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Ypsilon Electronics Hyperion monoblock power amplifier its output impedance was relatively high for a solid-state design, at 0.35 ohm

Ypsilon Aelius monoblock power amplifier The output impedance was high for a solid-state design, at 0.44 ohm at 20Hz and 1kHz, 0.47 ohm at 20kHz.

AkitikA GT-102 power amplifier 0.2 ohm at 20Hz, 0.11 ohm at 1kHz, and 0.13 ohm at 20kHz

Constellation Inspiration Stereo 1.0 power amplifier The output impedance was low, varying from 0.11 ohm at 20Hz and 1kHz to 0.12 ohm at 20kHz.

Bel Canto Design Black ACI 600 integrated amplifier The output impedance at the speaker terminals was 0.2 ohm at all audio frequencies

Pass Laboratories XA25 power amplifier The output impedance (including 6′ of speaker cable) was low, at 0.08 ohm at 20Hz and 1kHz, rising very slightly to 0.1 ohm at 20kHz.

PS Audio Stellar M700 monoblock power amplifier The output impedance (including 6′ of speaker cable) was low, at 0.1 ohm at 20Hz and 1kHz.

Pass Laboratories XA60.8 monoblock power amplifier The XA60.8’s output impedance, including 6′ of speaker cable, was 0.07 ohm at 20Hz and 1kHz, rising to 0.1 ohm at 20kHz.

Dan D’Agostino Progression Mono monoblock power amplifier The output impedance was high for a solid-state design, at 0.44 ohm at 20Hz and 1kHz, rising slightly to 0.49 ohm at 20kHz.

Siltech SAGA power amplifier output impedance was higher than is usual for a solid-state design, at 0.23 ohm

Krell OLD KST-100 power amplifier output impedance was close at 0.15 ohms, rising to 0.2 ohms by 20kHz.

Emotiva XPA Gen3 two-channel power amplifier The output impedance, including the series resistance of the speaker cables I used, was very low at 20Hz and 1kHz, at 0.09 ohm, and rose only slightly at 20kHz, to 0.125 ohm.

Mark Levinson No.536 monoblock power amplifier The output impedance was low, at 0.1 ohm at 20Hz and 1kHz, rising slightly to 0.13 ohm at 20kHz.

Audionet Max monoblock power amplifier The output impedance was very low, at 0.045 ohm (including the speaker cable) at 20Hz and 1kHz, rising slightly to 0.07 ohm at 20kHz.

AVM Ovation MA8.2 monoblock power amplifier The output impedance (including 6′ of cable) was very low, at 0.08 ohm at 20Hz and 1kHz, rising very slightly to 0.1 ohm at 20kHz

Boulder Amplifiers 2150 monoblock power amplifier Measurements The output impedance, including 6′ of speaker cable, was extremely low, at 0.02 ohm at low and middle frequencies; this rose only slightly, to 0.026 ohm, at the top of the audioband

Bel Canto e.One Ref600M power amplifier output impedance, including 6′ of cable, was low, at 0.1 ohm from 20Hz to 20kHz.

First Watt J2 power amplifier The output impedance was relatively high for a solid-state design, at 0.5 ohm from 20Hz to 20kHz.

PS Audio BHK Signature 300 monoblock The output impedance (including the series resistance of 10′ of speaker cable) was on the high side for a solid-state amplifier, at 0.16 ohm at all audio frequencies

Ayre Acoustics MX-R Twenty monoblock power amplifier the output impedance was higher than usual for a solid-state design: 0.26 ohm at low and middle frequencies, rising to 0.28 ohm at 20kHz

Simaudio Moon Evolution 860A power amplifier The output impedance, including 6′ of speaker cable, was very low, at 0.09 ohm at low and middle frequencies, rising to 0.11 ohm at the top of the audioband.

Bricasti Design M28 monoblock power amplifier The output impedance, including 6′ of speaker cable, was very low, at 0.05 ohm at low and middle frequencies, rising to 0.06 ohm at the top of the audioband.

Constellation Audio Performance Centaur Mono monoblock power amplifier The output impedance was low for a bridged design, at 0.07 ohm including the cable at low and middle frequencies, rising slightly to just under 0.1 ohm at the top of the audioband.

Symphonic Line Kraft 400 monoblock power amplifier The output impedance was 0.04 ohms or less at low and mid frequencies, and under 0.07 ohms at 20kHz

Mark Levinson No.53 Reference monoblock power amplifier The output impedance at low frequencies was very low, at 0.06 ohm, including 6′ of speaker cable. This rose to 0.25 ohm at the top of the audioband

darTZeel NHB-458 monoblock amplifier The output impedance was high for a solid-state design, at 0.3 ohm (including 6′ of speaker cable) at all frequencies.

Lamm M1.2 Reference monoblock power amplifier The output impedance at the “1–6 ohms” setting was 0.15 ohm at low and middle frequencies, rising to 0.18 ohm at 20kHz.

MBL Reference 9011 monoblock amplifier The output impedance was very low, at 0.05 ohm at 1kHz and below, rising to 0.1 ohm at 20kHz.

Goldmund Mimesis 8 power amplifier output impedance: assessing this by measuring the voltage drop at a 1W level when an open-circuit load was replaced by an 8 ohm resistor gave a figure of 0.04 ohms at 20Hz and 1kHz. Repeating the measurement using a 4 ohm load, thus demanding twice as much current, gave a figure twice this, at 0.08 ohms. Though this is still inconsequential, it puzzles me, as does the fact that at 20kHz, the discrepancy was smaller, at 0.12 ohms (8 ohm load) vs 0.15 ohms (4 ohm load).

Soulution 710 power amplifier The output impedance, including the series resistance of 6′ of speaker cable, was very low, at 0.06 ohm at 20Hz and 1kHz, rising to 0.075 ohm at 20kHz.

Luxman B-1000F monoblock power amplifier The output impedance was 0.125 ohm at 20Hz and 1kHz, rising slightly to 0.15 ohm at 20kHz.

Electrocompaniet AW400 monoblock power amplifier The output impedance is specified as being below 0.01 ohm, which is very low. However, including 6′ of speaker cable, I measured 0.135 ohm at low and midrange frequencies, rising very slightly to 0.17 ohm at the top of the audioband.

Bryston 7B SST^{2} monoblock power amplifier The Bryston’s output impedance was low for a balanced design, at 0.11 ohm at low and middle frequencies, rising slightly to 0.17 ohm at 20kHz.

Aesthetix Atlas power amplifier The output impedance was relatively high for a design with a solid-state output stage, at 0.34 ohm at low and middle frequencies, rising very slightly to 0.36 ohm at 20kHz.

EAR/Yoshino M100A monoblock power amplifier Transformer Coupled It was lowest from the 4 ohm output, at 0.55 ohm across most of the audioband—equivalent to a damping factor of just under 8—rising to 1.05 ohms at 20kHz. The 8 ohm figures were 1.1 ohms, rising to 2.3 ohms at 20kHz, while from the 16 ohm transformer I measured 1.9 ohms at 20Hz and 1kHz, and a very high 4 ohms at 20kHz.

Adcom GFA-565 monoblock power amplifier output impedance was uniformly low—ranging from 0.02 ohms from the bass to the upper midrange and increasing slightly to 0.03 ohms at 20kHz.

Musical Fidelity 750K Supercharger monoblock power amplifier The output impedance was a little higher than usual for a solid-state design, at 0.11 ohm at 20Hz and 1kHz, rising to 0.22 ohm at 20kHz

Electrocompaniet Nemo monoblock power amplifier The output impedance measured higher than specified, at between 0.17 and 0.23 ohms

Accuphase M-2000 monoblock power amplifier The output impedance was a extremely low 0.035 ohm at 1kHz, increasing to a measured maximum of 0.038 ohm at 20Hz into an 8 ohm load, and 0.063 ohm at 20kHz into 4 ohms.

Halcro dm88 Reference monoblock power amplifier The dm88’s output impedance was very low across almost all the audioband, at 0.1 ohm including 6′ of multistrand speaker cable. It rose slightly at 20kHz, to 0.14 ohm

McIntosh MC501 monoblock power amplifier output impedance, I was surprised to find it to be lowest from the 8 ohm tap, at 0.08 ohm, and highest from the 2 ohm tap, at 0.13 ohm

Edge NL-12 power amplifier output impedance was 0.1 ohm over most of the audioband, rising to 0.3 ohm at 20kHz

Pass Labs X1000 monoblock power amplifier The output impedance measured between 0.18 and 0.20 ohms (the higher values were at 20kHz)

47 Laboratory 4706 Gaincard power amplifier I measured around 0.15 ohm at 20Hz and 1kHz, this dropping to 0.08 ohm at 20kHz

Hovland Radia power amplifier The output impedance was a low 0.2 ohm across most of the band, this rising inconsequentially to 0.26 ohm at 20kHz.

Krell Full Power Balanced 350mc monoblock amplifier the output impedance was 0.18 ohms at 1kHz and 20kHz, 0.15 ohms at 20Hz

Musical Fidelity Nu-Vista 300 power amplifier the output impedance was 0.08 ohms/0.04 ohms, left/right, respectively, across most of the audio band.

Linn Klimax 500 Solo monoblock power amplifier The output impedance measured a minuscule 0.043 ohms at 1kHz, increasing to 0.071 ohms at 20kHz, both assessed by measuring the voltage rise when an 8 ohm load was open-circuited. Using 4 ohm loads, this increased by a maximum of 0.01 ohm at 20kHz, to 0.081 ohm.

Mark Levinson No.33H monoblock power amplifier output impedance was extremely low at 0.02 ohms at 20Hz and 1kHz, rising slightly to 0.026 ohms at 20kHz.

Ayre AX-7 integrated amplifier Its output impedance was moderately high for a solid-state design, at 0.4 ohm across the audioband.

ASR Emitter II Exclusive integrated amplifier The ASR’s output impedance was very low: <0.08 ohm across the audioband (including the series impedance of 6′ of multistrand speaker cable).

Mark Levinson No.383 integrated amplifier The power-amplifier output impedance was very low at approximately 0.055 ohms across most of the audioband, this rising to a still negligible 0.065 ohms at 20kHz

Luxman L-509X integrated amplifier The output impedance at the speaker terminals was a very low 0.075 ohm at low and middle frequencies, rising to 0.11 ohm at the top of the audioband.

NAD C370 integrated amplifier output impedance across most of the audioband a moderate 0.16 ohm, this rising slightly to 0.19 ohm at 20kHz.

Ayre Acoustics AX-5 integrated amplifier The AX-5’s output impedance was a little high for a solid-state design, at 0.27 ohm across the audioband, this due to the circuit’s lack of loop negative feedback.

Roksan Kandy K2 integrated amplifier output impedance was a very low 0.09 ohm at low and middle frequencies, rising slightly to 0.15 ohm at 20kHz due to the usual series Zobel inductor.

Sugden A21ai The amplifier’s output impedance from the speaker jacks was a low 0.12 ohm at 1kHz, rising to 0.39 ohm at 20Hz and 0.25 ohm at 20kHz.

Krell FBI integrated amplifier The output impedance (including 6′ of speaker cable) was very low, at 0.08 ohm at low and midrange frequencies.

darTZeel CTH-8550 integrated amplifier The CTH-8550’s output impedance was quite high for a solid-state design, at 0.4 ohms in the bass, rising to 0.5 ohms at 20kHz.

Devialet D-Premier D/A integrated amplifier I measured 0.04 ohm at all audio frequencies, but thus includes the impedance of the 6′ of speaker cable I used for the test. But the fact that the D-Premier has an extraordinarily low output impedance

Naim Audio NAIT 5si integrated amplifier The output impedance ranged from 0.34 ohm at 20Hz and 1kHz to 0.36 ohm at 20kHz—a very slight increase.

Creek Evolution 100A integrated amplifier The output impedance at the speaker terminals was less than 0.1 ohm, including 6′ of speaker cable.

GamuT Di150 LE integrated amplifier The Di150’s output impedance was very low, at 0.09 ohm (including 6′ of speaker cable) at low and middle frequencies, and rose only slightly, to 0.11 ohm, at 20kHz.

Cyrus 6vs integrated amplifier impedance of 0.08 ohm at low and midrange frequencies, rising very slightly to 0.1 ohm at 20kHz.

Arcam Alpha 10 integrated amplifier The output impedance is comfortably low—under 0.06 ohms up to 1kHz, rising to a maximum of 0.16 ohms at 20kHz

Chord CPM 3300 integrated amplifier the source impedance was a very low 0.06 ohm, rising slightly to 0.08 ohm at 20kHz.

Jadis JA200 Mk.II monoblock power amplifier The output impedance (including 6′ of speaker cable) was very low for a transformer-coupled tube amplifier, at 0.18 ohm at 20Hz and 1kHz, and 0.14 ohm at 20kHz.

PrimaLuna ProLogue Premium power amplifier The output impedance from both output taps was significantly higher than that of the ProLogue Premium integrated amplifier. From the 8 ohm tap, I measured 9.2 ohms at 20Hz, 8.75 ohms at 1kHz, and 8.45 ohms at 20kHz; from the 4 ohm tap, the respective impedances were 4.7, 4.5, and 4.35 ohms.

Thöress 300B monoblock power amplifier The output impedance was relatively low for a single-ended design, at 1.9 ohms at 20Hz and 1kHz, rising to 2.4 ohms at 20kHz

Air Tight ATM-1S power amplifier the Air Tight’s output impedance was high, ranging from 3.6 ohms at 20Hz to 3.3 ohms at 20kHz. This suggests that the single output-transformer tap is optimized for a 4 ohm load

VTL Siegfried Series II Reference monoblock power amplifier he lowest impedance was in triode/MDF mode, at 0.88 ohm in the midband and 0.9 ohm at the extremes of the audioband. The output impedance rose a little with each reduction of feedback, reaching 1.36 ohms at 1kHz and 1.4 ohms at the frequency extremes in the LDF condition.

Sophia Electric 91-01 300B monoblock power amplifier output impedance was also high, at 5.6 ohms at 20Hz, 3.7 ohms at 1kHz, and 8–9 ohms at 20kHz

Lamm Industries ML3 Signature monoblock power amplifier Even without any negative feedback, the 4 ohm tap’s output impedance was moderately low for a single-ended design, at 1.55 ohms at low and middle frequencies, rising to 1.9 ohms at the top of the audioband.

Lamm ML2.2 monoblock power amplifier The output impedance also depended on the transformer tap used. From the 4 ohm tap, the impedance was a low 0.38 ohm at low and middle frequencies, rising to 0.5 ohm at 20kHz. As expected it was higher from the 8 ohm tap, at 0.7 ohm at 20Hz and 1kHz, and 0.8 ohm at 20kHz, and higher still from the 16 ohm tap: 1.35 ohms at all audio frequencies.

Octave Audio RE 290 power amplifier The output impedance was fairly high, measuring 2.2 ohms at 20Hz and 1kHz, rising slightly to 2.4 ohms at 20kHz.

Jadis SE300B monoblock amplifier The output impedance was 2.5 ohms at 1kHz, 2.7 ohms at 20kHz, and 0.76 ohms at 20Hz.

Jadis Defy-7 Mk.II power amplifier with solid-state amplifiers it is usually sufficiently low to be considered negligible: 0.1 ohm or under. With a number of tube amplifiers it can be rather higher. For example, the “1000”—a low-feedback triode amplifier made by Audio Innovations in the UK—has an output resistance of several ohms. This is by no means an isolated example. The Defy-7’s output or source resistance was quite constant over the audio band, ranging from 0.45 ohms at 20kHz, to 0.4 ohms, midband, and 0.45 ohms at 20kHz.

Allnic Audio A-5000 DHT monoblock power amplifier output impedance was impressively low for a single-ended tube design even from its 16 ohm tap, where it measured 1.6 ohms at 20Hz, 1.35 ohms at 1kHz, and 1.5 ohms at 20kHz. As expected, the respective impedances were lower from the 8 ohm tap: 1, 0.8, and 0.85 ohm.

Conrad-Johnson LP125M monoblock power amplifier The source impedance from the single pair of output terminals was a high 2 ohms at low and middle frequencies, rising slightly to 2.2 ohms at 20kHz

VTL MB-450 Series III Signature monoblock power The output impedance also depended on the amount of negative feedback and the operating mode. The lowest impedance was in triode mode with the maximum amount of feedback, where it measured 0.87 ohm in the midrange, rising very slightly to 0.89 ohm in the low bass and 0.9 ohm at the top of the audioband. With the lowest amount of feedback, these figures rose to 1.45, 1.5, and 1.53 ohms, respectively. Operating the MB-450 in tetrode mode gave a slightly higher source impedance, ranging from 0.95 ohm at 1kHz with the maximum feedback, to 1.95 ohms at 1kHz with the lowest feedback.

Balanced Audio Technology VK-55SE power amplifier the output impedance from the VK-55SE’s High and Med taps was high, at 2.7 ohms in the midrange, rising to almost 3 ohms at high and low frequencies

AudioValve Baldur 70 monoblock power amplifier The Baldur’s output impedance, from both the 8 and 4 ohm transformer taps, was lower than usual for a tubed amplifier, at 0.66 and 0.42 ohm, respectively.

Yamamoto A-08 power amplifier The right channel measured 3.2 ohms at 20Hz, dropping to 2.85 ohms at 1kHz and 2.16 ohms at 20kHz. The corresponding figures for the right channel were a little lower, at 3 ohms, 2.73 ohms, and 2.05 ohms, respectively, but these are still high in absolute terms.

Bel Canto SET 80 monoblock power amplifier The measured output resistance was 1.7 ohms, corresponding to a low damping factor of 4

Cary Audio Design CAD-805 monoblock power amplifier output impedance is 0.74 ohm

Cary CAD-1610-SE monoblock power amplifier impedance from the 8 ohm output was high at 3.4 ohms across most of the band, rising to 4 ohms at 20kHz.

Graaf GM 200 OTL power amplifier The output impedance was 0.79 ohms at 1kHz, 0.7 ohms at 20Hz, and 0.83 ohms at 20kHz

Audio Research VTM200 monoblock power amplifier output impedance also varied according to which tap was used: a reasonably low 0.35 ohm from the 4 ohm tap, rising to 0.6 ohm from the 8 ohm tap and 1 ohm from the 16 ohm tap

Sonic Frontiers Power 3 monoblock power amplifier output impedance a maximum of 0.17 ohm at 20Hz and 1kHz, increasing to a maximum of 0.22 ohm at 20kHz

Wavac SH-833 monoblock power amplifier The SH-833’s output impedance was on the high side, at 5 ohms from the 8 ohm transformer tap and 2.6 ohms from the 4 ohm tap. These figures were maintained across most of the audioband, with a rise at 20kHz from the 4 ohm tap to 3.6 ohms. Unexpectedly, the 20kHz source impedance from the 8 ohm tap dropped from 5 to 4.3 ohms.

EAR 890 power amplifier the EAR 890’s output impedance was around 1.5 ohms from its 16 ohm tap and 0.5 ohm from the 8 ohm tap

Wavelength Audio Cardinal XS monoblock amplifier output impedance was a very high 3.4 ohms at 1kHz, increasing to 5.9 ohms at 20Hz and 5.3 ohms at 20kHz.

Antique Sound Lab Explorer 805 DT monoblock power amplifier output impedances were high, at 7.2 ohms (16 ohm tap), 3.65 ohms (8 ohm tap), and 2 ohms (4 ohm tap)

Audiopax Model Eighty Eight monoblock power amplifier output impedance also varied with the control settings. It was lowest with them at full clockwise, but was still very high in absolute terms at 3.5 ohms across most of the audioband. Unusually, the source impedance dropped at low frequencies, reaching 1.8 ohms at 20Hz.

Air Tight ATM-211 tube monoblock power amplifier output impedance measured a high 3.7 ohms at 1kHz. Unusually, this decreased slightly to 3.5 ohms at 20Hz and 20kHz.

Hovland Sapphire power amplifier The output impedance from the 16 ohm tap was a very high 3.55 ohms across most of the audioband, rising to 4.3 ohms at 20kHz.

Wavelength Audio Gemini monoblock power amplifier The low-frequency source impedance with the 2A3 was 3.7 ohms (8 ohm tap) or 1.7 ohms (4 ohm tap); the midrange impedance was 3.1 and 1.5 ohms, respectively; while at 20kHz, the impedance rose to 4.6 and 1.8 ohms, respectively. The respective figures with the 45 tube were 3.2 and 1.5 ohms, 5.5 and 2.6 ohms, and 6.8 and 2.9 ohms.

McIntosh Labs MC2000 power amplifier The output impedance measured a maximum of 0.4 ohm—quite high in comparison with solid-state amplifiers but admirably low for a tube amp.

Nagra VPA monoblock power amplifier output impedance was quite high, varying between 2.23 and 2.26 ohms depending on frequency and load impedance (measurements taken at the 8 ohm output).

Manley Laboratories Stingray integrated amplifier The output impedance, however, was also high, ranging from a maximum of 3.87 ohms at 20Hz to a minimum of 2.83 ohms at 20kHz—values guaranteed to affect the amplifier’s frequency response through real loudspeakers.

EAR V20 integrated amplifier The output impedance from the 8 ohm speaker terminals measured a minimum of 0.77 ohms and a maximum of 1.14 ohms, the latter at 20kHz. The corresponding values from the 4 ohm taps were 0.47 ohms and 0.62 ohms—respectably low values for a tube amplifier. The output impedance at the V20’s tape-monitor jacks was 1k ohm with a 25 ohm source impedance, and 1.56k ohms with a 600 ohm output impedance, indicating an unbuffered tape output.

Audio Note Jinro integrated amplifier The output impedance from the single transformer tap was commendably low for a single-ended-triode design, at 2.5 ohms

Leben CS300 integrated amplifier The output impedance at 20Hz was similar from all three outputs at 3–3.5 ohms. It did differ at higher frequencies: at 1kHz, the impedance was 2.65 ohms (8 ohm tap), 2.25 ohms (6 ohm tap), and 1.8 ohms (4 ohm tap); at 20kHz, it was 2.4, 1.6, and 1.1 ohms, respectively.

Quad II Classic Integrated amplifier The output impedance was on the low side for a transformer-coupled tube amplifier, at 0.28 ohm at 20Hz and 1kHz, dropping to 0.2 ohm at 20kHz.

Woo Audio WA5 integrated amplifier The output impedance from the speaker terminals was also high, at 3.2 ohms across the audioband

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