Los Monos are coming

Here’s a sneak peak of the long-term push-pull project. The second monoblock is one cathode bypass capacitor away from being ready for playback. A bad tester tube took out the cap on one side during testing with a bang, but I’ll have replacements soon. Until then here’s the intro of the project write up.

The monkey on your back

Everything should be made as simple as possible, but not simpler.


There comes a time in every DIY builder’s life where he or she gets the urge to stretch beyond single-digit output power and single-ended amplification. There is no shortage of worthwhile projects to choose from: variations on Williamson, Mullard, or Dynaco push-pull topologies are easy to find discussed in forums and tweaked to compensate for modern parts. You can even find kits for something like the Dynaco ST-70.

When the double-digit power bug bit me I could not bring myself to abandon my usual no-feedback, triode output, class A comfort zone. This is the simplest (but not the only) path to good sound and my speakers are efficient enough. I’m also too lazy to do feedback math but that doesn’t mean open-loop, class A triode designs aren’t an engaging challenge. This build faced the following complications (which are common to many push-pull amplifiers):

  • Class A requires healthy current in the output stage: this needs to be balanced in the output transformer to preserve inductance
  • Two cascaded grounded cathode input stages is too much gain, but one stage is generally not enough
  • The input stage must have low enough output impedance to drive the triode output tubes

For the most part, my solutions to the challenges strive for simplicity. As is often the case in tubes and life, simplicity in some areas is traded for complexity elsewhere. This push-pull amp has only two stages, the outputs are cathode biased, and it requires only three tubes per channel. To make this seeming simplicity possible, I used solid state helper circuits on PCBs. While these helper circuits are not technically complex, they drive up the parts count and require some measurement and adjustment.

Here is the conceptual topology for Los Monos:

Pictured is a two-stage triode output push-pull amplifier. The output stage is garter biased and the voltage gain and phase splitter stages are combined in a folded cascode long tail pair. This is all described below with a full schematic (showing lots more parts).

More of this write-up is on the way as soon as I’ve got both channels playing and glamour shots are taken!

A Mighty Cacahuate caught in the wild

Student “D” sent me some pictures of his Mighty Cacahuate project with a twist and it’s too unique not to share. D developed a PCB for his build and mounted all the amplifier parts to a top plate as would usually be done. Instead of a boring wooden box, D dropped this into a boombox enclosure for all-in-one listening. I love to see the creative use of a basic schematic I posted here on my little website.

We initially troubleshooted some wiring over email, mostly due to my omission of the details of heater wiring and pin numbers on the original schematic. Once sorted though, D says the amp started playing and sounding great without a hitch.

It was an amazing feeling the first time they powered on.


Careful there, D, that feeling is habit forming!

Update on the mono-blocks: left channel is done and right channel is coming together quickly. Write-up for the project is also underway. Looking forward to playing in stereo!

EL34 tester tubes, also considering springing for KT88s on this project
Night shot. Red glow through the vents is LEDs used as voltage references in CCSs inside.

2019 NAMM Show Transformer Stuff

This was a busy week, so all I have to post is a couple of quick pics of people I chatted with at the NAMM show in Anaheim (which I attend for my day job).

Kevin from K&K Audio was hanging out in the Lundahl booth on Saturday morning before the show. We chatted about the state of DIY, local building groups, and the NAMM show. I had never met Kevin before but we had an immediate connection over the DIY hobby. K&K is the USA distributor for Sweden-based Lundahl Transformers. Their C-core construction, multiple winding configurations, and super-detailed datasheets are unique in the tube building hobby. Lundahl products are also sought after for studio and pro line-level applications and microphones (hence the NAMM booth).

I also found a booth for Triad Magnetics at the show this year! This was the first time I’ve seen Triad at NAMM and I had to say hello and thank them for manufacturing such affordable and easy to source power supply chokes (which I use in almost everything I build). The Triad team said the show was going well and that they’d probably be back next year.

I had a whirlwind NAMM schedule this year and didn’t get to spend any real time at tube booths like JJ Electronics or Electro Harmonix. I did take some time to get scanned for CIEMs at Ultimate Ears, though. There may be a portable amp project somewhere in the future…

American Radio History

Today I came across a website project called American Radio History and it reminded me that the internet is an amazing resource for education and sharing/archiving information. American Radio History is an online depository for endangered antique electronics knowledge captured in the form of hobbyist magazines from the early 20th century (back when tubes were the dominant active device).

The site’s FAQ provides a glimpse into how it came about:

There is so much printed material about radio and television that is becoming harder and harder to find. Libraries are discarding (often to the recycle bin) many titles. Other collections are very limited in access so “the rest of us” can’t find information we want.

The site began over a decade ago when I found I was often being asked questions I could answer from my own library. So I went digital for all to see!

Just one person does most of the work. I have off-site backups at several locations under the custody of well-respected radio historians. I also have several hearing-impaired persons who help with the major flatbed and Atiz Bookscan digitizing projects.

Here’s a short list of some of the dozens of relevant magazines for DIY tube reading:


Radio TV Experimenter


It is not difficult to find schematics in our hobby, but it is somewhat more challenging to find schematics accompanied by articles that reveal their inner workings. That is one advantage still held by professional publications like the audio magazines cataloged by American Radio History.

Bench tips: organizing resistors

There is a new post at Hackaday detailing a very cool 3D printed resistor organizer. Resistors are an indispensable component for DIY tube projects and we typically collect a large variety of both resistance values and power ratings. If you’re into boutique parts and Holco/Riken/Caddock flavors, it is even more overwhelming. On the upside, resistors are pretty cheap to buy in bulk; but if you buy in bulk and don’t have a way to organize things, you end up with a tangled mess of kinked-up leads and unmarked parts.

That was me a couple years ago: always buying extras and just tossing them into a parts organizer with too few cubbies and not enough labels. Eventually I got sick of repeatedly buying the same values because I was too lazy to sift through my inventory. Doing a little reading on hobby forums, I came across a really cheap and useful solution: trading card binder pages.

Trading card pouches are just the right length to store resistors without having to bend up the legs. This is best for 1W and smaller sizes, so I still have some storage for power resistors. But I also don’t buy power resistors in bulk very often (because I’m cheap), so there’s less to store. Go for either tape and reel (see above) or keep your loose resistors in baggies:

The parts baggies you get from Mouser/Digikey are already labeled, so you just need to fold it with the value facing up and slide it into the card slot. Easy peasy. Best of all, the trading card pages and the binders themselves are easy to find. The only downside to this resistor storage solution that I’ve found so far is the high likelihood of dumping everything on the floor if you pick the binder up upside-down (true story, twice). I’m currently on the lookout for a Hello Kitty trapper keeper with a zipper to solve this. For science.

Bench update: the push pull mono-block project made its first music through a speaker yesterday. I’m looking forward to cranking out the second amp and posting the project for others.

Opamp + tube RIAA board art

A recent discussion on diyaudio.com reminded me of the opamp-based RIAA idea I shared last summer. It turns out that someone else has done something similar and reports very good results. Koifarm was after a more integrated build with phono, streaming, and line level all in a box, but the basic idea of using an opamp to perform the RIAA corrections and a tube to provide some/all of the voltage gain is the same. We differ just in how it’s integrated: I’m after a simple RIAA module with outboard tubes while Koi was going for an all-in-one.

Here’s where I’ve landed so far on a board to contain the opamp bits and bobs:

The opamp runs from the 6.3Vac heater winding that would be included on any tube-centric power transformer, meaning there are no special windings or an extra transformer to power the solid state section. The output of the RIAA module would be fed to a simple tube gain stage of your choice. Here’s a grounded cathode application, but keeping the tube off the board means there’s tons of flexibility.

So, will it work? Koifarm thinks so and he’s a pretty prolific phono preamp builder. I’ve also already tested the same RIAA correction scheme in the battery powered phono project. I’m saving up a few designs to place a board order, but I’m hoping this RIAA module would make for a relatively simple and fun build this year.

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?