AliEx amorphous, Hashimoto HL20K6 + Sony TamRadio line output transformers

L>R: 6AH4 + AliEx, 12B4A + Sony/Tamradio and 6AH4 + Hashimoto HL20K6

The last time I saw my buddy J-Rob was pre-COVID, but we keep in touch via email and text messages. During one of those hello/how you doin' moments, the chat progressed to line preamp transformers and how the $100/pr. amorphous jobs from AliExpress perform. 

I used to own a pair of Tango/Hirata NP216N nickel line OPTs, which were featured in my Sound Practices issue 17 homebrewer article. In hindsight, I don't think I gave them a fair chance so this was a good opportunity to revisit the topology.

SE171A Amp Remastered

My dad would've turned 97 today. To celebrate his birthday, the video clip above features a recording of his favorite violinist, Nathan Milstein, playing Presto from Bach's Sonata No. 1 in G minor for unaccompanied violin. While he was doing post-graduate studies at The Juilliard School in the mid 50s, he was fortunate to have attended concerts of the great violin virtuosos of that era - Jascha Heifetz, David Oistrakh, Nathan Milstein, etc.

He would later recount that Nathan Milstein's performance of the Glazunov violin concerto was the most unforgettable! Even if he never got a chance to add this violin concerto to his repertoire, he conducted the Manila Chamber Orchestra with me playing the Glazunov violin concerto in 1987. The music of the slide clip above is the middle movement from our performance.

Before I bore everyone reminiscing about my dad and our life as musicians, let's get on with the subject at hand. The amp featured in this upload uses a directly heated triode power tube that was introduced around the time my dad was born.

JE Labs Choke-Capacitance Coupled SE2A3 amp

Canon P + Canon 50mm f1.8 LTM 
Foma 100 in Rodinal

Inspired by my plate choke-loaded 76 line stage preamp adventures, I was curious how a similar driver circuit configuration would sound in an SE2A3 amp. So I pulled out my Radiotron Designer's Handbook, 3rd Edition to gain a better understanding of choke-capacitance coupling (aka plate choke loading) vis-à-vis resistance coupling.

Below are highlights of relevant information I gathered.

morrison micro 2A3, reimagined + Kasuga Wireless KA6625ST output transformer

Joe Roberts' piece, "I Never Met a 2A3 Amp That I Didn't Like," from Sound Practices Issue 15, is very persuasive. I built jc morrison’s micro 2A3 amp soon after I received the issue via snail mail fresh from the printers in the mid '90s. After rereading the article in ‘07, I got inspired to make a Radiotron SE2A3 variant.

!!!WARNING!!!

The voltages found in the following circuits can be lethal! Build at your own risk!

jc morrison's micro

That '90s morrison micro build used the Angela Universal Power Transformer with a 760 VCT @ 200 mA (very conservatively rated) HT secondary + a GZ37 rectifier for each channel. The thought of my 2A3s broiling way over the plate dissipation was daunting. Even if Chinese 2A3s were only $10 a pop then, my finger tips tingled in anticipation of a fireworks display each time I flipped the toggle switch on. But the sound through Audio Note UK 3K output transformers was delicate yet exuberant, with a touch of mystery! 

Since my Altec 755As in open baffles required more power, or so I thought then, the power and output transformers were expropriated for the classic JE Labs SE300B project. 

Fi "X" by Don Garber
from a Sound Practices ad

 In 2017, I had wanted to build a 6SF5>2A3 amplifier tribute to the Fi "X" in memory of Don Garber 😢 but a busy schedule got in the way. 

During the March 2020 lockdown, I was motivated to review my audio DIY to-do list which led to a long-overdue parts bin inventory. I turned up a 700 VCT @ 150 mA power transformer pulled from a junked Scott 99D mono integrated. This iron is not a stranger since I've used a pair in my PP2A3 mono-blocks in the olden days. I dug up a couple of 6C8G grid capped/octal based twin triodes as well.

6SL7 vs. 6C8G 
transconductance (gm)= 1600 umhos (both)
gain (mu) = 70 vs. 38
plate resistance (Rp) = 22,500 vs. 44,000
max. plate current (each plate) = 2.3mA vs. 3.2mA

Despite the equal transconductance, the 6C8G isn't a drop-in equivalent of the 6SL7. Aside from the different pinout, the mu and Rp are lower. When the triode sections are paralleled, the Rp is closer to a medium mu/gm triode like a 76. 

morrison micro, reimagined

This 2-stage direct coupled circuit is a reimagined morrison micro using a 6C8G front end. The paralleled triode sections draw about 3.4 mA and the 510 ohm cathode bias resistor is bypassed with a 100uf @ 16V cap to maximize input sensitivity. 

Chinese 2A3s typically sell for $100 a pair nowadays

Each 2A3 is AC heated by a Hammond 166M2 (2.5V @ 3A) filament transformer. The operating points are well within the modest specs of the Scott 99D power transformer.

Output Transformers

The fullest potential of an amplifier circuit can't be realized if it's inhibited by a mediocre output transformer.

Hashimoto H203S

Transformer design can be a contentious issue. Audio forums contain at least 30 years' worth of intense arguments amongst gurus with regards to winding techniques and materials - EI laminations, C-core, double C-core, silicon steel, nickel, amorphous or nanocrystalline (aka Finemet). Unfortunately, there have been no definitive conclusions and the debates continue.

James 6123HS

Despite my limited knowledge of electromagnetic induction, I've done lots of serious listening evaluation of various transformers + a bit of basic testing through the years. My ears developed an affinity for the design ethos of Japanese transformer companies like Hashimoto, Nature Sound, Noguchi (now General Transformer, Co. Ltd), Tamura, Tango, etc. I also lament the now-defunct line of James budget and mid-priced output transformers from Taiwan, which sounded akin to their neighbors to the north.

Nature Sound NS70SE

I don't think the skill and expertise is unique to the Japanese. My empirical mind tells me that they just improved upon what they learned from reverse engineering classic American and European audio equipment, analogous to Canon and Nikon copying and innovating upon Leitz and Zeiss cameras and lens blueprints post WWII. 

Tamura F7002 Permalloy

The transformer art is also not lost stateside. Based on my very positive experience with custom-wound nickel plate chokes from Intact Audio's atelier, my hunch is, dave's output transformers are probably wound to similar standards since he's had ample exposure to Nature Sound, Tamura and Tango iron due to his long association with the NY Triode Mafia scene. 

Tango/Hirata NY15S 

Regrettably, I haven't had much luck with budget-priced heavy iron. The ones I've tried usually present my 'pet peeve' tonal curve: boosted upper bass, recessed midrange, a spiked upper midrange to lower treble, followed by a sharp roll off in the top end. To some ears this kind of voicing sounds captivating. But to be honest, it's a musical disaster to mine. 😞  

Budget Output Transformers 
Noguchi PMF-10WS, Edcor GXSE15-16-3.5K, Noguchi PMF-15WS

When I can't afford a top of the line Sumo-sized output transformer, I compromise on full power at the bottom end and settle for a well-designed Bonsai-sized iron with a gradual and smooth roll off beyond the audio band. It's an insurance that my midrange will sound clean due to musically consonant upper harmonics. 

Noguchi (now G-trans.co.jp) Finemet

So far, all the Japanese budget transformers I've tried have not failed me in this regard. Some even exceeded my expectations, like the entry-level Noguchi Finemets above.

Kasuga KA-6625ST

For this project I used the Kasuga Wireless KA-6625ST. This is an entry-level output transformer, which is less than half the size and weight of the Audio Note UK 3K from the 90s. Weighing in at a lean 1.1kg, it can be configured  to reflect a 2.5K, 3.5K or 5K primary Z via the multiple secondary taps. 3K-3.5K has become my de facto load for the 2A3 and 300B, only because my ears prefer it. 

Alas, these transformers aren't available in the USA. But if you happen to be in the vicinity of their Akihabara store, you can pick them up for about $82 apiece.

Kasuga KA6625ST Square Waves
L > R: 100Hz, 1kHz, 10kHz

Although not musically definitive, the square wave pictures presented above indicate good circuit and transformer behavior. Considering that there's no corrective negative feedback loop in this DC SE2A3 amp, it hardly pales (except @100Hz) in comparison to the 100Hz, 1kHz and 10kHz square waves from the classic push-pull EL84 (with negative feedback correction) Dyna ST35 amp, renowned for its fine sonic quality and wide bandwidth ultra-linear output transformers. 

4.34V rms squared/8 ohms = 2.35W

The amp puts out 2.35W at 1 kHz into an 8 ohm load before clipping (lower trace) with an input sensitivity of ~ 1.2 V rms (top trace). If memory serves me right, I measured about 3W per channel from my '90s morrison micro build. About 20% of power was lost due to my cooler operating points. 

Sonic Palate

Connected to my Altec 753Cs in the main system or the Altec 755As in Silbatone cabs in the near field system, the DC SE2A3 amp doesn't have the body, complexity, intense richness, dynamic impact and bandwidth of my Bordeaux-like heavyweight amps which employ lower Z driver circuits + top of the line Japanese OPTs. Instead, it reminds me of the sweet and fruity flavors, earthy spices, color, subtlety and finesse from my White Burgundian Simple 45, which was the main reason I was forever drawn to this archaic method of building amps!

Enjoy the music!🍷

Darling Amp, JE Labs Style

It's been 20 years since jc morrison started hosting the new york noise audio design exhibition at his Hoboken, NJ loft. One of the highlights of that year's exhibit was Darlingfest '99 as reported by Jeremy Epstein in the final issue of Sound Practices. The Darling amp was the brainchild of Bob Danielak and was published in Sound Practices Issue 15.

Jeremy's octal driven Darling amp made a lasting impression on me but I never got a chance to build my own version due to a busy performing schedule.

While I was organizing my workshop in the basement this past winter, I found power and output transformers from a Sony TC500A tape recorder + other parts to build a Darling amp.  

The chassis cleaned up nicely with a wire brush. Then I drilled, cut and punched the necessary holes before spraying a fresh coat of silver Hammerite.

I chose the 6F5 hi-mu triode with a cool grid cap as the input/driver tube.

To qualify as a Darling amp, the indirectly heated 1626 transmitting triode  output tube is de rigueur.

The TC500A donated a healthy 6CA4 rectifier.

!!!WARNING!!! 

The voltages found in this circuit can be lethal, build at your own risk!!!

My interpretation of a Darling amp is ready for testing and listening!

Bench Testing

Both channels driven

P = V²/L

2.415 x 2.415 = 5.832/8 = .729 or 729mW per channel

Hum and Noise = 1.8mV

100Hz

1kHz

10kHz

I did not expect this level of performance from the cheap looking Sony OPTs, which I also verified while listening to the amp!

Bass extension is surprising from this 3/4 watter! It is deeper and tighter compared to my Tango'ed SE171A amp. However, in the midrange, the true Directly Heated Triode shines. The 1626 was just a touch veiled in comparison. Although it sounds more powerful than the similarly rated SE171A, it also doesn't overload as gracefully. I remember hearing a similar phenomenon when Steve @ Angela Instruments asked me to build the Simple 5691 > EL34 SE amplifier. The Simple 45/2A3 clips with more finesse. Is the cathode causing the relative lack of transparency and harder clipping?

The days of $5 Type 45s and 2A3s are long gone. But NOS 1626 tubes can still be acquired for under $10 a piece. Since it has a cathode, with careful wiring less than 2mV noise is achievable on AC heating which is quiet enough for me even on headphones. There's no need to hunt for hard to find 25-75 ohm, 2W hum nulling pots. That's just a couple of reasons as to why a Darling amp derivative is one of the cheapest and easiest routes to a DIY SET amp nowadays.

Did I mention that it also does a great job driving headphones?

headphone adapter/attenuator schematic for 
efficient headphones ex: Grado SR80, Koss KPH30i/Porta Pro
Lower efficiency/high impedance classic headphones like the 600 ohm AKG K240 Monitor, 
Sennheiser HD420, HD6xx and Orthodynamic/Planar magnetic: Echo TDS-16, 
Fostex T10 and Yamaha YH-1, can be driven directly from the speaker terminals

Happy listening!

Morikawa SE2A3 in Jose's Listening Room

Back in the 90s I used to hang out with Ding and Jose at Hamfests in the suburbs of Philly. Jose isn't your typical audio nerd who subscribes to the latest is best mentality. He goes for value for money and although not a DIYer, has an eye and ear for quality design! Even if he has a Technics SP10 + Sumiko MMT turntable combo, most of his listening nowadays is through a high quality pre-bitstream a Denon DCD 1500 and a stable platter Pioneer PD65.

Having heard that a well designed SE2A3 amp is a synergistic match for his Altec 604-8Gs, he spotted and won this Chuyu Morikawa designed Audio Professor SE2A3 amp on eBay. However, the amp had two issues. The input level controls were no longer functioning, and worse, the amp was unstable manifested by extra-musical popping and whistling sounds. Since we're practically neighbors now, I took it home for a check-up.

Under the hood 

Beautiful workmanship, typical of a handcrafted amplifier from Japan

After installing a dual ganged 100k Alps audio taper input pot, I realized that the cause of instability was the step down transformer the eBay seller sold with the amp. It was hardly stepping AC down to 100V. During the amp's residency here at JE Labs it was plugged to a General Radio Variac set to 100VAC. It was stable and quiet. Fortunately Jose also has a Variac on stand by, which he acquired during our Hamfest hunting days.

6Z-DH3A

The amp uses 4 x (2 for each channel) 6Z-DH3A, UX-6 based, high-mu triodes as input/driver tubes. This tube is electrically equivalent to an octal based 6SQ7. AFAIK, there are no plug-in compatible US or European equivalents. 

The input/driver circuitry is configured in SRPP, very similar to what I did in the JE Labs SRPP 45/2A3, albeit with textbook operating points.

5 Volts rms to an 8 ohm load at onset of clipping = 3.125W per channel

Luxman SS5B-2.5k OPT

100Hz

1kHz

10kHz

These square waves show that the Luxman SS5B 2.5K primary SE OPTs are of very high quality. Not much information is available on the internet regarding this transformer aside from completed auctions, but I've encountered this model in schematics and pictures in Asano's two volume tome on tube amplifiers.

Since Luxman SE OPTs were no longer available during the Sound Practices era in the 90s, I would date this amplifier to the 80s, at the latest. That's before Cary introduced the 6SL7 SRPP driven 300B or 2A3 AES SE-1 kit amp to the mainstream market.

I built SRPP circuits back in the day, but was never a fan of this topology. However, this amp made me reconsider my stance. Now I'm asking myself - could it be the textbook operating points and/or the 6Z-DH3A input/driver tubes which made this experience memorable? 

I truly enjoyed listening to this amp, thanks Jose!