The CMOY headphone amplifier was original created by Chu Moy on his now-defunct headphone website HeadWize. The amplifier was simplicity itself: a single op-amp powered by a single 9V battery and a "virtual ground" consisting of two resistors from battery V+ to ground. The virtual ground biased the op-amp's idle point at a votlage of half the battery, 4.5V for a 9V battery, which allowed the op-amp to operate in a +/- swing fashion between the maximum battery voltage in one direction and ground in the other. The signal currents from the headphone load were returned to the virtual ground resistors, each of which were bypassed with large capacitors to shunt the signal currents to the power rails (AC ground).
People quickly figured out that the metal tin Altoids (R) mints come in were the perfect low-cost box for their new low-cost CMOY amp. The CMOY headamp did remarkably well for having so few parts! CMOYs are often the first headphone amplifier people buy, just to get their feet wet with headphone amps after buying a new headphone.
Over the years people have made modifications to the basic CMOY headamp to address some of its shortcomings. One major one, which is used in the agdr Audio CMOY, is to simply use two 9V batteries rather than one, which provides a "real" ground between the batteries. Virtual grounds cause several problems that greatly increase noise and various forms of distortion. When the large return currents from the headphone load flow through the virtual ground they create a small signal-level voltage, which in turn gets mixed back in with the input. Currents flowing through a simple resistive divider virtual ground actually change the voltage! Instead of being 4.5 volts you might get 4.1 or 5.5. By using a real ground there is full isolation between the heavy output currents going to the headphones and the tiny input currents coming from teh source.
Another major problem with CMOYs that is addressed with the agdr CMOY is that the op-amps used were never designed to handle the high currents and high amounts of capacitance present with headphones and headphone cables. To solve that CMOY designers often have to add fairly large resistors in series with the output, like 10 or 20 ohms, to isolate the load capacitance and reduce the output voltage swing (the resistor forms a voltage divider with the headphone impedance). And output resistor unfortunately seriously affect the headphone damping factor (a bad thing). Both of the chips used in theagdr CMOY were deisgned by TI specifically to be able to handle headphone currents and load capacitances. One of the few "standard" 8-pin DIP op-amps that was designed to drive headphone loads is the NJM4556A, the chip used in the output stage of NwAvGuy's O2 headphone amp. But that is an old chip now. The new TI chips in the agdr CMOY provide as much (OPA1688) or more (OPA 1622) current and do it at much lower distortion and noise levels.
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