The Tom Christiansen Audio HPA-1 is a world-class headphone amplifier. It provides ample power, ultra-low distortion, and exceptionally high dynamic range and easily drives even the more challenging headphones on the market.

The HPA-1 has no sonic signature, thus offers a non-fatiguing and precise rendition of the source material.

Tom Christiansen Audio products are designed from the ground up as retail products. They are not available as DIY products. The HPA-1 is covered by a 3-year warranty (parts & labour) from the time of purchase.

Key Features
Specifications
Circuit Topology
Performance Graphs

Key Features of the HPA-1
- World class performance as characterized by ultra-low harmonic distortion, extraordinarily low intermodulation distortion, and ultra-low noise floor.
- Two inputs: XLR (balanced) and RCA (unbalanced), selectable by front-panel switch.
- One output: ¼" (6.3 mm) phone jack.
- Two gain settings: 0 dB (1×) and 12 dB (4×) to accommodate sources ranging from balanced pro/studio sources to media players, phones, and tablets. The gain is selected by a switch on the front panel.
- Protection against excessive DC voltage on the amplifier output. This protects your headphones in the case of a catastrophic amplifier failure.
The HPA-1 delivers true world class performance at a very attractive price. It provides up to 1.0 W (1000 mW) of output power at ultra-low harmonic distortion, exceptionally low intermodulation distortion, with an ultra-low noise floor. It is mainly due to its exemplary performance on these parameters that the HPA-1 is able to reproduce the music exactly like the artists and recording engineers intended.

The ultra-low distortion results in a clean and crisp sound. This is especially evident in the rendition of instruments that challenge many ordinary amplifiers. This includes instruments such as cymbals, hi-hats, snare drums, and saxophones, which the HPA-1 renders superbly. The ultra-low distortion design of the HPA-1 also allows you to hear the sounds normally only experienced in live performances, such as the body cavity resonance of an acoustic guitar on the leading edge of the string attack.

It is noteworthy, that high levels of intermodulation distortion activate the fight-or-flight response in humans. Thus, it stands to reason that a low level of intermodulation distortion is crucial to a pleasant listening experience. This is another area where the HPA-1 excels. Thanks to its extremely low intermodulation distortion – in particular on multi-tone tests – the HPA-1 renders the music with an incredible realism and an amazing sound stage without resulting in listening fatigue.

Finally, the ultra-low noise floor of the HPA-1 is responsible for the complete absence of sound during pauses in the music – even when used with sensitive in-ear modules.

The HPA-1 is designed and manufactured in Canada. The raw circuit board is made in Ontario. The PCB assembly is performed in Calgary. The chassis is made in Japan with some mechanical parts sourced locally from Calgary.

A summary of the HPA-1 specifications is provided in the table below.

Parameter	Value	Conditions
Output Power	1.5 W	20 Ω
Output Power	1.2 W	32 Ω
Output Power	250 mW	300 Ω
Total Harmonic Distortion (THD)	< -130 dB	300 Ω, 100 mW, 1 kHz
Total Harmonic Distortion + Noise (THD+N)	0.0003 % (-110 dB)	300 Ω, 100 mW, 1 kHz
Intermodulation Distortion (IMD) - SMPTE	< -108 dB	60 Hz + 7 kHz @ 4:1, 300 Ω, 100 mW
Intermodulation Distortion (IMD) - DFD	< -112 dB	18+19 kHz @ 1:1, 300 Ω, 100 mW
Intermodulation Distortion (IMD) - MOD	< -112 dB	917 Hz + 5.5 kHz @ 1:1, 300 Ω, 1 mW
Multi-Tone Intermodulation Residual	<- 140 dBr	AP 32-tone, 300 Ω, 100 mW
Gain	0 dB (1x) +12 dB (4x)	Selectable by switch
Input Sensitivity	2.8/0.7 V RMS	Low/High Gain, 32 Ω, 200 mW
Slew-Rate	±17 V/µs	Symmetric
Output Impedance	35 mΩ	1 kHz
Channel Separation	98 dB	1 kHz
Channel Imbalance vs Volume Setting	±0.57 dB	0 – 60 dB attenuation
Common-Mode Rejection Ratio (CMRR)	> 70 dB	1 kHz, typ.
Common-Mode Rejection Ratio (CMRR)	> 60 dB	1 kHz, typ.
Dynamic Range (AES17)	130 dB
Signal-to-Noise Ratio	132 dB
Residual Mains Hum & Noise	1.25 µV RMS	A-weighted
Mains Voltage Requirements	80–264 V AC 47–440 Hz
Size	140 x 55 x 210 mm	(W x H x D)
Weight	850 g
Designed and Manufactured in Canada

The signal path of the HPA-1 is balanced all the way to the volume control. Only the last ~40 mm of the signal path from the volume control to the output is single-ended. This ensures the best immunity against interference from mains hum and other interference sources.

The heart of the output stage in the HPA-1 is a high-speed, high-current, current feedback operational amplifier with extremely low intrinsic distortion. To further increase the performance of the output stage, the HPA-1 uses a precision audio opamp in a composite amplifier configuration to perform error correction on the output stage. The HPA-1 reaches its stellar performance level, in part, due to the use of this error correcting architecture. It is also due to this architecture that the HPA-1 is free of any sonic signature of its own, thereby allowing you to hear the music the way the artists and recording engineers intended it.

The block diagram of the HPA-1 is illustrated below.

The circuit board layout has been optimized for audio performance. Thus, the circuit board is a 4-layer PCB, which features the elaborate use of planes and pours to minimize the error voltages that develop across the various signal conductors and ground connections. The circuit uses Susumu RG-series precision metal film resistors with ±0.1 % tolerance at critical nodes in the circuit. The only capacitors in the signal path are two Nichicon MUSE UES-series bipolar capacitors. They make no measurable contribution to the sonic signature of the HPA-1.
Outline
Total Harmonic Distortion (THD)

The graph below shows the THD+N vs output power with 300 Ω load. The amplifier delivers 250 mW at the onset of clipping. Note that the sharp jumps (aside from when the amplifier clips) are caused by range switching in the Audio Precision APx525 audio analyzer used for the measurement. The THD+N vs output power plots mostly show the THD+N floor of the measurement system.

Repeating this measurement with 32 Ω and 20 Ω load results in the following:

Even at these heavier loads, the THD+N of the HPA-1 is still below the measurement limit of the APx525. The HPA-1 reaches clipping at 1.2 W and 1.5 W into 32 Ω and 20 Ω, respectively.

Headphones with an impedance below 32 Ω have started to appear in the market. Thus, I characterized the performance of the HPA-1 with various loads, including 12 Ω. The result is shown below.

As seen on the graph, it is only with 12 Ω load that the THD+N of the HPA-1 exceeds the noise floor (measurement limit) of the Audio Precision APx525 audio analyzer.

The THD+N vs frequency plots for 100 mW into 300 Ω and 100 mW into 32 Ω are shown below. Note that the measurement bandwidth was changed to 60 kHz to capture at least three harmonics of the test signal. This also increases the noise bandwidth, hence the THD+N, of the measurement.

Intermodulation Distortion (IMD)

Single-tone distortion tests, such as THD+N measurements, are useful to determine fundamental amplifier performance and behaviour. However, some criticize these tests as being too simplistic to properly describe the amplifier's behaviour when presented with a more complex signal, such as music.

Intermodulation tests address this by using two or more test tones. The gold standard among these tests is the 32-tone test developed by Audio Precision. I was among the first to use this test for amplifier characterization, and other vendors have started to follow suit.

The Audio Precision 32-tone IMD test uses 32 test tones of equal amplitude, logarithmically spaced in frequency. The phase of the test tones have been designed to provide a crest factor of 10 dB, which is quite representative of music. The signal itself sounds a bit like an out of tune pipe organ.

The graph below shows the result of the 32-tone IMD test with the HPA-1.

The combined amplitude of the 32 tones is the 0 dB reference, and corresponds to 100 mW into 300 Ω. The intermodulation products - i.e. the "grass" between the test tones - are distortion products from the amplifier. These should be as low as possible. As seen in above graph, these distortion products are 140 dB (10,000,000×) lower than the reference level, which is truly stellar. Thus, even with a complex input signal, the HPA-1 does not add any audible colouration to the input signal. It is a truly transparent amplifier. The transparency of the HPA-1 is supported by the two-tone IMD tests as well.

Siegfried Linkwitz has proposed a 1 kHz + 5.5 kHz IMD test, which he argues to be highly indicative of the perceived sound quality. He bases his argument on the fact, that IMD products in this measurement fall in the frequency range where the ear is the most sensitive (see the Fletcher-Munson curves for more detail). This seems like a reasonable argument, so I measured the HPA-1 accordingly.

The result of this measurement is shown below. Note that due to a limitation in the signal source of the APx525, the frequencies used must be an integer multiple of each other. Thus, my test signal consisted of 917 Hz (5500/6) and 5.5 kHz at equal amplitude. I performed this measurement at 1.0 mW as this is representative of typical listening levels with most headphones. The result is shown below.

Aside from the third order intermodulation product (d3), the IMD products are indistinguishable from the noise of the measurement equipment. Excellent!

The two more traditional IMD measurements are the SMPTE 60 Hz + 7 kHz (4:1 amplitude ratio) and the 18 kHz + 19 kHz (1:1 amplitude ratio). The former is often used to tease out thermal issues within the amp. The latter often reveals lacking loop gain (so basically lack of control) towards the higher end of the audio frequency range.

The SMPTE IMD test result for the HPA-1 is shown below.

Only the second (d2) and third (3d) order IMD product extend above the noise floor.

The result of the 18+19 kHz IMD measurement for the HPA-1 is shown below. Only the third order IMD product (3d) extends above the noise floor.

Noise

With its integrated power supply, the HPA-1 does show a little bit of rectifier hum below -130 dBV as shown in the plot below. The total integrated noise measures 1.25 µV (A-weighted). Needless to say, the residual mains hum and noise is well below audible.

Some may worry about the presence of ultrasonic noise from the switching power supply. There is no need to worry, however. The plot below shows that the output of the HPA-1 is absent of ultrasonic noise.

Amplitude Response, Channel Separation, Common-Mode Rejection, Etc.

For completeness, I measured the amplitude response of the HPA-1 for its two gain settings along with its gain flatness. The results are shown below.

The channel separation as indicated by the crosstalk between channels of the HPA-1 is shown below. The channel separation is dominated by the little bit of shared ground impedance in the 1/4" phone output jack and exceeds 98 dB within the entire audio band.

The volume control in the HPA-1 is a high-end ALPS RK271-series "Blue Velvet" potentiometer, which is well-established in the audio industry as a state-of-the-art volume pot. Yet, some are still concerned about channel imbalance in potentiometer-based volume controls.

To address these concerns, I measured the channel imbalance and attenuation as function of potentiometer rotation. I turned the volume manually for 30 seconds with as constant a rotational velocity as I could. I measured the resulting gain and channel imbalance throughout this time. The result is shown below.

As seen in this graph, the channel imbalance is within ±0.57 dB throughout the attenuation range of 0-60 dB. It is only at the last few degrees of volume knob rotation near the minimum setting that the channel imbalance exceeds this value.

The common-mode rejection ratio of the differential input of the HPA-1 is shown below. As shown in the plot, mains hum that appears common-mode on the differential input will be rejected by over 70 dB (3,200×).

Transient Response

The measurements above all characterize the amplifier in the frequency domain. Transient response measurements describe how the amplifier performs in the time domain.

Amplifier stability is paramount, and achieving good stability in an amplifier featuring a high-speed composite output stage can be a significant challenge. Thus, I characterized the transient behaviour of the HPA-1 for various reactive loads.

The oscilloscope screen shot below shows the transient response of the HPA-1 when presented with a 10 kHz square wave and loaded by 300 Ω in parallel with 470 pF of capacitance. This load is representative of a pair of Sennheiser HD-650 with the stock cable and a 2-3 m (~10') extension cord.

As shown above, the transient response of the HPA-1 is exemplary and shows no tendency to overshoot.

Increasing the capacitance to 1.0 nF (Sennheiser HD-650 with the stock cable and an 8 m (~26') extension cord) still shows a clean response.

With 4.7 nF (Sennheiser HD-650 + stock cable + 45 m (~147') extension cord), a very slight degradation of the transient response is observed in the form of a slight smearing of the edge. The amplifier still shows no signs of instability, as no ringing or overshoot is present in the measurement.

Finally, when loaded with 10 nF, the amplifier finally shows slight overshoot and ringing. Note, however, that to obtain such capacitive load in actual use would require approximately 100 m (300') of extension cable on a typical pair of headphones. It is also noteworthy that the HPA-1 remains stable even with 100 nF capacity load. Needless to say, I have full confidence in the stability of the HPA-1.

Another important stability consideration of an amplifier is its stability as it enters and exits clipping. I, thusly, measured this for the HPA-1. The three oscilloscope screen shots below show the output of the HPA-1 with a sine wave input. The amplitude is gradually increased until it is 1) just below clipping, 2) at the onset of clipping, and 3) clipping hard.

The HPA-1 shows a very slight blip as it exits clipping, but remains completely stable, even when driven hard into clipping.

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