J E Sugden Masterclass LA-4 line preamplifier Measurements

Sidebar 3: Measurements

I measured the Sugden Masterclass LA-4's performance with my Audio Precision SYS2722 system (see the January 2008 "As We See It"). The maximum gain for the balanced inputs to the balanced outputs was 20.75dB, and a little higher from the unbalanced input to the unbalanced output: 21.4dB. The preamplifier preserved absolute polarity (ie, was non-inverting) with both balanced and unbalanced inputs and outputs. (Its XLR jacks are wired with pin 2 hot, the AES convention.)

The LA-4's unbalanced input impedance was lower than specified, at 12.5k ohms at 20Hz and 20kHz, dropping slightly and inconsequentially to 11.6k ohms at 20kHz. The balanced input impedance was 37.5k ohms from 20Hz to 20kHz. The unbalanced output impedance was extremely low, at about 0.6 ohm from 20Hz to 20kHz. (As this is much lower than the specified value, I checked this using two different methods, with the same result.) The Sugden's balanced impedance was higher, at 119 ohms at 20Hz, and 94 ohms at 1kHz and 20kHz.

The Masterclass LA-4's balanced frequency response into high impedances was flat from 100Hz to 200kHz, though the low frequencies rolled off slightly, reaching –0.75dB at 20Hz (fig.1, blue and red traces). The increase in output impedance at very low frequencies mean that this rolloff reached –1dB at 20Hz into the punishing 600 ohm load (fig.1, cyan and magenta traces). Fig.1 was taken with the LA-4's volume control at its maximum setting. At lower settings, the bass was still down by 0.75dB at 20Hz, but the ultrasonic output rolled off slightly, reaching –0.5dB around 150kHz. In unbalanced mode, with the volume control set to 12 o'clock, the output was again down by 0.5dB at 150kHz, but the low-frequency response was now flat down to 20Hz (fig.2). I looked at the circuit board, but could see no obvious reason for the differences in output impedance and frequency response. Both the balanced and unbalanced outputs appear to be based on high-performance Analog Devices OP275 op-amp chips, followed by complementary pairs of discrete devices.


Fig.1 Sugden Masterclass LA-4, balanced frequency response with volume control set to maximum at 1V into: 100k ohms (left channel blue, right red), 600 ohms (left cyan, right magenta) (0.5dB/vertical div.).


Fig.2 Sugden Masterclass LA-4, unbalanced frequency response with volume control set to 12 o'clock at 1V into: 100k ohms (left channel blue, right red), 600 ohms (left cyan, right magenta) (0.5dB/vertical div.).

I was also puzzled by the measurements of channel separation. Measured with the volume control at its maximum setting, the separation was good below 1kHz, at >80dB in both directions, decreasing to 60dB at 20kHz for both balanced and unbalanced operation (fig.1, blue and red traces). However, when I repeated the test at the same output level with the volume control set to 12 o'clock, the R/L crosstalk rose to –53dB at 20kHz (fig.3, gray traces), with the L/R crosstalk almost 20dB higher (fig.3, green trace). This is very unusual.


Fig.3 Sugden Masterclass LA-4, balanced channel separation at 5V into 100k ohms with volume control set to: maximum (R–L blue, L–R red), unity gain (R/L green, L/R gray) (0.5dB/vertical div.).

The Masterclass LA-4's wideband, unweighted signal/noise ratio, measured with the input shorted to ground but the volume control set to its maximum, was disappointing, at 56dB ref. 1V output (average of both channels, which were very similar). Restricting the measurement bandwidth to the audioband increased the S/N to an excellent 81dB, while switching an A-weighting filter into circuit further improved this ratio, to 92dB. Spectral analysis of the Sugden's low-frequency noise floor in balanced mode revealed a spurious tone at 60Hz in both channels. The power-supply transformer has a steel shield around it, but the 60Hz leakage will still be due to magnetic interference from the transformer. However, it was very low in level at –97dB (0.0014%). (It was 10dB higher in level in unbalanced mode.) A 120Hz component can be seen in the left channel (fig.4, blue trace), though at –110dB (0.0003%) this will be inconsequential and inaudible. As the power-supply–related spuriae in the LA-4's output are very low in level, all I can surmise about the disappointing wideband S/N ratio is that, with its extremely extended ultrasonic response, the LA-4 is susceptible to RF interference.


Fig.4 Sugden Masterclass LA-4, balanced spectrum of 1kHz sinewave, DC–1kHz, at: 2V (left channel blue, right red), 0V into 100k ohms (left cyan, right magenta) (linear frequency scale).

Fig.5 plots the percentage of THD+noise in the Sugden's balanced output into 100k ohms. The THD+N rises below 2V output due to the fixed level of noise becoming an increasing percentage of the signal level, and the LA-4 clips (ie, when the THD+N reaches 1%) at 8V (fig.4), which is well above the maximum level it will be asked to deliver with typical power amplifiers. The balanced output clipped at a still-high 6.5V into 600 ohms, and for unbalanced input to unbalanced output, the LA-4 clipped at 9V into 100k ohms. I measured how the Masterclass LA-4's distortion changed with frequency at a very high level, to be sure that the reading was not dominated by noise. It remained consistently low throughout the audioband into both 100k ohm and 600 ohm loads, with no rise in the top octave (fig.7), though the THD was a little higher from the balanced outputs than the unbalanced, as suggested by figs. 5 and 6.


Fig.5 Sugden Masterclass LA-4, balanced distortion (%) vs 1kHz output voltage into 100k ohms.


Fig.6 Sugden Masterclass LA-4, unbalanced distortion (%) vs 1kHz output voltage into 100k ohms.

The spectrum of the distortion in both balanced and unbalanced modes comprised almost pure second harmonic (fig.8), but this was very low in level. Intermodulation distortion was also very low, the second-order difference product at 1kHz with an equal mix of 19 and 20kHz tones lying at just –97dB (0.0014%) (not shown).


Fig.7 Sugden Masterclass LA-4, balanced THD+N (%) at 5V into 100k ohms (left channel blue, right red) and 600 ohms (left cyan, right magenta).


Fig.8 Sugden Masterclass LA-4, balanced spectrum of 1kHz sinewave, DC–1kHz, at 2V into 100k ohms (left channel blue, right red; linear frequency scale).


Fig.9 Sugden Masterclass LA-4, balanced HF intermodulation spectrum, DC–30kHz, 19+20kHz at 2V into 100k ohms (left channel blue, right red; linear frequency scale).

I was puzzled by the difference between its balanced and unbalanced operations, but otherwise, the Sugden Masterclass LA-4 measured well.—John Atkinson

J E Sugden & Co. Ltd.
US distributor: Tone Imports

tonykaz's picture

Sugden is one of those Brit Companys I never got round owning any of, dammit, I just plain ran out of Time before my Audio Business World stopped turning.

Still, I've been curious about the Sugden A21 and the entire Range.

If I was still trying to do HighEnd Audio I'd certainly like a Corner of my Shops featuring the Sugden Range. I'd even do all those Colors.

Sugden always seemed like a PS Audio kind of Company that never made it to the USA or anyplace I've ever been.

The Product Looks substantial. If I happened upon the Sugden people at one of the Shows, I would've bought some "on the Spot".

I wonder if it's at good as it looks, hmm. I hope to find out, one day.


Tony in Michigan

georgehifi's picture

Strange distortion measurements differences.

Balanced transformers? could it be saturation with higher level if it's got them??

Cheers George

monetschemist's picture

... or what? Or is it just me? In any case, wow, kudos to Sugden for making something I want to go and find and touch.

I love the GH quote from RD in the last paragraph. I've been listening to fine music, often intensely, on decent or better equipment, for more than 40 years now. I've never quite got to the point where I find myself wishing I had kept component XYZ because I loved the sound so much; but if I don't understand, I sympathize at least!

Ortofan's picture

... the best choice, according to Doug Self.

tonykaz's picture

Thick shielding for the Power Transformer.

Construction looks substantial.

Sugden needs an Up-Dated Photo Album of their Manufacturing .

Dam nice looking Gear.

Tony in Michigan

Ortofan's picture

... electrolytic capacitors from - Samwha.

Bogolu Haranath's picture

JA2 used Revel Ultima Salon2 for this review ....... One of the best loudspeaker designs ....... Has been in continuous production since 2008 ........ Those speakers would be a great reviewing tool for associated equipment :-) .........

tonykaz's picture

Yea, I think his are Vintage, not the Asian versions.

Bob Katz sold his Ultima Revels ( not to me, I tried to buy his but was too late ). Bob switched to DynAudio.

Seems that JA1 & JA2 are not! Better get it right, or else....

1) JA1 is JA,

2) JA2 is JCA

I hoping to know more about Sugden, this just might ( or might just ? ) be a rare "Everyman's" Brand of Class A Recommended Components for us "Poor Man's Audiophile" types. ( or "Stereophile types" if Canadian ).

Maybe our intrepid Robert Scryer meets Sugden's Canadian Rep. from BC and works out an A21 Review Sample to compliment the already positive appraisals from Stereophile's Management Staffers. Fingers Crossed !!

Tony in Michigan

ok's picture

effectively dealing with basic philosophical as well as practical issues of testing methodology. The “bypass mode” cannot literally be what its name might suggest – since there still is a "source" output stage with its own impedance/EQ characteristics not always compatible with the main amp's input and therefore a half-baked preamp in potential need of revision (what kind of exactly?) – but necessary nevertheless for the evaluation process. Any part of a given circuit can be omitted (as is actually the case with SET, OTL, zero-feedback, DC-coupled etc designes) as long as it allows for a functional output, more often than not in order to highlight certain kind of "detail" at the expense of another; it has even happened to me once to suddenly start “hearing things I had never heard before” as an unexpected result of some gross hardware malfunction..

jmsent's picture

It's hard to understand why Sugden would make a fully balanced preamp; i.e., 4 identical gain channels from input to output, when they only have one balanced input. What would be the point? A full balanced design is twice as complex as a single ended one but gives you literally nothing advantageous over a single ended design without being fed by a balanced signal. I'm also skeptical that the preamp even operates this way. From the posted photo, the volume control appears to be an ALPS dual gang motorized potentiometer. But for full balanced operation, you need 4 gangs, not two, and a control for that would be physically much larger. So unless they've created some novel approach to fully balanced operation, I'm inclined to believe that there's balanced to signal ended conversion going on at the balanced input, and single ended to balanced conversion going on at the outputs. BTW, many preamps with balanced inputs and outputs operate in this fashion.

scott.w's picture

This review for me was a primer on a variety of topics. Read it 3x. My fave stereophile of all time; going back many moons.