Magnavox 8601, JE Labs Style

Realistic 42-2109 phono preamp upgrade

Even if I was a Radio Shack devotee in the early 80s, I didn't pay much attention to this RIAA phono preamp because it didn't have high-end audio pretensions. As the saying goes, ignorance is bliss.😆

scanned from page 36 of the 1989 catalog

This unit was first introduced in the 1980 catalog (p. 22) as part number 42-2101 at $19.95, then as part number 42-2109 for $24.95 in 1989, and made its final appearance for $27.95 on page 103 of the 1992 catalog.

Realistic MKVII idler-driven turntable, a future feature😊

With the resurgence of interest in LP playback, it was rediscovered by aficionados and has developed quite a following. No ICs or op-amps are to be found inside. It's a simple discrete circuit, which minimizes signal processing.

!!!WARNING!!! 

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

The circuit is a basic two stage cascade + a negative feedback RIAA EQ looped from the output back to the input. It is very similar in topology to its tube forebears - the Shure M65 and the phono section of the Dyna PAS.

Typically, I don’t subscribe to a wholesale approach of replacing caps but this is a very simple circuit that was built at a certain price point. Since I had some decent quality parts surplus from my CD player and DAC modification projects, I replaced the electrolytic capacitors in the signal path with film type WIMAs and increased the filter capacitance in the power supply section.

In stock form, I was already impressed by this preamp's listenability when passing through my loctal tube line stage. The parts upgrade tidied up the sound further, but it wasn’t a magical transformation.

In terms of midrange presence and transparency, low level detail, micro and macro dynamics, this preamp isn't the equal of the Shure M65 or a refreshed stock Dyna PAS phono section. Its main advantage over its vacuum tube forebears is low output impedance. This could plug into the 10k input Z of the Nobsound NS02g/JLH 1969 with no loss of frequency extremes. I won't recommend that with the M65 or PAS. There's also a slight graininess in the midrange that wasn't ameliorated by parts upgrades. That said, I don't know if one can find a "plug 'n play" phono preamp in the $100 range (or possibly more?), which will provide as much musical pleasure!

Hi-Fi for the Deck: Rpi3B >TDA1543x4 DAC > Simple Triode SE6V6 amp > EV Sonocaster

Spring and Fall are the best seasons to enjoy food and drinks on our deck. To complement the beautiful atmosphere, I assembled an e-z to set up and dismantle hifi system!

Music Source

This stack consists of a Raspberry Pi 3B loaded with Volumio music player/streamer tethered to a Signstek USB to SPDiF converter which connects to a Muse TDA1543x4 Non-OverSampling DAC.

Volumio offers a wide choice of radio stations streaming MP3 files up to 320 Kbps. The Radio Paradise channel has even started broadcasting FLAC files. 👍

Amplifier

In the late 80s, I built my first amplifier from scratch using transformers from a cosmetically challenged Dyna SCA35. I used this paraphase circuit from the Dynaco Transformer Catalog with a 6SL7 driving a pair of push-pull 6V6s. Musically, I preferred this amp to my then highly modified Dyna ST70 and pair of MKIVs.

with 6F6Gs courtesy of John Piro

Since I always had reservations about the 1626 triode's musical abilities, I converted the JEL Darling Style amp to use triode connected 6V6s. Wow!👍

!!!WARNING!!! 

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

Input sensitivity = 0.3V, no need for a preamp
This amp also does a good job of driving my collection of orthodynamic headphones via adapter.

 2.825 Vrms into 8Ω = 1W of triode power per channel 
I'm not worried about the slight overshoot/ringing, rumor has it, the Sony/Tamradio OPTs have nickel content 
also, the last time I saw a similar square wave was with a pair of Nature Sound NS70SE OPTs.

Triode connected 6V6s mimic directly heated triodes much better! 🎻🎸

Speakers

This pair of Electro-Voice Sonocasters came from the flea market. I guesstimate the sensitivity to be in the mid 90s dB @ 1W @ 1M. Driven by 1.4 triode watts per channel from the SE6V6 amp, I sure won't be annoying the neighbors. 🤣

The 8" drivers are unmarked but look very much like MC-8s - not quite an Altec 755C or Pioneer PIM8L in terms of refinement or realism, but not too shabby either + a lot more rugged!

Happy Listening!🍻

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!

John Linsley-Hood Class A Headphone Amp

Having been a happy user of a John Linsley-Hood (JLH) 1969 Class A amp cloned by Nobsound, I was intrigued by the JLH Class A headphone amp boards featuring discrete circuitry (no ICs or op-amps) advertised on eBay. In the midst of the 2020 lockdown, I ordered this $25 pre-assembled board, which arrived in a padded pouch within 10 days. I also got a surplus 24 VCT @ 2A power transformer for another $15.

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

Typical of Chinese kits, this board didn't come with instructions or any type of documentation. But the board is marked for an experienced DIYer to decipher and set up in no time. Unlike the original JLH 1969 design, which needed DC blocking capacitors at the output due to the single rail supply, this headphone amp module was updated to work with a dual rail (+/-) supply so the output capacitors can be omitted.

 

Unfortunately, I couldn't get stable zero DC offset at the output terminals. Trimming for zero volts after 30-45 minutes of warm up was not a guarantee that in another 30 minutes it'll still be zero - I've measured as much as 0.3V.😞

Since I didn't want to risk frying my headphones and classic speakers, I installed 2500uf @ 16V electrolytic blocking capacitors at each channel’s output. Then, I bypassed the electrolytic caps with hermetically sealed 4uf paper caps just like I did to the Nobsound NS02g.  

Top trace = audio generator
Bottom trace = amp output

In spite of the DC blocking caps at the output, the square waves still show excellent bandwidth, transient response with no trace of ringing. 

Before clipping, I measured 250mW/channel @ 4 ohms, 500mW/channel @ 8 ohms and 950mW/channel @ 16 ohms, 1.2W/channel @ 32 ohms,  395mW/channel @ 150 ohms, 190mW/channel @ 330 ohms and 115mW/channel @ 560ohms. I pretty much exhausted the resistor values from my parts bin to emulate the nominal impedance of typical headphones. 

The 115mW output into a 560 ohm load shouldn't worry headphone enthusiasts since this amp drove my Austrian AKG K240/600 ohm Monitor cleanly beyond SPLs my ears could tolerate. It also had no issues driving less efficient ortho-dynamic headphones like the Fostex T50RP MKII below, courtesy of my friend Nate. Thanks, buddy!

This amp definitely outclassed the Rat Shack mini amps I'm fond of. Its class A design's purity and transparency puts it on another league. Unfortunately, Class A amp designs require a more substantial power supply, which is incompatible with portability. 

The character of this amp is exactly like that of its bigger brother, a fatigue-free solid state amp that almost (but not quite) succeeds in sounding like a good tube amp!

BTW, did I mention that it can also drive my Altec 755As in Silbatone cabinets?

Cheap Thrills from Radio Shack + more

It's been three years since I blogged about Radio Shack's demise. I miss browsing at a Rat Shack store even more.😞 Through the years, a number of audio products from the Radio Shack Catalog  have caught the attention of budget-conscious Hi-Fi enthusiasts. 

I've already covered the 40-1354A 5.25" full-range driver and 40-1310 super tweeter in a previous entry. Below are capsule reviews of a few more Radio Shack audio goodies I've played with. Aside from the cool historical factor, IMHO, most can still equal or even outperform many entry-level audio gear today.

Realistic Pro 30 Headphones

The Realistic Pro 30 was introduced in 1981 for $39.95. Its price was reduced to $34.95 in 1983 and then to $29.95 in 1984. It made its final appearance in the 1986 Catalog. Instead of the usual dynamic drivers, these headphones were fitted with planar magnetic drivers, very unusual at this price point then and now.

It was one of the favorites in the orthodynamic round up thread started by wualta in headfi.org 15 years ago.

I got this pair very cheap because it needed new cables. In stock form, it sounds very smooth and refined. It can be tailored to 21st century tastes via judicious damping. The felt dot boosts high frequencies while the additional felt pad focuses the audio band a bit. New ear pads can improve the bass response since the flattened originals are no longer giving a good seal. Yup, speaker tweaks are applicable to headphones too!

Even if this is still a work in progress, I already prefer the sound of my modified Pro 30 to my Grado SR80. If you're patient, these turn up for around $30/shipped on eBay. 

32-1100A Stereo Mixer

 

I never got into DJ'ing/mixing but I couldn't resist this Stereo Mixer at a flea market for a couple of bills. It can be powered by 4 x AA batteries or a wall wart that can produce 6VDC at 35 mA. This mixer sports two magnetic phono preamps, 2 microphone inputs + 1 line level, tape out, a preamp output and a 1/4" headphone jack. No ICs or op-amps in the signal path as far as I can tell.

The headphone amp sounds very clean and has plenty of power for modern low Z headphones but may run out of steam driving higher Z or less efficient planars. I've even used this as a preamp in my near-field system with musically satisfying results!

41-2109 RIAA phono preamp

This outboard phono preamp was available in various permutations from the 70s to the early 90s. The circuit topology is very similar to the the above mixer but the sound is more dynamic and detailed due to the higher voltage rail powering the discrete circuit. 

41-2109 schematic

Some audiophiles claim that it can hold its own against tube phono preamps. It's good, but not THAT good. When pitted against my stock but refreshed Dyna PAS 3, it offered less resolution and was tonally more opaque. It was totally outclassed by my octal and loctal preamps. That said, I don't think you can find an RIAA phono preamp that can beat this bone stock unit for under $100.

 

Optimus LX4

Available only between 1995 and 1996, the LX4 achieved audiophile credibility because of Dick Olsher's review. It used the Linaeum monopole tweeter which provided a lot of air in the high frequencies without the sizzle of run of the mill domes. Unlike the paper coned Minimus 8 below, the LX4's Kevlar coned 4.5" woofer’s medium efficiency isn't as good in tracking micro dynamics. I voiced my original Simple 45/2A3 amp using a pair of Altec 755Cs in open baffles cross referenced to a pair of LX4s in the mid 90s.

Even if this was, and still is, a smooth and coherent sounding speaker at its price point, I wouldn't go as far as choosing this over the BBC LS3/5A. 

JE Labs Test Bench

A test bench isn't complete unless it has the facility to 'sound test' a freshly minted audio project or a newly repaired audio component.

Minimus 8

While I was a high school junior, I owned a pair of Minimus 8s driven by a Realistic SA-10. This speaker evolved from the Solo 103, which used various incarnations of the classic Fostex/Foster 103 4" paper coned full-range driver.

I got this ca. 1982 pair at Goodwill. Considering their diminutive size, these speakers are quite efficient at 89dB/1W/1M. To offset the upper midrange/lower treble tilt, these sound their best mounted flush against the wall. It's an easy load even for a 250mW per channel amp and provides nice background music while soldering.😉

The Realistic Stack

Top Right: Realistic TM-150, center right: Realistic SA-150,
center left: Realistic SA-10 (31-1982A)

Introduced in 1985, the TM-150 and SA-150 tuner/amp combo was riding on the mini-sized components trend of that era. The AM/FM TM-150 tuner has good sensitivity and can pull stations even in my basement shop via a simple omni antenna. 

 The SA-150 amplifier can trace its roots to the Germanium-equipped SA-175 amplifier from the late 60s, followed by the SA-101 from the mid-late 70s, which was succeeded by the SA-102 in 1980. Towards the end of the SA-102's production run, the accounting department tried their hand at circuit design and grafted an LA4440 IC chip amp on the same chassis.😡 This topology was carried over to the SA-150. Surprisingly, it doesn't sound bright or grating. On the contrary, it sounds dark and grainy.😞 It's ok for background music and has a value for the money MM phono stage.

Schematic of the SA-150 RIAA phono preamp
which can be accessed via the tape out
Note the similarity to the 42-2109 phono preamp 

The SA-150 was cosmetically altered in later years and was still available as the RCA SA-155 in the 2002 catalog.

Realistic SA10

Since I owned a Realistic SA-10 amplifier in my youth, I got inspired to search for another sample when I noticed the similarity of its circuit topology to the John Linsley-Hood 1969 Class A amplifier .

JLH 1969 Class A amplifier

Realistic SA-10 (31-1982A) amplifier section

However, the resemblance is only superficial since the JLH 1969 operates in pure Class A and the output stage is quasi single-ended while the Realistic SA-10 is a four transistor push-pull amplifier operating in Class AB1.

!!!WARNING!!! 

The voltages found in these circuits can be lethal! Proceed at your own risk!

Left: later version with IC chip amp
Right: earlier version with discrete solid state circuit

During its long life, the SA-10 was also subjected to cost cutting measures. The later 31-1982B sported IC chip amps in the output section.

Radio Shack could justify their use of IC chip amps on paper since I measured 700mW per channel from both versions. But the similarity ends in the test bench. Even if a different IC chip amp was used, the later 31-1982B also sounds dark and veiled just like the SA-150, whereas the all transistor SA-10 (31-1982A) has a more vivid midrange that reminds me of tubes.😊 

Loudness compensation bypassed + cap upgrades
31-1982A schematic

To be honest, I don't think the cap upgrades made a significant improvement in sound. I changed the output capacitors to protect my speakers and headphones in case of DC output in the speaker terminals due to leaky caps. However, I highly recommend bypassing the loudness compensation circuitry to remove the "Bose Wave Radio" crud.😆

With my DIY headphone adapter attached, the SA-10 (31-1982A) also does a good job as a headphone amp. The separate left and right level controls don't bother me and if I need more inputs I can press my 42-2112 audio source control selector into service.

 

42-2112 audio source selector

This source selector will extend one stereo input to three.

33-1009

If your amplifier doesn't have a headphone jack and puts out 2 watts per channel or more, you need this headphone coupler. This device has attenuating resistors inside to protect your headphone drivers (and your ears 😆) from getting fried.

Realistic SA101

The SA-101 was the unit that got away in '79 because it equalled the cost of my SA-10 + a pair of Minimus 8s. This amplifier hails from the generation of discrete solid state circuits. It has an RIAA phono preamp + two line level inputs, a balance control and 1/4" headphone socket. It's the ideal amp for a retro vinyl LP + headphone set up!😎

The amplifier circuit is virtually identical to the SA-10 (31-1982A) except for the increased voltage and beefed up power supply. Thus, my Fluke 87 measured 4V rms to an 8 ohm load or 2000 mW per channel. This may not sound like much but it drove most of the speakers in my collection to musically satisfying levels. Just like the SA-10 (31-1982A), it has some warmth and depth in the midrange reminiscent of tubes.😊

Why would I listen to the SA-101 and the early version of the SA-10 rather than their later IC chip amp brethren or the Class D chip amp I got from Parts Express a couple of years ago? 

First of all, I'm not a bass head and I don't listen at ear splitting SPLs. I also put more emphasis on sonic refinement which is easier found in flea powered amps (tube and solid state) designed by engineers who honed their skills (and ears) during the vacuum tube era, exemplified by Mr. John Linsley-Hood! 

Postscript

Ms. JE Labs' Office/Sewing Room Set Up

Tish’s office system consists of a Lafayette LA25A stereo integrated amp, fed by a Magnavox CD2000 CD player  + a Kenwood KT880D tuner. The components are flanked by a pair of Akai SW132 Jet Stream speakers.

The LA25A amp was OEM'd in Japan for Lafayette in the early 70s. This unit is very similar to the Lafayette LA25 sans magnetic RIAA phono input.

The Akai Jet Stream is a compact transmission-line (TL) cabinet manufactured in the Philippines in the early 70s. It supposedly contains a Fostex/Foster 103 driver variant. Unfortunately, I can't confirm this without disassembling the woodwork. 

This is a pretty good sounding speaker. The upper midrange forwardness inherent in the Fostex/Foster 103 driver is neutralized by the TL loading even if there's still not much activity below 90 Hz.

The speakers belonged to Tish's dad. He was a Filipino cardiologist and these Akai SW132s were the rear speakers of a quadraphonic system gifted to him by a patient. He also left behind a pair of Jensen Triettes and a Rek O Kut R34 belt drive turntable, which were part of his stereo rig while doing his fellowship in Cook County, Illinois in the early 60s. 

According to Tish, he was also into photography and developed his own negatives. Unfortunately, I never got to meet him. We would've enjoyed talking shop over ice cold beer since we shared a lot of common interests!