Over two decades ago, I ventured into single channel music reproduction to trace the roots of high fidelity. From that whole experience, I developed an aesthetic quest for a certain purity and honesty in sound, which runs parallel with my other hobby of black and white film photography! Sadly, I had to leave that particular mono set up in the attic in 2017.
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| L>R: 6AH4 + AliEx, 12B4A + Sony/Tamradio and 6AH4 + Hashimoto HL20K6 |
After going home with me to Manila and back stateside, I finally found time to try the JS 4120Ns in my preamp. After adjustment of voltages and operating points, I was impressed by the increased detail and resolution brought forth by replacing the Allen-Bradley plate load resistors while retaining the sonic aura I've come to treasure from my Bruce Berman-derived 76 > 1/2 6SN7 line stage circuit from Sound Practices Issue 13.
Unfortunately the JS 4120Ns have been out of production for quite a number of years. π
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| NOTE: According to the 7N7 data sheet, it has a maximum cathode to heater (filament) voltage rating of 90V. To get around the 142V at the cathode, I lifted the filament potential 65V above ground via a voltage divider network in the B+ supply. Hence, 142 - 65 = 77, which brings it back to spec. |
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| 100 Hz, 1 kHz and 10 kHz square wave output from 2 x Hammond 156C plate choke loaded 1/2 7AF7 + 1/2 7N7 cathode follower |
I tried the budget-friendly Hammond 156C plate load chokes in the circuit above on my loctal preamp as well as on my main preamp with the 76 line stage below.
Detail improved but I missed the micro dynamics and organic quality in the midrange of the JS 4120Ns. That said, I'm now using this topology in the nosTDA1541tube DAC in my main hifi set up. Meanwhile the loctal preamp line stage has been reverted back to its original state with Allen-Bradleys.
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| NOTE: This choke saturates instantly past 8 mA! |
Since the Hammond 156Cs are easy on the wallet (just like the 125ESEs I wrote about decades agoπ), why not try and judge for yourself? If you hear potential, then proceed to the nickel plate choke-equipped circuit below.π
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| Custom wound Intact Audio 250H nickel plate chokes |
I was so impressed with Dave's nickel plate chokesπ such that I ordered a pair for myself so I could use the JS 4120Ns for other tubes and circuits. I also confirmed that in order to get the most out of the plate load choke topology, one needs a competent winder, and nickel should be part of the equation!
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| Even if I always used more rugged 6SN7 GTA and GTB types in the cathode follower section, when I built this preamp over two decades ago, the power supply was configured so that the filament potential was about 75V above ground. |
I've always admired the sound (or more appropriately, the lack thereof) of nickel permalloy, having owned the Tamura F7002 and Tango NY15S since the 90s. Even if these output transformers are hefty beasts, which helps the low frequencies, it's not their bass response that captivates the listener. Instead, they present a very natural midrange replete with rich and harmonious overtones, which are virtues I also hear with Finemet.
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| 100 Hz, 1 kHz and 10 kHz square wave output from an Intact Audio nickel plate choke loaded 76 + 1/2 6SN7 cathode follower |
After praising the merits of exotic core material, I wouldn’t want readers to get the impression that nickel or Finemet are the quintessential elements to great sound. Tube choice, circuit topology and operating points, parts selection, etc. not to mention the synergy of components in the hifi chain, all contribute to the sonic stew. That's why I encourage audio hobbyists to listen and experiment so that they can carve their path towards their own musical satisfaction.
Although this video is merely a proof of concept, I got inspired and used my Sony A7II + Voigtlander Ultron 28mm f1.9 LTM lens mounted on a tripod rather than my usual iPhone. So plug in your Koss Porta-Pros or Sennheiser HD6XXs and (hopefully) enjoy!πΆπΉπ»
Maybe π€ I'll talk about my plate choke adventures with SE amps next time!π
Before taking the preamp out of the system for a thorough check up on the test bench, I searched for a couple of tube shields from my parts bin.
Voila! The unusual hum and noise vanished.π
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.π
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| scanned from page 36 of the 1989 catalog |
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| Realistic MKVII idler-driven turntable, a future featureπ |
Tube rolling is a fun aspect of the audio hobby. It only gets tedious when audiophiles start splitting hairs about a VT231, 5692, etc. I draw the line when an audiophool claims, "...my line stage preamp resolves the R train rumble under Carnegie Hall better when I use a 6AS7 in place of the original 6SN7..." π
Those two tubes may be pin compatible BUT a power transformer π₯melt-downπ₯is imminent since the 6AS7/6080 draws 2.5 amps @ 6.3V. That's over 4 times more filament current drawn by a 6SN7, which only draws 0.6 amps @ 6.3V.
π£ π£ π£ Rolling tubes is dangerous unless you have a working knowledge of electronics to comprehend tube data/specs. π£ π£ π£
It's been a while since I listened through Telefunken 12AX7 smooth plates on an RCA phono. Since I used to graft this circuit on a Dyna PAS preamp, I ordered a pair of 12AU7/12AX7 > 6SN7/6SL7 adapters to relive the experience on my RCA octal preamp.
Back in the 90s, Steve @ Angela Instruments had a boatload of Raytheon 5755s and shared a sleeve with me. Since I never found a project requiring these tubes, I got a pair of 5755/WE420 > 12AX7 adapters to listen to them in the Shure M65 clone. Piggy-backed to 12AX7>6SL7 adapters, they also worked nicely with the octal phono preamp.
Despite slight differences in transconductance and plate impedance between the 6SL7/7F7 and their younger 9-pin siblings (ECC83, 12AD7, 5751 and 5755), there was no visible variation in square waves nor frequency response aberration when I bench tested those tubes in both circuits.
Admittedly, loctals don't look as glamorous as their octal cousins with bakelite bases. But I'm very fond of them to the extent that I built and use a full function loctal preamp. I've been adopting unloved loctals at radio shows and hamfests. Essentially the industrial looking 7F7 and 7N7 are rebased 6SL7 and 6SN7, respectively! My only issue with loctals is, the tube pins are prone to corrosion. This is easily remedied by a wire brush + DeoxIT before use.
I purchased these tube adapters from the seller xulingmrs. They are very well made and fairly priced. Shipping cost and transit time from China was also reasonable. Highly recommended!
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| Hamingredient inspired chassis π |
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| Shure M65 |
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| Shure M65 schematic |
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| One channel shown |
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| 1 kHz stock RIAA EQ |
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| 1 kHz LAR modified RIAA EQ |
It's been three years since I blogged about Radio Shack's demise. I miss browsing at a Rat Shack store even more.π 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.
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.
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.
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!
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.
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| 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.
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.
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.
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.π
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| 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.π‘ 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.π It's ok for background music and has a value for the money MM phono stage.
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| 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.
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| JLH 1969 Class A amplifier |
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| 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.
The voltages found in these circuits can be lethal! Proceed at your own risk!
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| 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.π
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| 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.π
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| 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 π) from getting fried.
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!π
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.π
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!
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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!
