OPA1688 Super CMOY single-chip headphone amplifier with 2x 9V batteries, real ground, and a headphone relay

A mint-tin CMOY headphone amp like no other!  Uses the brand new Texas Instruments OPA1688 headphone driver chip with 75mA output per channel, 3x the output of most op-amps.  Ultra low distortion with extremely low idle current for long battery life.  Uses 2 9V batteries with a real ground and "solid state relay" technology - no virtual grounds or rail splitters to distort the sound. 

Do a search for "CMOY" headphone amplifier on eBay and you will get screenful after screenful of the same thing.  Mint-tin amps using various older DIP (dual inline plug-in) op-amp chips with small maximum output currents in the 20mA range. Chips never intended to drive headphones.  Many require a resistor in series with the output to maintain stability, which alters the damping factor and reduces the available output voltage swing and power delivery.   All of them, until now, rely on "virtual grounds" using rail splitter chips to try turning one 9V battery into two.  Even the few that use 2 9V batteries just put them in series and still use a virtual ground.  This new Super CMOY uses a real ground between the two 9V batteries to form a true +/-9V power supply.  Optical-mosfet solid state relays with a tiny 0.0125 ohm "on" resistance cut the battreries off if they get low or one gets disconnected.

Texas Instruments has just released a brand new generation of headphone chip, available only in a surface-mount package.  This chip has many benefits over the older chips:

This CMOY's motherboard uses an OPA1688 surface-mount chip, which can provide up to 75mA per channel output, enough for most headphones. The input facing the volume pot is the FET inputs on the OPA1688 chips for extremely low DC output offset and neglibile source resistor input distortion.

The agdr Audio OPA1688 CMOY has a very unique feature for ultra-high quality sound: a real ground rather than the virtual "fake" ground found in most other CMOY headamps, similar to the real ground used in the NwAvGuy headphone amplifier.  This CMOY uses two newly-released optical-mosfet solid state relays with a tiny 0.0125 ohm "on" resistance in the power management circuit to cut off the batteries and protect the headphones if one battery is missing, shorted, discharged, or open.  The Super CMOY includes a headphone relay circuit on the output to eliminate all of the turn-on and turn-off thumps normally found in headphone amplifiers.

Up until now most SSRs have had high "on" resistances of 10 - 100 ohms or more and high LED minimum currents. Using the two SSRs solves a problem with the NwAvGuy power management circuit of the two mosfets turning on and off at slightly different times. The two SSR LEDs are in series which guarantees both power rails turn on and off at exactly the same instant in this CMOY, The circuit uses the just-released TPS3701DDCT comparator chip from TI that pulls an amazing small 8 microamps (8uA) of idle current. At that rate the 600mAh lithium batteries would last 85,714 hours in standby!

The OPA1688 Super CMOY can use regular throw-away 9V batteries (primary cells), NiMH rechargeable "9V" batteries (can be purchased as either 8.4 actual volts or 9.6V), or lithium ion "9V" rechargeable cells (7.4V actual).  The Super CMOY doesn't recharge batteries, so you would use the batery manufacturer's recommended charger offline for battery charging.  Most of those chargers are fast chargers that can fully re-charge the battery in 2-3 hours.

For an interesting history of the CMOY headphone amplifier, read here.  A dual-chip (parallel) version of the Super CMOY PC board is also available.

For ordering information and prices, please see here..

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