Actually a person that purchased your amp originated this thread. I just copied him on your request…hopefully he will correct. I am sure this wasnt intentional…
Alex
The thread title is already fixed It is already fixed. With this thread and 9 hours at Type One Diabetes Conference and going to get a drink. Life is way to short argue about specs
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Awesome. Thank you. I know it wasn’t intentional and I didn’t take it personally. But it is important to me that someone searching for the “Tom Christiansen Audio HPA-1” can find this thread, hence my desire to get it fixed.
Thanks to @angstorms for starting this thread. I appreciate it.
I’m more than happy to answer any questions related to the HPA-1 here.
Tom
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Tom, no worries. I understand the importance of branding, glad it was corrected. I saw Torq did the change while I was driving people home from the conference.
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Yup, thanks I dont like people to post my stuff without at least asking or at least give the authors info…I just linked to the thread,
Thanks and whats this “retiring” bit?? Are you just going to disapper into the sunset?
Alex
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It looks like a good amp. I’d be interested to know more from your point of view.
I always find it fun when an amp designer speaks a bit to the development history of their product. I know there are quite a few amps using the LM4562, so I’d be curious what sets your design apart from others as well. Why SE on a differential design, the choice of opamps, etc.?
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@Torq - Thank you for sharing the Kippel test. Testing and honing your listening skills is important indeed. In addition to the Kippel test you link to, I suggest having a look at Harman Kardon’s training software: http://harmanhowtolisten.blogspot.com
I believe it was developed by Floyd Toole and Sean Olive at Harman.
Yet another approach would be to look at the current science on the topic. There are quite a few AES papers on the audibility of things (and I’ve been meaning to perform a comprehensive literature review for ages). Belcher (1978 if I recall correctly) found that some can hear THD down in the -80 dBc range (0.01%), while others are much less sensitive to THD. He argued for a multi-tone test to be used. In his case “multi” was three, so a 3-tone test. I’ll see if I can dig out the paper.
I’ve argued for some time now that we should use the Audio Precision 32-tone test, which presents 32 tones of equal amplitude, logarithmically spaced in frequency. A piece of equipment which shows very little intermodulation distortion residual (the ‘grass’ between the tones) should be sonically transparent.
I’ve been using that test since I bought my APx525 back in 2014 and other manufacturers have started to follow suit (if they provide measurements, that is). You can see the result of the multi-tone test of the HPA-1 below.
You’ll notice that the IMD products are more than 140 dB below the combined total amplitude of the tones (the 0 dB reference line). This exceeds the dynamic range of human hearing, thus I think it is entirely accurate to call the HPA-1 a transparent amp.
Tom
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Almost literally … per my “I’m sailing away” thread. But that’s a discussion for that thread.
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The LM4562, which was later rebranded as the LME49720, is a very high precision opamp. It provides stellar performance (low THD, even into a low-impedance load, low noise, reasonably low DC offset, etc.) at a very competitive price. The OPA1612 does provide lower noise and lower distortion, though not meaningfully lower. It is also considerably more expensive. Furthermore, the OPA1612 contains an internal compensation circuit, which results in a phase wobble in the open-loop frequency response of the opamp. This makes the OPA1612 harder to use - especially in a composite amplifier, such as the output stage of the HPA-1.
In a composite amplifier, a precise amplifier is used to provide error correction on a less precise amp. In case of the HPA-1, the output stage is a 300 MHz, high output current, current-feedback line driver. It actually provides quite low THD on its own, but its DC precision needs improvement for it to be used as a headphone driver. The LM4562 takes care of that and further reduces the THD of the output stage. Using the LM4562 in a composite loop with the line driver also drastically lowers the output impedance of the amp. These things combined is what drives the performance of the HPA-1.
Also - in the interest of full disclosure - I do have a warm spot in my heart for the National Semiconductor parts. I used to work for National Semiconductor. I worked for three years in their Precision Amplifiers Group (designed the LMP2021) before I was reorganized into their Precision Timing Group, where I worked on the LMK03000, LMK04000, LMK04800, LMX2581, and some other parts. I was with National/TI for a little over a decade.
That said, I like the old Burr-Brown guys and their work too. The OPA16xx are from the former Burr-Brown (gobbled up by Texas Instruments in 1999; TI gobbled up National in 2012).
Anyway… At the <-120 dB THD level, things like resistor materials matter. I use ±0.5% and ±0.1% tolerance metal film resistors to ensure that the resistors do not add any distortion of their own. Similarly, the one capacitor in the signal path is a Nichicon Muse UES-series bipolar capacitance, which offers exceptionally low distortion.
The ALPS RK271-series “Blue Velvet” volume pot was chosen for a few reasons:
- It’s very well known and well-regarded in audiophile circles
- It provides a good feel, which leads many to associate the HPA-1 with quality (as they should based on its measured performance)
- It offers excellent channel matching
- It is available (albeit as a custom order item from ALPS) in a low-impedance version (lower noise)
The main drawback of the RK271-series pot is that it does not offer any way to mechanically secure the pot to the PCB (aside from the six pins that connect the electrical signals). This means the pot has to be secured mechanically somehow or the use of the pot will cause the solder joints to fail over time. Thus, I added an internal sub-panel or bracket to hold the pot and to provide stress relief on the 1/4" output jack.
Neutrik recommends that the output jack is supported by a panel or bracket, but many manufacturers go without it (as you can see in the various tear-downs of amps). You can see the bracket in the picture below.
The switches are of high quality and sealed so they cannot become contaminated over time. The switches are not in the signal path - all the signal switching is accomplished by relays. They should last a lifetime.
The relays used for signal switching have gold-plated switch contacts. They were selected for their ability to reliably switch low-level signals. The relays are low sulphide types and are sealed, which means that the risk of switch contamination is extremely low. Unfortunately, that also means that the HPA-1 contains about $30 worth of relays! They’re decidedly non-cheap!
Even at the $899 price point, I cannot justify the design and setup cost of a custom designed chassis, so I went with the nicest chassis I could get for a reasonable sum of money. Even at the relatively low quantities (QTY = 100 per order), they’re still surprisingly expensive. That said, even with its “upper class military” ruggedness (thanks @lost33), the chassis is pretty darn nice. The surface finish is consistent and smooth. The front and rear panel have a slight matte texture, which presents really well. The internal length of the chassis is accurate to within ±0.18 mm (which is amazing!), though the manufacturer commented that they’re usually more precise than that. 
The PCB is mounted on standoffs that are pressed into machined blind recesses in the aluminum bottom. This ensures that the chassis is grounded (it’s a safety thing).
Regarding the features: The feature set was developed from a market analysis I conducted last year. I had about 100 respondents indicate their preferences and expectations. To my surprise, and contrary to my expectations, a vast majority did not want a 4-pin XLR output, but did want an XLR input.
Going with a 1/4" output only also made it feasible to squeeze the amp into the current chassis. Had I needed to support both 1/4" and 4-pin XLR outputs, the chassis would have needed to be larger (-> higher cost, higher cost of packing materials, higher shipping cost, etc.) And, honestly, there aren’t really any meaningful technical advantages of the 4-pin output. Sure. You can get higher output power, IF the output driver supports it. And you get to skip a few mΩ of shared ground impedance. But that’s it.
Now from a marketing perspective, it’s another story. The 4-pin connector is large and in charge and screams “this is a serious amp” more so than the 1/4" connector. And should I choose to release an even-higher-end amp than the HPA-1, it will certainly have a 4-pin output (and be in a larger, heavier, and prettier box, etc.).
I really wanted to hit the sweet spot on the price/performance ratio with the HPA-1, so I chose to spend money where it would provide the most benefit: Circuit board, electronic components, and circuit board assembly. The raw board is manufactured in Ontario, Canada (just outside of Toronto). I buy genuine parts from authorized distributors and deliver them and the circuit boards directly to my assembly house, located a 15-minute drive from my house in Calgary, Canada. They do amazing work.
The chassis is nice but not outlandish. Similarly, the packaging is nice but not outlandish. I pack the HPA-1 in a 200-lb rated white cardboard box. The HPA-1 is braced by two foam end caps. The white box then goes into a plain cardboard box (with proper foam bracing) for shipping.
I hope I’ve struck a good balance between an “engineer’s amp” and an “all-marketing” amp. Time will tell.
You can find the HPA-1 here: https://www.tomchr.com/products/hpa-1
Note that I updated the page last week, so it now shows all the performance measurements in the “Performance Graphs” tab rather than as a downloadable report.
Tom
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That was a great read, and I thank you for writing such a detailed response!
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Excellent review. Great detail.
I agree with you…The HPA-A costs $899, and there are other alternatives out there, maybe not as well built but with great specs as well.
For $899 this amp has a two year warranty, the $400 THX amp from Monoprice has a 3 year warranty!
So if an amp is being touted as having great parts then why doesnt it have at least a 3 year warranty.
Schitt Audio, the guys with the sappy verbiage, offer a 5 yr warranty on their more costly stuff and a 2 yr warranty on the lower cost stuff.
The missing XLR output for me is a big thing. After spending bucks on 4 pin xlr cables and not being able to use them is an issue for me, maybe not for you?
To me specifications are great with this amp, and the listening experience I had was superlative, but there is more to a purchase than the numbers.
Alex
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I can easily change the warranty term to three years if that leads to a perception of greater value. That’s a 5-second change on my website. I fully expect my products to last.
The two-year term was the result of my market analysis as well. I asked what term people wanted and the majority said two years. Some expected longer, others shorter, but the average (and mode) was two years.
When you buy my stuff, at least you know what you’re getting.
Tom
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Just wanted to say well done for this, I love that you are even open to any feedback. This response clearly shows how strongly you believe in your build and others should as well, bravo!
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My warranty is now as follows:
- Product price: $0 – $499: One year
- Product price: $500 – $999: Three years
- Product price: $1k+: Five years
The rest of the conditions remain the same. You can read my warranty and return policy here: https://www.tomchr.com/pages/warranty-returns
Tom
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Great stuff. This is a win win for everyone.
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Well that’s much better…
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@tomchr’s children, siblings.
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I have the PASS WHAMMY design class a amp and upgraded to opa627 after auditing many other opa amps- the TCA HPA-1 uses a more glaring in voices OPA amp- but I’m sure Tom has made it so it’s not so glaring in male and female vocals- I would love to see if it beats my WHAMMY w/OPA627 - the return fee of 15% restocking makes it REALLY hard for me to take a chance- the price is no big deal at $899 but restocking fee is too steep to try at home- how else are we suppose to shop at home? Maybe Tom could weigh in- im using the Topping D90 and I just ordered a Bottlehead Crack w/sb and upgraded tubes- just to have a tube amp around for my HD600s - but I would love to get the HPA-1 here at my home and see - I just don’t want to loose money if I decide not to keep it after only 2 weeks of trial period. Maybe Tom will have mercy and extend some allowance. I live in New York and Covid is keeping so many glued at home.
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The two-week return period starts when you take delivery of the amp. You then have two weeks to decide whether you want to keep it. If you decide to return the amp, it will need to be shipped back no later than 14 calendar days after you took delivery of it, so if you took delivery on June 1st, you will need to ship it back no later than June 15th. 336 hours should be enough to allow you to decide whether to keep the amp. Please use a tracked and insured service as you’re on the hook if the amp gets lost in transit.
I will process your refund within 48 hours of receiving your return, usually faster. Basically, I need enough time to determine the amount of reconditioning needed. You’re on the hook for all shipping costs and the restocking fee.
The restocking fee pays for me to recondition and test the amp, and for the loss I will incur when I sell it as B stock. I can’t with good conscience claim the amp is new when it’s been sold once already…
Some may find this policy strict, but it is clear and upfront. Returns cost money. That money has to come from somewhere. I can either make the person who returns the amp pay for it, or I can increase the prices of my products and distribute the cost of returns onto everybody. There’s no free lunch, unfortunately.
In more positive news: I am now able to offer rather incredible shipping rates on UPS and FedEx shipping. These shippers will get an amp to you in a few days, and overnight in many cases. Canada Post (and USPS and other postal services) take quite a bit longer these days, but packages are getting through.
You can find the HPA-1 here: https://www.tomchr.com/products/hpa-1
Tom
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