J5 that connect to the symbol "_/_".
• 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).
• 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).
Gain Adjustment
You may wish to set a gain not listed in the Bill of Materials. Flat-response gain of the cMoyBB can be calculated as follows:
Gain (voltage) = Av = 1 + (R4/R3)
Gain (dB) = 20*log(Av)
Higher gain creates a more stable opamp circuit. Most opamps are stable with a gain of 3+. If you encounter unexpected behavior while trying other opamps, try a higher gain.
cMoyBB Frequency Response Calculator - This Java applet simulates frequency response of the cMoyBB based on all component values. Gain is calculated in decibels (dB). The "Bass boost OFF" curve is fully applicable to a standard cMoy circuit.
Gain (voltage) = Av = 1 + (R4/R3)
Gain (dB) = 20*log(Av)
| Headphone Impedance | Suggested Gain (Av) | Number of Batteries | Virtual Ground IC's |
|---|---|---|---|
| 8-40 ohms | 2-3 | 1x9V | U2 and U3 |
| 41-120 ohms | 4-6 | 1x9V or 2x9V | U2 |
| 121-600 ohms | 6-8 | 2x9V | U2 |
Higher gain creates a more stable opamp circuit. Most opamps are stable with a gain of 3+. If you encounter unexpected behavior while trying other opamps, try a higher gain.
cMoyBB Frequency Response Calculator - This Java applet simulates frequency response of the cMoyBB based on all component values. Gain is calculated in decibels (dB). The "Bass boost OFF" curve is fully applicable to a standard cMoy circuit.
Optional Modifications
DC Coupling / LOD Attenuation:
Capacitors C2 serve to AC couple the audio signal to the operational amplifier. 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. DC coupling is highly recommended when using line-level input signals as it provides sufficient input attenuation to prevent overdriving of the opamp when using a weak battery.
To use DC coupling, replace C2_L and C2_R with 330 ohm resistors (or use 1.6k for LOD attenuation).
It's good practice to measure source DC offset before using a DC coupled amplifier. DC offset at either channel should not exceed 20mV with bass boost on. Offset can be measured with a voltmeter set to millivolts: Place the black probe on the output jack ground and touch the red probe to the left or right output.
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.
Virtual Ground:
The cMoyBB is normally assembled with a single Texas Instruments TLE2426 "rail splitter" placed in spot U2. These highly accurate rail splitters have been used in CMoys since 1998 for their superior performance over resistive voltage dividers. The majority of users will find listening satisfaction from a single TLE2426, but in some cases (very low impedance headphones played at very high volumes), more current is demanded than any CMoy can continuously supply. If you require even higher volumes, a second TLE2426 can be added to double current handling.
This modification has several downsides:
• Battery life is reduced by 10-15 hours (~50%)
• Higher cost
• Little to no benefit with headphones above 64 ohms
• Not recommended for use with dual 9V batteries
• Prolonged listening at higher volumes causes hearing damage!
To install dual rail splitters, place a second TLE2426CLP in spot U3 and use a 0.1uF ceramic capacitor for C5.
Dual 9V Batteries:
Increasing the supply voltage to 18V enables higher volumes when driving high impedance headphones (typically 64 ohms and above). Two batteries in series will drain somewhat faster than a single battery. This modification also adds weight to the amplifier and adds slight difficulty when changing batteries due to the very tight fit. Most people with 120 ohm headphones and less are pleased with a single 9V battery, therefore, we only recommend using 18V if you find 9V to be insufficient.
Two pads labelled "18V" reside between the normal V+ and V- battery pads. To install dual battery connectors, connect a black lead from one 9V connector to an "18V" pad, and the red lead from another 9V connector to the remaining "18V" pad. You will be left with one red and one black wire. Connect these wires to V+ and V-, respectively.
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.
This modification is rarely recommended due to higher cost and possibly decreased performance. Using long wires in this part of the circuit negatively impacts THD and the SNR of the amplifier. Also, most users ultimately turn the knob to 0 or 100%, thereby defeating the purpose of the modification. Best performance and value is attained from the standard toggle switch.
Capacitors C2 serve to AC couple the audio signal to the operational amplifier. 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. DC coupling is highly recommended when using line-level input signals as it provides sufficient input attenuation to prevent overdriving of the opamp when using a weak battery.
To use DC coupling, replace C2_L and C2_R with 330 ohm resistors (or use 1.6k for LOD attenuation).
It's good practice to measure source DC offset before using a DC coupled amplifier. DC offset at either channel should not exceed 20mV with bass boost on. Offset can be measured with a voltmeter set to millivolts: Place the black probe on the output jack ground and touch the red probe to the left or right output.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.
Virtual Ground:
The cMoyBB is normally assembled with a single Texas Instruments TLE2426 "rail splitter" placed in spot U2. These highly accurate rail splitters have been used in CMoys since 1998 for their superior performance over resistive voltage dividers. The majority of users will find listening satisfaction from a single TLE2426, but in some cases (very low impedance headphones played at very high volumes), more current is demanded than any CMoy can continuously supply. If you require even higher volumes, a second TLE2426 can be added to double current handling.
This modification has several downsides:
• Battery life is reduced by 10-15 hours (~50%)
• Higher cost
• Little to no benefit with headphones above 64 ohms
• Not recommended for use with dual 9V batteries
• Prolonged listening at higher volumes causes hearing damage!
To install dual rail splitters, place a second TLE2426CLP in spot U3 and use a 0.1uF ceramic capacitor for C5.
Dual 9V Batteries:
Increasing the supply voltage to 18V enables higher volumes when driving high impedance headphones (typically 64 ohms and above). Two batteries in series will drain somewhat faster than a single battery. This modification also adds weight to the amplifier and adds slight difficulty when changing batteries due to the very tight fit. Most people with 120 ohm headphones and less are pleased with a single 9V battery, therefore, we only recommend using 18V if you find 9V to be insufficient.Two pads labelled "18V" reside between the normal V+ and V- battery pads. To install dual battery connectors, connect a black lead from one 9V connector to an "18V" pad, and the red lead from another 9V connector to the remaining "18V" pad. You will be left with one red and one black wire. Connect these wires to V+ and V-, respectively.
Tip: When using dual batteries it's extremely helpful to use a thin profile "Economy" 9V battery connector for the second battery, rather than another (very thick) "Premium". Changing batteries is much less troublesome with this configuration. Suggested 9V connectors are listed in the BOM.
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.
This modification is rarely recommended due to higher cost and possibly decreased performance. Using long wires in this part of the circuit negatively impacts THD and the SNR of the amplifier. Also, most users ultimately turn the knob to 0 or 100%, thereby defeating the purpose of the modification. Best performance and value is attained from the standard toggle switch.
Tip: Try to minimize wire length. Unnecessarily long wires will add inductance and capacitance to the feedback loop, potentially causing unwanted noise or oscillation.
