HPA-1 Headphone Amplifier – Tom Christiansen Audio


  • total harmonic distortion (thd)
  • intermodulation distortion (imd)
  • noise
  • amplitude response, channel separation, noise rejection common mode, etc.
  • transient response

total harmonic distortion (thd)

Reading: Hpa-1 headphone amplifier

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. TCA HPA-1: THD+N vs Output Power (300 Ω, 1 kHz, 20 kHz BW)

repeating this measurement with a load of 32 Ω and 20 Ω results in the following:

TCA HPA-1: THD+N vs output power (32 Ω, 1 kHz, 20 kHz BW)

even with these heavier loads, the thd+n of the hpa-1 is still below the measurement limit of the apx525. The HPA-1 achieves clipping at 1.2 W and 1.5 W into 32 Ω and 20 Ω, respectively.

Headphones with an impedance of less than 32 Ω have begun to appear on the market. therefore, i characterized the hpa-1’s performance with various loads, including 12 Ω. the result is shown below.

TCA HPA-1: THD+N vs output power (20 Ω, 1 kHz, 20 kHz BW)

as seen in the graph, it is only with a 12 Ω load that the thd+n of the hpa-1 exceeds the noise floor (measurement limit) of the apx525 audio precision audio analyzer.

below are the thd+n vs. frequency plots for 100 mw into 300 Ω and 100 mw into 32 Ω. note that the measurement bandwidth was changed to 60 kHz to capture at least three harmonics of the test signal. this also increases the bandwidth of the noise, hence the thd+n, of the measurement.

TCA HPA-1: THD+N vs output voltage, load impedance (12, 20, 32, 50, 300 Ω, 1 kHz, 20 kHz BW)

intermodulation distortion (imd)

Single tone distortion tests, such as thd+n measurements, are useful in determining the fundamental performance and behavior of the amplifier. however, some criticize these tests for being too simplistic to correctly describe the amplifier’s behavior when presented with a more complex signal, such as music.

Intermodulation tests solve 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 one of the first to use this test for amplifier characterization and other vendors have started to follow suit.

the 32-tone audio accuracy imd test uses 32 test tones of equal amplitude, spaced logarithmically in frequency. the phase of the test tones is designed to provide a crest factor of 10 db, which is fairly representative of music. the signal itself sounds a bit like a detuned pipe organ.

The following graph shows the result of the 32-tone imd test with the hpa-1.

TCA HPA-1: THD+N vs Frequency (300 Ω, 100 mW, 60 kHz BW)

the combined amplitude of the 32 tones is the 0 db reference and corresponds to 100 mw into 300 Ω. the intermodulation products, ie the “grass” between the test tones, are distortion products of the amplifier. these should be as low as possible. as seen in the graph above, these distortion products are 140 db (10,000,000×) lower than the reference level, which is truly stellar. therefore, even with a complex input signal, the hpa-1 does not add any audible coloration to the input signal. it’s a truly transparent amp. hpa-1’s transparency is also supported by imd two-tone testing.

siegfried linkwitz has proposed an imd test of 1khz + 5.5khz, which he says is highly indicative of perceived sound quality. he bases his argument on the fact that imd products in this measure fall in the frequency range where the ear is most sensitive (see fletcher-munson curves for details). this seems like a reasonable argument, so i sized the hpa-1 accordingly.

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The result of this measurement is shown below. Please note that due to a limitation in the apx525 signal source, the frequencies used must be an integer multiple of each other. therefore my test signal consisted of 917hz (5500/6) and 5.5khz at equal amplitude. I made this measurement at 1.0mW as this is representative of typical listening levels with most headphones. The result is shown below.

TCA HPA-1: THD+N vs Frequency (32 Ω, 100 mW, 60 kHz BW)

Apart from the third order intermodulation product (d3), the imd products are indistinguishable from the noise of the measurement equipment. excellent!

The two most traditional imd measurements are smpte 60hz + 7khz (4:1 amplitude ratio) and 18khz + 19khz (1:1 amplitude ratio). the former is often used to troubleshoot thermal problems within the amplifier. the latter often reveals a lack of loop gain (basically lack of control) towards the upper end of the audio frequency range.

The smpte imd test result for the hpa-1 is shown below.

TCA HPA-1: Multi-Tone IMD (AP 32-tone, 100 mW, 300 Ω)

only the second (d2) and third (3d) order imd product extends above the noise level.

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 background noise.

TCA HPA-1: 1 kHz + 5.5 kHz IMD


With its integrated power supply, the hpa-1 shows some rectifier hum below -130 dbv, as shown in the diagram below. the integrated total noise measures 1.25 µv (a-weighted). Needless to say, the hum and residual network noise are well below audible.


Some may be concerned about the presence of ultrasonic noise from the switching power supply. however, there is no need to worry. the graph below shows that the output of the hpa-1 has no ultrasonic noise.

TCA HPA-1: 18+19 kHz IMD

amplitude response, channel separation, common mode rejection, etc.

For completeness, I measured the hpa-1’s amplitude response for its two gain settings along with its gain uniformity. the results are shown below.

TCA HPA-1: Residual mains hum & noise

The channel spacing indicated by the crosstalk between channels of the hpa-1 is shown below. channel separation is dominated by some shared ground impedance at the 1/4″ phone out jack and exceeds 98 db within the entire audio band.

TCA HPA-1: Residual hum and ultrasonic noise

The volume control on 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 potentiometer. however, some are still concerned about channel imbalance in pot-based volume controls.

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To work around these issues, I measured the channel’s unbalance and attenuation as a function of potentiometer rotation. I rotated the volume manually for 30 seconds with as constant a rotation speed as I could. I measured the resulting gain and channel imbalance over this time. The result is shown below.

TCA HPA-1: Amplitude Response

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

The hpa-1 differential input common mode rejection ratio is shown below. as shown in the graph, mains hum appearing in common mode on the differential input will be rejected by more than 70 db (3200×).

TCA HPA-1: Gain flatness

transient response

The measurements mostly characterize the amplifier in the frequency domain. transient response measurements describe how the amplifier works in the time domain.

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

The following oscilloscope screenshot shows the transient response of the hpa-1 when presented with a 10 kHz square wave and loaded with 300 Ω in parallel with 470 pf of capacitance. This load is representative of a pair of Sennheiser HD-650s with the original cable and a 2-3m (~10′) extension cable.

TCA HPA-1: Channel separation

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

increasing the capacitance to 1.0 nf (sennheiser hd-650 with the standard cable and an 8 m (~26′) extension cable) still shows a clean response.

TCA HPA-1: Channel imbalance vs volume pot rotation

at 4.7 nf (sennheiser hd-650 + original cable + 45 m (~147′) extension cable), a very slight degradation of the transient response is observed in the form of a slight smear on the edge. the amplifier still shows no signs of instability, as there is no ringing or overshoot in the measurement.

TCA HPA-1: Common-mode rejection ratio

finally, when loaded with 10 nf, the amp finally shows a slight overshoot and hum. note, however, that to obtain such a capacitive load in actual use approximately 100 m (300′) of extension cable would be required in a typical pair of headphones. It is also worth noting that the HPA-1 remains stable even with a capacity load of 100 nF. Needless to say, I have full confidence in the stability of the hpa-1.

TCA HPA-1: Transient response (300 Ω || 470 pF)

Another important consideration in the stability of an amplifier is its stability going in and out of clipping. I therefore measured this for the hpa-1. The three oscilloscope screenshots below show the output of the hpa-1 with a sine wave input. the amplitude gradually increases until 1) it’s just below clipping, 2) at the beginning of clipping, and 3) it’s clipping hard.

the hpa-1 exhibits a slight flicker when it comes out of clipping, but remains completely stable, even when heavily clipped.

TCA HPA-1 transient response (300 Ω || 1.0 nF)TCA HPA-1 transient response (300 Ω || 4.7 nF)

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