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August 2001
Enjoy the Music.com Review Magazine
Herron Audio M-150 Amplifier
Review by Karl Lozier
Click here to e-mail reviewer

M150 Front Panel

  I had barely finished listening evaluations and writing the review for Herron's excellent first consumer efforts. That was the tubed preamp model VTSP-1A and the tubed phono preamp model VTPH-1; reviews can be seen by clicking on our archives section and scrolling to Herron. He mentioned to me that he had developed an amplifier that was about to go into production. I called to see when production models might be available for review. Herron replied he would send a pair for review soon. He then really surprised me by mentioning that his amplifiers were solid state! Seems that some of his thinking paralleled mine, as we both usually preferred the sound of good-tubed amplifiers. However we both had serious reservations about long-term reliability. His viewpoint reflecting the reputable manufacturer backing up a new product, mine was reflecting some annoying past experiences. My reaction for the past many years has been to choose a preamp with really good tubed sound and mate it with solid-state amps that offer a neutral sound with a high-end response lacking in edginess and harshness. Keith Herron explained that his reaction was to develop a solid-state amplifier that equaled or at least comes close to equaling the sound quality of the better tubed amps. I couldn't wait to get my hands on a solid-state amp that had the sound qualities of good-tubed gear. After all, essentially everything he'd told me about his tubed gear turned out to be basically accurate. Though exceptions exist, most of us expect tube amps to have liquidity throughout the mid and high frequencies, a sense of presence, and a "bloom or richness" in the upper bass to lower mid-range.

M-150 Inside The Herron M-150 amplifiers arrived a couple of days before promised delivery time. I was under the impression that a few other reviewers and some dealers received units at about the same time. Unpacking them gave me a couple more surprises. They were small; they were light and visibly looked rather unimpressive and with Herron's usual row of green lights. As with the other Herron units, in ascending order the lights indicate power on, the voltage light indicates the circuitry is activated, the output light indicates ready for listening with output relay engaged. Herron has provided very comprehensive protective circuitry that has no effect on audible sound quality. A bad cable connector plug proved the protection's value to my system during a recent cable review.

Close inspection revealed that the front panel and case are identical to the other Herron units and cost effective. With his initial products Herron has tried to control costs with things that have no effect on sound quality. No cost cutting is evident with engineering and parts. In this instance there are some small cooling fins added on the rear panel as well as a RCA type input jack and impressive output posts for the speaker connection and an on/off switch. Years ago I had a custom built solid-state amplifier made for me; its transformer weighed more than the entire Herron amp and just its massive filter capacitors would probably fill the Herron case. I was leery of these small amps, so different from what I've been accustomed to.


M150 Back Panel

When I fired up the units and started listening leery turned into a bit of disappointment. The entire top end was clean, clear and very detailed and actually about as liquid as most tubed amps. There was no bloom and little fullness or warmth to balance the top half of the spectrum. The sound was the opposite of my BEL amps, which if anything are a bit overly warm or full sounding. Where oh where was the tubed amp sound Herron had enticed me with? I gave the units weeks of break in time resulting in only a slight improvement. Different cables helped a bit, particularly the Kimber Select #1011 added a small but perceptible degree of warmth and fullness. The amps were running at what I call stone cold and the sound was a bit like that too. Unfortunately Herron's superb preamp is all too revealing and flaws are uncovered easily. It was time to call Keith Herron.

I called and said to him, "Keith, when I received your preamp for review, sooner or later most things you told me about it turned out to be true". "Now with your new amplifier only some things you told me are turning out to be true, overall it sure doesn't resemble an excellent tubed amplifier". He kind of laughed and responded by saying something to the effect, "I've been wondering when you'd call, a number of the early run of the amplifiers somehow got sent out with the critical bias misadjusted and you, Michael Fremer, Art Pfeiffer and a few dealers got the out-of-spec-units - send them back to me and be prepared for a huge change that I'm sure you'll like." I returned the units to Herron and by the time they were returned to me I got to see Fremer's review, which pretty well mirrored my unwritten conclusions. Knowing Herron's dedication to outstanding neutral sound quality at reasonable pricing, I was surprised to hear that some units were released without undergoing and passing critical testing. Who said that we reviewers always receive the best units that have been checked thoroughly and then rechecked? Herron was understandably embarrassed about the situation. "It sure brought me down to earth after the rave reviews and awards the preamps had recently garnered," said he.

What about the sound of the returned and now correctly biased amplifiers, you ask? The difference correlated well with the difference between night and day - like WOW, can a simple change in bias adjustment do all this? The answer is, yes it can and does! The bias adjustment for the Herron is unusually critical because of its unique circuitry design that has a feed forward component of the signal from every amplification stage to the output stage to greatly reduce distortion. If you've heard of crossover-notch distortion affecting sound quality at relatively low levels, when accurately biased, Herron claims his amplifier to have zero crossover-notch distortion. Adjusting the bias a bit higher than ideal (or lower), in this case actually raises distortion while lowering sound quality.

Class A biased designs would often respond differently and distortion would go down as well as power output and only temperature would increase. Originally the amplifiers' heat sinks became barely warm but the cases remained cold. Eventually after all upgrades were completed, the case adjacent to the heat sinks would become slightly warm. This has to be one of the coolest running amps. While the case warmed up slightly, the sound warmed up as fully and richly as could be desired. The original lean sound quality had disappeared completely. The transparent and detailed upper-mid and higher frequencies remained and seemed even slightly more pristine than ever and still had that liquid quality to them. Had Herron actually created a solid-state amplifier that sounded like a very good-tubed amplifier?

As good as it is, and it is far better than just good, it still is not "a solid state tubed amplifier". It has most (but not all) of the richness and fullness of tubes without their typical slight loss of control in those critical mid and upper bass frequencies; in fact with a number of tubed amplifiers that loss of tight control can be an appealing quality but not an accurate one. In the lower bass range can there be more than a couple of tubed amplifiers at any price that can compete with a number of solid state amps (Herron's M-150 included) in the lowest reaches of bass extension and with tight control? Tubed amps are famous for what they can do in the so-called presence range. 

The Herron gives a great deal of the presence range's presence, but not as much as tubes often do. Does some of the sensation of presence with many tubed amps result from the sense that the higher frequencies (above the presence range) are relatively subdued? They certainly are not with the M-150 amplifiers and neither do they appear to have any added grain or edginess. As time passes I've come to think that just maybe the solid state Herron sounds more tube-like than its stable-mate tubed preamplifier. While these many musical qualities abound, observations revealed a smooth and truly grain free response and no tendency to exaggerate faults (almost unique for solid state) though all the while clearly revealing them more so than ever, but not adding to them.

Its unveiled transparency can let it be a revealing tool for evaluating subtle differences in cables, wires and accessories. Meanwhile even the subtlest nuances of musical enjoyment are presented in a musically natural way even at very high sound pressure levels. This is true for all except those rather rare loudspeaker designs that present impedances around one or two ohms at low frequencies. These few are simply not really compatible with the Herrons, nor with most other amplifiers. In his design it would have meant sonic compromises for the far more common speakers rated at 4 ohms or higher. With 4 to 8 ohm speaker loads, dips down to 1 or 2 ohms at higher or middle frequencies will not cause any operational problem.

I was getting used to the Herron M-150's while finishing the "cable shootout review" which can be found elsewhere in our Internet magazine. Whether it was the differences in fingering on a guitar or banjo or the differences between guitars, all the nuances seemed to be clearly and naturally revealed on recordings such as the newly released Classic's remastered in 24-bit/96kHz technology, The Weavers' 1963 Reunion at Carnegie Hall [Classic # 24/96 DAD 1041]. Background comments by members of the group, foot stomping and audience applause were as natural sounding as I've ever heard. There are no specific passages that would stress a good amplifier; here I just had to listen and relax and see if I had the feeling that, "yeah, this sounds just like it must have sounded thirty-eight years ago". It did.

To check how well the Herron did with more demanding material, I often used Reference Recording's new Copland album [RR # 93CD] which features the HDCD process. This is a tremendous album, excellent playing and conducting and an outstanding recording; don't sit around thinking that something in SACD or DVD-A is going to be out next week with better versions of these excellent American pieces. The finale of the third symphony is an almost staggeringly stunning achievement in sound recording. In addition to the beginning selection, the famous Fanfare for the Common Man, listen to the remainder of the album. After that, you can show off by replaying the last cut, the final movement of the symphony. What a workout and the Herron amps handled it all with ease. The great passages for horns and brass with tremendous dynamic contrasts - it's all there, plus some of the best closely mic'ed bass drum whacks of recent memory. I really did not have any recordings that seemed to bother the Herrons.

I can also recommend Classic's 24/96 remake of the rather famous Casino Royal soundtrack [DAD 1033]. Check out the outstanding horn/orchestral passages on the first cut. A very interesting aside occurred when I received Telarc's DSD [80433] The Film Music of Jerry Goldsmith for review. I had recently reviewed Telarc's DSD [80535] Mega Movies; check our music archives section. Each album has the main title music from the movie Air Force One. This comparison was particularly intriguing because one recording was done in a concert hall and the other in a studio (Abbey Road, London). You'll find a great difference in perspective between them, very interesting! I have every intention of finding out why the London Symphony Orchestra, which I heard live while writing this review, sounded much more like the studio recording!

I have a great deal of trouble trying to tell our readers how good the sound quality of the Herron M-150 amplifiers is when I don't have anything to really compare it with. My much older BEL amplifiers are simply not in the same league, though they've been my favorite solid-state amps (used as monoblocks) for some (too many?) years now. I thought that I had my relativity problem solved as a friend had just purchased a pair of used tubed amplifiers that many insiders (and our Senior Editor, Dick Olsher) consider one of the very, very few "great" amplifiers, name unimportant here. My friend thought they needed only some tubes replaced, which we did. So we hooked them up in my listening room and zilch. Off to the factory they went for complete checkup/tune-up; but it'll be awhile before I can hope to compare them with Herron's M-150s. Possibly there'll be a future update comparison. There's simply no point in trying to compare them with typical amplifiers in their approximate price range. For readers with interest in the story of the "upgrading of the M-150s", see the addendum at the end of this review article, a letter from Keith Herron.

At this point in the review, a few hours earlier today I returned from three solid days in Daytona Beach, the "other home" of the London Symphony Orchestra. Every other year the orchestra resides there typically performing series of three concerts during two weekends and also almost every "combination or breakup" of orchestral members during the usual twelve day stay. You can attend all-brass programs, all-string programs, trios, quartets and so on; including other artists at this International Festival there are more than sixty performances during this time. Again I came to the conclusion that we're really not close to equaling the experience of a great orchestra in a decent concert hall in our homes.

This year my attention was particularly drawn to and repeatedly struck by the almost unbelievable sense of depth and presence with no loss of clarity and detail from the winds and horns. Visually they appeared to be about thirty-five feel back from the conductor. Never have I heard such a sense of depth combined with detailed presence as I did with this series of concerts. My wife and I were seated in the eighth row center (two to three rows further back than usual) four rows in front of us were Sedrick Harris, his wife and out of town friends. Sedrick is a former French horn player, now with Immedia Audio whose product lines include Herron Audio. Sedrick has replaced his far more expensive tubed amplifiers with Herron's (not because of business reasons).

During intermissions I told Sedrick that I was having a bit of trouble concluding my review of Keith Herron's M-150 amplifiers. Specifically I mentioned that I couldn't decide whether the Herrons sounded as if they were superb solid state amplifiers with some added qualities usually attributed to tubes, or did they actually sound more like superb tubed amplifiers with some qualities usually ascribed to the best solid state examples. I asked Sedrick what he thought. His response was in effect, "I don't really know; they just sound like music to me". At this time I can't add significantly to that, so I could close by saying "ditto". Ultimately I decided to call the Herron M-150 amplifier a truly outstanding solid-state amplifier that also has some attributes of good tubed units. Ultimately, with time and comparative experience with other solid-state and tubed amplifiers, I may be able to call it one of the great amplifiers. When looking for a new amplifier, tubed or solid-state, make certain the M-150 is on your must-audition list.


Adendum/Manufacture Comments

As mentioned in the review article, this is the story of the upgrading of the M-150s as relayed to me by Keith Herron.

Biasing changes:

1) We have made some changes in the biasing circuits and procedures for setting the bias. The biasing of the M-150 with its multi stage to output design is extremely critical and distortion will rise if the bias is set either too high or too low and it has to be set under precise operating conditions. We have recently obtained a Hewlett Packard 330A Distortion Measurement Set, which makes setting the bias much easier. (Pre-drivers need to be covered so they retain heat during the biasing procedure.)
It now takes the M150s approximately 5 minutes to reach the bias Q point needed for the crossover notch to be eliminated as transistors (ones not on the heat sink) need to reach steady state operational temperature. The amps will go through considerable sonic changes the first 5 minutes and to a lesser degree for the first hour of operation. 
Yes, the M-150s run warmer now with the bias increased.

2) The location of the distortion cancellation loop needed to be adjusted with the change in output stage bias. We now use epoxy to hold this loop in its optimized position. This loop cancels effects from electromagnetic and electrostatic fields on the amplifier circuitry.

3) Stacking: Best sonic performance can be achieved with the units separated or side-by-side rather than stacked. This minimizes electromagnetic interaction between the units.

4) Input filter: We have added an rf shunt capacitor at the input of the M-150 which reduces low level radio frequency signals which can contaminate the audio signal as it enters the first stage of the amplifier. The result was improved clarity from top to bottom. Karl sez: Herron sent the shunt capacitors to me and I installed them before returning the units for final checkup. He gave very explicit installation instructions to me including a photo. I called him and said, "these look like non-polarized caps, why would they need to be installed a certain way as in the photo?" "Because we surprisingly found they sounded better in this direction was his reply." I thought I knew what and where to hear any resulting improvement. Surprised again, there was a very noticeable improvement in clarity and detail over the entire audible spectrum!

5) Speaker protection: The M150 will trip off temporarily and stay off as long as a sufficiently high enough level of d.c. is present at the input to produce significant d.c. at the output of the amp (allowed rate/time limits are determined by a differential/integrator circuit). The voltage light will blink during this condition. The d.c. servo will gradually take out small d.c. offsets. The M150 will deliver up to (very near up to but not including clipping) rail to rail on a 10 Hz square wave (and it is square) as long as there is not d.c. components in the signal. This criterion serves well for delivering music without nuisance trips in an audio system. I do stand behind the design as it does what I want it to in an audio system and the extra protection may save the listener's speaker system when some of those unexpected blasts of low frequencies and d.c. comes from a bad connection (or other catastrophe) at an input source.

This "rebiasing" package is being offered at no cost (other than shipping) to all of our customers and dealers.

Best Regards,

Keith Herron


Tonality 100
Sub-bass (10 Hz - 60 Hz) 100
Mid-bass (60 Hz - 200 Hz) 100
Midrange (200 Hz - 3,000 Hz) 100
High-frequencies (3,000 Hz on up) 100
Attack 100
Decay 100
Inner Resolution 99
Soundscape width front 98*
Soundscape width rear 98*
Soundscape depth behind speakers 98*
Soundscape extension into the room 98*
Imaging 94*
Fit and Finish 94
Self Noise 100
Value for the Money 100

* In my listening room, because of room problems and less than ideal loudspeaker placement, translate a rating of 98* to mean as good as any similar type of product listened to under the same conditions has ever performed as accurately as I can remember.



Configuration: Single-channel (monaural)
Frequency Response: 20 Hz to 20 kHz 0.1 dB
Output Power: 150 Watts @ 8 ohms
                     275 Watts @ 4 ohms
Gain: 24 dB
Input impedance: 47k Ohms
Speaker fuse: AGC 6 amp supplied
                   (value selectable by user)
Absolute Polarity: Non-inverting
Power Requirements: 100, 120, 230, 240 VAC. 50/60 Hz. 
Standard IEC power connector
Toroidal transformer taps internally selected for appropriate voltage
Dimensions: 19" x 4" x 10.5" (WxHxD)
Warranty: 5 years, parts and labor
Suggested Retail: $5,895.00 per pair



Herron Audio
12685 Dorsett Road, #138
St. Louis, Missouri 63043
Voice: (314) 434-5416
Fax: (314) 434-6629

Website: www.herronaudio.com 
Email: keith@herronaudio.com













































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