Showing posts with label High-Fidelity. Show all posts
Showing posts with label High-Fidelity. Show all posts

Saturday, June 5, 2021

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 by JE Labs

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), TamuraTango, 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 correctionDyna 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!🍷



Saturday, April 17, 2021

Marcel Breuer Designed House + Altec 604C + Infinite Baffle

  

This beautiful mid century modern home aka the Snower residence was designed by Bauhaus architect Marcel Breuer and posted by midcenturyhome, whose finely curated photostream I follow on Instagram.


Aside from my interest in mid century modern architecture, the Altec 604C mounted prominently in the living room caught my eye.😄


I've also seen a Fisher 50C preamp (bottom) at FLW's Fallingwater. Unfortunately, interior photography was prohibited there when I last visited. 😞 


The Snower house + hifi installation was already brought to my attention in the comment section of architects, music and hifi  by a blog visitor many years ago. Thank you!

from Dwell.com

More Infinite Baffle Installations



I blogged about this installation before, but this picture offers another perspective of the 2-way horn speaker system installed in Case Study House #9 designed for John Entenza by Charles and Ray Eames and Eero Saarinen.

Scans from How to Build Speaker Enclosures


An office stereo hifi


John Hilliard's home hifi system



Hi-Fi system details

I'm pretty sure all these wall mounted speaker systems were formidable music reproducers! However, ever since soundstaging and imaging took precedence over sound quality, infinite baffle loading fell out of favor. 😔



Saturday, April 3, 2021

Lar Retro-resto Shure M65 Phono Preamp


Hamingredient inspired chassis 😉

In the early 2000s, I tried to unload a half dozen or so of my used/test good Telefunken 12AX7s on eBay. The buyer promptly requested for a return claiming that they didn't test well in his TV-7. Was that a sign...? 😉 
So instead of re-listing, I stashed them away. I had forgotten about their existence until they showed up during a 2020 lockdown clean-up spree. 

Shure M65

I met my buddy Lar through Audiomart in the late '80s. He's a trained sound engineer and audio hobbyist who shares my passion for the classic Austin Mini.👍 We hang out and talk shop at ham radio festivals, audio and radio shows. He's firmly committed to tube phono circuits employing active RIAA EQ and posts his projects at Audiokarma and Lenco Heaven.

Shure M65 schematic

The classic Shure M65 "chrome cutie" (as Lar fondly calls them) is one of his favorite platforms for modification. Keen eyes will notice the similarity of this topology to the phono section of the classic Dynaco PAM-1 and PAS-2/3 tube preamps, albeit operated at a lower B+ voltage and with AC heated filaments.

One channel shown

The 0.47uf ceramic disc input coupling caps, C1 and C2 + the 1M shunt resistors, R3 and R4 are discarded. Likewise, the 100k positive feedback resistors (R11 and R12) are eliminated, which I also used to do to my Dyna PAS preamp hacks. IME, this procedure improves transparency. The most significant performance upgrade is Lar's recomputed RIAA EQ capacitor values (see the above schematic) tweaked for flatter response.

I didn't have a Shure M65 but had enough parts to clone one so I could put my rediscovered Tele 12AX7s into good use in the midst of Covid-19 lockdown.


This one chassis configuration sounded fine on first turn on but had an annoying low level hum I couldn't eradicate. 


After trying all possible grounding schemes, I yanked the power transformer out and built an outboard power supply. Problem solved!


My power supply is slightly more sophisticated than the stock M65. The full wave rectified B+ is filtered by a 20H/20mA choke followed by a 47uf/450V cap, which splits to separate left and right channel rails with additional decoupling and filtering. The tube filaments are DC heated. 

1 kHz
stock RIAA EQ

Using the stock RIAA EQ capacitor values, lower frequencies sound bloated with a noticeable treble boost as noted in the Shure M65 literature, which is also visible in the above 1 kHz square wave.

1 kHz
LAR modified RIAA EQ

The 1 kHz square wave is squarer and the phono preamp sounds so much better with Lar's tweaked cap values!

I don't think I was ever able to derive this level of musical satisfaction from any of my past 12AX7 phono efforts using active RIAA EQ. I believe the sonic purity stems from the simplicity of the circuit itself. The Dynaco PAM-1 and PAS-2/3 circuits have an additional cap after the negative feedback RIAA EQ tap + elaborate switching and a balance control in the PAS, all of which can cause veiling. 

In spite of the Shure 5000 ohm output impedance spec, this is still a relatively high output Z device. When connected directly to the 10k ohm input of my Nobsound NS-02g amp, the loss of extension at both frequency extremes was discernible. It sounded best driving a 100k ohm load using low capacitance cables, 3 ft. long, max! 


This phono preamp synergized very well with my favorite magnetic cartridges with conical stylus - Nagaoka MP10, Pickering XV15, Shure M3D and M7D, amongst others. So after a couple of months of enjoying the Lar mod M65, I decided to build it on a nicer (if a bit cramped 😁) cast aluminum box with two pairs of inputs.

Instead of boring you with audiophile adjectives, I'll just say that my RCA passive RIAA EQ octal sounds effortless and airy, while the EAR 834P derived loctal is detailed and dynamic and the Lar retro-resto Shure M65 is sweet and alluring! 


This phono preamp is a most welcome addition to my tools for enjoying LP reproduction. I highly recommend following Lar's phono preamp adventures at Audiokarma! 

Bravo Lar!👏



Friday, March 19, 2021

Snell Type JIII Part 1 - Deconstructed > Re-Foam

 In the late 80s, I could have gotten a Snell Type E or J after unloading my Magnepan SMGas. But I loved the sound of EMI and Decca recordings, for which I give credit to BBC-trained sound engineers, so I invested in a pair of Quad ESL57s and Rogers LS3/5As instead. I was such an Anglophile that I even became a partner in a company that imported Art Audio Amplifiers and Spendor Speakers to the Philippines. Unfortunately, we didn't make money...but that's another story.🤣

 
During a moment of nostalgia, I chanced upon this pair of Snell JIIIs online. I messaged the seller expressing interest. After two weeks of silence, I made a polite follow-up and was surprised to receive a reply that they were still available. According to the seller, he's had several offers from tire-kicking flippers but he chose to sell to me because I sounded like an enthusiast who'd give them a good home.😄


Typical of speakers from this era, the foam surrounds have disintegrated. Other than that, this is a very clean pair with consecutive serial numbers just like when they were sold in high-end audio salons in the late 80s to early 90s.


While waiting for the replacement foam surrounds, I thoroughly cleaned all vestiges of old foam from the cone and speaker frame. This was the most tedious and time consuming step in the re-foaming process but essential for a successful result!

 Meanwhile, I also took advantage of the down time to analyze what makes this speaker system tick. 

Deconstruction


Peter Snell was a very gifted speaker designer. Due to his untimely demise at age 38 in 1984, speaker models that were manufactured during his lifetime have developed a following. Peter Qvortrop of Audio Note UK was a big fan of the 2-way Type E, J and K models and continues their legacy in the Audio Note E, J and K using custom drivers and boutique parts.

The Snell E and J had front mounted reflex ports while the K was a sealed box/acoustic suspension design, which employed a different woofer. A rear firing 3/4" dome tweeter was added to the EII model, while the reflex port was moved to the back of the cabinet in both the EII and JII.

Vifa D26TG-05-006

The EIII, JIII and KII designation denotes the switch from a Tonegen 1" doped silk dome tweeter to the Vifa D26TG-05-006 6 ohm 1" dome pictured above. A tweeter level control was also added on all models. 

According to the Snell Type JIII brochure, the Vifa D26 tweeter handles an octave of upper midrange frequencies from 2.7kHz + another two octaves of harmonic overtones.


Even if the JIII was manufactured posthumously, it retains the cabinet dimensions and proportions of the original model. After many years of experimenting with open bafflesAltec 614 and 618 and  cabinets, I'm a firm believer in the acoustic superiority of Rubenesque front baffles over Victoria's Secret slim waifs.😆 



Was the use of plywood (top, bottom and sides) + chipboard (front baffle and back panel) chosen for acoustic reasons? Regardless, the cabinet is of pretty high quality construction with particular attention placed on rigidity - see the brace from the front baffle to the back panel in the picture below.



Good quality air core inductors, electrolytic and film capacitors or a combination of both + an L-pad for tweeter level adjustment were used in the crossover network.



The high-pass is a straightforward 12 dB/octave 2nd order slope hinged at 2.7kHz, as previously mentioned. On the other unit, there was an additional electrolytic cap in the high-pass section with a higher total capacitance reading. This confirms Snell's claim of matching their units to within +/- 0.2dB based on a reference model in their factory. 

Likewise, the low-pass section is also a 2nd order slope + an EQ/contour network for the Vifa woofer (see the LCR combo highlighted in red above). Explaining how this EQ works is beyond my technical ability. My empirical mind just knows that it "shapes" the woofer's frequency response. A similar, albeit more complex, crossover + EQ can be found in the BBC LS3/5A

Since all the capacitors measured close to their indicated values, there was no rationale to change or upgrade. Besides, I want to hear these speakers as close to how they were originally designed.


Vifa 8" M21WN-07-04

AFAIK, the Vifa 8" M21WN-07-04 was the only woofer used in the J, JII and JIII. Based on the 49Hz-20kHz +/- frequency response specified in the JIII brochure, and cross-referenced to my Musical Instrument Frequency Chart, this drive unit/cabinet combo was tasked to cover six and a half octaves of musical information starting from the bass frequencies up to midrange fundamentals. 


Re-Foam


After judiciously sifting through audio sites on the net, I chose the $25/pr. foam surrounds from Simply Speakers because they were recommended by audio hobbyists who exhibited good ears and common sense. Check their re-foaming video for a straightforward and confidence-inspiring tutorial.



 The Vifa M21 voice coil is essentially self-centering. So don't even consider cutting or removing the dust-cap to shim the voice coil. It'll be ruined because the cone + dust cap is one whole piece. 

 

Not bad for a first attempt in replacing foam surrounds, huh? 😉


Since time immemorial, I've been evaluating drivers bare naked (or mounted in a simple baffle) to hear their potential. After 25 years of enjoying Altecs, these drivers sound very different...🤔  


Snell JIII in room RTA (Onyx iPhone app
Snell JIII specs

I have to live with this speaker system for a while and play lots of tunes 🎻♫🎶♬🎺and will report back in Part 2!😊


See Part 2 and Part 3