JE Labs

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 baffles, Altec 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

Friday, March 5, 2021

Parallel Single Ended 6SN7 [Headphone] Amp

Ever since I grafted a 6SN7 on the line stage section of my hacked Dyna PAS preamp, I've been hooked on the "octal sound." Could it be the higher heater current draw boiling more electrons and/or larger stature that give it a 3-D quality, richer tonal color, detail, dynamics + more, compared to their 9-pin/noval siblings?

For these reasons, I've longed to hear how the ubiquitous 6SN7 could sound as a milliwatt amp. The Covid-19 2020 lockdown was a good opportunity to try "off the cuff" experiments. An unbuilt Chinese 6V6 preamp kit I bought at an Amazon closeout languishing in the basement for a couple of years provided a nice pre-punched chassis + power transformer for this project.

Ideally, the 3850 ohm plate resistance of the two triode sections of a 6SN7 connected in parallel should drive a 12K primary Z. I considered getting a pair of Hammond 125BSE output transformers, which have the flexibility of reflecting back a 2.5K, 5K or 10K primary Z but I already had a pair of these 5.5K:4 ohm SE OPTs in my parts bin that I was sure would fit under the chassis.

!!!WARNING!!!

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

The circuit is a simple 2-stage cascade - 1 triode section of a 6CG7 functions as the input/driver stage R-C coupled to paralleled 6SN7 triode sections.

The HT supply output of the 6X5 rectifier feeds a 22uf/450V capacitor, which splits into left and right channel B+ rails via two 5H/50mA chokes filtered by separate 47uf/450V caps, provide clean power to the amp. I rectified the filament supply to run at 6VDC for minimum hum and noise on headphones.

Square waves at 100Hz, 1 kHz and 10 kHz
Top trace = audio generator
Bottom trace = amplifier output

Due to the primary impedance mismatch previously noted and the bonsai stature of the transformer itself, these square waves show limited extension at both frequency extremes. Subjectively, this was more apparent via speakers rather than on headphones. I should point out that the absolute lack of ringing is indicative of a competently-wound transformer.

Input sensitivity = 0.9 V RMS

Output = 2.408 V RMS/ 8 ohm load = 720 milliwatts/channel

Between 16 ohms > 600 ohms, I measured 3.5 --> 4.2V RMS output, which was more than enough power for the most inefficient ortho-dynamic headphones I have in my collection.

This amp draws the listener to focus on the midrange in which most musical activity transpires. It made the JLH HP amp sound comparatively veiled in the midrange, which shifted the listener's attention to the frequency extremes where the JLH actually does pretty well.

Despite the superior bandwidth of the output transformers fitted to the Darling amp, JEL style, the PSE6SN7 amp displaced it musically! The Darling amp just couldn't keep up with the DHT-like harmoniousness and graceful overload characteristics of the PSE6SN7.

Lush and caramel flavored midrange, nuanced dynamics, silky highs and well-defined bass (within limits) are the virtues of this Lilliputian amp. Perhaps due to the high output impedance and low damping factor, this amp can portray the sophisticated qualities I've experienced from the SE171A, 10/VT25 and Western Electric 205D amplifiers!