Sutherland Engineering Little Loco phono preamplifier Measurements

Sidebar 3: Measurements

Ron Sutherland refers to his Little Loco phono preamplifier as a "transimpedance" amplifier, which means that it amplifies a current rather than a voltage—ie, the input signal is a current and the output signal is a voltage. Phono preamplifiers almost all use voltage-amplification circuitry. However, as a moving-coil cartridge inherently generates an output current, sourced from a very low impedance, it makes sense for a moving-coil phono preamplifier input stage to operate in current mode rather than voltage mode. I measured the Sutherland Little Loco using my Audio Precision SYS2722 system (see the January 2008 "As We See It"). However, as the Loco's input is not referenced to ground and its input impedance is effectively a short circuit, I had to improvise some changes to my usual procedure. If I connected the AP's balanced output to the Little Loco's input, the resultant current would be very high and might damage the preamp's input circuitry.

As I had when I measured the B.M.C. MCCI phono preamplifier for our June 2013 issue, I soldered a 10k ohm resistor in series with pins 2 and 3 of a male XLR plug. (The two 10k resistors were matched to within 2 ohms.) I then soldered a 10 ohm resistor between pins 2 and 3. The Audio Precision's balanced output stage would thus see a high 20k ohm load, but the signal presented to the Sutherland preamp would be typical of an MC cartridge. I then used a second cable terminated with a female XLR plug at the source end and an RCA plug at the other, with the XLR's pin 2 signal connected to the RCA's center pin and its pin 3 signal connected to the shell of the RCA. This RCA plug was connected in turn to each of the Little Loco's input jacks. Pin 1 of the XLR was connected to the preamplifier's chassis ground terminal. This setup attenuated the Audio Precision's signal level by 67dB, a 1kHz signal with a level of 1V resulting in 437µV at the preamplifier's input.

The module for each channel of the Sutherland preamplifier has plug-in resistors to set the overall gain. It ships with 3k ohm resistors installed for medium gain; replacing the 3k ohm resistors with 1k ohm equivalents reduces the output voltage by 6dB, or removing those resistors increases the output voltage by 6dB. These resistors are tightly matched: I measured 2997 and 2992 ohms for the left channel and 3001 and 3004 ohms for the right channel. So what was the Loco's gain with these 3k ohm resistors?

Ron Sutherland has a strong opinion regarding the specified gain of a transimpedance amplifier. In an email he told me "I think that measuring and spec'ing the Loco 'as if' it is a voltage-gain amplifier (albeit of very low input impedance) is useless. Worse, it invites a confidence in a number for comparison to voltage-gain amplifiers that is misleading. Fundamentally, voltage-gain and transimpedance amplifiers are different and have different units of gain. There is a matter of adhering to technical correctness. I understand that the reader wants (demands) a voltage gain in dB. That is like a car reviewer being forced into spec'ing a Tesla's miles-per-gallon.

"The real question; is there the right gain for the cartridge of interest? The answer is No for moving-magnet and moving-iron cartridges or the use of a step-up transformer. The answer is Very Likely Yes, for most moving-coil cartridges."

So with Ron's opinion in mind, I found that a balanced input of 500µV at 1kHz resulted in an output voltage of 415.4mV—ie, a gain of 58.4 mpg . . . er, 58.4dB, which is indeed appropriate for a moving-coil cartridge. The Sutherland preamp preserved absolute polarity and its input impedance was indistinguishable from a short circuit. The output impedance was a low 200 ohms from 20Hz to 20kHz. The Little Loco offers both very accurate RIAA equalization and very close channel matching (fig.1).

1019SuthLocofig1

Fig.1 Sutherland Little LOCO, response with RIAA correction into 100k ohms (left channel blue, right red) (0.5dB/vertical div.).

The Little Loco's unweighted, wideband S/N ratio, measured with the input shunted with a 10 ohm resistor as recommended by Ron Sutherland in his email, was 47.8dB, ref. 1kHz at 500µV. Restricting the measurement bandwidth to 22Hz–22kHz increased the ratio to a respectable 64.8dB, while switching an A-weighting filter into circuit increased it further, to 72.85dB. Spectral analysis of the Sutherland's low-frequency noise floor with the input shunted by a 10 ohm resistor (fig.2) indicated that random noise components were low in level, though some supply-related spuriae are visible in this graph.

1019SuthLocofig2

Fig.2 Sutherland Little LOCO, spectrum, DC–1kHz, of output ref. 500µV input with input shunted with a 10 ohm resistor (linear frequency scale).

These spuriae can also be seen in fig.3, which shows the spectrum of the Little Loco's output with the input fed 1kHz at 5mV. I suspect that these supply-related artefacts are due to the high-impedance balanced adaptor cable I constructed picking up interference from the Sutherland's power supply, even though the preamp has a steel top plate and chassis. (The levels varied when I moved the adaptor cable around; this graph was taken with the cable positioned to give the lowest levels.) The input signal level for this graph was 20dB higher than the standard MC level of 500µV because at lower input levels, the distortion harmonics were obscured by noise. Even so, while the second harmonic can now be seen, it lies at just –94dB (0.002%). Reducing the load impedance to the punishing 600 ohms increased the level of the second harmonic to –80dB (0.01%), and the third harmonic was now the highest in level, at –74dB (0.02%). But these are still too low in level to be a problem.

1019SuthLocofig3

Fig.3 Sutherland Little LOCO, spectrum of 1kHz sinewave, DC–10kHz, into 100k ohms for 5mV input (linear frequency scale).

Fig.4 shows how the Sutherland Little Loco handled an equal mix of 19 and 20kHz tones, at an input level of 600mV. This is 21.5dB higher than the standard MC level at these frequencies, but the difference product at 1kHz lies at just –80dB (0.01%), and there are no higher-order intermodulation products visible. This is a very linear circuit.

1019SuthLocofig4

Fig.4 Sutherland Little LOCO, HF intermodulation spectrum, DC–30kHz, 19+20kHz into 100k ohms for 600mV peak input (linear frequency scale).

I must admit that measuring a preamplifier with a current-mode input stage when my test system is optimal for conventional voltage-mode input stages makes me break out in a sweat. But Sutherland's Little Loco appears to be a well-engineered device, its measured performance confirming that it is perfectly appropriate for use with low-output moving-coil phono cartridges.—John Atkinson

COMPANY INFO
Sutherland Engineering
455 East 79th Terrace
Kansas City, MO 64131
(816) 718-7898
ARTICLE CONTENTS

COMMENTS
Anton's picture

If it weren't for streaming, I would have never engaged with a computer to interface with my Hi Fi. Download, Winn this AFLAC that...I was never gon' do it.

Now, this carn sarned complicated stuff hits me right in my 12 inch!

So, to use this preamp, I need a random cartridge, chosen in the hope that maybe it will work with my system, perhaps, and also get a different wiring set up for my tonearm...as you say: "The shield and ground of the phono cable must be isolated from the plus and minus conductors, which eliminates certain cables and tonearms from consideration. There's nothing a user can adjust to optimize the circuit for a certain cartridge—no adjustment of load resistance, in other words. You're almost entirely limited to moving-coil cartridges, and apparently, even among moving-coils, some combinations don't work at all. Got a cartridge you want to use? Plug it in, hold your breath, and see what happens."

That is so inconvenient, I don't know how us vinyl aficionados can resist!

Whole new layers of inconvenience, expense, and complexity.

Sign me up!

I would buy this if it were plug and play...can we beg a primer on how the Hell to get a system ready to install this crazy thing?

Ortofan's picture

... caliber should settle for nothing less than the CH Precision P1/X1 combo. It has both voltage and current mode type inputs, so that you can switch back and forth between them in search of the ultimate sound quality until you drive yourself silly.

https://www.ch-precision.com/en/products/source/p1
https://www.ch-precision.com/en/products/extra/x1
http://ch-precision.com/images/reviews/1704_Stereophile_P1.pdf

Bogolu Haranath's picture

The Rolexs of phono pre-amps .......... CH Precision and Soulution (see TAS review, Soulution 755) ......... Swiss Precision :-) ...........

Bogolu Haranath's picture

"Drive yourself silly" and get into a nervous breakdown :-) .........

Bogolu Haranath's picture

Parasound Halo JC3+ phono pre-amp ....... $2,995, Stereophile Class-A :-) ...........

Anton's picture

Think you'd be able to tell them apart by listening?

Saying something like "Nagra BPS: $2459 phono preamp, Stereophile Class A :-)" is meaningless.

Bogolu Haranath's picture

I don't know ...... You can read the reviews and decide ......... JC3+ provides several user adjustments in the back panel ....... It would be nice if, Stereophile did a comparison review :-) .........

Bogolu Haranath's picture

HR said " ... best commercially available phono pre-amp, I have used - Period" about JC3+ :-) ........

Anton's picture

Did you buy it?

Bogolu Haranath's picture

I no longer listen to any analog playback gear (sad!) ........ I was trying to make some helpful suggestions to fellow comrades (audiophiles) :-) ........

JRT's picture

The transimpedance amplifier provides extremely low load impedance at its input.

Phono cartridges have an optimal load impedance for flat response to high frequency. Jim Hagerman provides a good explanation at the following link.

http://www.hagtech.com/loading.html

Stereophile has provided an industry profile of Jim Hagerman at the following link.

https://www.stereophile.com/content/industry-profile-jim-hagerman-hagerman-audio-labs

JRT's picture

https://www.electronicdesign.com/analog/whats-all-transimpedance-amplifier-stuff-anyhow-part-1

JRT's picture

https://www.passdiy.com/project/other/zen-i-v-converter

Mökö Koo's picture

http://www.hagtech.com/loading.html

Sadly mr. Hagerman makes an error. Formula for Ropt should be:

Ropt = √(L/C)/2.

Without the divider 2 we get peaking frequency response with ringing in the time domain.

Graham Luke's picture

...The cleanest industrial design chic yet!

JRT's picture

As an alternative, one might try using a Bob's Devices Cinemag Sky to step up the voltage and provide impedance matching between a moving coil phonograph cartridge and a moving magnet phonograph preamplifier.

https://www.bobsdevices.com/product-page/sky-cinemag

Similar:
https://www.stereophile.com/content/bobs-devices-cinemag-1131-phono-transformer

JRT's picture

RIAA filter ... phono equalization

http://www.hagtech.com/equalization.html

Reference inverse RIAA filter

http://www.hagtech.com/pdf/riaa.pdf

Ortofan's picture

... buy a well-designed phono preamp that works with both fixed and moving coil type cartridges.

https://www.lehmannaudio.com/phono-stages/black-cube-se-ii.html

https://www.lehmannaudio.com/assets/products/black_cube_se_II/reviews/EN_review_Hifi_News_02_2010_SEII_screen.pdf

https://www.analogplanet.com/content/lehmann-audio-black-cube-se-ii-mmmc-phono-preamp-reviewed

Bogolu Haranath's picture

Lehmann Audio Decade phono pre-amp ($2,099) was better rated Class-A by Stereophile :-) .........

Ortofan's picture

... only a 3% increase (according to Hi-Fi News) in sound quality, then by all means be my guest.

tonykaz's picture

Everyone can have an Opinion.

The above Summary Opinion needs qualifiers:

Best $3,800 Analog Money Spent for folks that can spend $50,000 a year on audio hobby purchases. ( about $4,000 per month considering Sales Tax )

Circuits like this demand an appropriate Transparent Audio Cable costing near $5,000 for the entry level priced one.

Well, dammit, this Circuit makes ALL phono Cartridges sound wonderful, even cheap ones statement reveals the reviewer's system deficiency, despite the extravagant costs of the gear.

Superb Phono Cartridge transducers have their own unique voice which is beautifully revealed thru well chosen Pre-Amps, is this review suggesting that Today's Pre-Amps with Phono are no dam good?

I'll suggest that it's the Vinyl being played is not so good. ( along with the Pre-Amps being offered )

Superb Vinyl Analog is super Pricy ( and a pain in the Schiit'r ).

Tony in Venice

Anton's picture

Didn't you used to be the world's biggest seller of Monster Cable?

No way I am gonna believe you, dude, You sold your credibility!

;-D

He didn't say it made all transducers sound the same, he said it appeared to allow them to sound better than they did with the other type of phono preamp.

If you found an amplifier that lowered the noise floor of a system, would you say that it made all preamps and sources sound more engaging, or would you think it made all other components sound the same?

I am wondering if you read the review: did you interpret what he said to mean that it made all cartridges sound the same? try it again and read to the end.

tonykaz's picture

Monster's early days. I bought in Deep

You make good points about these things. He does suggest compelling results.

I checked the Sutherland Company's engineering report to find them a detail oriented Vinyl Playback optimiser .

I too worked with optimizing Vinyl playback and had a range of Phono Step-up transformers and amplification devices and circuits.

So, these guys are making MC circuits as did Electrocompaniet ( the best of my era ) , Audio Research and Conrad Johnson, Ortofon with their thousand dollar transformer for their top of the line MC transducer. All the above plus many more Manufacturers going deep into Low output MC step up circuits.

Vinyl now is populated by an obsessive cult ( which I was once a member ).

I suppose that I over-reacted with the reviewer's claim of best $,$$$ spent. ( an extravegent exaggeration , in my opinion )

I'm suspicious of the worshipful Wilson people, wondering how their claims could be relevant for everyman Stereophiles.

I could regret even reading that Sutherland Review, considering my outspoken position against all things Vinyl. The best money spent claim triggered my angst.

Sorry, Mea Copa

Tony in Venice

Anton's picture

This is all good natured ribbing, of course.

You know the difference between a cult and a religion?

The number of members.

As of this year, vinyl is a bigger 'cult' than CD.

Vinyl is a fun part of the audio hobby, why denigrate it? People enjoy vinyl at all price levels.

Do you troll auto enthusiast forums and admonish people that driving really fast costs millions of dollars so they and their street legal car cult should shut up and shut down their obsession?

Do you whine at wine forums that the finest wines cost tens of thousands of dollars per bottle, so they should quit with the cultish talk about finding great 50 dollar bottles?

Well, maybe you do, but you shouldn't!

;-D

Sit back, enjoy some nice copa and cheese, pop open a fine bottle of wine in your favored price range, and get off the vinyl whiner train.

Or, better yet, do us a big favor....go mewl on the digital review posts about the cult of digital and those 15,000 dollar CD players and 80,000 dollar DAC stacks.

Go tilt at the 50,000 dollar amps, while you are at it! Take down the entire hobby, man!

We all have our points of price ridiculosity. There are times I will agree with you, too! Sometimes, I read show reports that list prices and just shake my head. For fun, watch the show videos and pay extra attention when a manufacturer is asked the price of his piece of kit...there is always a pregnant and embarrassed pause, as if they are wondering if the interviewer will laugh, be outraged, or ask who he's kidding. If these guys were playing poker, we'd own them. They know it's silly, too.

Cheers, man.

Bogolu Haranath's picture

"Music and passion were always in fashion at the Copa .... Copacabana" :-) ..........

misterc59's picture

I may have missed something in these comment sections over time, but I have never seen anyone troll digital on this site. Good luck getting your point across to you know who. I, probably like yourself, listen to most if not all musical mediums (there's a term for what I just said, but don't know what!) Anyway, happy listening, even if it's through a paper 6x9 speaker in a '57 Chevy!

Cheers,
Terry

rschryer's picture

Yep, it's: "polyphonical".

Feel free to use the term liberally. :-)

Mökö Koo's picture

Using the current-mode in phono amp means that all the cartridge current is sinked by the short circuit in amp's input leaving no current to run in the cable capacitance. As a result the filter that acts on the cartridge signal is now of the LR type instead of the usual RCL type. So, what mostly limits the choise of cartridges is the corner frequency of this filter which must be large enough. It is calculated by the formula: f(-3dB) = R/L/(2π), where R,L = resistance and inductance of the cartridge coil. E.g. for Ortofon Quintet Black whose R and L are 5Ω and 6.2µH we get: f(-3dB) = 128kHz. By contrast, for Ortofon 2M Black which is MM cartridge and whose R and L are 1.2kΩ and 630mH we get: f(-3dB) = 303Hz!

X