ALLNIC HT 2500 Head Amplifier Nuvistor Tube preamp
Allnic HA 3000

WHAT'S OLD IS NEW AGAIN: ALLNIC HT 2500 HEAD AMP:

Lately, I have been snug as a bun in a rug on the audio side, no audiophilia nervosa, no pondering about the greener grass on the other side of the audio fence, thinking that I may as well retire from the audio swirl and just enjoy spinning records with my Allnic H 3000 and Allnic L 5000 DHT preamp. These products have given me enormous pleasure from my records. I have written consumer reviews of these, and thought my fifteen minutes of fame was up.

I was surprised when David Beetles called me and said that KS Park wanted me to try his new nuvistor head amp, the HT 2500. I have to admit that I saw this component on the Allnic website but had no special curiosity about it. I thought it might just be for consumers who otherwise couldn't find the scratch for the Allnic H 1500 or H 3000, or favored solid state or non-Allnic branded phono units, and was a little surprised that Mr. Park would be manfacturing a component that competed with his own excellent and very quiet input transformers. I have felt no lack of satisfaction with the input trannies of the H 3000, either in theory or in practice. However, knowing that Mr. Park is an audio polymath, I found it a little intimidating that he would believe that the HT 2500 needed some attention and assessment and agreed to try it out. It also put me back into the murky swirl of controversy that boils around the topic of input transformers and head amps in general. Those who already know about head amps and MC input coils can skip past the SIDEBAR.

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WHY A DEDICATED HEAD AMP?

Head amps are basically flat amps for very low level signals. In the olden days, when vinyl was supreme and the low output MC cartridges began their heyday, the MC cartridge tended to have small coils, low impedances and very low outputs. Circuits tended to be too noisy for these small signals, so cartridges were sold with the idea that they should have a voltage step up transformer to amplify the small signal voltage quietly for a signal level adequate for an RIAA phono stage,

In the 70's and early 80's, many high end manufacturers started offering separate active head amps to perform the step up function, with relatively quiet circuits and transistors. Many of the Japanese manufacturers made elaborate and expensive separate head amps. As the 80's merged into the 90's, and vinyl was becoming "obsolete", most of the preamps rolled the head amps into their full function preamps, and eventually rolled the phono stages out completely as CD gained market share and vinyl was considered the province of the audio dinosauors. The era of the lavish head amp ended with a whimper.
Now that vinyl has enjoyed a major resurgence, what's old is new again. The issues of transformer coupling of LOMC cartridges vs. Active head amp stages is in the audiophile chatter on a regular basis. There is a wide choice of high performance MC cartridges and they are not all ultra low output. Some newer manufacturer have been making dedicated head amps, rather than just add ons to phono preamps, and its seems that Mr. Park has taken on the mantle of constructing a true high end, elaborate and state of the art head amp once again.

SWIRLING CONTROVERSY:

HEAD AMPLIFIERS, YUCK!

Is there a true ideal amplification device that is high gain, low noise, yet renders the cartridge's capabilities in a maximally transparent but musical manner? Probably not. It seems that head amps were devised for the purpose of revealing shortcomings in amplification devices due to the huge gain required and the signal to noise problems of the tiny signal. If an amplification device reveals a low gain MC cartridge, then the amplification device will also likely reveal its own shortcomings in the process. So head amps are most definitely about making hard choices and accepting comprimises somewhere amongst those choices.

PROBLEMS WITH COILS, OR ARE THERE?

Coils can be used to convert current to voltage, and can thus be used as voltage amplifiers with moving coil cartridges. They are "cold", passive amplifiers in the sense that they do not use an external power supply, you basically are listening to wire wrapped around metal. By their nature, they are wide band filters.
Some observers say that coils remove some information for this reason. However, input coils also add information by an inevitable ringing, however slight. Input coils can also introduce phase shift.

In a really nice coil synergistic with a particular cartridge, these effects can be minimal. Also, the penumbra of the coil itself, much like turntable resonance and tube warmth, can be quite lovely and musical in its own right. As long as there are no hum fields to pick up, coils are a completely quiet voltage amplifier, which is one of their main virtues.

Coils act in conjunction with the MC cartridge through a "complex impedance" relationship rather than a flat resistance. When using coils, you are always listening to the binary of the larger input coil and the MC cartridge coil. In this sense, you are never really hearing the unalloyed merits of the cartridge.
Like a binary star, the MC cartridge and the coil are engaged in a gravitational waltz. There is a constant exchange of gases between the larger star and the smaller star. The waltz can be a thing of beauty. However, you can't see the smaller star in its unmolested, unattached space with the benefit if its own full grace and beauty.

Some observers believe that these characteristics of the MC input coil can mask the purest rendition of the cartridge itself, its dynamic capabilities, detail and air or atmosphere.

On the other hand, some experienced old analog salts believe that the very low output cartridges cannot be optimally heard without the quietness and participation of a good coil cartridge match.

I won't bear judgment either way because I love the sound of a nice coil, but whether it is completely honest to the capabilities of the MC cartridge itself is a matter of opinion.

PROBLEMS WITH ACTIVE AMPLIFICATION OR ARE THERE?

Active amplification in a head amp requires the use of a power supply, either battery or plugged in. Also, "active" amplification device choices are restricted to transistors or tubes.
You would think that head amp construction is an ideal job for transistors because of transistor's low noise potential, wide bandwidth, ability to match pairs, ability to render dynamics and stable operation over time. Certainly many of the great head amps of the analog heyday were transistor, and some are still highly sought after. Transistors, of course, can glare and dessicate, and these traits can be passed through the entire amplification chain. Some head amps use battery powered power supplies to reduce the inevitable power supply noise problems.

The other amplification alternative is tubes. Tubes are a hard sell in this small signal amplification phase. They need to be very low noise and high gain. Also, tubes tend to age in ways that make them noisier with time and use. This is not noticable in preamps so much. In head amps, it can be distressing to go through cartons of expensive tubes to find those that are matched and quiet, only for noise to re-emerge with moderate usage.
Also, sound characteristics that may display as bloom and warmth in a regular line preamp or amplifier can become a flabby spare tire in a head amp, obscuring detail in a wash of bloaty harmonics.

Ideally, active head amps will preserve the complete information and phase integrity of the signal created by the cartridge. However, they carry the penalty of added noise, either from the power supply or the amplification device itself.

To their credit, active head amps present a simple resistive load to the cartridge rather than a complex impedance seen by a MC coil input, thus they offer lower potential for phase shift and ringing than input transformers. Ideally, an active stage will allow the full capabilities of the cartridge itself to convey with its speed, delicacy and atmosphere.

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NUVISTORS:

There is a pretty nice, succinct write up on nuvistors on wikipedia: http://en.wikipedia.org/wiki/Nuvistor. The nuvistor Ampex MR-70 was considered one of the best if not the best sounding studio reel to reel tape recorder ever made. There were perhaps 50 samples of the MR-70, all more or less custom made.

Conrad Johnson made a couple of nuvistor based head amps in the 1980's, the HV1 and HV1a and HV2. The circuit boards were suspended with rubber bands for microphonics of the nuvistors, and the sound was considered good.

Musical Fidelity has used nuvistors in its "Nuvista" and "Trivista" product lines The Musical Fidelity nuvistor units are sought after as used product.

Suffice it to say, nuvistors were the last gasp of the vacuum tube industry facing impending obsolesence from semi conductors.

Nuvistors were an attempt at miniaturizing tubes to compete with transistors. They are small, thimble like metal tubes that needed complete vacuum controlled chambers for assembly and production. They were mostly used for TV's of the time.

GENERAL CONSTRUCTION OF ALLNIC HT 2500:

The Allnic HT 2500 comes with two chassis: 1. power supply with an umbilicus. 2. the separate housing that contains the nuvistor tubes, attaches the phono input and output cables, and contains the knob for setting resistance seen by the MC cartridge. Resistance choices are 50, 100, 200 or 500 ohms and should pretty much cover the 5-20 times multiplier rule for "active stage" input resistance with virtually any MC cartridge (cartridge internal impedance multiplied by 5-20). The choice of resistance generally effects the impression of "air" and upper midrange transparency with an MC cartridge, with higher resistance used to get more high frequency effect. My cartridges, the Harmony Mg and the Benz MC3, have impedances of 50 and 30 ohm respectively, so I used the 500 setting.

The power supply is large and heavy for the use involved. Peeking through the grill shows dual mono construction with two separate power transformers and an additional pair of smaller transformers which are either filament transformers or chokes. There are at least two conventional tubes per channel used as voltage regulators. Otherwise, I didn't dissect the chassis to look deeper.

Looking through the much lighter nuvistor chassis shows two nuvistors per channel and some capacitors on a circuit board.
Nuvistors are reputedly the longest lived of vacuum tubes, supposed to have a life span of 100,000 hours of use without degradation. That would be three hours of "turned on" time a day for about 100 years. However, don't ask me to come back in fifty years to verify this fact. Nuvistor tubes seem to come up regularly on ebay for very reasonable prices.

NOISE ISSUES OF ALLNIC HT 2500:

Nuvistors have noise issues. Because of their small size and narrow grid pitch, nuvistors are subject to microphony. Nuvistors are sensitive to hum pick up. Also, nuvistors traditionally have had socket problems due to the small pins used. As a rule, any vacuum tubes used with the tiny signals generated by MC cartridges have had issues with tube rush.

Nuvistors to their credit have very high signal to noise ratios compared to conventional tubes. Combined with high gain (mu), this makes them very suitable for low signal, low noise amplification if their microphony and hum issues can be resolved.

Mr. Park manufactures his own tube sockets, and has made the sockets for the nuvistors used. Allnic also has used its own gel absorber technology to minimize microphonics in tube bases. Rapping the nuvistor chassis as hard as I could with my knuckles produced no audible artifact of any kind through the speakers, indicating that the technology used to prevent microphonics from the nuvistors in the HT 2500 is highly effective.

Placing the nuvistor chassis at least six inches away from electrically active transformers seems to resolve any significant hum issues in my audio chain. The HT 2500 should not be stacked with its own power supply, because some hum will become evident.

The Allnic H 3000 phono stage, using its built in cartridge input transformers, has a nearly tomb like silence. With the HT 2500 attached to bypass the input coils and go directly into the H 3000 MM inputs, there is definitely audible nuvistor breathing and mild rush that can be heard through the bass panels of my speakers when I put my ears up close to the panels, compared to the complete silence when the MC input coils are used instead.

I use planar speakers in a near field listening arrangement, I am probably about seven feet from the speakers when listening to source material.
However, from my listening position, using either the .6mV Harmony Mg cartridge or the .3mV Benz MC3 cartridge, the noise was not an issue at moderate to loud listening levels and was lower than the noise experienced from the quietest touch down groove of my quietest record. I suspect that the HT 2500 is use-able with cartridges as low as .2mV. Below a cartridge sensitivity of .2mV is anybody's guess, but I do not have a .2mV cartridge to test.

I would imagine that the quietness of the coil input versus the obvious additional noise would be subject to individual evaluation and preference. I think that the performance benefit of the HT 2500 is compelling enough to make the noise issue with a moderate gain MC cartridge moot. If one wants the most compelling of black silences between cuts, than one should probably be listening to digital, anyway.
If one wants the most compelling of musical playback once the signal is engaged, a certain small amount of subliminal noise, as with vinyl, seems a small price to pay.

There is nothing barring the use of the HT 2500 with an input coil as well. A low winding, low phase shift, low ringing 10 db coil with a .1mV cartridge would elevate the level to about a .25mV equivalence.

I think after listening to the HT 2500 with high quality medium output MC cartridges, however, that using a coil with the HT 2500 input would be a long run for a short slide. An ultra low output MC cartridge with a coil attached to the HT 2500 would just seem superfluous, I am at a loss to think it would guarantee any kind of enhanced sound quality, because there are now so many high quality medium to high output MC cartridges on the market that could be used with a head amp.

Nominally, the HT 2500 has 30 db of uncorrected output amplification. However, the HT 2500 with the MM sounds much louder than the 30 db input coil option available on the H 3000. There are issues that relate to voltage amplification and overall impedance with input coils that could reduce audible output of an input coil. However, with the HT 2500, you are not left asking "where's the beef." The total amplification in conjunction with the H 3000 MM phono input would be rated at a total of about 70 db.

The HT 2500 does not seem to need much break in to sound optimal. However, it does benefit from a long warm up before using it. Warm up of at least an hour with an additional 20 minutes of "signal" play seems optimal from a cold start. When played stone cold, the HT 2500 sounds a bit "squawky" or "squeally", almost like a vocoder, but this characteristic dissolves rapidly with warm up.
With the alleged long life of the nuvistors some users might be tempted to just leave it on all the time, but I would leave that up to them. I just got into the habit of turning it on a long time before planning a listening session, and not feeling bad if I left it on over night.

SO HOW DOES IT SOUND?

I had a phone conversation with Mr. Park when he visited David Beetles earlier this year. Mr. Park speaks very passable English and is able to communicate his audio concepts quite well, unlike my complete obtuseness at Korean or any other language. He generally stated that the HT 2500 was devised in order to bypass the sonic compromises caused by the complex impedance presented by input coils. Also, Mr. Park consults his demanding Korean audio society as a sounding board for his creations. The classical music lovers in his Korean audio society wanted something that had even greater transparency, dynamics and detailing than was afforded by coils, thus the work with nuvistors and the genesis of the HT 2500.

Unexpected to me after using the H 3000 with its excellent input coils, the sonic result of the HT 2500 is just spectacular.

I have never heard a nuvistor based product before. My early assessment after a good period of warm up, was that the HT 2500 nuvistor device has the attack, dynamics, frequency extension, bass grip and detail of transistors but with the spaciousness and tonal capture of tubes. However, it doesn't have the greying, glaring, flattening, etching or drying of transistors, nor the bloat and overhang of tubes when tubes are used for small signal amplification.

My listening comparisons are with the input coils of the H 3000 phono preamp using my available cartridges.

The HT 2500 creates a much higher level of "image lock", greater width and depth, and much greater image density combined with greater image subtlety compared to the MC coil inputs of the H 3000. The image density created by the HT 2500 is reminiscent of high quality reel to reel tape, but the vinyl is deeper and smoother sounding than tape.
When playing Stowkowski's phase 4 Scheherazade, The strings and basses on the right stage hung off the speaker like a dense beehive of instruments. Having moderate size planar speakers, I can never recall anything hanging off of one speaker with that level of density and resolution, certainly not to that extent with the MC coil input of the H 3000.
So, right and left separation seems more complete with the HT 2500, giving vivid, deep performances to the extreme right and left.

When using the H 3000 input coils rather than the HT 2500, from many records, I could elicit an ever so slight phase-i-ness from the soundstage, I could move my head slightly and cause the image to roll around a bit. Not so with the HT 2500 in the chain, the image addressing is incredibly tight and immobile in the sound field with the HT 2500. Even my digital sources are less focussed sounding than vinyl with the HT 2500 in the chain.

There were occasional records when using the MC coil inputs of the Allnic H 3000 where I thought the cartridge wasn't tracking quite so well, or that possibly there was a tiny wobbling from master tapes that I could discern. One such record is Steven Stills eponymous album. It sounds great, but seems to have a slight dynamic pumping effect and phase drift when using the input coils. I thought this might be due to tape aberrations from the master. When heard through the HT 2500, all of these small anomalies disappear and the imaging locks in tight as a drum with no perceptible variations of any kind, absolving my turntable, cartridge and the master tapes.
Is this stuff I have heard on some records the phase drift audible from the coils? I have no idea, but there is no doubt that the imaging precision with the HT 2500 in the chain is significantly superior to the H 3000 using the input coils

OTHER CHASSIS:

Because the HT 2500 stimulated my curiosity about head amps in general, I managed to get a hold of an Accuphase C-7 transistor head amp. This is one of the good transistorized head amps from the Japanese heydey, before the Godzilla foot of digital stamped out vinyl. It is a predecessor of the Accuphase C-17 headamp, which was one of the best head amps ever made and sold for $2000 in the mid 1980's.

The Accuphase C-7 is a beautiful product in its own right. It is extremely transparent sounding, and creates a similar sense of image lock compared to the HT 2500. However, the C-7 sounds as dry as desert wind, and does not have the dimensionality or image size of the HT 2500. The Accuphase C-7 sounds precise, but lacks the moisture and heat of either an input coil or the HT 2500.

I would imagine that choosing the C-7 vs. a good input coil would be cartridge and electronics dependent, as opposed to the HT 2500, which is an easy tie breaker with the input coils of the H 3000. Whatever tube character that the HT 2500 has in the way of tone capture and fullness is preferable to the Accuphase C-7 in this audition with my particular cartridges.

I have heard some of the all tube phono preamps that included tube head amps in other system a few times. I thought they sounded good, but if ever there was a place to exercise restraint with a nice transistor, it would have been at the input of those units and toss out a tube or three. The HT 2500 needs no such comparable restraint.

POST PRE AMPLIFICATION VS. PRE PRE AMPLIFICATION:

There seems to be a popular objectivist dogma that adding a chassis is always worse than not adding a chassis or stage. My Clearaudio Harmony Mg has a .6mV output, which can produce a moderate output by itself through the MM input of the H 3000 without the input coils. I hooked the Harmony Mg this way directly into the MM inputs bypassing the coils. Cranking up the preamp to accommodate the lower level output certainly sounds good, but sadly is not nearly as good as using the HT 2500 in the pre pre amplification stage with a lower main preamp volume level.

Apparently, protecting the delicate ember of signal from the MC cartridge and carefully elevating and preserving that small signal on a tsunami of clean power prior to the RIAA stage is far more effective in consolidating detail, dynamics and image density than the post RIAA technique of just cranking up the preamp volume.

So the complications of adding a chassis and set of cables seems to be worth the added complexity of the head amp. You can't amplify what you never properly preserved or lost in the first place.

DIFFERENT PHONO STAGE:

I have a MM phono stage in my Yamaha RX Z9 home theater receiver. I tried connecting the HT 2500 through that phono stage rather than the Allnic H 3000. It reminded me of why I paid the bucks for the H 3000. The HT 2500 cannot "cure" a lower echelon phono stage. Whole swatches of air and dynamics dissolved with the Yamaha RX Z9 MM cartridge input compared to the Allnic H 3000. Nontheless, the sound wasn't bad by conventional analog standards. I would call it 60's Dynaco push pull pentode type sound. What surprised me is that with the transistor MM stage and the HT 2500, there wasn't even the tiniest hint of any kind of harshness. I think the HT 2500 would do very well with a modern, high performance solid state MM phono preamp and I hypothesize the HT 2500 would tend to mollify a sense of transistor overkill in a MM transistor phono stage when used that way.

DOES THE HT 2500 RESOLVE THE ACTIVE VS. PASSIVE COIL DEBATE:

Audiophiles, like most people, like arguing too much for this ever to happen. You read about died in the wool "active stage" guys who after so many years hear a magical coil/cartridge combo and become hard core "coil" aficionados. You also hear about "coil junkies" who have every high end coil out there, and hear some "active" stage and are converted the other direction.

With the HT 2500, I won't lose any more sleep wondering if there is some magic coil/cartridge out there. I guess I find this active stage so compelling I am not inclined pursue coils any more. The HT 2500 also allows me a clearer window on my MC cartridge for its own innate performance characteristics without worrying about the endless, multiple permutations of coil/cartridge optimization.

However, I also can't listen to every coil/ cartridge combo much less every phono stage variation on the market, so I have no actual way of knowing for sure, so let the debates rage on and the blood and guts fly.

DROOLING, BUG EYED FAN BOY:

I have written reviews of two Allnic products, the H 3000 phono preamp and the H 5000 DHT preamp. I bought these and reviewed them because I liked them. David Beetles and Mr. Park are aware these are favorable reviews. This review of the Allnic HT 2500 is more of an "out of left field" review. Mr. Park sent me the HT 2500 because he wanted my opinion of it. It was to be sent on to a proper audio critic at an online audio rag after I spent some time with it.
Before receiving the HT 2500, I was not curious about it, had no special interest in it, and without hearing it, would probably not have bought it. David Beetles didn't say anything special about it except that it had some "tube rush". I wasn't expecting all that much compared to the input coils of the H 3000.
Now, having heard the HT 2500, I realize how exquisitely sensitive the cartridge/head amp interface is in optimizing the overall performance heard by the vinyl system. Whatever isn't preserved at this level just doesn't exist any more, or if it is altered adversely, it is altered and magnified that way forever in the audio chain.

I am buying the Allnic HT 2500. I was supposed to forward it to the reviewer, but I am going to keep it. After hearing it at the head of the MC cartridge chain, I just can't do otherwise.

I suppose that after three positive consumer reviews of Allnic products in a row, I am coming off as a drooling, bug eyed fan boy. Since my audition of the Allnic HT 2500 was as an unsolicited try out on an unpaid piece, I wouldn't blame anybody for thinking that way, I probably would myself, but it would really be too bad. My system already qualifies in the "high end" zone. The Allnic HT 2500 in my system nontheless is like a turntable, tonearm, cartridge and phono stage upgrade all in one. It has made me realize that my turntable is a deeper, more stable platform than I thought, that my cartridges and tonearm are capable of greater information retrieval than I thought, and that the vinyl medium itself is capable of much denser, clear and dynamic performance than I thought. The fact that the HT 2500 can kick the performance of the H 3000 phono stage upstairs again speaks volumes. Drooling or not, I think the HT 2500 is more than worthy of an audition in any high end vinyl system.