A visit to J-Rob's and the Silbatone 755A box by Dr. Stefano Bae

OB/755A

Despite the fact that Joe Roberts and I both revere the 755A, he never thought highly of my open baffle set-up. To this day, I'm still baffled as to why he featured my home-brewer article in Sound Practices. 😉 I enjoy the engulfing planar-like airiness of the 755A/OB combo which Joe finds lifeless, boring and lacking in snap. 

OB + 755A

Could the dip in this RTA trace be indicating a lack of snap? 😜

Custom-built Altec 618 + 755A

J-Rob custom 618 cabinet

  Back in the mid-90s, I owned a pair of original Altec 618s. Those sounded boxy and colored, perhaps due to their lightweight construction. So I traded them for a pair of early sand casted 811 horns...😧 Oh well, live and learn. 😁

When my friend Chong gave me a pair of well braced vintage DIY 618 boxes (allegedly built by an ex-WE engineer and used to house a pair of WE755As), I understood the snap that ticked the Joe-man.

ex-J-Rob Altec 618 + 755A

This is the nicest and most modern looking RTA trace in this survey! Loaded with Altec 755As, the sound is snappy and dynamic while retaining that warm and realistic sheen in the midrange, which evades most speakers marketed in the past 50 years. Although some listeners may still find that quality a vintage coloration...

A visit to J-Rob's

People don't always have to agree to remain friends

Case in point: Joe seems to prefer the Japanese 755 - Lafayette SK98/Hamlin/Pioneer PIM8L - over the ferrite 755C. However, if push comes to shove, I'd take the 755C over the various permutations of the PIM8/20L even if I played a role in reintroducing its virtues to the world. Maybe it's just my sentimental attachment to the 755Cs since they were my very first high-efficiency drivers?

J-Rob sez, "I shouldn't even be talking to you since you like 755Cs." 😆

When I visited Joe in the summer of 2018, he had a pair of 4 cubic foot WE753A look-alike cabs that house Western Electric WE728Bs driven by a spruced up Dyna SCA35 which was recently replaced by a J-Rob cloned Peerless A100 PP 6A5G amp built during the Sound Practices era. Signal source is a Mac Mini>Tidal>Audirvana>Wavelength Audio Brick DAC.

Despite the natural high frequency roll off of the 12" WE728B, it sounded very full-range because of its rich overtones. That is, overtones as musicians use it to describe musical instruments. A high quality violin, piano, or any musical instrument for that matter, possesses rich overtones and this has nothing to do with high frequency extension. Even FFT analyses have failed to identify why del Gesus and Strads sound harmonically richer and have greater carrying power in a good acoustic space compared to a painstakingly crafted modern violin.

I was looking forward to hearing the Silbatone SE300B amp on the floor but Joe hasn't gotten around to re-wiring it for 120V, and also to LPs, but the Gates CB500 was waiting for a resurfaced idler wheel. Then he showed me his next project - Klangfilm horn repros (center background) + Altec and JBL large format compression drivers.

Finally, I was introduced to the pair of Altec 755As perched on top of the WE753C-like cabs. I was quite impressed with the sounds emanating from the 755As!

This 755A/box combo was featured in the Silbatone exhibit at the Munich High-End 2018 Show. Prior to that, Joe spoke fondly about the box and gave DIY hints in this Audiokarma thread. I was hoping that he'd be inspired to update his Junkyard Jukebox blog for a formal presentation of this cabinet design before he went on a sabbatical from the internet.

Since there's no sign that he'll be back anytime soon, with his permission, I took it upon myself to write about this 755A cabinet breakthrough.

Silbatone/Dr. Bae WE/Altec 755A box plan
Copyright Silbatone Acoustics

I re-drew the box plan based on a quick sketch Joe uploaded in the Audiokarma thread. Please note that this cabinet plan is published solely for the Audio/DIY community as a public service of Silbatone Acoustics. 

Under no circumstances should this be used for commercial purposes! 

Let's see if that works...truth be told, I am well aware that there have been a few enterprising individuals who have benefitted commercially from projects published in this blog. So I'm not holding my breath.

John Piro Clone Build
Interior walls + back lined with two layers of Kimpak

I sent the drawing to my buddy John Piro shortly after my Altec 753C cabinets were delivered.

Can anyone distinguish JP's clone from the original Silbatone box? 

Dr. Bae designed Silbatone box + 755A

J-Rob comments:

"The cabinets were designed by genius Silbatone engineer Dr. Stefano Bae as a group project for the Silbatone-sponsored DIY/Western Electric forum in South Korea. 100 pairs were quickly sold out, immediately driving up the world price for 755As. The original uses Silbatone's custom silver litz wire, which is said to be an essential ingredient of the original recipe, and top shelf WBT connectors.

Over the years Silbatone designed and made literally dozens of experimental boxes, including back horns for up to four (755A) drivers per side. This was probably the most successful, although rather modest.

My analysis of this untuned little cabinet is that it falls under the 'small box with a hole in it' design paradigm."

Unlike before when I felt the need for at least SE300B power to make the OB/755A sing, now I'm enjoying near-field sounds from my JP cloned Silbatone/Dr. Bae boxes + Altec 755As driven by an SE46 amp with tiny Noguchi FM6WS Finemet OPTs.

The SE2A3dx which normally drives the Altec 753Cs in the main listening area had no trouble driving the Altec 755As in Silbatone boxes to moderately high SPL levels.

 Even without the silver litz wire and high tech WBT connectors, I heard all the virtues of the slant 618 box + open baffle combined = detail, snap, macro and micro dynamics from the 618 box + airiness and lack of vintage boxy colorations from the open baffle.

The Dr. Bae/Silbatone cabinet transported the 70 year old 755A design to the 21st century in a monitor-sized package!

First Philippine Triode Festival June 15-16, 2019

Makati City, Philippines

Featuring a G.I.P. Laboratory + Harana Audio collaboration

GIP 597A field coil high frequency horn

GIP CH-65 midrange horn + GIP 555W field coil compression driver

GIP 815 = 15" field coil woofer loaded in Harana Audio V-Vent II bass cabinets

+

Flea-powered triode amplifiers built by Filipino enthusiasts and guests from our Southeast Asian neighbors.

The Enablers

more info

Hashimoto H-203S from Kasuga Wireless

In September 2018, I lamented the demise of Noguchi Transformers. Fast forward to November, my brother-in-law visited Tokyo, and was kind enough to pick up a couple of transformers at Kasuga Wireless.

How to get to Kasuga Wireless

Kasuga Wireless

Address: 1F, New Akihabara Center, 1-16-10, Kotobuki, Chiyoda-ku, Tokyo 101-0021

Hashimoto H-203S

The H203S was installed as a 3.5k primary Z output transformer in the je2a3 mono integrated amp. 

Testing

Top trace = audio generator

Bottom trace = amplifier output

100Hz

1kHz

10kHz

5.324Vrms to an 8 ohm load = 3.5W 

In the past, I've used Hashimoto transformers for a pair of SE2A3 and a pair of SE10 mono-blocks I built for friends. I know these transformers are well designed, are of high quality construction and sound excellent. Thus, before Hashimoto ceases production, I had to add it to my collection. 

My mono SE2A3 integrated is ready for prime time!

Now I have to find a nice circuit for a pair of Kasuga KA6625ST OPTs. 😉

Altec 753C

After many years of raving about the Altec 32 horn, the only stone left unturned was using it as it was originally designed, that is, with the compression driver oriented to radiate upwards.

Last year, I received a message from John Piro, whose beautifully crafted preamp project was previously featured in this blog. He sent me pictures of his interpretation of the legendary WE753C using an Altec 414-16B woofer + Altec 32A + Faital Pro driver. 

Altec 414A + 32C + 802D + 20275 

I was very impressed and requested if he could build a pair for me. We finally met at the Kutztown Radio Show last September. He and his wife had dinner here at home when he delivered my beautiful cabinets!

A Western Electric 753C used as a monitor at WABF in the early 50s

As an homage to the legendary Western Electric 753C, which was a major inspiration for this project, I henceforth christen this speaker system as the Altec 753C.

Altec 2-way XO redux

I've settled on running the 414A woofers in full range mode (ala J-Rob) and replaced all the crossover resistors with non-inductive 12W Mills. This took away the last vestiges of grain in the upper frequencies.

RTA Graphs

Altec 2-way

Altec 614 + 414A + 32C + 802 with GPA34258

The above RTA was taken from this blog entry

VS

Altec 753

Altec 753 + 414A + 32C + 802D with 20275

I could've lived happily with my Altec 614 or repurposed 861 (aka Madrid) cabs + the inverted 32 horn situated on top. However, with proper horn orientation, the sound was taken to another level of refinement. Now the blend between the 414A and horn is practically seamless and I bask in the airiness of the top end, which I attribute to significantly improved high frequency dispersion, precisely as envisioned by Western Electric engineers.

In spite of the stellar midrange performance of the Open Baffle mounted Altec 755As, I started the Altec 2-way project in the late 90s to address the following issues:

1. More efficiency
2. Extend the frequency extremes
3. Less congestion in dynamic peaks with complex musical passages

After 20-some odd years, those goals have been realized. The Altec 753C is pretty much end game!

Thanks to my buddy Joe Roberts, who explained the intricacies of proper 32 horn orientation. The idea was carefully stored in my mind. I just had to wait for the right opportunity to implement it. And of course, a big thanks to my friend John Piro (aka RCA NUT), craftsman extraordinaire!

Let's hear some music. 😊

Ikea Knarrevik

Ikea Knarrevik 

I use it as:

component stand

turntable stand

speaker stand

Best $10 I ever spent on Hi-Fi! 😉

John Linsley-Hood 1969 Class A Amplifier aka Nobsound NS-02g

Original JLH1969 Class A amp schematic

The John Linsley-Hood 1969 Class A amplifier circuit is a classic solid state design that is carefully studied by audio designers. Even the great Nelson Pass revisited this circuit when he embarked on low wattage amplifier designs.

It has been described as one of the most tube sounding solid state amplifier designs, which is not surprising since John Linsley-Hood's references were tube amplifiers. Here's a link to the 1969 article published by Wireless World.

Since I am a DIYer at heart, I originally considered buying a pair of these circuit boards on eBay. But when I spotted a built unit for $170/shipped, I calculated that by the time I factored in the cost of a power transformer and a massive heat sinked chassis, it didn't seem wise or cost effective to go the DIY route.

The JLH 1969 Class A clone I got is branded Nobsound NS-02g but I've seen the same chassis and guts in silver or gold sporting a Krell logo!

It is very compact- a hair over 13" wide x 8.25" deep and 3" high- but it weighs 12 lbs. due to the heavy gauge chassis, massive heatsinks and power transformer. Parts quality is surprising for a $200 amplifier requiring discrete components. Input impedance is set by an Alps Blue Velvet 10k Ohm logarithmic potentiometer. There are no IC chips in this amp, just a couple pairs of transistors, a handful of resistors and capacitors + a lot of copper wire wound in the toroidal transformer. This is a true Class A design from the late 60s built at the end of the second decade of the 21st century.

When I first fired up the amplifier, I heard a buzz when the input level control was at maximum. I traced this to the IEC socket which was not grounded. Strapping the center pin (green cloth insulated wire) to ground took care of the buzz.

!!!WARNING!!! 
The voltages in this circuit are potentially lethal! Proceed at your own risk!

Set Up and Adjustments

NOTE: Red probe to test point, black probe to chassis ground

Idle Current

Out of the box, the amp sounded pleasant but I was not hearing anything magical. I was suspicious of the meager 0.5 amp AC current draw at 120V I monitored on my GenRad Variac. So removed the top cover to study the circuit and determine the purpose of the two potentiometers on each channel's PC board. 

The pot towards the middle of the PC board is for idle current adjustment. Since I didn't have a manual and couldn't find one on-line, I took the cue from this video by a JLH 1969 Class A kit amp builder.👍 His circuit board looked similar enough, thus, the proper voltage must be closer to about 20V(?). I measured 26.5V and 26.2V on my unit. So I turned the pot clockwise until I got about 24 volts DC on both channels. The AC current draw was now over 1 amp and I started feeling heat emanating from the chassis. 

AC Balance

The next step was to adjust AC balance. I loaded each channel's output with an 8 ohm, 20W wire wound resistor and then injected a 1 kHz sine wave at the input jacks. I adjusted the AC balance pots on both channels while observing the clipping behavior closely for best symmetry - both phases of the trace should "square off" simultaneously.

Tweaking the idle current

137℉ @ the heatsink

I checked the idle current once more and listened. 26V yielded almost 10 watts per channel on the bench but I felt the sound can be sweeter. At 21.5V the amp sounded excellent. But the chassis was running physically too hot. Just to be conservative, I backed off a bit and actually found the sweet spot between 22V - 24V. I also made sure there's always ample ventilation around the amp.  

On the Test Bench

Top trace = signal generator

Bottom trace = amp output

100Hz

1kHz

10kHz

Not perfect, but nice and clean square waves from a half a century old solid-state design

Input sensitivity = .25 Vrms > ~ 6.8 watts per channel rms into an 8 ohm load with both channels driven

Tweaks

A slight glare in the upper midrange was ameliorated when I by-passed the 2.2uf input capacitors with 0.47uf Vitamin Q "Hyrel" PIOs and the 2500uf output capacitors with 10uf KBGs. Before reassembly, I also replaced the red LED pilot lights with blue LEDs. Much easier on the eyes especially in a dimly lit room.

 The Nobsound clone amp spent most of its time in the system above, driving a pair of Altec 755Cs in 618 cabinets and also a pair of moderately efficient Radio Shack LX4s. I also listened to it in the main system driving the Altec 2-way. In both cases it never failed to sound sweet, fatigue-free, with a tube-like quality and depth in the midrange, maybe not the tightest bass but definition was good + airy highs.

The only time it betrayed its solid-state identity was when I spun an LP of Shostakovich Symphony No. 11 in the main system. It went into nasty clipping during orchestral peaks like a typical solid-state amp. Maybe it was asking too much from a Class A amp with a measly 7 watts rms per side to cope with the 16 ohm load presented by the Altec 2-way? However, with the same piece and on the same speaker system, my 4 watt per channel Stereo SE2A3dx with Tango NY15s went into overload with grace and composure at a similar SPL.

It is a testament to John Linsley-Hood's engineering and hearing abilities that his 50-year-old circuit is still a very capable music maker. Even if it can't replace any of the tube amplifiers in my collection, this is a much more listenable amplifier than the handful of contemporary Class D switching amps I've heard thus far.