@ Fosi Audio .... I have been testing the ZA3 for several weeks with the purpose being end use of a headphone Amp mainly for hard to drive headphones like the Susvara, HE6seV2 and Modhouse Tungsten and others.. I purchased two of the ZA3 amps and I built 2 interface cables, one cable has a 4 pin XLR/banana and the other cable has banana/banana connection... for the first test I connected a Hifiman HE Adapter Box that presents the ZA3 amp with the following.. ( hifiman source )
Specifications:
Input: Speaker Connector
Output: 4 pin XLR
Resistors in Parallel: 10 Ohm
Resistors in Series: 25 Ohm
The configuration presents the amp output stage with a 10ohm shunt.. and the 4pin XLR connection to the headphones.. The amps are setup for Mono with one amp driving the left and the other amp driving the right headphone drivers... Both amps with the volume knob fully to the clockwise position ( full amp drive ) for the source I used two methods... a dual 3 pin XLR to 4.4mm balanced cable from ifi Audio used for balanced input to the amps and 4.4mm to a fiio BTR7 and for direct balanced connection I utilized the M15s via balanced 4.4mm LO (variable) ... with the BTR7 being wireless I can use an Iphone or the M15s ... the advantage device is the M15s as it has capable EQ so the sound quality can be fine tuned to personal preferences... and it can be connected via cable or wireless... in wirless its like the ZA3 setup has a remote control... so its nice to have that option... another connection option is the USB input on the BTR7..
For the two ZA3 amps I swapped out the left/right op amps with Sparkos op amps... the result is an airier sound but still well balanced FR from bass to treble...
The Headphone used to test this configuration was mainly the Hifiman HE6se V2 ..... sound quality is very nice, full FR from bass thru treble.. nice sound stage and instrument separation... couldn't ask for much more and at the price it is actually fantastic...
I also tested this configuration with a single ZA3 with the results pretty much the same.. I doubt a A/B test would allow someone to pick out the different setups... and there is plenty of power from a single ZA3 or a dual/mono ZA3... more than enough headroom..
For those concerned about the fact that the amps exhibit a Frequency response load impedance dependency where the FR can be presented at an elevated power on frequencies above human hearing...
If you examine the Frequency Response graph its noted that with a 4 ohm load the FR is " nice and flat " and with the 8ohm load " overshoot " "This means depending on your speaker impedance, high frequencies may be exaggerated a bit."
So I decided to utilize a different cable to interface the headphones... this cable has banana to 4 pin XLR but at the amp output posts I put a 5 ohm shunt resistor across the positive - negative posts and the cables banana plugs into the amp output posts... so the amps output is now presented with a 5 ohm shunt and no resistors in line to the headphones... again this setup sounds absolutely great
To the Fosi Engineers... you have the capability to create a purpose built headphone amplifier as I demonstrated with my interface cables... an opportunity is present for another product line...