LSA VT-70 integrated amplifier Measurements

Sidebar 3: Measurements

I performed the measurements of the LSA VT-70 with my Audio Precision SYS2722 system. Although potentiometers on the top panel allow the bias currents for the output tubes to be adjusted, I understand that these currents are set by the manufacturer before the amplifier leaves the factory. Nevertheless, before I started the testing, I checked the tube bias with the front-panel meters after a 30-minute warm-up; with no signal and the volume control set to its minimum, the bias was very close to an indicated "100" for each tube.

The LSA VT-70 preserved absolute polarity, ie, was noninverting, from both of its output transformer taps, from the headphone output, and from the preamplifier output. The maximum voltage gain at 1kHz from the loudspeaker outputs into 8 ohms was 37.4dB from the 8 ohm tap, 36.17dB from the 4 ohm output tap. The maximum gain from the headphone output was 41.4dB and from the preamplifier output, it was –0.23dB, ie, slightly lower than the input level. The single-ended input impedance is specified as a high 100k ohms. I measured a still-high 42k ohms at 20Hz, 44k ohms at 1kHz, and 39k ohms at 20kHz.

The preamplifier output impedance was a low 2 ohms at high and middle frequencies, though it rose to 265 ohms in the low bass, presumably due to the presence of a series coupling capacitor. The headphone output's source impedance was a high 470 ohms across the audioband, which means this output will not be optimal for use with low-impedance headphones.

Although the VT-70's loudspeaker output impedance is specified as 8 ohms or 4 ohms, I suspect that these actually refer to the nominal values of the output transformer taps. The source impedance of the 8 ohm tap was a very high 5.1 ohms at low and midrange frequencies, rising to 7.4 ohms at the top of the audioband. As expected, the impedance was lower from the 4 ohm tap, at 2.7 ohms at 20Hz and 1kHz, 3.8 ohms at 20kHz. The variation in the LSA-70 amplifier's small-signal frequency response from the 8 ohm tap with our standard simulated loudspeaker (fig.1, gray trace) was an audible ±2.4dB. From the 4 ohm tap (not shown), the variation was smaller, at ±1.75dB, but this will still be audible. Into resistive loads (fig.1, blue, red, cyan, magenta, and green traces), the VTA-70 started to gently roll off in the top octave, with then a sharp dropoff above 45kHz.

922LSA70fig01

Fig.1 LSA VT-70, 8 ohm tap, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), and 2 ohms (green) (2dB/vertical div.).

Fig.1, taken with the volume control set to its maximum, features excellent channel matching. The channel gains were less well-matched at lower settings of the volume control, with the right channel almost 1dB higher in level than the left when the control was set to –10dB. This difference decreased to 0.3dB with the control set to –20dB.

The VT-70's reproduction of a 1kHz squarewave into 8 ohms (fig.2) was superbly square, which correlates with the extended low frequencies seen in fig.1. However, the ultrasonic rolloff lengthened the risetimes of a 10kHz squarewave (fig.3), and the small peak just above 40kHz in the frequency response is associated with some ringing on the tops and bottoms of the waveform.

922LSA70fig02

Fig.2 LSA VT-70, 8 ohm tap, small-signal 1kHz squarewave into 8 ohms.

922LSA70fig03

Fig.3 LSA VT-70, 8 ohm tap, small-signal 10kHz squarewave into 8 ohms.

The VT-70's channel separation (not shown) was >60dB in both directions below 2kHz but decreased to 40dB R–L and 46dB L–R at 20kHz. The unweighted, wideband signal/noise ratio (ref.1W into 8 ohms, taken from the 8 ohm tap and measured with the input shorted to ground) was a good 76.7dB (average of the two channels). This ratio improved slightly, to 77.8dB, when the measurement bandwidth was restricted to 22Hz–22kHz, and to 90.1dB when A-weighted. The S/N ratios at the 4 ohm taps were around 1.5dB greater. Spectral analysis of the low-frequency noisefloor while the LSA's 8 ohm tap drove a 1kHz tone at 1Wpc into 8 ohms (fig.4, blue and red traces) revealed a low level of random noise, and the AC-related spuriae at 60Hz and its odd- and even-order harmonics lay at or below –80dB. This graph was taken with the volume control set to its maximum. Peculiarly, with the control set to –20dB, the odd-order AC-related spuriae were all higher in level.

922LSA70fig04

Fig.4 LSA VT-70, 8 ohm tap, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms with volume control set to its maximum (left channel blue, right red) and to –20dB (left green, right gray) (linear frequency scale).

LSA specifies the VT-70's maximum power as 35Wpc into 8 ohms (15.44dBW), which is what I would expect from a push-pull pair of EL34 tubes operated in Ultralinear mode. (The traces in figs.5–7 were taken with the left channel but with both channels operating.) With our usual definition of clipping—when the THD+noise reaches 1%—and with both channels driven, I measured a clipping power of just 7Wpc from the 8 ohm tap into 8 ohms (8.45dBW, fig.5) and the same from the 4 ohm tap into 4 ohms (5.45dBW, fig.6). Relaxing the clipping definition to 3% THD+N gave output powers of 25.3Wpc from the 8 ohm tap into 8 ohms (14.0dBW) and 28.45Wpc from the 4 ohm tap into 4 ohms (11.53dBW). The shape of the traces in these graphs suggests that the amplifier uses a limited amount of loop negative feedback. With the 4 ohm tap driving 8 ohms (fig.7), 1% THD+N was reached at 19.5Wpc (12.9dBW) and 3% THD+N at 27Wpc (14.3dBW). More significantly, the THD+N was lower at low powers than it had been with the load matched to the nominal transformer tap.

922LSA70fig05

Fig.5 LSA VT-70, 8 ohm tap, distortion (%) vs 1kHz continuous output power into 8 ohms.

922LSA70fig06

Fig.6 LSA VT-70, 4 ohm tap, distortion (%) vs 1kHz continuous output power into 4 ohms.

922LSA70fig07

Fig.7 LSA VT-70, 4 ohm tap, distortion (%) vs 1kHz continuous output power into 8 ohms.

Fig.8 shows how the percentage of THD+N in both channels varied with frequency from the 4 ohm tap into 8 and 4 ohms at 4.9V, which is equivalent to 3W into 8 ohms and 6W into 4 ohms. The THD+N was higher in the right channel (red and gray traces) than the left (blue, green) and as, shown in fig.7, was lower with the higher load impedance. The THD rose at low frequencies due to the onset of core saturation in the output transformer. The THD+N percentage at the same voltage from the 8 ohm tap (not shown) was about 5 times as high as it was from the 4 ohm tap.

922LSA70fig08

Fig.8 LSA VT-70, 4 ohm tap, THD+N (%) vs frequency at 4.9V into: 8 ohms (left channel blue, right red), 4 ohms (left green, right gray).

The distortion waveform from either tap (fig.9) suggests that the second harmonic was dominant. However, at low frequencies, the third harmonic was the highest in level (fig.10), which again will be due to the output transformer. Note that the right channel's second harmonic in this graph is almost 20dB higher than in the left channel. This might have been due to the output tubes' bias currents not being sufficiently closely matched in that channel, except that these appeared identical when I checked the bias before testing. It is more likely, therefore, that one of the right channel's output tubes had aged out of spec. (In a push-pull circuit with perfectly matched devices, there is no even-order distortion.) However, at the same output voltage at higher frequencies (fig.11), the second and third harmonics are equally high in level in both channels, each close to –60dB (0.1%).

922LSA70fig09

Fig.9 LSA VT-70, 8 ohm tap, left channel, 1kHz waveform at 3W into 8 ohms, 0.18% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).

922LSA70fig10

Fig.10 LSA VT-70, 4 ohm tap, spectrum of 50Hz sinewave, DC–1kHz, at 1Wpc into 8 ohms (left channel blue, right red; linear frequency scale).

922LSA70fig11

Fig.11 LSA VT-70, 4 ohm tap, spectrum of 1kHz sinewave, DC–10kHz, at 1Wpc into 8 ohms (left channel blue, right red; linear frequency scale).

Despite the relatively high level of harmonic distortion, intermodulation distortion was only moderate in level. With the VT-70's 8 ohm taps driving an equal mix of 19 and 20kHz tones at 1Wpc peak into 8 ohms (fig.12), the 1kHz difference product lay at –66dB (0.05%). The higher-order products at 18 and 21kHz were only 4dB higher in level.

922LSA70fig12

Fig.12 LSA VT-70, 8 ohm tap, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 1Wpc peak into 8 ohms (left channel blue, right red; linear frequency scale).

The LSA VT-70's measured performance is typical of a conventional tubed design using a push-pull pair of EL34s for its output stage and with only a small amount of loop negative feedback. The lowest distortion will be obtained from the 4 ohm output transformer tap, especially driving impedances >4 ohms, with no significant reduction in maximum power.—John Atkinson

COMPANY INFO
LSA/Underwood HiFi
underwoodwally@aol.com
(770) 667-5633
ARTICLE CONTENTS

COMMENTS
Jack L's picture

Hi

Don't hold yr breath on Chinese-made tubes. Lower cost shorter life-span. No miracles !

Such 6-month short short warranty should have already hinted the consumers what would happen sooner or later.

FYI, the 50-plus-year-young Telefunken ECC83s & then-brand-new made-in-Geat-Brtain Mullard ECC82s used in my phono-preamp for some 7 years, operating quite many hours week-in week-out. They still make me good classical music - nooo problem.

Brandnames speak !

Listening is believing

Jack L

Bill Leebens's picture

Ir's worth noting that a great many audio products are only warranted for 90 days until the warranty is extended when the product is registered with the manufacturer.Those warranty terms don't apply to tubes included with a product---
that six month warranty on tubes is the de facto standard---Chinese, Czech, Russian, whatever. The longest warranty I know of for tubes included in a product is one year, and that was only for the small signal tubes. I've seen power output tubes with as little as 90 day warranty coverage.

Jack L's picture

Hi

Really? So please name a few brandname audio products in the marketplace today only provide "90 days" warranty.

I am all ears

Jack L

Jack L's picture

Hi

Guys, No politics in this open venue please.

As posted in Stereophile forums since day one a few years back, I am on 'quality" only.

While we are still on this tube amp, suppose you were the owner of this amp, just arrived & put on test run - it broke down in no time!
What woud you feel ? You would still love its design/quality control ?????

It could be built in the Silicon Valley, CA, or the Brazilian rain forests or whatever. It "happened" to be made-in-China ! This unfortunately reflects on the "ethnics" issue as seen by some politically knowledgeable readers here.

In fact, I fixed a few "high-end" tube power amps made-in-China, using bigboy tubes, e.g. 300B & 2A3 just within a few years & surely I know how good were those made-in-China were. Even after upgrading it with expensive audiophile-grade caps, interwiring etc etc as requested by their ownes, the upgraded sound quality did not impress me at all vs tube amps built in Japan/USA which I auditioned before.

I quote basing facts being an electrical engineering guy.

Listening is believing

Jack L

johnnythunder1's picture

Hungarian music (Bartok string quartets) on my French, Canadian, German, Italian and British system.

Jack L's picture

Hi

I love Chinese ethnic cuisines bigtime, not those cheapie 'alien' chinese food in foodcourt takeawys or in foodcups !! Like tubes, money talks !!!

Yes, I pretty like Brahms' music, particularly his Violin Concerto in D Major. Soooo romantic !!

His Hungarian Dance No. 5 is also my favourite, not the other Dances though.

Listenig is believing

Jack L

JaredBeebe's picture

I noticed the Crazy Eddie reference or Easter Egg, does this reference "The Mote in God's Eye" by Larry Niven and Jerry Pournelle?

X