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Nelson Pass has a tube design available

It’s no secret that I admire Nelson Pass both for his design skills and for what he gives to the DIY audio hobby. Unfortunately, us vacuum tube enthusiasts are mostly left out in the cold when Pass flexes his design muscles. That is until Burning Amp 2017 when he presented a pre-amp using the Korg NuTube.

Ok, so it isn’t the first “tube” that comes to mind when we think thermionic emission, but hey, it’s got a vacuum at least!

The Korg NuTube is a twin “triode” made by adapting vacuum florescent display technology to audio applications. Just like a DHT, it has an anode, a grid, and a directly heated cathode. The principals of operation (i.e. emission from cathode to anode modulated by a grid voltage within a vacuum envelope) are essentially identical to the little glass bottles we all know and love.

However, the low-voltage miniaturized technology requires certain compromises. With a max dissipation of only 1.7mW, the NuTube is limited in the maximum anode voltage and this forces positive grid operation, requiring a buffer to drive the inevitable low impedance. Like wise, the high plate impedance also necessitates a buffer on the output for most applications.

This is exactly what we see in Pass’s design, using his signature CCS-loaded JFET follower style buffer stage at the input and output. The result is a very compact and relatively low-voltage tube preamp with around 16dB of gain (as designed). Best of all, you can pick up a board and parts at the DIY Audio Store here!

I had the opportunity to hear this preamp not long ago in a nice system (Magnepan speakers, tube and solid state amplifiers) and compared it with an Alex Cavalli designed tube buffer and a MOSFET-based preamp. The Pass preamp sounded fantastic in this good company.

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Preamp project nearing completion and NYT “listening bars”

The linestage with built-in phono project is just about ready to get a first listen and testing. At this point all that’s left is chassis assembly and wiring the in/out/switching. Builds always take longer than expected, usually because I can’t stand doing the same thing the same way more than once. There are a couple of new circuits, PCBs, and approaches in this one that will be detailed in the full write-up as soon as I’m satisfied with the sound.

On another topic, I read an interesting article in the New York Times the other day about Japanese-style listening bars popping up in Los Angeles and New York. These are “cafes with high-end audio equipment, where patrons listen to vinyl records, carefully selected by a bartender, from a record library behind the bar.”

High-end audio showrooms must be suffering the pinch of online retail, making it more difficult for enthusiasts to find a place to experience equipment that they, often as not, don’t intend to purchase in the first place. So why not tap into the same kind of clientele in a no-sales-pressure environment and make your money on drinks instead of cables? The fact that vinyl is up and product ownership is down helps make the case.

This would potentially require a substantial investment in equipment (I’m assuming more expensive or esoteric systems draw more people in), but clever manufacturers might be keen on the advertising potential of getting their product/brand in front of consumers who appreciate listening and quality audio. If you’ve got a regular crowd, they might even appreciate regular changes to the system.

I’ve personally been on the casual look-out for an opportunity to build a system for a local brewery. People who appreciate local beer might appreciate other local products (I mean, I like local beer and the farmer’s market). Putting a visually interesting amplifier in a setting that it would be used and seen by the right kind of customer sounds to me like an excellent marketing investment/experiment. The brewery gets both good sound and a unique/local touch to their atmosphere.

If only I could find the time to build more than one amp every couple months…

Read the New York Times article here.

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Letters to WTF: how do you test a build?

Q: Hi, I’m working on a schematic from your website. How do you usually test your circuit, as you go, or once everything is wired?

This is a great question. The short answer is that it depends. On a simple build with just one or two stages and passive loads and power supply filters, I will probably finish all my wiring and then power up and test. On a complex build with things like active loads, multiple bias voltages, or regulated power supplies, I will test as I build. In both cases, my general testing process is fairly similar.

  1. Connect the project to a variac or light bulb current limiter (if available).
  2. With only rectifiers installed (no other tubes), power on and measure B+ voltages. These will be higher than the voltage levels with the rest of the tubes installed, but should be in the ballpark.
    • 2a If using any circuits on PCBs, I will test before installing in a chassis if my external power supply and loads allow it.
  3. Install preamp tubes and measure bias points to be sure they’re in the right ballpark. If fixed or directly biased output stages, measure bias levels. The B+ is still a little high at this point.
  4. Install output tubes and dummy loads, and measure current draw and bias point. The B+ should now be at roughly the calculated level. Adjust bias if needed.
  5. Connect to cheap speakers and debug hum/noise. Let the project run for extended periods of time and generally abuse it a bit.
  6. Hook-up to the main system and crank it!

At each step, any trim pot adjustment appropriate to the stage would be adjusted as needed. Typically I will have one digital multi-meter (DMM) on the B+ at all times and additional meters to measure individual tube bias. I use alligator clips and connect/disconnect meters with projects powered down. Don’t poke around live amps if you can help it!

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An Amplifier Primer: Technical Terms for Beginners (Headphonesty article)

Early tube computer modules with some of the female mathematicians and physicists who worked on the projects

I just finished an article for Headphonesty on headphone amplifier basic terms. It focuses on building block concepts and aims to make technical marketing copy more comprehensible by users without an electronics background.

Here are the topics covered:

  • Single-ended and push-pull
  • Classes of amplification (A, B, AB)
  • Output coupling methods (direct, capacitor, transformer)
  • Amplifying devices (tubes, transistors, ICs)
  • Negative feedback

This article is probably a good introduction to articles on WTF Amps like the Output Stages page or the various headphone amplifier projects.

Read An Amplifier Primer on Headphonesty here!

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Modern Quality Amplifier Conceptual Outline

May is a very busy month for me personally so I have only a short update this week. My last post was about the Quality Amplifier, a forerunner to the well-known Williamson amp. I proposed a modernized 6V6 A2 version based on the same topology that could probably crack double digit power.

In the conceptual outline above we have a MOSFET rather than a tube as a concertina splitter to save heater power. The unity gain MOSFET splitter feeds a tube differential pair to generate enough voltage gain to drive the outputs. MOSFET grid drivers with a CCS load help the differential stage cope with the drastic impedance changes at the output tube grids in A2 operation. Outputs are wired as triodes, of course.

The A2 drivers will go on a PCB. I also have one designed for the MOSFET splitter, though that’s simple enough to wire any way you might want. We’d need just three tubes per channel: a pair of 6V6s and a dual triode driver. My driver pick at the moment is probably the 5965 (or a pin-compatible 12AT7 if it needs to be new production).

I’m also happy to report that progress is being made on the preamp project. Hopefully I’ll have pictures to start sharing in the very near future!

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The Quality Amplifier

Having found an irresistible deal on a pair of Hammond 1620A output transformers ($35 each), I have started some preliminary research on suitable amplifiers to build around them. These transformers have a 6.6k primary and are rated for 20W. This is around the power and impedance used in classic amplifier topologies like the Mullard 5-20 or the Williamson Amplifier. Both would probably provide blameless performance, but I’ve got this incorrigible itch to do things the hard way.

Dennis Grimwood’s website Optimized Electron Stream has a great collection of articles and reading. In particular, his history of the Williamson Amplifier caught my attention. According to Grimwood, the Williamson amplifier was an evolution of a design published as The Quality Amplifier in Wireless World in 1943 (and updated in 1946).

Note you can find the archive of Wireless World back issues here at American Radio History.

The Quality Amplifier was the work of WT Cocking (who was also a prolific writer about valve electronics). In contrast to other contemporary designs using interstage transformers, Cocking exclusively used RC coupling between stages and a concertina phase splitter at the input. Much time is spent in his Wireless World articles detailing the care and feeding of capacitors, something we take for granted today.

Several potential triode output stage configurations are detailed in the 1946 article:

  • the original push-pull PX-4, producing 4W (or 8W with higher supply voltage)
  • push-pull PX-25, producing 12W
  • push-pull 6V6G triodes, producing 2W

All of the designs recommend MH4 valves in the phase splitter and driver stages, but list the 6C5 as an alternative. The 6C5 is a forefather of the modern 6SN7. None of the variations use global negative feedback. Here’s an example schematic showing the topology:

Apart from the appealing simplicity, I note the coupling caps needed at the input and between the stages. This is one more RC coupling than used in the Williamson, but there’s no global negative feedback to complicate the phase shifting. The placement of the concertina is also interesting here. By splitting phases before the drivers, rather than after such as seen in the Dynaco ST-35, Cocking is getting more gain per phase. Given the limited Mu in the tubes of the day, this was probably necessity.

This brings me back to the Hammond 1620As. I’m not going to be building with PX-4s or PX-25s, but the 6V6G that put out only 2W has had some spec bumps since Cocking’s day. We also have the benefit of transistors to assist us in squeezing out a few more watts and otherwise modernizing parts of the original design. Specifically, I’m eyeing the A2 grid lines provided on the 6V6 datasheets…

The 6V6GT triode-strapped is praised for its tone but lamented for its limited power. The Cocking Quality Amplifier looks like a great template that, with a few modern touches, will minimize the 6V6’s weaknesses and maximize its strengths. The push-pull loadline above looks like about 10W triode, a five-fold increase over the original application!

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Removing DIY barriers

It seems to me that there are three fundamental obstacles for beginners in the DIY tube hobby:

  • Layout and connection of component parts for best hum/noise performance
  • Choice of parts for correct and safe ratings/types/etc
  • Chassis fabrication and layout

Complete kits with chassis, parts, PCBs, and the whole ball of wax hit all of the points, but they are a daunting investment in both time and parts. See great examples from Bottlehead or Elekit. In a baby-steps approach, I’ve begun experimenting with putting entire circuits on a PCB design (image shows the El Estudiante). This addresses the first point.

I have ideas on ways to tackle the other challenges that minimize capital requirements and keep the hypothetical business idea agile and scalable (brushin off the old business and supply chain lingo). It might even be enough to turn into a respectable side-hustle. Hopefully I’ll be posting more on what I’m calling “quarter kits” in the near future.

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Guest editor at Headphonesty

Headphonesty is a digital magazine dedicated to high-fidelity headphone audio culture. Outside of designing and building a few heapdhone amps, I’m personally a regular high fidelity headphone user, especially now that I have a toddler in the house. I recently connected with the chief editor at Headphonesty on Reddit and agreed to lend a hand with an article on headphone and amplifier impedance.

Unlike speakers, which are really rated at 4 or 8 ohms 99% of the time, headphones have a wide spectrum of impedance ratings. From 16 ohm in-ear-monitors to 100k+ ohm electrostatic headphones, matching sources and loads is a confusing aspect to the hobby that’s often done through trial and error or hearsay. This article written by Trav Wilson and edited by yours truly seeks to explain some of the concepts in an easy-to-read headphone-centric way.

Read Headphone Impedance Demystified on Headphonesty here!

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Line stage + phono build underway

In a recent post I set a goal for myself of creating a couple of preamp designs that included both line stage and phono preamp circuits. The first of these builds is now underway! Because of the number of input and output jacks as well as switching and volume controls, I’m using a different style chassis than the all-wood apron approach in most of my builds. The face plate and rear of this enclosure are aluminum with wood (walnut here) used as accent panels on the sides.

I’m emphasizing the sleeker look by mounting all of the transformers inside the chassis. The power supply is mounted to a section of aluminum c channel that also serves to section off all of the AC power from the rest of the chassis (which will carry sensitive signal circuits). So far so good. We’ll know if the approach to shielding and layout is effective once it’s powered up and playing. Placement for the signal portion will be finalized after I’ve mounted the front panel controls into the 3/8″ aluminum flat. I expect that to be a bit of an adventure…

The generic circuit for this build is below (final values will be published once it’s tested). Tracing the path of a phono signal: the resistor loaded input stage feeds the RIAA correction filter which feeds a gyrator loaded output stage. I’m using a gyrator as a flexible load to allow for tube swapping as well as a low output impedance device to effectively drive the follow control that follows after the selector switch. The volume control feeds a transformer loaded 6H30.

The Edcor GXSE 15k:600 output transformers are an experiment here. Reading through others’ experiences and measurements, I think the 6H30 is going to be a suitable driver with good bandwidth if it’s given enough current. I expect to do some experimenting with loading the secondary.

All in all, this will be all 9-pin current production tubes and parts. If the execution works out as well in real life as it does on paper, it will be a great, relatively-affordable preamp build.

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