Line Magnetic Audio LM-518IA integrated amplifier Measurements

Sidebar 4: Measurements

I measured the Line Magnetic LM-518IA using my Audio Precision SYS2722 system (see www.ap.com, and the January 2008 "As We See It"). Before starting the testing, I followed the instructions in the manual and left the amplifier turned on for 15 minutes; then, without any signal, I adjusted the bias of each 845 output tube to the correct value. (Very little adjustment was necessary.) Next, I short-circuited the inputs and, with the volume control at its maximum, adjusted the hum-balancer potentiometers to give the lowest measured noise for each channel. I performed a full set of measurements from each output-transformer tap.

The LM-518IA preserved absolute polarity from each output tap (ie, it was non-inverting), and its input impedance was very high, ranging from 59k ohms at 2kHz to 75k ohms at 20Hz. The maximum voltage gain, measured into 8 ohms, varied with the output tap, but was appropriate for an integrated amplifier: 40.2dB (16 ohm tap), 38.6dB (8 ohm tap), and 36.5dB (4 ohm tap). The output impedance varied slightly with frequency and load impedance, but was approximately one-quarter the nominal value of each output-transformer tap: 3.8 ohms (16 ohm tap), 2 ohms (8 ohm tap), and 1.1 ohms (4 ohm tap).

These impedances are relatively low for a single-ended-triode amplifier. As a result, the modification of the LM-518IA's frequency response due to the Ohm's law interaction between this impedance and that of our standard simulated loudspeaker remained within limits of ±0.8dB (4 ohm tap), ±1.2dB (8 ohm tap; fig.1, gray trace), and ±2.1dB (16 ohm tap). The traces in fig.1 were taken with the volume control set to its maximum—note the excellent channel matching, which was preserved at lower settings of the control. The frequency response into resistive loads is flat from 20Hz to 10kHz, with only a mild rolloff below 20Hz. The LM-518IA has an excellent output transformer. And though the secondary of this air-gapped transformer has a well-defined resonance at 60kHz, the associated response peak is low in level, particularly in the left channel, and that level doesn't rise with increases of load impedance.

Fig.1 Line Magnetic LM-518IA (8 ohm tap), volume control set to maximum, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green) (2dB/vertical div.).

Though a degree of overshoot can be seen in the LM-518IA's reproduction of a 1kHz squarewave (fig.2), the 10kHz squarewave (fig.3) reveals just one cycle of damped ringing. Note also the flat tops and bottoms of the waveform in fig.2, which correlate with the well-extended low frequencies seen in fig.1.

Fig.2 Line Magnetic LM-518IA (4 ohm tap), small-signal, 1kHz squarewave into 8 ohms.

Fig.3 Line Magnetic LM-518IA (4 ohm tap), small-signal, 10kHz squarewave into 8 ohms.

Even after the hum-balancer adjustment, the Line Magnetic's noise floor was higher than is typical for a tubed design. Measured with the inputs shorted but the volume control set to its maximum—the worst-case condition—the unweighted, wideband signal/noise ratios, ref. 2.83V into 8 ohms, were: 57.4dB (16 ohm tap), 59.7dB (8 ohm tap), and 61.5dB (4 ohm tap). Switching an A-weighting filter into circuit improved these ratios to, respectively, 65.6, 67.4, and 68.8dB, which will be barely adequate with high-sensitivity loudspeakers. Fig.4 indicates that while there are some power-supply–related spuriae, the highest in level being at 120Hz, this noise is predominantly random. The blue and red traces in fig.4 were taken with the LM-518IA driving 1W into 8 ohms with its volume control at the maximum setting; repeating the spectral analysis with the volume control at its minimum setting (fig.4, cyan and magenta traces) didn't lower the noise floor. This suggests that the noise occurs after the volume control, which means that, unlike other amplifiers, reducing the volume won't improve the S/N ratio. The channel separation at 2kHz was 80dB, L–R, but 60dB, R–L, these decreasing at 20kHz to, respectively, 58 and 40dB.

Fig.4 Line Magnetic LM-518IA (8 ohm tap), spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms with volume control at: maximum (left channel blue, right red), minimum (left cyan, right red; linear frequency scale).

Figs. 5 and 6, both taken from the 4 ohm tap, respectively show how the THD+noise percentage changes with output power into 4 and 8 ohms. The LM-518IA is specified as offering a maximum power of 22Wpc (13.4dBW into 8 ohms, 10.4dBW into 4 ohms), and these graphs indicate that this power is achieved at just over 3% THD+N when the load impedance is the same as the nominal output-transformer tap. These graphs also indicate that, as usual with a SET design with minimal or no loop negative feedback, the THD+N increases linearly with output power above a couple of hundred milliwatts. Unusually, the LM-518IA is less linear when the load is higher than the nominal tap value (fig.6), and more linear when the load is lower than the nominal value (fig.7). The rather ragged appearance of the trace below 1W in fig.7 is due to the presence of very low-frequency "flicker noise."

Fig.5 Line Magnetic LM-518IA (4 ohm tap), distortion (%) vs 1kHz continuous output power into 4 ohms.

Fig.6 Line Magnetic LM-518IA (4 ohm tap), distortion (%) vs 1kHz continuous output power into 8 ohms.

Fig.7 Line Magnetic LM-518IA (16 ohm tap), distortion (%) vs 1kHz continuous output power into 8 ohms.

This can also be seen in fig.8, which plots the THD+N percentage from the 8 ohm tap at 2.83V into 8 and 4 ohms. The distortion was lower into 4 ohms (cyan and magenta traces) than into 8 ohms (blue, red). But, as expected from the LM-518IA's SET topology, the distortion is heavily second-harmonic (fig.9)—even at low frequencies (fig.10), when I would have expected some third harmonic to appear due to the onset of core saturation. Some third harmonic does appear when the load impedance is lower than the tap value, though it's still lower in level than the second harmonic (fig.11).

Fig.8 Line Magnetic LM-518IA (8 ohm tap), THD+N (%) vs frequency at 2.83V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta).

Fig.9 Line Magnetic LM-518IA, 1kHz waveform at 1W into 8 ohms, 0.58% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.10 Line Magnetic LM-518IA (8 ohm tap), spectrum of 50Hz sinewave, DC–1kHz, at 1W into 8 ohms (left channel blue, right red; linear frequency scale).

Fig.11 Line Magnetic LM-518IA (8 ohm tap), spectrum of 50Hz sinewave, DC–1kHz, at 2W into 4 ohms (left channel blue, right red; linear frequency scale).

The LM-518IA's decreasing linearity at high frequencies (fig.8) will not be a problem when the load is the same as or less than the nominal value of the transformer tap. The second-order difference products at 1kHz (fig.12) lie at –60dB left and –70dB right (0.1% and 0.03%, respectively). But at higher powers, with a load impedance higher than the tap value, that product reaches 1% (fig.13).

Fig.12 Line Magnetic LM-518IA (8 ohm tap), HF intermodulation spectrum, DC–24kHz, 19+20kHz at 1W peak into 8 ohms (left channel blue, right red; linear frequency scale).

Fig.13 Line Magnetic LM-518IA (4 ohm tap), HF intermodulation spectrum, DC–24kHz, 19+20kHz at 5W peak into 8 ohms (left channel blue, right red; linear frequency scale).

I must admit, I was biased against the Line Magnetic amplifier before I began measuring it—its model number is similar to that of one of my least-favorite audio op-amp chips from my DIY days, National Semiconductors' LM318! But as I got deeper into the testing, the more impressed I became. Yes, as it's a single-ended-triode design, its bent transfer function leads to a nonlinear signature dominated by second-harmonic distortion. And it's noisier than I like to see. But for such a design, it offers high power. The only unusual aspect of its behavior is that, whereas I usually recommend that a tube amplifier be used with the lowest-value output-transformer tap that gives adequate loudness, the opposite is the case with the LM-518IA. Using its 16 ohm tap with 8 ohm speakers will give the lowest distortion, though the subjectively benign second harmonic will then be joined by some third harmonic.—John Atkinson

COMPANY INFO
Line Magnetic Audio Co. Ltd.
US distributor: Tone Imports
ARTICLE CONTENTS

COMMENTS
RobertSlavin's picture

This amplifier may sound very good but, personally, I wouldn't want glass tubes whose contents were 3140 degrees fahrenheit in my living room. It sounds too dangerous to me, setting aside how much heat they would generate.

Even if the amplifier had tube cages (it would have been nice to have seen them in this review), I suspect the cages themselves may get so hot to be dangerous to the touch.

Robert

John Atkinson's picture
RobertSlavin wrote:
This amplifier may sound very good but, personally, I wouldn't want glass tubes whose contents were 3140 degrees fahrenheit in my living room.

When I tested the amplifier, it was sitting on my test bench just 2 feet away from me. I got a tan!

John Atkinson
Editor, Stereophile

SET Man's picture

Hey,

Beside making a nice space heater in winter and turn your room in to sauna in summer! A pair of 845 tubes are also make a great night light! Very pretty brightly warm glow.

On a serious note, I like 845 tubes. A well designed and executed one can sound amazing with power to drive more than horn speakers.

Metalhead's picture

I heard this at pitchperfect in LA with Devore speakers. Unlike Herb I have a small speaker=small sound bias. I fell in love with big horns and stats decades ago. Having said that it was a fantastic combo and I was certainly impressed with the demo. Enough so that I am going to to try to get chair time in front of it again and perhaps spring for it.

Heat is welcome here in Alaska so no worries on that front.

Wonderful review that captures what this integrated can do.

lo fi's picture

What a beautiful amp.

iosiP's picture

I currently read most ampilfier reviews in Stereophile (well, less the digital stuff) and most of the time I give more attention to JA's measurements. Now I've read many conclusions like (quoting from memory) "This amplifier behaves quite well for a low-powered SET design", "Yes, as it's a single-ended-triode design, its bent transfer function leads to a nonlinear signature dominated by second-harmonic distortion. And it's noisier than I like to see. But for such a design, it offers high power." and other similar qualifications.
Now do I have to remind you the underpowered Audio Note Jinro or the similarly grossly underpower Wavac SH-833?

So here comes my question: why would I opt for a tubed amplifier that sounds good "for it's class", i.e. with qualifications (or even does not deliver a shade of what it promises), generates heat, is a danger for kids and pets and uses... well, consumable output devices when the same amount of money can get me a decent - even class A - SS amp that would reproduce faitfully the signal that is fed in?

Milesian's picture

Having run the LM518-IA for close to a year in a house with three cats a dog and occasional grandchildren, I can attest that no creatures were harmed during the playing of my music. Nor was my listening room overbearingly hot during the sweltering Ontario Summer. Come on folks get serious, you do realize you can turn it off when you're not listening right? I'm more afraid of the electric stove and light bulbs in my house.

Marc210's picture

In the past, I've listened extensively to two tube amps, ARC Classic 30 and Yves Cochet ALP2, and many others briefly.
And it's no surprise I prefer ss even as pre and of course digital to analog.

james's picture

but the filaments get to 3140°F, not the glass itself. I could be wrong. I have one of these and it puts out some heat, but I don't feel it until I'm flipping a record.

I agree with Herb's review of this amp. It is fantastic

doak's picture
jmsent's picture

depends on the current in the filament. In this case about 3.25 amps. That's the same as a pair of KT 88's so the overall heat output of a push pull amp using those tubes would be about the same. The worst burn I've ever gotten from a tube was when I inadvertently grabbed onto a 6CW4 nuvistor in a tv tuner. Those tubes may be tiny, but they get hot as blazes because there's no surface area for heat dissipation.. At least the 845 has a huge glass envelope and has a filament with a large surface area.

james's picture

Hey, Line Magnetic...please make a phono stage to match the 518 and the gold series DAC!

MusicT's picture

Been running one of these for 18 months. I've not found the heat to be any issue at all - wondering if the reviewers are making good humor at the expense of accuracy. (i.e. I'm got a tan. ??) It does get warm but even in a small room, I've not found it bothersome.

Wonderful amp indeed. I picked this over PrimaLuna which I see you have rated as Class A in your annual roundup.

Patrick Raffin's picture

would you recommand an adress un Europe, in China, in America ?

Miyabi's picture

Hello! You can easily find Line Magnetic line up in Moscow, Russia.
Here the store, you can buy it: www.mvert.ru

piebia's picture

Dear All
I know that the new 508 has been released. did you have listened it?
How abaout the comparison with the 518 one matching with devore gibbon 3xl.

Thank you
Peter

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