Audio Research VT-150 monoblock power amplifier Measurements

Sidebar 2: Measurements

I measured a voltage gain of 10 (20dB) from the VT-150's 4 ohm output tap, 14 (22.9dB) from its 8 ohm tap, and 18.6 (25.4) from the 16 ohm tap. This increase in voltage gain at different output transformer taps occurs because the taps have different turns ratios (the number of turns of wire on the secondary winding in relation to the transformer primary winding). Input sensitivity (the input voltage required to drive the amplifier to clipping) was 2.4V RMS (balanced), which is ideal for the high-gain LS5. Input impedance measured a high 215k ohms across the band.

The VT-150's output impedance was nearly 1 ohm (0.99 ohms) at any audio frequency measured at the 16 ohm tap. This value decreased to 0.56 ohms at the 8 ohm tap, and 0.3 ohms at the 4 ohm tap. These are relatively low values for a tubed amplifier, particularly from the 4 ohm tap, meaning that the amplifier's balance will not significantly track a loudspeaker's impedance variation.

The VT-150's frequency response (not shown) was flat throughout the audio band, but showed slightly different high-frequency rolloffs depending on the transformer tap selected. From the 8 ohm tap, the response was down 0.05dB at 20kHz; from the 4 ohm tap, the response dropped by 0.15dB at 20kHz—still a negligible rolloff. The VT-150's 1kHz and 10kHz squarewave responses at low signal levels were excellent. There is very little rounding of the edges, and just a hint of overshoot on the 10kHz signal (fig.1).


Fig.1 Audio Research VT-150, 10kHz squarewave into 8 ohms.

THD+noise levels (fig.2) measured less than 0.1% THD+N across the band at low signal levels into any impedance. The VT-150's THD vs output power plots are shown in fig.3. The solid line is into an 8 ohm load (from the 8 ohm tap), with the VT-150 delivering 150W (21.8dBW) at the 3% distortion level. Four ohm and 2 ohm plots were made from the 4 ohm tap, with each delivering exactly 100W at 3% distortion (17dBW and 14dBW, respectively).


Fig.2 Audio Research VT-150, 4 ohm tap, THD+noise vs frequency at (from top to bottom): 4W into 2 ohms, 2W into 4 ohms, and 1W into 8 ohms.


Fig.3 Audio Research VT-150, distortion vs output power into (from bottom to top at 70W): 8 ohms, 8 ohm tap; 4 ohms, 4 ohm tap; and 2 ohms, 4 ohm tap.

Fig.4 is an FFT of the VT-150's output when driving a 4 ohm load at two-thirds rated power with a 50Hz sinewave. The distortion is odd-order, as would be expected from a push-pull tubed amplifier, with the third-harmonic highest in level. The higher harmonics decrease in level with increasing order—a characteristic that seems to correlate with good sound. At higher frequencies (fig.5), the VT-150's distortion waveform showed the distortion to be primarily second-harmonic with some noise. The distortion waveform looked nearly identical at 2W into 4 ohms, but showed an increase in third-harmonic when driving a 2 ohm load (fig.6).


Fig.4 Audio Research VT-150, spectrum of 50Hz sinewave, DC–1kHz, at 67W into 4 ohms, 4 ohm tap (linear frequency scale). Note that the third harmonic at 150Hz is the highest in level, at –46dB (0.5%).


Fig.5 Audio Research VT-150, 1kHz waveform at 1W into 8 ohms, 4 ohm tap (top); distortion and noise waveform with fundamental notched out (bottom).


Fig.6 Audio Research VT-150, 1kHz waveform at 4W into 2 ohms, 4 ohm tap (top); distortion and noise waveform with fundamental notched out (bottom).

The VT-150's intermodulation spectrum is shown in fig.7. This plot is an FFT of the VT-150's output when driving an equal mix of 19kHz and 20kHz at two-thirds rated power into a 4 ohm load. The IM products with this demanding signal are much higher than those typical in most solid-state amplifiers, but lower than in single-ended tubed units.—Robert Harley


Fig.7 Audio Research VT-150, HF intermodulation spectrum, DC–22kHz, 19+20kHz at 67W into 4 ohms, 4 ohm tap (linear frequency scale).

Audio Research Corporation
6655 Wedgwood Road N., Suite 115
Maple Grove, MN 55311
(763) 577-9700

Ortofan's picture

... 40dB below the fundamental, one has to wonder whether you're listening to the music or to the distortion?
Is it those relatively high levels of both harmonic and inter-modulation distortion that contribute to the allegedly "magic" sound quality?

Did RH ever compare the VT-150 with the Threshold S/550e?

Bogolu Haranath's picture

These ARC VT-150 mesurements are somewhat similar to recently reviewed ARC Ref.160M and ARC Ref.160S (reviewed in Feb. 2020 issue) :-) ........

Bogolu Haranath's picture

BTW .... A rose by any other name still sounds (smells) the same :-) .........

Bogolu Haranath's picture

Sound to die for (S2D4) :-) ........

JRT's picture

Not sure how that VT-150 SE differs from the VT-150, but it's interesting regardless.

More commonly the output transformer of an "ultralinear" configured amplifier (eg Dynaco MkIV) simply includes taps on the primary to connect to the screen grids, so those taps share the same B+ power supply voltage with the anodes, which requires suboptimal voltage on the anodes to prevent too much at the screen grids, and that compromise often results in lower than optimal voltage at the anode and higher than optimal voltage at the screen grids with shortened life and degraded performance. This amplifier (at least the SE edition of this amplifier) takes the better approach of providing a separate center tapped winding for the screen grids, allowing those to be operated at reduced power supply voltage than the anodes, which significantly reduces nonlinear distortion and lengthens usable life of the output tubes. Also there are four separate cathode feedback tapes, one for each output tube. For an unchanged level of global feedback, utilizing cathode feedback tapes further reduces nonlinear distortion and reduces output impedance by including the transformer in a local degenerative feedback loop with the output tubes' cathodes.

Lorton's picture

I had this amp, worked well on my watt puppies, wounder how it would sound today and if it will hold against current ARC.