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DIY Modifications - v2.02

Feature Substitutions and Omissions

Bass Boost cMoy DC Jack Bypass Pins • 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).

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)

Headphone Impedance: Suggested Gains
• 8-32 ohms: Av = 2-4
• 33-80 ohms: Av = 5-6
• 81-250 ohms: Av = 7-8
• 251-600 ohms: Av = 9-10

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:
Replace C2_L and C2_R with 330 ohm resistors. 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. DC coupling is highly recommended when using line-level input signals. 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.

Use 1/8W resistors instead of 1/4W. Physically smaller resistors will reduce lead length, for improved performance.

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". 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 rare cases (very low impedance headphones played at very high volumes), more current is demanded than any CMoy can continuously supply. If you demand even higher volumes, add a second TLE2426 (component U2) in parallel with the first to double current handling.

This modification has several downsides:

• Battery life is reduced by 10-15 hours (~50%)
• Higher cost and assembly difficulty
• Little to no benefit with headphones above 32 ohms
• Prolonged listening at higher volumes causes hearing damage!

Bass Boost cMoy v2.02 with dual TLE2426CLP's     Bass Boost cMoy v2.02 with dual TLE2426CLP's     Bass Boost cMoy v2.02 with dual TLE2426CLP's

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.
Vishay/Alps Potentiometer Pinout for cMoyBB Panasonic Potentiometer Pinout for cMoyBB