Rogue Audio RP-7 line preamplifier Measurements

Sidebar 3: Measurements

I measured the Rogue RP-7 using my Audio Precision SYS2722 system (see the January 2008 "As We See It"). The RP-7's volume control operates in 0.5dB steps, with the maximum level indicated as "192" on the front-panel display. All of its outputs—balanced, single-ended, and headphone—preserved absolute polarity (ie, were non-inverting). The balanced input impedance measured a very high 174k ohms at 20Hz and 1kHz, dropping very slightly to 162k ohms at 20kHz; the unbalanced input impedance was lower, at 24.5k ohms at 20Hz and 1kHz, and 21k ohms at 20kHz. The RP-7's maximum voltage gain into 100k ohms was 15.33dB for both balanced input to balanced output and unbalanced input to unbalanced output. Measured at the headphone output, the maximum gain was a very high 30.2dB into 100k ohms, which is probably why HR noted some noise issues with headphones.

The Rogue's output impedance is specified as 50 ohms. However, I found that this varied with both output and frequency. Measured at the headphone output, the impedance was 1.6 ohms at 20Hz, 0.8 ohm at 1kHz, and 1.2 ohms at 20kHz. The unbalanced output measured a very low 0.8 ohm at 20kHz and 3.5 ohms at 1kHz, but rose to 2.44k ohms at 20Hz, this presumably due to a DC-blocking capacitor in series with the output. The respective balanced line-output impedances were 1.4 ohms, 12.4 ohms, and 5.64k ohms. As a result of the increased output impedance at low frequencies, the RP-7's response drastically rolled off below 400Hz into 600 ohms, reaching –3dB at 200Hz (fig.1, cyan and magenta traces). The RP-7 should not be used with power amplifiers having an input impedance much below 30k ohms. Into 100k ohms, however, the RP-7 offers a flat, wide-bandwidth response with an upper –3dB frequency of 170kHz (fig.1, blue and red traces). Fig.1 was taken with balanced operation and the volume control set to "192." Commendably, the response didn't change with the setting of the volume control or with unbalanced operation. The frequency response at the headphone output was also flat and extended, though the –3dB point dropped to a still-high 120kHz.


Fig.1 Rogue RP-7, balanced output, volume control at max, frequency response at 1V into: 100k ohms (left channel blue, right red), 600 ohms (left cyan, right magenta) (1dB/vertical div.).

Channel separation was similar for balanced and unbalanced operation, and somewhat asymmetrical, at 80dB L–R at 2kHz and 97dB R–L, these respectively decreasing to 60 and 78dB at the top of the audioband. The unweighted, wideband signal/noise ratio, measured at the line outputs with the inputs shorted to ground but the volume control set to its maximum, was only moderate, at 68.3dB ref. 1V output. This improved to 76.1dB, when the measurement bandwidth was restricted to 22Hz–22kHz, and to 80dB with an A-weighting filter switched into circuit. Fig.2 shows spectral analyses of the Rogue's low-frequency noise floor while its balanced output drove a 1kHz tone at 1V into 100k ohms. The random noise-floor components lie at –100dB, with the primary noise contribution from power-supply–related spuriae.


Fig.2 Rogue RP-7, balanced output, volume control at max, spectrum of 1kHz sinewave, DC–1kHz, at 1V into 100k ohms (linear frequency scale) (10dB/vertical div.).

Rogue specifies the RP-7's maximum output voltage as 18V balanced and 9V unbalanced. Figs. 3 and 4 respectively show how the percentages of THD+noise in the balanced and unbalanced outputs vary with voltage. With clipping defined as when the THD+N reaches 1%, the RP-7 clips at 24V balanced and 12V unbalanced. These very high clipping voltages were maintained into the punishing 600 ohm load. The fact that the traces in these two graphs slope downward from left to right below 1–2V means that the distortion actually lies below the noise floor at these lower output levels. When I measured the percentage of THD+N against frequency at 1V into 100k ohms, it didn't change with frequency (not shown).


Fig.3 Rogue RP-7, balanced output, distortion (%) vs 1kHz output voltage into 100k ohms.


Fig.4 Rogue RP-7, unbalanced output, distortion (%) vs 1kHz output voltage into 100k ohms.

As with Rogue's RH-5 preamplifier and headphone amplifier, which Herb Reichert reviewed in "November 2017, the distortion is predominantly the subjectively benign second harmonic at a low –76dB (0.015%, fig.5). This graph was taken from the single-ended output; with the balanced output, the second harmonic lay at –80dB (0.01%). Fig.5 was taken with a low-frequency fundamental; when I repeated the test with a 1kHz signal, while the second harmonic was still the highest in level, there were now some higher-frequency enharmonic tones present (fig.6). Fig.6 was taken with the volume control set to "192." Reducing the setting to –20dB ("152") and increasing the input level by 20dB dropped the levels of these spurious tones by the same 20dB (fig.7), which suggests that the circuitry is picking up these tones ahead of the volume control. It turned out that this problem was fixed with v2.2 of the RP-7's firmware. Repeating the spectral analysis with the new firmware eliminated the spurious tones (fig.8).


Fig.5 Rogue RP-7, unbalanced output, volume control at max, spectrum of 50Hz sinewave, DC–1kHz, at 2V into 100k ohms (linear frequency scale).


Fig.6 Rogue RP-7, unbalanced output, volume control at max, spectrum of 1kHz sinewave, DC–1kHz, at 2V into 100k ohms (linear frequency scale).


Fig.7 Rogue RP-7, unbalanced output, v2.1 firmware, volume control at –20dB, spectrum of 1kHz sinewave, DC–1kHz, at 2V into 100k ohms (linear frequency scale).


Fig.8 Rogue RP-7, unbalanced output, v2.2 firmware, volume control at –20dB, spectrum of 1kHz sinewave, DC–1kHz, at 2V into 100k ohms (linear frequency scale).

Keeping the volume control at "152," I looked at the spectrum of the RP-7's single-ended output while it reproduced an equal mix of 19 and 2kHz tones at 1V into 100k ohms, again with the new firmware (fig.9). The high-order intermodulation products are very low in level and the second-order difference product at 1kHz lies at –70dB (0.03%). The picture was similar from the balanced output, though the difference product dropped to –66dB (0.05%).


Fig.9 Rogue RP-7, unbalanced output, v2.2 firmware, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 1V peak into 100k ohms (linear frequency scale).

As long as it's driving a power amplifier with a high input impedance, Rogue's RP-7 offers generally respectable measured performance. Although HR auditioned the RP-7 with the earlier v2.1 firmware, he used sufficiently low volume control settings that the spurious high-frequency tones would not have had audible consequences.—John Atkinson

Rogue Audio, Inc.
PO Box 1076
Brodheadsville, PA 18322
(570) 992-9901

tonykaz's picture

Azure is an expensive type of Blue, almost Lapis Lazuli. Isn't it.

For $ 5 Grand I'm wondering why they didn't use variable color LEDs like Chord would use? Maybe they're Old-School "Blue" is pricy and rare type designers.

...they sure did use "thru-hole" Circuit Boards which are far more repairable than those dam surface Mount boards which require High Magnification Surgery Tools ( and sober hands ) to repair.

"Alert & Water" as descriptives are a "First" in Audio Reviewing but "Transparent" is now Officially Worn-Out and can safely be discarded along with Carbon Fibre Record Cleaning Brushes.

Does this design present Musical Density along with it's Alertness?

Dam fine Five Star Appraisal here, bloating with exotic visuals that leave me in a Psychotic "tube rolling" wonderment.

Each HR writing is like a Sunset over the Gulf of Mexico: differing "colorings and flavorings", always beautiful and memorable .

Tony in Michigan

ps. the Audiophiliac is writing & explaining the New Rules Now.

ps. 2) Shabby Suits & used record stores in florida are Audio's version of Amish

tonykaz's picture

What is a Chinese Zippo worth?

Zippo gets it's high Value because it's Made in Pennsylvania, supported in Pa and has it's traditional value as part of American Culture. A Chinese Zippo isn't a Zippo!

A Chinese KEF isn't a Maidstone KEF made by Raymond Cooke.

A Chinese ROLEX is available at a small fraction of SWISS ROLEX Price.

Have a look at those Belgium Browning Guns and their Market Value levels compared to the Later Brownings made elsewhere.

The Market determines Prices

Chinese made stuff earns substantially lower Market acceptance.

Outfits that take their Proud Brand to Asia for Manufacture are deceiving their customer base who are not likely to remain loyal.

Asian Brands, like KIA, are building trust and Market Base thru Quality, Design and Support. KIA expanded their Manufacturing by building Cars in USA.

Asia sourcing is about TPP costing local jobs so that Corporations can avoid costs and capture significant short term profit: Quarterly.

People that support Asian Off-Sourcing do so because they've been bought ( purchased, owned ) by the Corporates, they serve Corporates not the customer.

Asian sourced stuff is worth less because it destroys it's purchaser.
It probably has negative value.

TPP is a Legacy Gift from Obama, dammit. ( My Obama )

Tony in Michigan

funambulistic's picture

Mr. Tony - who in the heck are you talking to? I would assume "Steve G." is referring to Mssr. Guttenberg, but he has nothing to do with this article, has not posted anything on Stereophile since, what June of last year and, if anything, has 2-3 posts per year (at the most). You are more likely to find him hanging around CNET or his YouTube channel, which, from what I have seen, he is very active on. I would hate for another one of your "Chinesium" tirades fail to reach your intended target because said target is not haunting the Stereophile comments section (especially comments on an article he did not pen). Good luck!

tonykaz's picture

Mr.HR's writings are read by all Reviewers, everywhere. Of course Steve G is here, reading and making contributions.

Steve G & Mr.HR are almost the same Philosophy, one is the leading man of letters and one the leading man of Visuals.

They together are Audio's Greatest Team. ( Steve & Tyll were, sadly, no more )

I'm a Stereophile participant, not a CNET. I support getting Stereophile into all Libraryies and all Barber Shops, Stereophile is the Center of the Audiophile World, Stereophile is Everyman's publication. I am a "Stereophile"!

I don't work in Publishing and am not Staff of Stereophile or connected in any way since being an AUDIO Mag. Advertiser in 1980s when I had a business relationship with L.Lovechio and Distributing Importer of HFN&RR in the 1980's.

Tony in Michigan

ps. I am a Patreon subscriber to Steve G.

grantray's picture

Maybe spend more time typing up rambling ranty stuff like this with liked-minded folks in the comments sections of Youtube or WSJ.

ok's picture

according to the "dominance/submission dialectic" the master gradually becomes more and more dependent on the servant’s mind and work, whereas the latter in turn becomes more and more potent and skillful till he finally realizes that he no longer has any use for his lazy old master – but Hegel never actually made it to our former brave new world..

tonykaz's picture

I'm feeling a rather thin understanding of your observation.

Are you suggesting "dialectic" is a debate? or investigation? or something else entirely.

English words can have too many meanings for safe usage in short technical paragraphs.

Tony in Michigan

ok's picture

since "dialectic" is actually an ancient greek term which in this particular hegelian context means "interaction" (between master and servant) or "swapping" (of economic and political power that is..)

mmole's picture

...but I always appreciate a Marxist analysis of a line-level preamp.

ok's picture

since most of the aforementioned are usually vinyl-nostalgics, in a manner of speaking :-}

The Don's picture

Why is gain important if the Sit-3 will start to clip at around 3 Volts and most preamps put out over that? Wouldn't the actual performance of the volume control be a wild card in this as most seem to work better at say 2 o'clock setting then 9 o'clock? Thanks.

grantray's picture

I saw him perform at the San Francisco Herbst Theater almost exactly two years ago. His performance of the Debussy Preludes as well as an encore of Scriabin was inspired, leaving all of us in the audience slack-jawed when the last note went silent and he stood for a final bow. It was amazing to watch his process as he paused before each piece and reflected on it before starting. He seemed to be conversing with himself on the complexity of the spirit and intent of the thing he was to create before striking the first note, in the way a visual artist pauses and prepares for the first mark on a surface. It was the kind of performance that demands a long walk afterwards. Next time he comes through New York on another tour, you should definitely go see him.

Ortofan's picture

... all that's required to produce a reference grade product for a Stereophile reviewer.

Bogolu Haranath's picture

Love (of tube sound) is blind (to measurements) :-) ...........

John Atkinson's picture
Ortofan wrote:
Apparently "generally respectable measured performance" is all that's required to produce a reference grade product for a Stereophile reviewer.

Stereophile reviewers don't see a product's measured performance until after they have written and submitted their review. So if they decide something is a reference product, this is based on their auditioning alone.

John Atkinson
Editor, Stereophile

Bogolu Haranath's picture

May be Stereophile could review the new Parasound JC-5 stereo-amp? :-) ..........

Ortofan's picture

... perhaps the reviewers should have the opportunity to make follow-on comments after they've seen the measurements. For example, would the reviewer still deem this unit to be of "reference grade" once it became known that the levels of distortion limited its resolution to only about 10 bits? Or is the effect of the added "second harmonic sauce" so beguiling that the measured performance can be ignored? If so, then why bother using such a device to listen to any hi-res (let alone CD-quality) recordings? Maybe there's an untapped market for recordings made in a 10-bit/96kHz format?

Bogolu Haranath's picture

Regarding 10-bit resolution (SNR) ......... Analog source media, vinyl and analog cassette tape have max resolution 10-bits ........ Analog reel to reel tape has max 12-bit resolution :-) .........

Bogolu Haranath's picture

BTW ..... Mp3 320 Kbps and AAC 256 Kbps have about 8-bit resolution :-) ........

ok's picture

distortion bottlenecks render any electronic “resolution” concern virtually pointless. God only knows what certain "reviewers" try to pass through their borrowed metaphors; picking up audio gear based on third party subjective accounts (and to a lesser degree on individual measurements) is in fact no different than picking up gear based on placebo, intuition or pure luck.

rt66indierock's picture

Give reviewers the measurements first then let them justify their impressions. It would be interesting to read how Herb justifies this product as reference level knowing the specifications.

Ktracho's picture

For me the question is, what is more important, to please electronic measurement equipment, or to please my ears? Also, consider that my ears don't function the same way as electronic measurement equipment. In other words, electronic measurement equipment is not a faithful model of how my ears function. Certainly, very high measured levels of distortion will be reflected by poor sound, but increasingly lower measured levels of distortion are not necessarily correlated with increasingly stellar sound. At least that's my opinion.

Ortofan's picture

... of distortion become too high to be acceptable, in your opinion?

Ktracho's picture

In my opinion, it's not a useful question. For example, how do you compare the sound of a truly great LP with a few unfortunate scratches through a great sound system vs. a cheap cell phone playing an MP3 file? More importantly, my answer would not be a useful guide for anyone else. For example, some may not be able to hear the difference between a $100 sound system and a $10000 one. Would it be foolish for someone to only spend $100 even though the distortion level in my opinion is too high?

rt66indierock's picture

This device is not quiet enough.

mememe2's picture

let's face it - unfettered love for all things with tubes.If one were to scour all of Stereophile's tubed equipment reviews( from beginning to present day) it would be hard to find many negative reviews of said equipment. Those reading should expect nothing less than approval for almost any tubed gear under review. It is what it is.