The Lightning-Quick HeadAmp GS-X mk2 Headphone Amplifier
Some audiophile gear is like fine jewelry, but lacks teshnical competence. Some audiophile gear is butt-ugly, but gloriously competent at revealing the music.
The HeadAmp GS-X does both insanely well.
HeadAmp GS-X mk2 ($2795 w/Alpha pot; $2995 w/DACT stepped attenuator; blue and red color options +$200)
The HeadAmp GS-X mk2 is a fully balanced, solid-state headphone amplifier with left and right channels remaining separate all the way back to the dual power supplies and transformers. The headphone amp and power supply live in separate, and exquisitely finished enclosures, connected by a beefy umbilical with keyed, twist-lock connectors at either end.
The front panel has controls and connections for (left to right): Pre-Amp on-off, which enables/disables rear panel pre-amp outputs; Gain select for Low (0.9dB), Medium (12.5dB), and High (21.2dB); 4-pin XLR female for balanced headphones; two combo connectors for headphones with two 3-pin XLR balanced connections, or for two headphones with 1/4" TRS plugs; volume control (either Alpha or DACT stepped attenuator); and input select for XLR balanced or one of two RCA unbalanced inputs.
Rear panel has connections for (left to right): Right and left female XLRs for balanced input; two pairs of RCA inputs for unbalanced input; loop outputs for XLR balanced or RCA unbalanced inputs; and the power supply umbilical input.
The matching power supply unit is simple having a single on-off switch on the front, and IEC power input module and power output jack for the umbilical on the rear.
A Little History
The original design for the GS-X mk2 started with Kevin Gilmore's "A Pure Class-A Dynamic Headphone Amplifier" originally posted in 2001 on the now defunct HeadWize forum. This was Justin's DIY introduction to building amplifiers; he sold the amp in 2002his first amplifier sale. Some time later, Kevin Gilmore designed a similar, more powerful headphone amplifier he named the "Dynahi", at which point folks began to call his earlier "Pure Class-A Dynamic Headphone Amplifier" the "Dynalo"a much more palatable moniker.
Justin eventually took the amp to market as the Gilmore V1, then with a nicer enclosure the Gilmore V2. In 2005 Justin again made significant improvements and the GS1 was born heralding in a new look reflecting HeadAmp's serious venture into commerce. In 2006, Kevin Gilmore refreshed the design for the HeadAmp GS-X to separate the power supply from the amplifier and add balanced ins and outs. And finally, in 2012 the current GS-X mk2 hit the scene. Improvements over the GS-X include:
- Higher power supply voltages.
- New Gilmore Dynalo+ module design, one of the feature is increased output current.
- Larger power supply transformers.
- New circuit board layouts.
- Added a 2nd RCA input.
- Added a pre-amp output on/off switch.
- Improved 3-way gain switching - got rid of long wires, added relay instead.
- Option of Alpha 21mm potentiometer or the DACT stepped attenuator.
- Added a 4-pin XLR balanced output, changed 2x 3-pin XLR and 1/4" stereo outputs to combo jacks.
- Improved build quality (thicker, polished front panels, laser engraving, powdercoated body).
Let's take a quick peak inside the box.
The schematic above is the original Kevin Gilmore 2001 design. This is a dual differential input (JFET) with class-A biased push-pull output using four pairs of of bi-polar transistors in emitter follower configurationa design which can deliver loads of current with authority.
The current GS-X mk2 topology remains fundamentally the same, but there have been a number of improvements over time that include: higher power supply voltages; more output current; and improved circuit board layout.
I'm not qualified to do a deep dive into the circuit above, but will provide a simple overview. Audio comes in at the left side of the schematic on the pin named "In", which is applied to one input of the dual differential JFET amps labeled Q1 and Q2 for the negative side, and Q3 and Q4 for the positive side. The output of these amps feed the output power amplifier at bipolar transistors Q8 and Q12 respectively. Constant current sources Q11 and Q5 bias the output of the differential amps such that the DC value of current through Q12 and Q8 collectors always provides a voltage drop across R15 which keeps output transistors on at all times (Class-A operation) and negates cross-over distortion. (Cross-over distortion occurs when both positive and negative output transistors are momentarily off when the audio crosses 0 Volts.)
The configuration of the dual differential input amps requires these devices are carefully matched in performance. Justin told me that previous incarnations of the amp used matched FET complementary pairs in a single package. These parts became unavailable, so currently all JFETs for these circuits are matched by hand. He claims he actually gets better matching with this method, but it is more time consuming.
The amplifier's power output is found at the join of the eight resistors of the emitters of the output devices. You'll notice another line off the output that goes down the page and across to the left to meet at the junction of R27 and R29. These resistors are part of a network around IC1 that form a DC Servo. This circuit ensures there is no steady state DC being amplified and sent as a potentially damaging DC current through the headphones.
One very important thing to note in the above schematic is that there are no capacitors or inductors in the signal path, which helps to keep problems of shifting phase in check. The only caps in the circuit are power supply filter caps, and the time constant setting cap on the DC servo.
A Couple of Notes on Operation
Though not quite in the sense that single output device, class-A amplifiers like those from Nelson pass are, the GS-X mk2, in my mind, is a purest amplifier. There are no frills here, and no accommodating electronic trickery for the sake of convenience. For example: if you are feeding the amplifier unbalanced signals, there is no phase splitter to create a mirror image inverted signal to drive the modules on the inverted side of the balanced connections. The balanced connections to your headphone will still work, and it's safe to plug them in, but they will be driven in single ended mode only. The reason for this is that phase-splitters will never give you a perfect mirror image of the original signal, and that's something that Justin and Kevin don't tollerate. "Besides," as Justin told me, "if you can afford this amp, you will likely also be in a place to purchase a commensurate balanced source to mate with it."
Perfection. Simple, elegant, gloriously finished perfection.
I've known Justin Wilson, HeadAmps founder and CEO, for going on 15 years now. I can't tell you how many times, when reading his internet postings about how hard he struggles with suppliers, I've said to myself, "Justin! No one takes perfection that seriously. It's unachievable." It slows him down; people end up with very long wait times. In the end though, despite my concerns, he does manage it. Every HeadAmp product, from the extraordinarily compact Pico Slim to the gorgeous Blue Hawaii electrostatic amplifier, are virtually museum quality in their construction and finish. Pull the lid off, and you'll see the same radiant expression of perfection. The build quality and finish of the GS-X mk2 is simply beyond reproach.
I spent a little time on the phone chatting with Justin and asked him how he could sustain such a long and arduous journey with this intense focus on quality. His answer:
"Well, I'm not married."
Ha ha ha! Pretty hysterical. He went on to say if he had a wife and kids he simply wouldn't have the time for this strong a focus on product quality. Further, he said if he was a better marketer he might have stronger sales, but that's just not in his skill set. Quality he understands though, and he believes product quality will slowly, but surely, bolster the reputation of HeadAmp product and keep ongoing sales strong.
When asked to give some examples of this drive for quality on the inside of the box, Justin offered a couple of examples:
In earlier incarnations, most of the mounting hardware for circuit boards and the like were mounted using stand-offs that required bolts through the case panels attached in place with lock washers and nuts. Justin, on rare occasion, found these to come loose in shipment, which caused rattles and, at worst, loose hardware within the enclosures that might cause shorts and damage. To ensure this simply wouldn't happen, he changed the design to include PEM nuts (threaded nuts that are embedded in the metal panels), which made loose hardware much less likely.
On previous versions of the amp, audio was sent to the front panel gain switch and routed back to the amp boards on flying leads. On the GS-X mk2, the front panel gain control switch controls relays below each amplifier module for gain switching, which significantly reduces the path length for audio signals. He said it added about $24 in parts to the finished amp, which was not reflected in a price hike and would likely not result in increased sales. His nature is such that it didn't matter: He knew how to improve the amp, so he improved the amp. His assumption is the value would eventually accrue to the reputation of the brand.
Well, for what it's worth, I don't know how much it effects the sound quality, but I can tell you that measurements of the amp show extraordinarily low noise and cross-talkmeasures sensitive to things like longer audio path length and power supply noise.
The point here is that he has made numerous improvements over time, yet the basic price-point of the amplifier has remained remarkable steady. This amp is essentially a bargain; most any other high-end audio manufacturer producing an amplifier of this impeccable quality would charge at least $1000 more.
On the down side, the relentless drive for quality has Justin rejecting things like front panels due to flaws causing significant wait times for those placing orders. Couple that with an unexpectedly high order volume and at times the wait for GS-X mk2s have exceeded a year! Fortunately, Justin has got production dialed in fairly well at the moment, and wait times are around 3-4 weeks at the time of this writing.
Alrighty, let's have a listen.