Hammond 125ESE
For years I have been searching for a truly affordable SE OPT that is flexible enough for experimentation with various triode output tubes. Last year I found a DIY SE2A3 amp project featured in MJ 2/00 using a Noguchi universal OPT [pri.Z = 2.5K, 3.5K and 7K], the PMF10WS for JPY 6,800. They even have a lower priced model, the smaller, PMF 6W for JPY 3,500 for people who want to build an amp around a 71A or 112A. Unfortunately they do not export and their fax no. specifically states "not used overseas". This is a clear indication of a well developed market in Japan that caters to the audio hobbyist that makes it unnecessary for manufacturers and dealers to look beyond their shores to thrive.
Even if the SE/DIY revolution in the USA started in the early 90s, the availability of a true high-fidelity airgapped SE OPT for under $100/pr. is practically nonexistent. This could be one of the main reasons why in such a huge and progressive country, proportionately, the number of audio/DIY enthusiasts remain small.
The Audio Note UK "experimenter series" at $75/each was a pretty good performer but has been unavailable for quite a while and it still would have cost over a hundred bucks for a pair. The early Hammond transformers I tested [125E and 1627SE] back in '97 had some issues that fell short of my expectations. Although the 1627SE had pretty good bass extension [due to the massive laminations], the upper midrange to high frequency was very rough, whereas the 125E without an air-gap just didn't have anything below 150Hz with more than 20ma. of DC unbalanced current flowing and forget about the upper range. But that was 4 years ago and I am well aware that the 1627SE is a popular choice for satisfied builders of my SE 300B project while the 125E is a favorite for my Simple 45/2A3 and Bob Danielak's Darling SE 1626 [Sound Practices 15]. I can just assume Hammond has improved the performance of these products based on the market success of these models.
By chance, I stumbled by the new air-gapped 125ESE at $35/each while surfing Angela Instruments' website. After some deliberation I whipped out my credit card and ordered a pair. I also took the chance to say hello to Steve since I have not seen him or the shop for almost two years. Within a day or two they arrived at my doorstep. The 125ESE is an open frame unit weighing a little over 3 lbs and uses M6 silicon steel laminations according to the data. It is indeed twice the size of the original 125E but instead of solder tags, it came with long color coded leads.
Here is a picture of the 125ESE beside a Tango U808 for size perspective. It actually reminded me more of the Tango U708 in terms of size and weight.
The primary is adjustable between 2.5K, 5K and 10K and claimed to handle 80ma. of DC unbalanced current. There was no data on inductance [relevant for bass performance] but it looked promising enough despite my reservations about cosmetics.
I installed them [2.5K mode] on my current test mule, a stereo SE2A3 [schematic below] amp to play background music in my second system and had the surprise of my life because I was actually hearing music and found myself spinning various LPs and CDs. After a while I noticed one channel had a very slight tendency to spit out sibilance in the upper frequencies and confirmed it by switching cables between channels and it remained on the same side.
The amp was taken to my test bench for basic power output, frequency response measurements and I injected square waves to see how the OPTs were behaving. Sure enough OPT 2 had a slower rise time, an overshoot and some ringing at 10kHz, even at 1kHz the overshoot is quite evident [not a good sign].
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| OPT 2 @ 10kHz |
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| OPT 2 @ 1kHz |
However OPT 1 at 1kHz and 10kHz compared favorably with the Tango U808.
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| OPT 1 @ 1kHz |
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| OPT 1 @ 10kHz |
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| Tango U808 @ 1kHz |
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| Tango U808 @ 10kHz |
[Note: top trace is the input signal from the audio generator and bottom trace is the amplifier output].
In spite of my reservations about OPT 2, the 10khz performance was still better compared to the early '97 1627SE and significantly better than the 'ungapped' 125E across the entire audio bandwidth.
Obviously, what I heard [and confirmed on the scope] was a serious mismatch between the two 125ESEs, that is why it can be detected through critical audition. If I had two identical units like OPT 2, I would not have detected the anomaly, although I surmise that the sound would not be as clean as having a pair that measured like OPT 1.
I brought the amp to NYC so that my friend Ding can offer another perspective and I could also hear them through 604-8Gs. He was very impressed and thought they were great value for the money although he did mention hearing something slightly funky about the high frequencies on one channel. On some material he found it more extended but on other tracks it sounded a bit rough. But he qualified that it was not really very apparent and only noticeable when he listened for it. He also made me promise to bring the amp on my next NYC trip because he really liked it. Meanwhile I had already emailed Steve requesting for another pair of 125ESEs because of my earlier findings and wanted to get a handle on quality control. By the time I returned home, the package was already in my apartment.
The first pair was replaced by the second pair. I turned on the amp to play background music for about an hour and then started serious listening evaluation in my main system with the Altec 2-way speakers. I used another SE2A3 amp with Tango U808 OPTs as a reference point. The circuit is virtually the same in both amps with bi-plate 2A3s running at 18W plate dissipation and the U808 set to 2.5K mode [usually set to 3.5K, but I wanted to limit variables]. I listened to a variety of music ranging from LPs by Sinatra, Ella, Billie, jazz/big band, 80s pop/new wave, to Mahler sym. # 5, Bartok concerto for orchestra, Mozart string quartets, Ravel piano trio and DAT masters of live performances I recorded.
Do not be discouraged by the specified +/- 1dB, 100Hz-15kHz bandwidth because it was easily exceeded in my listening evaluation and test bench measurement results. I could hear down to 50Hz, perhaps the fundamental was already rolling off but the bass note overtones [harmonics] were right, which is very important to hear good bass definition. The U808 offered more air and space indicating a higher bandwidth ability and gentler treble roll off, it also sounded more controlled and composed in the bass. If I was to nitpick, I would say that the U808 has the edge in the ability to resolve detail and more transparent overall with a slight tendency to sound dry and clinical. In spite of a bit more grain across the audio spectrum, the 125ESE always delivered the emotional impact of any pre-recorded material. This is what I deem as high-fidelity in the true sense of the word, honoring the music. The relative lack of transparency will not be apparent unless the units are pitted against superior and more expensive units.
Assuming OPT 2 was a fluke, I give the 125ESE my highest recommendation. It should not take that much time nor effort for Hammond to keep quality control within the standards set by OPTs 1, 3 and 4 while keeping the same cost. Based on the impressive performance noted, the 125ESE deserves metal shrouds to cover the core even if the 2 - bolt mounting tab arrangement is kept. This way it can be painted giving DIYers some flexibility with aesthetics. I also no longer endorse hacking PP OPTs from vintage tube integrated amps for use with my Simple 45/2A3 project; the 125ESE will perform much better in this application.
If I had the time I will not hesitate running 300Bs with these transformers. But watch the current, even if it is specified to take 80 ma. the core may still saturate [just a hunch, based on the 100Hz square waves I saw], you may be better off backing down to around 60-70ma. For those who really prefer their 300Bs hot, the 1627SE is a safer bet and will assure greater bass extension.
Given the performance and flexibility the 125ESE offers at such modest cost, it should bring the joy of SE DHT amplification to a wider audience and the unavailability of affordable OPTs is no longer an excuse not to build your own SE amp. Place your order for 125ESEs at your favorite dealer so that Hammond will continue improving their products and come out with quality transformers like the 125ESE for DIY enthusiasts. While you are at it, snap up the remaining American made 2A3s, 50s, 45s, 10s/VT25s, 71As before they dry up and watch out for vintage paper cone alnico drivers, because you will need them.
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