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.

Altec 32 Horn Nomenclature

This Altec 32 horn update was prompted by this must read article from Electravolt tracing how the Altec 32 horn evolved from the Western Electric 32A.

Altec 32A

Cast aluminum + longer throat

Altec 32B

Plastic + shorter throat

pics borrowed from sound-impression

Probably a very short run, rarely seen in the market

Note: no ribs

Altec 32C

Same as 32B but with re-enforcing ribs

Note: re-enforcing ribs

Topping D30 vs. Xiang Sheng DAC 01a

This installment is long overdue since I got distracted by a flea market find, a Yamaha T-2 FM tuner, which was aligned to factory specs by Punker X, highly recommended tuner tech! Yes, I am eating my words from this past post since I've been enjoying WRTI, WPRB, WXPN, amongst others. FM broadcasts just sounds more analog with a human DJ than streaming from the internet.

But I haven't ditched my Raspberry Pi 3B+ set up since it has its purpose.

If you are looking for a DAC to hook up to a CD transport to playback Redbook CDs STOP and GO there! However, if you need an affordable DAC for your Raspberry Pi, laptop or server, you may proceed.

Cirrus Logic CS4398

Both these units use the Cirrus Logic CS4398 chip, a well-respected Delta-Sigma type DAC that was introduced in 2003. But it is the implementation of the DAC chip which sets these two units apart.

Listening tests were done using music files mastered to CD Redbook standards. These include live concerts, CD projects and music from favorite CDs I transferred to a USB thumb drive. Streaming sources were internet radio stations via the latest version of moOde audio or Volumio loaded to a Raspberry Pi 3B+ with an Allo DigiOne HAT. I also listened using USB connections and arrived at the same conclusion.

The Topping D30 is a highly acclaimed DAC and measures great according to meter readers. It sounds pleasant, I didn't hear anything objectionable. But compared to the Xiang Sheng DAC-01A, the D30 sounds dry and lifeless. Maybe that's what others will deem as accurate? 

A hundred and twenty something dollars buys a neatly stuffed PCB with Wima capacitors in the audio stage. The PCB card slides into a nice anodized aluminum case. But Topping only provided an SMPS wall wart. Hmmm...

For about the same amount of money the Xiang Sheng DAC-01A is built on a solid metal chassis. It sports a real power supply based on an R-Core power transformer that provides an 80VDC rail for the 6N3/5670/WE396A tube buffer/cathode follower blocked by Wima coupling caps.

In spite of the decent quality workmanship, the blue LED pilot light on my unit was flickering since I got it and died a couple days later. But it was an easy fix.

Since I was planning to use it mainly as a DAC, I bypassed the volume control by setting the two jumpers to connect 2+3 and b+c. To enable the volume control, connect 1+2 and a+b. With this set up, it can also function as a stand alone line stage preamp.

The only upgrade I did to the DAC-01A was replace the stock 6N3 tube with a US made NOS 5670 (about $7 a pop), which gave a more refined sound. I regret unloading my 70s date code stash of WE396A/2C51s years ago...I can't afford to buy those back.😞 Anyway, I don't think it would have made the sound 10 times better.😉

Sonically, the DAC-01A trounces the D30. This unit sounds enticing. Maybe others will consider this colored because of the tube buffer. But to my ears it is A LOT more musical! Music IS THE BIG picture folks, leave the details for laboratory analysis.😄 

The unit has two sets of outputs; the normal out, which bypasses the tube buffer, gives up a bit of the dignified poise but still sounds more authoritative than the D30. Although I'm pretty sure the tube output stage will be a delight to pick on by scope jockeys. 😄

BTW, did I mention it also has a headphone output?

Coda

Inasmuch as I laud the Xiang Sheng DAC-01A as great value for the money, if you're in the market for a DAC to playback Redbook CDs, I'd persuade you to hit the flea markets and thrift stores to hunt for an ancient TDA1541-equipped CD player. Even an early 90s CD player with the cheaper TDA1543 chip will give more musical satisfaction than the DAC-01A. However, if you need a Hi-Res capable DAC for the Raspberry Pi music player, computer or server, this is hard to beat at this price point.

Listening Room - Minimalist no more?

Among the audio systems of my friends, this is the only set up that has no LP playback ability. Neither does the owner have interest in DVD-A, SACD or a music server/streaming device - only16 bit linear PCM sampled at 44kHz red book mastered CDs, period!

Since our last visit, a Northstar CD transport replaced a CEC TL51X, while another belt-driven CEC TL5 CD transport is being contemplated. The NOS (Non-Over-Sampling) Zanden Signature DAC equipped with a double crown Philips TDA1541A-S2 chip remains. Now you know why I hunt for unloved TDA1541 CD players from the 80s.😎

The most significant change in this system, however, is that
the Phy-HP Co-Ax speakers have been replaced by a pair of field coil powered Western Electric descendants - G.I.P. Monitor 1s.

According to the owner, the JE Labs line level preamp +
SE10 mono-blocks give life to GIP Labs! He is such a good friend. 😉

dynakit stereo 35

Before 2018 drew to a close there was a buzz on various audio forums about the auction of a vast collection of audio gear in the northeast. My attention was caught by lot number 4, a seemingly unmolested Dynakit Stereo 35. I bookmarked the site and registered for bidding.

Fast forward to mid December, my wife and I drove through the new Tappan Zee Bridge aka Gov. Mario M. Cuomo Bridge

and scenic Merritt Parkway (Route 15) to collect my loot, which included the stereo 35.

Feuillantine  - layers of dark chocolate mousse,
hazelnut buttercream, chocolate cake with
hazelnut wafer crunch

On our way home, we let the rush hour traffic subside by exiting in Pleasantville, NY. As a reward for my wife's enduring patience, we had dinner at Jean-Jacques and took home this decadent Feuillantine.

As mentioned in this JEL Arkiv entry, my passion for vacuum tube amplification was sparked by a Dynakit Stereo 35 I found in what was left of Radio Row in downtown NYC in the early 80s. Unfortunately, I had to unload it in the 90s to finance Tango and Tamura SE output transformers for my SE-DHT amp projects. It was a sentimental journey to reacquire this amp.

Removing the cage reminded me of the days when I serviced classic tube gear at Angela Instruments in the 90s. I quickly recalled the routine of checking for shorts in the multi-section capacitor and identifying continuity in the transformer windings. After those static tests were established, the amp was plugged into my trusty GenRad Variac and I slowly applied AC voltage while carefully monitoring the current draw. My hunch was right, this was a healthy amp that just needed a lot of cleaning and a little updating.

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

Dyna Stereo 35 Schematic

The secret to the ST35's success was its elegantly simple circuit design. It uses a 7247 twin triode which is essentially half of a hi-mu 12AX7 + half of a med-mu 12AU7. The hi-mu section functions as the input voltage amplifier, while the med-mu is configured as a cathodyne-phase inverter driving a push-pull pair of cathode biased 6BQ5/EL84 pentodes connected to wide bandwidth Dynaco Z565 ultra-linear output transformers. The judicious use of both positive and negative feedback made this a very stable amplifier.

Aside from replacing the original mylar coupling caps with polypropylene Wonder Caps in typical 80s fashion, the only mod I ever did to this amp was bypass the "green ceramic disc cap" (C1) and filter network (R1 & C2) in the input. This idea came from page 67 of Bascom H. King's review of the Mark Levinson ML10A preamp + ML9 amp in the August 1985 issue of Audio.

 Since I wanted to keep this amp as original as possible, I avoided removing parts and cutting traces like I did as a Dynaco hacker in the 80s.

This is the stock wiring of the RCA input jack. Audio signal + is connected to solder tag No. 1.

I unsoldered the wire from solder tag No. 1 and soldered it directly to the input grid, pin 7 of the 7247 input/driver tube. From above it looks stock but now the audio signal goes directly into the input grid (shunted to ground by R2) of the hi-mu voltage amp section. This mod effectively bypasses the filter network (C1, R1 & C2) which protects the amplifier from a preamp that produces DC offset at the output. If your preamp passes DC (or you aren't sure) DON'T DO this modification.

Since the 90s I've been convinced that paper in oil (PIO) caps sound more like live music. Although I'm not 100% sure whether the surplus mil-spec West-Cap/Astron cap combo I installed are PIO, the amp doesn't sizzle like it did with the original mylars.

Dynaco skimped on the power supply of the ST35 by using solid state rectifiers and a wire wound resistor in the 𝜫-filter. The least I could do was replace the original 60Ω, 5W resistor with a proper C-354 choke from Dynakit.

Minimal phase shift @ 100Hz = excellent low frequency response

Fast rise time @ 1kHz with just a hint of overshoot

Again a tiny overshoot @ 10kHz with minimal and well damped ringing

9.89 Vrms squared = 97.8121/8Ω load = 12.23 watts per channel
before clipping, both channels driven

stereo 35 sans cage

Speakers I've used in the past with the Dyna Stereo 35 include DIY mini-monitors using Radio Shack components, BBC LS3/5As and original Quad ESLs. It had a hard time driving a pair of Magnepan SMGas I briefly owned in the mid-80s, so I unloaded them. 😉

I'm listening to jazz tunes with the stereo 35 driving Altec 755Cs in 618 cabs as I type. It doesn't have the mid-range magic nor does it present micro-dynamics like my Simple 46, but this fine PP EL84 comes pretty close.

Just like Feuillantine, the dynakit stereo 35 is yummy!