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!

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.

A cheap sleeper tube thought experiment

One day I was combing through tube characteristics using the parametric search function in the Tube Data Sheet Locator desktop app and I came across an interesting 7-pin triode. As luck would have it, I happened across a couple at a swap meet about a week later when the tube type was still fresh in my mind. I bought them, thinking they might be interesting to experiment with.

The 6AF4 is a 7-pin indirectly heated triode. I found the 6AF4 interesting because of its fairly low amplification factor (Mu of 15), decent transconductance, and high perveance. These characteristics suggest that a really simple and fairly low voltage preamp may be within reach. Check out the datasheet here (link to PDF).


I’d power this with a pair of 48V SMPS in series and wire the two 6AF4 heaters in series to be powered by a 12V SMPS. This would bring PSU costs and size down significantly. Voltage gain here would be about 10 and output impedance should be a little under 2k ohms.

Because resistor loading will still provide a lot of gain, even with a low Mu of 15, this version uses a matching transformer to step down gain and output impedance. Voltage gain here would be 2 or 3 and output impedance should be a couple hundred ohms. Edcor makes affordable candidates for output transformers in this application (WSM series). I’d also look for second hand matchers in the 2:1 to 5:1 range.

As far as a loadline and bias point, the blue blob area looks pretty good to me. Note how low the supply voltages are here. These low voltages make parts both cheaper and smaller. While I like the lower output impedance and fanciness of parafeed, the resistor load would probably sound a little tubier. It would also be simpler, cheaper, and more compact.

Whelp, I seem to have talked myself into it…

Designing for DIY

At a recent audio swap meet, I had the chance to meet Matt from Toolshed Amps (check out his great looking work here!). We talked quite a bit about tubes and audio design and our different approaches to the same goal (quality sound). It was interesting and relevant enough for me to want to share some thoughts here on the blog as well.

On the surface, the differences between what Matt and I create seem obvious. Matt favors classic triodes like 2A3, 1626, or 45. Supporting components include tube rectifiers, big can caps, and Magnequest (!) iron. His amplifiers are housed in meticulously handmade chassis with intricate etching. In short, Toolshed Amps lives up to its name and the cottage industry tradition of passionate small-batch craftsmanship. I love it.

WTF Amps is a DIY-focused project first and so I try to design with other builders (not just end-users) in mind. In addition to quality audio, this creates some hobby-specific goals that guide many of my design decisions. At times there is even conflict between these goals:

  • Parts availability and flexibility
  • Novel and exploratory circuits
  • Simplicity and intelligibility

I like looking for NOS tube hidden treasures and am always hunting for a deal on second-hand transformers. When I publish a design to be replicated by others though, I have to be cognizant of the availability of the parts I specify and whether alternatives exist. You’ll find more Hammond/Edcor iron in my designs than Tango/Magnequest not just because of costs, but because they’re widely available. Similarly, although I love the 5965 tube, I’ll probably specify a 12AT7 because they’re in current production. Where I favor easy-to-find parts, I still design (and write) for flexibility in upgrades or tube substitution.

I believe that the DIY tube hobby (like most hobbies) is a journey. As we progress in the hobby and our repertoire of concepts and circuits grows, the uncommon and novel designs are what keep us building and learning. Building leads to experience and self-evaluation, which leads to conceptualization and experimentation (side note: andragogy is the method and practice of adult learning). I should note that playing with new circuits and approaches are as much for myself as they are for the readers!

The last guiding principle (simplicity and intelligibility) is often at odds with the need to explore new things. If I publish a DIY design, I would like to be able to explain it in a project write-up as well. Some of this is accomplished when I’m researching topologies but complex projects (even if the component parts are simple) are a daunting task. The urge to push the design envelope is always there, but I’ve learned to take baby steps and rely on conceptual stepping-off points for published projects. This is good general advice for the hobby as well. Don’t rush it; build what you know and iterate.

So in summary, do I want to build an A2 DHT amplifier with Tango iron, 274B rectifiers, regulated everythings, TVC attenuator, and a rosewood enclosure? You bet your butt. Do I respect guys like Matt who do (and do it well)? Darn straight. But this kind of all-out end-game amplifier wouldn’t quite fit with my DIY-friendly design goals. On the other hand, WTF Amps will try to get you as close as possible to building one of these yourself with available parts and easy-to-understand write-ups. The last mile is just up to you.

Workshop toys and a different style chassis

I recently spent some time in the garage workshop on a non-tube project I’ve been meaning to do for quite some time. Up to now, my chassis have all been a mitered box with a lip to inset the top and bottom plates. I cut the lip with a straight edge and a hand-held router. This works fine and produces good results, but setting it up for the cuts was a chore. The better solution for this kind of rabbet with a fixed width/depth is a router table, so that’s what I built!

My new table includes a white-board top, inset aluminum plate for the router, and t-track to adjust the fence distance. The fence has adjustable stops so I can get close to zero clearance and add shims to plane edges or remove small amounts of width from boards. This will all make cutting the same chassis I’ve been doing much quicker. It also opens up some new chassis possibilities:

Here’s a different style chassis that I’ll use in the next preamp build. The sides are still wood and include a lip to inset the top and bottom, but the front plate is 1/4″ aluminum and the back is an aluminum c channel. This will make drilling for jacks and controls much easier than using a 3/4″ wood panel. Making the same enclosure with decent precision would have been much more difficult without the new router table.

More updates to come as I begin this preamp project!

New batch of boards!

Another batch of boards just arrived for some of my upcoming projects and experiments. These include:

  • MOSFET driver for A2 output stages
  • FW rectifier+voltage doubler combo on a compact board
  • MOSFET cap multiplier and CCS for DHT filaments
  • Shunt cascode boards v2 (slight layout changes to previous version)
  • MOSFET “gyrator” anode load
  • Low voltage bipolar supply (LM317 and LM337) for opamps

Some of these will be used for upcoming phono and line-stage builds while others are daydreams that I’ll get around to eventually 🙂

Reminder: extra PCBs are available for free or for purchase to my Patreon supporters

New page: gyrators

Ok, so what we often call a gyrator is not technically a gyrator. This page is named after the circuit popularly referred to as a gyrator, not an actual gyrator.

I’m developing a little PCB for a simple gyrator circuit to be used in an upcoming integrated preamp project (2nd stage of phono circuit: needs gain and low enough output impedance to drive a volume pot). The thing that’s most intriguing to me at this point is how a gyrator lets you set an anode voltage rather than anode current (but still maintains a high impedance for AC). On paper, this looks more flexible in rolling compatible pinout tubes than setting a current. And what the heck, it’s a new circuit to try!

See the new page here!

Combining Line Level and Phono

I love boxes with tubes sticking out as much as the next DIYer. Generally, the more boxes, the better in my mind. In practical domestic life however, lots of specialized chassis (beautiful as they may be) don’t always translate well to limited space or the aesthetic considerations of cohabitants (AKA: WAF).

The phono and line-level functions are a good place we can look to consolidate our pretty enclosure collection. The voltage levels are manageable, the current requirements are usually low, and the tubes used are not especially large (in most cases). The question then is how best should we integrate something like a phono preamp and a line level preamp.

The schematic above gives an idea of the approach I intend to take for this kind of phono+line level project. A phono signal travels through an RIAA section sandwiched by two gain stages. This is attached to one input of a three way switch; the other two inputs at the switch can be used with a CD player, streamer, or other source of your choice. The output of the switch feeds a volume control, which in turn feeds a transformer-loaded single ended output stage.

Using a transformer on the output allows us to set a nice low gain for the line level section. Although a CD player probably won’t need it, some vinyl recordings and cartridges benefit from a small boost (e.g. 2x voltage gain, 6db). The transformer also allows us to step down our output impedance, much like the cathode follower in the Muchedumbre project. Of course, line level output transformers that can be used in a series feed configuration are not usually cheap.

I have a pair of Lundahl 1660 AM transformers to be used in this project. These run around $500 a pair (via kandkaudio.com). They are a well-known transformer for exactly this application. I have also purchased a pair of Edcor GXSE 15k:600 transformers ($40 a pair) as a budget-minded comparison. The transformer ratios are similar (4.5 or 5 to one) and both can be used in series-feed applications. While the Lundahl datasheet is very detailed, you may have some trouble getting inductance and DCR specifications from Edcor.

This is a tale of two preamps. I intend to design and build two all-in-one preamps with the same overall topology, but different tubes and parts. One preamp will be built using NOS tubes and high-end parts, while the other preamp will be built using current-production tubes and every-man components. I’m very excited to hear how the two projects compare and to be able to publish more than one option for people looking for an all-in-one preamp project.

More to come on this topic as I work-out the circuits and parts choices!

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.

-Einstein

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!