This is a review and detailed measurements of the Tom Christiansen Audio (TCA) HPA-1 headphone amplifier. It was loaned to me by its designer and member @tomchr . The unit I received has production PC board and components but the case is not the final one. The HPA-1 has a final cost of US (?) $899 but there are discounts for preorders and first set of units. I let Tom give that detail. Production is slated for January.The HPA-1 has a rather elegant, calming front face:While symbols are used there are only two switches for setting gain and input type so not hard to figure out.The volume control has a very nice feel which I appreciated. The switches have long travel which makes it easy to know if you have or have not pressed them.The back panel has what you expect:XLR input is mandatory in any premium audio product and that is what I used for my testing.Inclusion of AC power supply is always appreciated as it nicely declutters your desk/power strip. It also gives the unit additional heft so it doesn't move around on your desk.In use, the case didn't seem to even rise above room temperature. So heat dissipation is not a concern.As usual, we start with our dashboard view with unity gain (what goes in, is what comes out as far as voltage):Distortion levels are beyond even remotest threshold of hearing at -130 dB. By itself it would set the SINAD to 130 but since there is noise both in my analyzer and HPA-1, it gets set at 119 dB. This puts the HPA-1 right at the top of the chart:What embarrassment of riches with so many high performance headphone amplifiers!Dynamic range is excellent at 4 volt output and very good at 50 millivolts (simulating what you need with very sensitive IEMs/headphones):Frequency response is flat to 40+ kHz as we would expect:Most important measurement for any headphone amplifier is power versus distortion and noise. Here it is into 300 ohm load:Low gain mode reduces noise and works to the threshold of measurement (the analyzer can do better but I don't want to change it and invalidate earlier measurements in this graph). High gain produces 200 milliwatts, doubling my requirement of 100 milliwatts into 300 ohms. It goes beyond Drop/Massdrop THX AAA-789 in unbalanced mode.Power is still good but more limited into 33 ohm load:Noise and distortion is exceptional just the same as 300 ohm.Channel balance relative to volume control position was exceptionally good for an analog volume control:Not sure if this is representative of larger production sample or not.Output impedance was excellent at just 1.1 ohm:Edit: additional tests as requested in the thread.Multitone:I warmed up the unit prior to testing for 10 minutes. Baseline performance did not change. However, I noticed some occasional spikes. I traced this to BNC connectors of my headphone lead to the analyzer! Just sitting there, it would change its impedance enough to cause a spike. We are talking such sensitive measurements that seemingly breathing on the instrumentation the output can change! Anyway, until I sort this out, I am not posting the graph.I started my testing with Sennheiser HD-650 headphones. The sound here was wonderful. Plenty of dynamic range, detail, bass, etc. With well recorded content you just want to sit there and keep cranking the volume up.Next I switched to 25 ohm and inefficient Drop Ether CX closed back headphones. There was plenty of power here as well, eliminating my doubt that there may not be. The difference between HPA-1 and something more powerful is that you get scared of pushing the volume to max. Here, I was able to go to max volume but just for a second or two.Lastly I tested with my Hifiman HE-400i. As with HD-650, the sound was a delight with ample volume, dynamics and detail.The Tom Christiansen Audio HPA-1 provides exceptional measured performance. It has no weakness subjectively or objectively. It is fully transparent and as such, joins an exclusive club of half a dozen headphone amplifiers that provide such superlative performance. Cost is up there though, second only to Benchmak HPA-4. So you need to judge whether the look & feel together with superb support from Tom is worth the premium or not.------------As always, questions, comments, recommendations, etc. are welcome.Been too busy with testing to get a haircut. Need to go and get it done today before I start to look like a hillbilly. So pleaseusing: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/