Stacked SMPS PSU for tubes

I used a 48V switch mode power supply in the El Estudiante headphone amp and am pleasantly surprised with the quiet background and relative simplicity. When it comes to higher voltages though, you will not find many AC/DC switch mode power supplies at vendors like Mouser or Digikey. While a beefy low voltage DC supply could feed a DC/DC booster (see Millet’s 10W booster project and various eBay listings), I’d like to find something that is more easily repeated by others with parts from major PSU manufacturers.

I came across the idea of stacking low voltage SMPS supplies in a couple places and the idea intrigued me as a scalable and affordable approach to creating B+ (see here and here). The Meanwell EPS-15 48 are regulated and isolated AC/DC supplies that sell for about $8. Stacking six of these would supply 300mA at about 300V.

This schematic shows the general outline of what I’d like to try with a single-ended amplifier. The V1 supplies produce the anode to cathode voltage for the output tube. These are referenced about 100V above ground by the V2 supplies. The input tube’s B+ is a combination of the V2 and V3 supplies. All in all, this would call for seven 48V supplies, plus another low voltage supply for heaters. The final cost would be somewhat less than a traditional transformer and CLC filter, but there are more interesting reasons to try this.

All the supply gymnastics make it very easy to direct couple the two stages. In this example, Q1 is a gyrator load and Q2 sets the reference voltage. This would also allow us to drive the output tube into A2 operation. My choice for output tube here would be a 6V6: a really sweet sounding triode that otherwise doesn’t produce much in A1 operation. Power would still be low (around 2W), but that would be plenty for headphones or enough for high-efficiency speaker systems.

2018 year in review

I can’t for the life of me remember a louder year than the one that ends next Tuesday. Personally, professionally, politically, it was a hectic twelve months. In retrospect, concrete and measurable goals (AKA resolutions) are probably what kept me alive and sane amidst the chaos. I’m no self-help guru or productivity genius, but I feel like I have survived a sink-or-swim situation over the past year, so I’ll share some thoughts on living with a hobby in the real world. First, I’ll pat myself on the back for my accomplishments:

Finish master’s degree:

  • Done! The last year and a half were a challenge (my daughter was born spring of 2017), but I somehow found the energy, motivation, and focus needed to finish my MBA. I hope that being done with school will now free up the mental space for other parts of life (family, friends, projects).

Post to blog every week:

  • 99% successful! I missed one or two weeks and not every post was laser-focused on tubes, but I’m happy that the blog and website survived a hectic year for me.

Exercise regularly:

  • Exercise is an important ingredient for mental focus for me personally. I also have my exercise equipment in the same room as my electronics workbench and the multi-tasking kept audio projects moving forward (albeit slowly). My explicit goal for this year was to get into the 1,000lb club (weightlifting) and I did it with some room to spare.

I’m a big fan of quadrant prioritization to keep daily life in perspective (this idea is credited to Eisenhower, I believe). The gist is that we should rank our goals as high/low importance and high/low urgency and pursue them accordingly. I draw one of these diagrams at least once a week:

If we spend our time in quadrants I and II, we’ll get the important stuff done. The hard part is understanding your goals well enough to see the difference between urgency and importance. There is plenty of subjectivity to this importance/urgency classification and not everyone would come to the same conclusions.

For me, tube projects are a solid quadrant II item and that isn’t going to change this year. However, being done with school now means that high urgency and high importance things are screaming a little less loudly. That lets me spend a little more time on long-term goals.

This is all just a long way of saying that I intend to build more this coming year. On the short list are:

  • EL34 push pull mono-blocks (see pic for one very near completion)
  • Transformer-coupled line stage (to pair with above, probably DHT)

And on the list of schematics-in-waiting are:

  • EL84 push pull stereo amplifier
  • Line stage with integrated phono
  • SMPS-powered 6V6 SET headphone amp
  • Battery-powered experiments

On top of the workbench and test equipment projects that builds usually create, I’ll be happy if I can finish four of the above in 2019. I’d really like to return the website to its roots as a practical resource for projects and reading for beginners in the tube audio hobby. As I told someone else recently, my goal with this is creating the resources that didn’t exist when I started in the hobby. While this may not be urgent, it is important!

Simple discrete CCS

You’ve already read the CCS and loadlines page that discusses how a constant current source affects a tube’s behavior. You have also seen CCS’s made from depletion mode MOSFETs and LM317’s in projects like the Papa Rusa and El Estudiante (respectively). These don’t go into the actual operation of a CCS, so here’s a post illustrating a simple discrete CCS.

A NPN bipolar junction transistor (BJT for short) is a three terminal device, like a tube triode:

The BJT collector, base, and emitter are roughly analogous to a tube’s anode, grid, and cathode. With a vacuum tube, we set a bias between the grid and cathode and generally the grid is held at some negative voltage potential relative to the cathode. With a NPN BJT we set a bias between the base and emitter and generally the base is at a positive voltage potential relative to the emitter for current to flow.

With a tube, current flow is limited by a cathode resistor (also serving to set bias). We can make a simple CCS out of a triode using a cathode bias resistor and referencing the grid to ground. The impedance at the anode (where we’d connect the load) is the anode impedance plus the impedance of the cathode resistor multiplied by Mu + 1. In other words:

Z = Rp + Rk * (Mu +1)

The tube CCS requires a fair amount of voltage across it to function. This limits the practical applications and is probably why you really don’t see it very often (and it’s usually pentodes used as a CCS when you do). The impedance seen by the load and the current handling are also pretty low compared to what can be achieved with evil, no good, dirty, rotten solid state.

Like the triode above, we can use a resistor in the emitter of a BJT to set the current at the collector. The difference here is that the base needs to be positive relative to the emitter for it to be ‘open’ (letting current flow). This means we cannot use ground as the base’s DC reference like we did with the triode:

The humble LED has a stable voltage drop that varies little with the amount of current through it. They also tend to exhibit very low noise. These qualities make LEDs nice voltage references for transistors (as well as cathode loads for tubes).

We feed the LED through a resistor (R2) from a voltage source (B+ or other auxiliary supply) to limit the current. The amount of current through the LED is not critical, typically in the neighborhood of 5-15mA. The voltage drop of the LED varies with type and color, but red LEDs typically average about 1.6V. This voltage drop is the voltage reference for the base of the BJT.

For the transistor to pass current, we need to turn it on by biasing the base positive relative to the emitter. The voltage required to “turn it on” is often referred to as the base-emitter drop and is in the neighborhood of 0.6-0.7V. With a red LED reference of 1.6V and a base-emitter drop of 0.6V, we would have about 1V across the emitter resistor (R1).

The value of the emitter resistor determines how much current passes through the BJT. We find the resistor value needed for our target current with regular old Ohm’s Law. If we want to set 2mA through the CCS, we need an emitter resistor of:

R = V / I

500 ohms = 1 volt / 0.002 amps

In practice, the emitter resistor may be replaced with a trim pot to allow for an adjustable current. The load impedance created by the transistor CCS is approximately the dc current gain (hfe) multiplied by the emitter resistor. For small TO92 transistors (e.g. PN2222), current gain can be a factor of a couple hundred. That means with a voltage drop of only a few volts, we can create an impedance of 100k+!

The above simple discrete CCS would work in the cathode of a tube stage, but we can also use PNP transistors in the same way as anode loads:

The above generic circuits require suitably rated parts to work in real life. The resistor feeding the LED often needs a high voltage and power rating if it is connected to B+. The transistor may also need a high voltage rating and/or a heatsink. In practical terms though, these are as simple as they look and make a good introduction to the inner workings of constant current sources/sinks. A common and quick improvement to the above examples is to cascode two transistors, multiplying the current gain, and easily pushing the impedance well over 1M.

Off-topic: Copyrights and Piracy

Piracy and copyright are topics close to my heart. In fact, my day job is in music publishing (digital, physical, and distribution). A recent post over at The Journeyman’s Toolbox provoked some thoughts on the topic of piracy. Reading through my blog, you’ll rightly come to the conclusion that I believe strongly in the importance of the creative process and sharing ideas and knowledge. I hope you find this slightly-off-topic discussion interesting. Back to circuits soon.

Rafael’s conclusion (see link above) that piracy often boils down to availability is a good one supported by research from NGOs and industry groups. Technology has spread across the planet faster than either secure distribution platforms or ethical/legal understanding of copyrights. In short the demand (for legal IP) is there, but the supply often is not. Today’s big challenge lies in the ‘flatness’ of the digital world and transitioning from piracy culture to one that supports creator rights. 

The forefathers of today’s copyright law (Statute of Anne in UK, US Constitution, Berne Convention) drew a direct line between authorship and ownership, progressively strengthening the rights of the creator. The idea was/is that creators need some financial incentive to create and protecting the right to (e.g.) reproduce or license a work gives original creators an exclusive commercial advantage. Technology, like copyright, has moved in the direction of empowering individuals through universally accessible distribution platforms (YouTube, WordPress, etc).

However, ethics and regulation have not kept pace with technology. As a result, we see continuous violations of copyrights without practical mechanisms for authors to combat it. Education is also lacking as users often have no idea they are violating someone else’s rights when they share, borrow, or download protected works. Finally, there is the question of consumer access to legal channels, the presence or absence of digital services or payment standards. These are practical drivers of pirated content that may eventually be solved with better services and updated laws (consider for a moment the pace of technology and the average age of legislative representatives).

In the USA, the SCOTUS asks four questions in weighing copyright/fair use cases: what is the purpose and character of the use, what is the nature of the underlying copyrighted work, how much is used in relation to the whole, and what effect does it have on the market for the work? It is this last point that is often most salient. Where pirated content is available and legal content is not, it is hard to blame the ‘pirate’ even if they are technically breaking the law. Pleading ignorance is an understandable excuse, though the platforms benefiting from the content and traffic (not the individuals themselves) should be held to a higher standard when it comes to copyright education and oversight.

Setting aside these practical factors, we are left with ethical issues. Specifically, many take the ethical stance that piracy preserves or democratizes culture. This is a common theme in copyright discussions that appeals to emotion, but it contains critical missteps in logic. The most fundamental flaw here is that copyright covers expression, not underlying ideas. Culture is not a library of distinct units (expressions); it is a collective understanding of society and the world (ideas). Copyright does not privatize ideas and therefore does not seek to regulate or control culture.

Dissolving copyrights as a legal/financial mechanism in order to declare artistic works public domain suggests that culture and monetization are mutually exclusive. While there might be some productive discussion with regards to copyright duration, the idea that author protection could be abandoned and result in a positive impact to the creative market ignores basic realities of survival and specialization. Today’s public domain cornerstones like Beethoven, Mozart, da Vinci, or Shakespeare would not have existed or endured to be appreciated without the contemporary financial support of patrons (i.e. monetization). Predictable and protected incentives, not spontaneous generosity or exposure, are required to liberate and motivate the creative spirit. 

Most worryingly, the piracy-for-culture argument assumes that no better solution to the practical problems (availability, education) can be had. In that way, piracy stands in the way of commercial/civil services that could serve the same public function without violating the rights of original creators (a win-win). Personally, I’m happy to see more subscription-streaming services (Spotify, etc) become available; even if they are not perfect, they are a step in the right direction. There is no denying though that these services compete with (intentional and unintentional) piracy for the hearts and wallets of would-be consumers.

While I empathize with the cultural demonetization viewpoint, it ignores too much economic context and history to hold up under the scrutiny of real life. Consuming culture is a two-way transaction for artists and audiences alike. Lofty ideals muddled with the vulgarities and economics of reality make art evocative and visceral. How can I ask an artist to put his blood down on paper if I’m not willing to open up my pocketbook in return?

Letters to WTF: Holiday Gifts for the Tube DIYer?

Q: A simple question – is it possible to purchase a kit from you? If not, are there decent kits you would recommend for a first-time tube build? My husband has built a couple of solid state preamps and power amps and is intrigued by tube ware.

A: Your husband is a lucky guy to have someone encouraging his hobby!

I don’t sell any kits for my builds at this point (though I do send out prototype PCB boards to my Patreon subscribers on occasion). I am happy to provide some recommendations for beginner-friendly tube projects and/or gift ideas, though. Some of these are PCB boards that require you to select parts (or leave your husband to do so afterwards). Many builders enjoy the process of picking out and sourcing parts, so this isn’t necessarily a bad thing  and you might include a ‘parts budget’ as part of the gift in that case. 

Tubecad.com makes some of the best documented and flexible board kits you can find for tube hobbyists.  In particular, the Aikido and CCDA designs have a great following and lots of user support on community websites like diyaudio.com:

Aikido Noval Stereo
TubeCAD Aikido (click to go to listing)
9-Pin CCDA PCB and User Guide
TubeCAD CCDA (click to go to listing)

Note the above let you add parts or order boards by themselves. Adding parts might be tricky for you to do without your husband’s input, though TubeCAD does a good job keeping the options and confusion to a minimum.

Here’s another PCB board (no kit) for a phono preamp (for turntables) that I can also recommend. The designer of this one is another well-known author on tube topics:

Valve Wizard Phono Board (click to go to listing)

Bottlehead is one company that gives you everything you need in a full kit. They have a lot of tube kit options at different price points. If your husband also listens to headphones, this company is especially well-known for their headphone amp kits (two options below, but explore the site to find more). 

Bottlehead Crack (click to go to listing)
Bottlehead Single Ended eXperimenter’s Kit (click to go to listing)

Bottlehead’s kits are pricier, but the documentation and the all-in-one nature add a lot of value for beginners. 

Lastly, Elekit is another Japanese company that does all-in-one kits. These are available through the diyAudio Store.  I don’t have personal experience with Elekit kits, but I have read a lot of good things (and the manuals I’ve seen look very well-done).

Elekit TU-8500 (click to go to listing)
Elekit TU-8100 (click to go to listing)

Hopefully you find something in your budget in the above links.  I think anything you do to show an interest in his hobby will be very well received!  

A PCL86 SET kit with chassis from the DIY Audio Store

The DIY Audio Store (part of the diyaudio.com forums) is now selling the Elekit TU-8100 PCL86 kit:

TU-8100.jpg

At only $275, this 2W output kit is about as low an entry point to all-in-one well-documented tube kits as you’re likely to find. It includes two inputs (rear and 1/8″ front) and is powered by a 12Vdc power brick. This keeps the amp small enough for even desktop usage (5.5″ square). According to the ad copy, SMD components (DC booster, etc) come pre-soldered.

The PCL86 is a 9 pin triode + pentode roughly equivalent to a 12AX7 and EL84 in a single envelope. See a datasheet here. It’s used in the Elekit in a traditional two stage cap-coupled single-ended arrangement. The output transformers are rated as a 7k primary and the circuit employs global feedback to squeeze some extra linearity out of the high gain input stage. See the Mighty Cacahuate for a similar design (no feedback and 6CG7 instead of 12AX7).

If you’re hoping to find a tube kit under the tree this year, add this one to your Christmas list. Elekit puts out some very cool products and purchasing through DIY Audio Store helps support one of our hobby’s precious resources.

I have no affiliation with Elekit or DIY Audio (other than sincere admiration).

Thomas Mayer’s new site

I’ve followed Thomas Mayer’s hobby website for a long time. He uses a tantalizing mix of high quality transformers and DHT tubes to build some beautiful audio devices. The tube of the month series is also a must-read review of odd-ball tubes and applications.

Now we can all see how much Mayer charges for his impressive tube builds. It’s about what one might expect based on the craftsmanship he clearly puts in and what the audiophile market supports in other products.

Check out vinylsavor.com here. If nothing else, browse the galleries and drool over the very Scandinavian glass, wood and metal work.

vinylsavor.png

There is now a NuTube portable amp kit

HA_KIT_All.jpg

Some recent digging around Pete Millett’s nutube.us website turned up this interesting page. What we see is a pocket size amplifier powered by AA batteries.

At first glance, it looks similar to a project you can find on this archive of Audio Mania magazine (Japan). Here’s the schematic showing the DC booster power supply:

nutube

The Nu:Tekt kit appears to use an opamp driver on the output rather than the FET combo shown above. With the exception of the small daughter board which appears to come assembled (I assume this is the booster), the kit looks to be completely through hole.

Little is revealed on Millett’s site, but I’ll be following this kit with interest!

Solid State Phase Splitters

The phase splitter is a critical step in a push pull (differential) amplifier. Because tubes don’t come in “p-types”, we feed the output devices signals that are inverted relative to one-another in order get one to push while the other pulls.

I’ve been finding solid state concertina-style phase splitters crop up here and there recently. A couple of days ago even the great Pete Millett got in on the action. Millett employs a JFET concertina splitter in his hybrid amp (a must-read, btw), but MOSFETs are also a good option for this application if you use parts with reasonable input capacitance.

Here’s a push pull schematic I’ve been marinating that illustrates the MOSFET concertina:

mosfet concertina pp

By using the MOSFET we’ve reduced the twin-triode count in a stereo push pull amp by one. The MOSFET will also let the splitter swing closer to the power rails, though in this particular case the 10BQ5 doesn’t really need a lot of voltage swing at its grid. The tubes shown are odd heater standards: 407A is 396A with a 20V heater and 10BQ5 is 6BQ5 with a 10V heater.

You can find a lengthier explanation of the RC step network between the 407A and MOSFET in Morgan Jones or buried in this diyaudio thread. In brief, the resistor divider sets the DC voltage at the gate of the MOSFET while the 0.1u cap bypasses the upper portion of the divider at AC frequencies so that we don’t lose any gain due to the divider.

Phono Preamp Heaters

Heater supplies, even with indirectly heated tubes, are a potential source of hum with high gain circuits like phono preamps. In a grounded cathode gain stage, the tube will amplify any signal it sees between the grid and the cathode. The tube doesn’t particularly care if that is an audio signal or an induced signal from some other part of the build. Indirectly heated tubes have a cathode sleeve around the filament heating it. The close geometry creates a happy little environment for coupling between the two. Eliminating this source of noise may require running heaters on DC rather than AC.

Here’s a simple schematic adapted from something Eli Duttman suggested for his modified RCA phono preamp:

12V dc heaters

This circuit (now on a PCB waiting for a phono build) uses a voltage doubler to turn a common 6.3Vac input into ~16Vdc which is then regulated to 12Vdc by a LM7812. The regulator is limited to 1.5A, but this is probably enough for any sane phono preamp’s heater demands (the pair of 12AX7 in the El Matemático require only 0.3A). This is one way of producing a DC heater supply.

I was recently discussing truly budget-oriented tube phono preamps with another builder. They proposed a $100 parts budget. The first place I’d look to start cutting costs in such a build is on the relatively pricey purpose-built power transformer needed for tube projects. In the case of a simple phono preamp like El Matemático, I’d try the following cost-cutting measures to the power supply:

  • Solid state 1N4007 rectification
  • Use a 115/230V isolation transformer like Triad N-68X in reverse (115V in, 230V out) for B+ @ $16
  • Use a 12V SMPS like Meanwell EPS-15-12 for heaters @ $7
  • Triad C-1X choke @ $10 and 220uF 350V+ caps @ $4 ea as CLC filter
  • Add RC to end of CLC filter to lower B+ and/or clean up residual ripple

We can greatly lower the cost of the B+ supply with the isolation transformer trick but it leaves us without a heater supply. Rather than a separate 6.3V or 12.6V transformer followed by a regulator circuit like the one shown above, I’d be tempted to experiment with a switch mode power supply like this Meanwell unit:

EPS15-12

The EPS15-12 supplies up to 1.25A at 12Vdc with 80mV of ripple (peak to peak). One need just supply it with mains voltage (85-264Vac). Power supplies like this switch at a very high frequency, which is why their transformers can be made so small. If that switching is audible, capacitively coupled between cathode and heater, additional filtering may be needed. Meanwell does not specify the switching frequency, but it’s very likely well above the 20hz-20khz range.

The final, potentially very affordable power supply, would look something like this:

very cheap psu