Review: Rogue Audio Cronus Magnum Tube amp

Category: Amplifiers

Rogue Cronus Magnum Review

I owned the Cronus for a couple of years before sending it into Rogue for the conversion to the Magnum edition and Mark O'Brien was extremely patient with my very many questions about the upgrade. His excellent reputation is more than deserved.

The Magnum upgrade to the regular Cronus amp consists of
- Larger power supply
- Power supply mods
- Polypropylene bypass capacitors
- Precision Dale-Vishay resistors in critical spots
- Upgraded binding posts
- Gold tube sockets
- Upgraded small signal tubes
- KT90 output tubes
- Magnum logo on faceplate

I did not order the upgraded small signal tubes as I have a good supply of various new and NOS ones. I also did not order the KT90s as I had various other power tubes I wanted to try (more later). It is to be noted that the output coupling caps are upgraded as well as bypassed; something not listed above.

System is Mac Mini Sourced from an external hard drive to an ART Legato Async Converter (all plugged into an APC S15 power conditioner/surge protector), to a MHDT Havana (with upgraded caps, fuse and tube), to the Cronus (upgraded fuses), to Aural Acoustics Model B speakers (3 way, time and phase coherent, 87db efficient). Cables are db Labs Essential USB, KCI Pegasus Digital IC (BNC-BNC), Jena Labs Symphony Analog ICs, and Kimber 12TC speaker cables (with Cardas un-plated Cu spades on the amp end and Eichmann Cu Bayonet plugs on the speaker ends). Power cords are Kaplan and Pangea.

I enjoy most types of music with classical and folk/traditional forms being my favorites and rock and jazz close behind. I use a wide variety of recordings from these forms which are both well produced and of average quality. Tonal and harmonic accuracy are of the utmost importance to me with bright strident sound being the most off putting quality. PRAT and frequency extension and balance are next in importance followed by soundstaging and imaging qualities.

I enjoyed the base Cronus but was bothered by a few things. One, insufficient dynamics, was certainly occasioned by a power mis-match with the Aurals which need more than the Cronus's 55wpc. Two, congestion in complex musical passages, was partly, but not entirely, due to the same power issue. The third, a bit of hash and harshness in the upper mid-range and highs, was about half down to the amp, with the other half being down to cheap speaker binding posts, which I changed after the upgrade.

In any event, the changes, subsequent to the upgrade, were dramatic.

Obviously macro dynamics are much more pronounced. The bass is more powerful, focused and extended. Both traits hold even using the Black Treasure 6CA7 and Genalex KT66, previously utilized by me in the regular Cronus, which are limited to their 50-55 wpc output capability. What was somewhat surprising to me however, is how much micro dynamics were improved. Low level resolution, in terms of both ambience and the differentiation of individual lines (including background ones which are now much more discernible), is concomitantly greatly increased. Orchestral crescendos are not at all blurred and sound effortless while rock has real punch and drive.

Another pleasant surprise was just how much more natural the tonality is: for example pianos have a greater realism and lack the clatteriness I heard with the non-Magnum. Highs are now very clear and possess greater extension. The one (relative) weakness of the unit is refinement that is less than that of the best. The best, of course, being significantly more costly.

All of these improvements, as one would imagine, were magnified with more powerful tubes. I tried three different types: Penta KT88SC, NOS GE6550 and Tung-Sol KT120. These tubes, as well as the 6CA7 and KT66, it must be noted, all require the use of socket extenders as they do not fit into the holes in the chassis top of the Cronus (regular or Magnum), which only accommodate EL34/KT77/KT90 tubes. Also, the KT120s, although raised an eighth of an inch above the chassis by the extenders, overhang the holes completely. To avoid any issue of heat build up inside the unit, I had the holes punched out by a friend about an additional 7/32" in diameter. (Note: new production units of both the Cronus Magnum and Atlas Magnum, as of mid Febuary, now have a re-designed top to accommodate large bottle power tubes, which a lot of people have apparently been experimenting with. That is customer responsiveness.)

The Penta KT88SCs, while initially enjoyable, possess the overly emphasized bass that I have almost always heard from amps using that tube type, which is not to my taste. I have always preferred 6550s. They have a more even frequency balance to my ears than KT88s, even though their bass is slightly recessed compared to the mid-range and the highs. NOS GE6550s are of course superb (and very expensive), with a mid-range that is tonally naturalistic and incredibly three dimensional, and highs which are extended and sweet. Although the Magnum's circuit can output 90wpc, I do not know what the output was with these tubes. The spec'd output for KT88 amps usually range from 60-80wpc per quad of tubes but can reach 100wpc; for 6550 amps 60-70 wpc seems to be the maximum. The Pentas did sound more powerful and dynamic than the GEs but not as good.

The relatively new Tung Sol KT120 is a very good tube. Capable of outputting up to 75 watts per tube they have the best frequency balance of the three types; with bass that is as powerful and extended as that from KT88s, and a mid-range and treble that is equally prominent. If they don't have quite the three dimensional images or sweetness of the NOS GE, they still sound more natural to me given their range and dynamism. They also throw a larger soundstage than any of the others. They also obviously put out the full 90 watts per side the CM maxes out at but sound even more powerful, given the extra headroom resulting from their coasting at that output.

I have had several audio friends over who were equally impressed with the CM and also lent it out to a couple of them. In both my system and those of two others, the CM was preferred to several other amps, both integrated and power, that cost significantly more. And in fact, with the KT120s, the CM sounded more powerful and more dynamic than one tube power amp rated at 100wpc, and another at 125wpc (both using KT88s).

In conclusion, as so many others have already opined, the Cronus Magnum is an excellent integrated amp and a superb bargain.

Associated gear
Mac Mini (with external Hard Drive), ART Legato asynch converter, Havana DAC, Aural Acoustic Model B speakers.

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