Modified HIFI A2 Beyer Clone Headphone Amp w/ Wired USB PCM2706 & ES9023 DAC

For sale is a used and modified, A2-PRO Beyerdynamic Reference Clone Headphone Amplifier marketed as a Clover Classical Amp with a newly installed and wired USB PCM2706 & ES9023 DAC. It is in excellent cosmetic condition overall. Some of factory listed product details are:   120 Volt Mains Input Only Dual headphone jacks for high and low impedance headphones (10 Ohm to 600 Ohm) Uses a Japanese Brand Signal Relay   
Regulators used are LM317 & LM337 devices Mains input has a built-in EMI filter Contains an ALPS "Blue Velvet" Potentiometer Has a Sealed Toroidal Transformer
Modification parts added are:
1 ea. USB PCM2706 & ES9023 DAC (installed and audio output is wired to the inner pair of RCA connectors) 2 ea. Burr Brown OPA2134PA SoundPlus™ High Performance Dual Audio Operational Amplifiers (replace stock op amps) 4 ea. 1/8 Watt 1% Vishay/Dale 10.0 kOhm Precision Metal Film Resistors (under PCB to Class A bias the two OPA2134PA Op Amps) 2 ea. Small red SPST, DIP Slide Switches (under PCB to bypass input caps - shipped in OFF position) 2 ea. Custom Silver Wire Jumpers with Silver Plated RCA Connectors 1 ea. 3 prong US only Power Cord
Modification  description is as follows:
The rear panel outside pair of RCA connectors provide the headphone amp's Audio IN and the inside pair now provide Audio OUT from the installed USB DAC. A pair of silver wire jumpers are included so you can "loop" the DAC audio OUT and the audio IN to use the USB DAC with the headphone amp. (See images to see the four possible hookup configurations with this setup!)
A pair of small red switches to bypass the amp circuit input caps are mounted under the PCB and are set OFF when the amp is shipped and the bottom panel must be removed to set these switches ON to bypass those input caps. Remember, the best cap is no cap! (IMPORTANT!: The source output driving the headphone amp input typically has zero DC offset since most CD players, etc. have signal output caps. If you are uncertain, you MUST measure for DC offset at the source's RCA outputs BEFORE sliding the switches ON!)
This modification biases the two high fidelity, Burr Brown OPA2134PA FET input op amps into single-ended (SE), class A operation. It is this biasing technique that helps provide a subtle, warmer and smoother, more musical presentation. This specific biasing process still remains part of Walt Jung's audio modifications and in the late 1980's, Nelson Pass, "The Old Man of Audio", commercially pioneered SE, class A, op amp biasing in the Threshold product, the Model Forte 2 Preamplifier.  I learned this biasing while at Threshold and have implemented it extensively for nearly three decades. (NOTE: This biasing is not "pseudo-class A", whatever that nonsensical, "alternative fact" means!) For an accurate, technical explanation of the "pull down" resistor biasing used on the op amps, please see the bottom of this listing.
The great sonic character of this amp should be apparent when first powered ON and will sound better after the unit has warmed up over a period of about 60 minutes or so. It will continue to improve a bit with a few days of continual "burn in". It is quite safe for this unit to remain powered continually and it is recommended that the buyer do so. The amp draws little AC current at quiescence, so let it "idle" ON, 24/7.
And if you're an audiophile that's considering this headphone amp, but are hesitant to buy because you're unfamiliar with my audio modification work, just know that I've been doing hi-fi equipment modifications for over 35 years. I have been mil-spec soldering and materials handling certified and have modified and sold over 85+ modified Creek OBH-11 headphone amps in the last five years. Headphone amp mods are far from new to me!
This is a great hi-fi item for the asking price, but it is still is a relatively expensive item, so be sure you want it before you buy. Please don't hesitate to ask questions! The headphone amp is guaranteed to work and a return is OK only for shipping damage. Calculated USPS Priority Shipping cost with insurance of an added $2.75 are for the US only.
Here's an explanation of how "pull down" biasing an op amp into single ended, class A operation is done and why it works:
This resistor "pull down" biasing technique is called "current sourcing" and is accomplished by directing a small, specific amount of current from circuit ground (or -V) past the PNP output transistor which is the device shown as the bottom half in the "push-pull" transistor pair on schematics depicting an op amp channel. (While either one of these output transistors can be bypassed, it is the PNP transistor that electronic engineers choose to bypass because it is somewhat less linear in operation than its NPN counterpart. This is because PNP charge carriers, called "holes", are less mobile in a transistor substrate and have slower rise times than the electron charge carriers of an NPN complementary device.) The bypassing shunts the collector and emitter of this transistor and effectively turns it "off". With this PNP transistor nonoperational, the NPN transistor now operates fully "on" due to the current guided through it and past the PNP device now effectively removed from the circuit. Thus, the op amp output stage becomes converted to true single ended, class A operation and can now take full advantage of their sonic benefits over complementary, class B operation. (Within the class A "current envelope" created, this biasing completely removes any output stage crossover (notch) distortion and this prevents half-wave, class B artifacts (meaning nasty, third and fifth odd order harmonics), from entering the supply rails which potentially reduces noise and distortion caused from the coupling of these extraneous distortions into sensitive power supply circuit nodes.) Another important, yet often overlooked benefit of current sourcing is its creation of a steady state, "thermal envelope", in which the entire op amp circuitry operates more stably due to the minimization of internal temperature fluctuation. This thermal stability is particularly important when the op amp is handling sustained, large amplitude signal swings which are the direct result of long duration, high level musical passages.