Look. I have to be somewhat responsive to market demands. If the market wants a large connector for the balanced input
AND there is a good technical reason for adopting such an input, then I will allow the chassis to grow. Consumers will then have to pay more as the product is now more expensive to produce.
There are many technical advantages (and some marketing advantages) of featuring a balanced input. In addition, many in my market survey indicated that they wanted a balanced input on an amp at this price point. They also wanted the Alps RK271 "Blue Velvet" pot. So I included those. To my surprise, very few indicated that they were interested in a balanced or 4-pin output on an amp at this price point, so away it went.
I am not aware of any technical advantage of a balanced headphone
output. There may be some high-distortion topologies where you can gain some advantage on paper, but I don't operate there. I design ultra-low distortion headphone amps. In my designs, the 1/4" output and the 4-pin output (if there is one) will provide the exact same stellar performance.
As I have pointed out before, I am not willing to compromise on performance or cost to support a marketing fad with no technical advantage. In a relatively compact headphone amp such as the HPA-1, adding a 4-pin XLR connector would cause the chassis cost to explode as the chassis would need to be wider. The PCB would need to be wider as well, which impacts both the cost of the raw PCB and its assembly cost. With a more compact connector for the balanced output, it is conceivable that such an output could be added without impacting the cost (too much).
An even-higher-end amp would need to be larger or it won't be perceived as serious (marketing). It also needs to provide higher output power (mostly marketing, some technical advantage in limited use cases) and feature a 4-pin output (marketing). It would need a more expanded set of features (mix of technical advantages and marketing). In such an amp, adding the 4-pin output basically adds the cost of the connector plus a little bit for assembly. Also, consumers expect to find a 4-pin output on such an amp, so it is needed due to marketing demands. It may be a fad, but it won't add much to the cost of the even-higher-end amp, so why not support it. The even-higher-end amp would be more expensive to produce, thus, require a higher price tag than the HPA-1.
A still-good-but-lower-end amp would have the differential input removed. This would allow the chassis to shrink. Unfortunately, this would also mean that the RK271 pot would be gone as it wouldn't fit in the chassis. One could entertain the thought of using an external wall wart for the power supply. That would allow the PCB slots in the chassis to be used (much smaller clearance needed due to the lower voltage in the chassis), which would reduce the chassis production cost. The still-good-but-lower-end amp would be possible to produce and sell at a price point below that of the HPA-1.
Tradeoffs, tradeoffs...
Any real-world engineering problem is a multivariate optimization. It's sorta like pushing on a balloon. You push in one place and it expands elsewhere. There may be an infinite (or at least high) number of good solutions that are all different, and good in different ways. There may be one global optimum (I've never had that happen). Or there may be no solutions. Add all the variables related to market demands/needs, marketing (consumer psychology), and cost, and watch the decision space grow exponentially. You may find that optimizing for engineering (as I did with the Neurochrome HP-1) results in a product that's very far from optimum on the other parameters.
What I'm doing with Tom Christiansen Audio is to strike a good balance between "engineering optimization" and "market optimization". I will openly admit that I'm biased in the "engineering" direction. I'm at peace with that.
I fully intend to keep my products rooted in science and engineering, but I will have to accept that to sell my products to people other than engineers, I will have to keep an eye on the marketing and consumer psychology aspects as well. I think the HPA-1 strikes a good balance between engineering and marketing. Others are free to disagree.
I hope I've shed some light on what goes into the specifications of an amp. If you're now even more confused, just remember that you're not alone. Multivariate optimization is not for everybody...
BTW: Tomorrow I'll increase the preorder price of the HPA-1 to $849. I expect to be able to ship before the end of the month. Once the HPA-1 is in stock, its price will settle at the final $899. There are 13 left at the preorder price. I'll start actively advertising these starting tomorrow as well.
Tom