Rogue Audio M-150 Monoblock Tube Power Amplifier
An intelligent and reliable design.
Review By Dick Olsher
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  What do the following items have in common: output transformer, ultralinear connection, triode mode, KT-88, phase-splitter, fixed bias, feedback, and a Suzuki SV650 racebike? Most of us would recognize and categorize all but the last item as tube amplifier design elements. Then you might ask what a racebike has to do with a high-end audio product such as the M-150? Rogue Audio's Mark O'Brien has been an avid bike racer for most of his life, though he tells me that Rogue Motorsports is in a bit of a transition spurred by injuries to a couple of racing friends. He's giving up on bikes and plans to do some (much safer) SCCA car racing next season. The point is that the mindset of racing in terms of parts and build quality is very much akin to that of high-end audio. And from where I sit it's clear that Mark has been able to import excellence in design and execution into his audio products.

In my opinion, there are far too many 40 to 60 wpc KT-88 based stereo tube amplifiers out there. Most of them sound euphonic, lack adequate bass control, and need to be carefully matched to a prospective speaker load. What I was looking for was a tube power amplifier with adequate power reserve that could partner any real-world speaker load, extract maximum bass definition from a typical bass reflex speaker, and reproduce harmonic colors and textures without significant editorializing. And just as important, a price tag well below the cost of new automobile. It's fair to say that the M-150 monoblocks easily fulfilled my wish list. And I'm inclined to think that 150 wpc (UL mode) also fulfills the Goldilocks principle — it is just right for all the speakers I have on hand. The icing on the cake is superb cosmetics and build quality. How about that 0.5-inch machined and engraved faceplate!

Technical Details
Mark O'Brien started designing his own tube gear about 20 years ago. Having worked at Bell Labs in engineering physics he had access to superb lab resources as well as lots of help from electronics engineers who had received their degrees during the golden age of tube electronics. It's obvious to me that Mark has learned his craft well. It starts with the monoblock layout. It's nearly impossible for me to accept an amplifier as Reference class unless it is of monoblock construction. Two totally independent and isolated power supplies and separate signal paths result in ideal channel separation and stable imaging under dynamic drive conditions.

Both the single-ended (RCA) and balanced (XLR) inputs are connected to a wide-bandwidth Jensen input transformer. Although it's strictly not necessary in the case of the RCA input, Mark believes that it does a neat job of eliminating potential ground loops and smoothing out the top end. One leg of the secondary is grounded, so that only the positive going portion of the balanced signal is used. The signal is then fed to a long-tailed pair phase inverter consisting of a 12AX7 dual triode. The driver stage is more sophisticated than what one typically sees in a 1950s vintage amp. It's a series-regulated push-pull circuit, also known as a Mu follower or totem pole, which uses a pair of 12AU7 dual triodes. The output stage is comprised of two pairs of KT88s operating in parallel push-pull configuration. The output transformers are quite beefy and feature an ultra linear connection. They use E-I laminate cores of grain-oriented silicon steel and are custom wound to a frequency response spec of 10 Hz to 50 kHz (+/- 1dB). The windings are bifilar and are interleaved to reduced leakage inductance. The output stage mode may changed on the fly from UL to triode by the mere flick of a switch on the back panel. Of course, maximum output power is reduced by about one half in triode mode.

Instead of the more common cathode bias, a fixed bias scheme is used. There are no cathode resistors, a dedicated –100 VDC bias supply is provided. This is a far more efficient biasing scheme than self bias since the entire B+ voltage is useable as plate voltage. A very nice touch is the built-in current meter. The quiescent current of each output tube may be adjusted individually using a set of pots located under a plate near the meter. A special tool is provided for that purpose. The entire process only takes a couple of minutes to complete if there are a few pots that need tweaking. Note that the bias is set on the low side at the factory as a precaution because it is a function of the AC main voltage. The recommended bias current is 40 mA and it should be set accurately only after each amp has been running for at least one hour. One knock against self bias is that it is not self-regulating as is cathode bias with tube aging. I found it necessary to tweak the bias a couple of time during the first week of use. After that, there was little drift; I would recommend a once-week bias check under heavy usage conditions and a bimonthly frequency for the average user.

The power supply is solid-state rectified and features voltage regulation for the front end preamp tubes. The other important feature, mandatory really, for a fixed-bias amp is the soft start circuit which ensures that high voltage is applied slowly while the bias supply kicks in. Each output tube is individually fused as an added safety feature to prevent runaway currents should the bias supply be interrupted.

The M-150 is normally shipped with Electro-Harmonix KT88s and vintage US brand preamp tubes. My samples were outfitted with GE 12AX7 and 12AU7 tubes. Rogue Audio would prefer to use current production preamp tubes, but have been dissatisfied recently with the quality of the Russian-made 12AU7. The output stage plate voltage is said to be near 600 VDC, which means that for a quiescent current of 40 mA, plate dissipation is about 22 watts — well below the rated maximum. At this operating point, KT88 lifetime is expected to be in the range of 2,000 to 3,000 hours.

Both 4 and 8-Ohm impedance taps are provided. These ratings should not be confused with the amp's internal source impedance which determines its damping factor. The source impedance is said to be about 0.4 Ohm — quite low for a tube amplifier with moderate levels of global feedback (about 18 dB) — and resulting in a damping factor of 20 into an 8-Ohm load. I'm so fed up with insulated EU style binding posts that look fancy but are difficult to tighten. Hence, it was really nice to see good old US style un-insulated posts of hexagonal profile that I can really tighten down on with a nut driver.

Sonic Impressions
My strategy for evaluating any power amp is to audition it with several speaker loads in order to obtain a balanced overview of its performance potential. Otherwise, the review process runs the risk of merely becoming a hit or miss proposition. The M-150 was first teamed up with the Esoteric MG-20, a speaker of fantastic mini-monitor like imaging capability. In fact, it really raises the bar in this regard. First order of business was to experiment with impedance tap options and UL vs. Triode modes. A valuable lesson I re-learn periodically is how critical it is to try all of a tube amplifier's impedance taps with a given loudspeaker. Most audiophiles simply select impedance taps that match the nominal impedance of their speakers. That could be the optimum choice, but not always. And in the case of the MG-20 with its 6-Ohm nominal rating, the optimum choice isn't clear. My finding was that performance off the 8-Ohm taps was surprisingly more dynamic vs. the 4-Ohm taps with sweeter sounding mids and a fuller bodied tonal balance.

It was a closer call in selecting the operating mode. Off the 8-Ohm taps, Triode mode gave a more relaxed presentation and timbre accuracy was judged to be a bit better relative to UL. It would appear that in Triode mode the distortion spectrum becomes a bit richer in second order harmonic distortion and would explain the observation that midrange textures were sweeter sounding. But, and this is significant, I missed the microdynamic intensity and drive of UL. The music's raw energy was slightly tamed, more polite if you will in Triode mode. Ultimately it seemed to be a choice between pretty vs. exciting sound, though I admit that my preference for a particular mode was also influenced by the choice of music. Therefore, you should feel free to experiment in this regard as it boils down to flicking a single switch on the back panel.

The M-150 needed about 30 minutes of warm-up to sound its best, after which its essential characteristics of clarity, image focus, and transient speed shone through in spades. Teamed up with the Prima Luna model Eight CD player, which erects a 3-D soundstage like no other player out there, the resultant soundstage was not only cavernous but featured a nicely layered depth perspective. Image outlines were fleshed out with palpable extension, but did not quite equal the reach-out-and-touch-someone realism of the much more expensive ($19,000 to be precise) Esoteric model A-100.

My first impression, which held up during subsequent auditions, was that at least with the stock tube complement the M-150 was not particularly tubey sounding. It didn't overly liquefy midrange textures, by virtue of its extended bandwidth, and it didn't romanticize the lower midrange. I could do without the euphonics, but I did wish for a more vivid harmonic color palette and a smoother presence range. There was no tube glare or gratuitous brightness, but a bit of textural grain broke through occasionally. It was time for tube rolling. In my experience, tube substitution is a necessity rather than a luxury with most tube amplifiers. My first avenue of attack was the 12AX7 in the phase splitter circuit because there was only one tube in this circuit and it was the first one in the signal path. My two favorites in this spot turned out to be the Telefunken “smooth plate” and the Sylvania Gold-pin 5751, which Mark O'Brien assured me would work fine in this circuit. The Telefunken extended the imaging virtues of the M-150 to a new level. It engendered a wonderful sense of space, a great feel for hall reverb, creamy smooth midrange textures, and expressive microdynamics. It was clear that the Telefunken was a truly great choice in this circuit. The Sylvania 5751 was also a winner, by virtue of its airy treble range, pure textures, and tight image outlines. Overall, I preferred the Sylvania by a slight margin due to its more neutral balance vs. the darker presentation of the Telefunken. But I did not achieve sonic nirvana until I replaced the driver circuit 12AU7s with NOS Mullard CV4003 box anode types. It took me a while to accumulate two pairs of the Mullard, but it was worth the wait. This is probably the finest 12AU7 type available today and it took care of the remaining items on my wish list; namely, more vivid harmonic colors and a smoother presence range. Now that's the way to make the KT88 output stage sing! The M-150 now delivered not only speed and stress-free dynamics, but also plenty of finesse. Complex passages were fully resolved and the music boogied forward with plenty of verve.

I've waited till now to reveal one truly amazing aspect of the M-150's performance. The MG-20, being a bass reflex design, had previously evinced fully satisfactory bass control and pitch definition only when driven by solid-state power amps such as the Silicon Arts ZL-120 monoblocks and James Bongiorno's Son of Ampzilla 2000. The M-150, especially in UL mode, grabbed hold of bass lines with an iron fist and dug deep with excellent extension. Its high damping factor was responsible for convincing bass control not only with the MG-20 but also with the two other speaker loads I threw at it: my own DIY Basszilla Platinum Edition Mk. II, also a bass reflex design, and Final Sound's 1000i ESL.

With the Basszilla Platinum Edition, I preferred the 8-Ohm impedance taps. The Triode mode sounded more tubey, more mellow, and less crisp than the UL mode. Most of the time I preferred the UL setting because of its enhanced speed and immediacy. With the 1000i ESL, I preferred the 4-Ohm taps and Triode mode for its smoother treble and sensuous mids. At no time did the M-150 have any difficulty driving the 1000i to satisfying levels. However, the resultant tonal balance was a bit dark, lacking in treble energy. I suspected that the M-150 was rolling off the 1000i's treble, due to the interaction of the amp's source impedance with the low load impedance, which touches 1.2 Ohm at 20 kHz. The measured frequency response of the 1000i driven by the M-150, with the measurement microphone centered on the tweeter panel, showed a treble rolloff starting at 10 kHz. The roll off reached a level of 3 to 4 dB at 20 kHz and implies an effective source impedance closer to 1 Ohm. To be fair, other tube amplifiers and even some solid-state amplifiers also exhibit similar load interactions with the 1000i ESL.

Conclusions
The M-150 is an intelligent and reliable design, which I'm very fond of from a technical perspective. But that's not all. Outfitted with the right preamp tubes, it is a high-resolution muscle amplifier combining finesse and brawn under one chassis. Its bass performance is astounding for a tube amplifier. And it has the power reserve to cope with most real-world speaker loads. The M-150 is absolutely a world-class amplifier, and considering its performance and build quality, its asking price is all the more remarkable. Highly recommended!

Specifications
Type: Monoblock tube amplifier

Company Information
Rogue Audio
P.O. Box 1076
Brodheadsville, PA 18322

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