Bad Hombre tube headphone amplifier in the wild

I recently received some pics from W of a Bad Hombre Mk2 build (12AU7 + ECC99). This one uses Lundahl LL1676 input transformers and LL2765 output transformers, and features a regulated B+ and a loaded front panel of controls. Very nice!

The LL2765 is a fine transformer with ECC99 output tubes in this circuit. It has a 5k primary and multiple output taps for 32, 150, 600 ohm headphones. The 5k primary will result in more power than the 8k in the original design, but it can also be used to reflect a higher impedance depending on what taps and headphone load one chooses (e.g. 300 ohm headphones on the 150 ohm tap for 10k load).

On a side note, I see Lundahl also now has a LL2774 3k primary headphone output transformer (16, 64, 300 ohm secondary taps). This is a very similar turns ratio but it looks like the LL2774 is available with a bigger gap in single-ended configuration for higher current output tubes. Great to see new options for headphone output transformers!

Multiple B+ voltages from a single transformer winding

For the current project (a line stage with added phono) I needed more than one B+ value. The difference between the two voltages I wanted and current drawn was too large for a single high voltage rail and a filter or regulator to drop the lower rail to the correct value. So I looked for ways to add a voltage doubler to a standard bridge rectifier. Turns out, there’s more than one way to skin a cat:

The “Millet Doubler” is detailed here and uses a single center-tapped winding. The entire secondary is rectified via a bridge rectifier, rather than the usual approach of grounding the CT and using a full-wave rectifier. The center tap voltage is then rectified and feeds the lower half of a stack of capacitors.

The “TubeLab Doubler” is something I found posted on; it is also discussed in depth here. This one uses a single winding without a center-tap. The doubled voltage rail is somewhat lower than what you’d get in the Millet Doubler, but still potentially useful especially with inexpensive isolation transformers.

I can only find a schematic of the “TubeCAD Doubler” (no discussion), but if you’re familiar with TubeCAD’s blog, it doesn’t look too unfamiliar. See a good article on multiple power supply voltages here. This one looks a little bit like a combination of the other two variations.

In the end I went with the second version because it allows me to use an isolation transformer (and because I found it before seeing the TubeCAD one). Of course a couple of wiring oopsies are being worked out before I can report back on the power supply or preamp it is intended to feed…

More local feedback

I’ve been on a bit of a feedback kick lately, researching both for projects and to add a page to the website about the topic. Local feedback, in particular, has piqued my curiosity (which is usually fixated on triodes and open loop circuits).

A tube’s grid is an inverting input (with output taken at the anode). This makes it a natural point to loop feedback from the anode. TubeCAD covers exactly this topic in-depth in Partial Feedback. But, my favorite variation on this type of feedback uses a P-channel FET in the configuration shown above. The FET does several nifty things here:

  • Provides low impedance fixed bias to the tube grid via the source, allowing for A2
  • Provides high impedance input for the preceding stage
  • Defines the feedback/gain in the circuit with source resistors R2 and R3

DiyAudio alias SpreadSpectrum provides details on a push-pull build using this circuit on his blog here.

E-Linear: an interesting kind of feedback

Oops, I bought another pair of output transformers. This iron is 60W rated push-pull with a 6k6 primary, so the natural pairing would be 6L6GC.

I’ve used the 6L6GC in the past (see the Luciernaga). Because it is common in guitar amplifiers, it’s an easy to find tube both new and vintage. While it’s no power-house in triode mode, the 6L6GC is quite capable in pentode or ultralinear.

Operating the output tubes as pentodes means higher output impedance and distortion. The Quality Amplifier from a couple weeks ago avoids the issue entirely by operating the outputs as triodes. The other approach, which is actually more common today, is to sacrifice some of the circuit’s gain to bring output impedance and distortion back down. That’s called (negative) feedback.

The question is how do we want to apply it?

This post won’t get into the nitty gritty of feedback (that deserves its own page on the website), but there are generally two prevailing approaches. Global feedback is what we see most often; this takes a signal from the output of the amp and wraps it all the way back to the input. The Williamson amplifier is the quintessential example of this.

The second approach to feedback is to use local loops. These affect a circuit just like global negative feedback, but are isolated to just a stage or two. Local feedback, because it involves fewer stages and phase shift, is more stable than global negative feedback. That means they’re generally simpler to employ.

The circuit above is a variation of what seems to be unofficially referred to as an E-Linear stage. Feedback from the output transformer primary is applied via the ultralinear taps directly to the load resistor for the input stage. This local feedback drastically reduces the output impedance of the 6L6GC.

The input stage is commonly a pentode because the high plate resistance is a benefit here to applying feedback. In this case, the input stage is a hybrid cascode, which still has a high “plate resistance” owning to the MOSFET upper device. That also gives us more options for the lower triode tube.

I like the simplicity of this circuit quite a lot. In Class AB, it looks like a good 25W should also be available with a pretty modest B+ (or a little less in Class A). Seeing as how I’ve got the iron on hand, I hope to give this one a test at some point!

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.

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.

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!

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!

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!