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.

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!

Schitt is getting into DIY, except they’re definitely not

Pic from Schitt product page

If you’ve missed the recent hubbub, Schitt is launching a line of drink coasters. It just so happens that these coasters double as unsupported DIY projects with scarce documentation.

The schematic shows a 6418 sub-miniature triode direct connected to an AB push-pull transistor output stage (cap coupled). Power is a blistering 30V to the tube and 15V to the output stage, cleverly derived with a bridge doubler and regulated with LM317s.

Schitt is hedging the product with extra coy advertising (AKA the Schitt Shtick) and reinforcing in several places that they are not a DIY company. Hence the product is a coaster, not a miniature hybrid amplifier. It will come at no surprise to Schitt when DIY documentation is created by early adopter hobbyist communities, I’m sure.

Some of the quoted specs:

  • Frequency Response: not terrible, but not exciting (like 10-100K, -1dB or so)
  • Power Output: much less than anything else we make (like, less than Fulla 2, maybe 400mW into 32 ohms, all in, 10% THD or so)
  • THD: about 0.5% at 1V RMS (6418 tubes) or about 1.5% at 1V RMS (6088 tubes)
  • IMD: didn’t bother measuring, this amp ain’t about measurement
  • Output Impedance: about 8 ohms (yes, 8, not 0.8, not 0.08), in case you didn’t get the memo, this ain’t a high-performance amp

What is it about Schitt’s non-committal and self-effacing copy that gets so many people so excited?  Why did I just buy a small lot of 6418s? Why are coasters already on the way to me?

Schitt, you sly dogs.

Edcor has new secondary options

Finding output transformers with turns ratios suitable for headphones used to be an exercise in futility (and endless eBay refreshing). As the market for high-end headphone amplification has grown over the past few years, it seems like Edcor has taken notice. I’m really happy to see some new options for high impedance headphone transformers on the Edcor website. Relative to vintage UTC or custom winding, these look like very affordable options:

edcor heapdhone OPT options.png

In the past I’ve resorted to matching transformers in a parafeed arrangement or speaker transformers with low impedance headphones. I look forward to trying some of these transformers in future builds!

The Nuvistor and Bob Katz’s Audio Blender (via Inner Fidelity)

Bob Katz has been writing a series of articles over at InnerFidelity for several years and they’ve recently taken a turn down a more experimental path. His most recent article details a device that mixes a transparent solid state signal and a Nuvistor signal biased to provide a distortion spectrum with just a small percentage of second harmonic. Check out his write up here!

nuvistor

A Nuvistor is a small metal and ceramic tube released by RCA just as transistors began supplanting vacuum tube technology in most electronics. They are a true vacuum tube with familiar triode operation and characteristics and an indirectly heated cathode. The most common Nuvistor in consumer electronics was the 6CW4 (high Mu) though there are several triode flavors and even a couple of tetrodes.

nuvistor cutaway.png

Because they were originally intended for radio and TV usage, Nuvistors enjoy very good bandwidth, low noise, and high gain (high Mu variants).  The metal envelope is integrated with the basing and the tube plugs into what RCA dubbed the Twelvar base. You can probably guess how many pins that had. With the Nuvistor, RCA also introduced the RCA Dark Heater, a lower temperature filament that guaranteed higher stability and less AC leakage. Despite this innovation, most Nuvistor heaters require around 1W to light (e.g. 150mA @ 6.3V).

The 8056 used in Bob Katz’s project has the following characteristics:

8056 characteristics.png

And the following very respectable plate curves:

8056 plate curves.png

With a modest Mu, low plate resistance, and very low B+, it’s no wonder Bob decided to marry this interesting tube to a solid state partner for his Blender. The 8056 heater requires 6.3V at 135mA. At this voltage and heater requirement, it’s close to being practical for modern portable devices. In their heyday Nuvistors were used in battery-powered and efficiency-critical applications like the US Space Program and military radios and communications equipment.

Would I ever build something with Nuvistors? It’s tough to say.  I’ve been on a casual hunt for tubes that might be suitable in a portable battery-powered application. Other candidates are the Korg Nutube or the sub-mini 6088. Like all things in this hobby, there are trade-offs. The Nuvistor 8056 heaters are hungry relative to these other options, but the other characteristics are very attractive. In all likelihood, I’ll try them all eventually. This is why I DIY.

The difference between a headphone amp and a preamp

This is a question that, as a beginner builder, confused me quite a bit. While it isn’t too hard to understand why a preamp cannot drive power-hungry low-impedance headphones, it’s less obvious what separates an amp that can drive headphones from a low gain line stage. Headphone amps and preamps often share the same small signal tubes, usually Class A, and often single-ended.

Here are the modifications I would make to the El Estudiante headphone amp to make it better suited to line stage duty. While a purposely designed line stage might perform better, I can’t think of a way to do a halfway decent tube line stage any cheaper or simpler. If you don’t go mad on caps, this costs less than the headphone version.

Output Stage

Power requires both voltage and current. How much voltage or current required for a given amount of power depends on the load you intend to drive. Remember:

Power = Voltage x Current

But also:

Power = Voltage² / Impedance

AND

Power = Current² x Impedance

To create power into low impedance headphones, we need current. This drives a lot of design decisions in tube headphone amplifiers. Common approaches to create power are push-pull output stages (eg SRPP, White Cathode Follower), output transformers, and solid state power buffering. The Estudiante creates the power required for low impedance headphones using the latter approach: a single-ended CCS-loaded MOSFET buffer. At a 100mA quiescent current, it can make about 150mW into 32 ohms:

0.1A² x 32 ohms x 1/2 = 150mW

(note RMS = Peak / √2)

On the other hand, with a 10,000 ohm input impedance on an amplifier, this current is unnecessary because the maximum ‘power’ is limited by the voltage, not the current:

24V² / (10,000 ohms x 2) = 25 mW

Now we don’t really look at power output per se in line stages and we’re rounding up the peak output voltage as half the power rail voltage, but it’s obvious that we don’t need all the current to drive the input impedance of an amplifier because we’re limited by voltage anyways. Consequently, we can lower the current in the MOSFET output stage to something that doesn’t even require a heatsink, making a preamp build that much simpler and cheaper.

With the LM317 CCS, we calculate the needed set resistor as 1.25V / Iq (where Iq is the idle current). A resistor of 100 ohms will give us 12.5mA idle current, which should be plenty for a reasonably low output impedance, but not enough to need a heatsink (I would probably still bolt my TO220 parts to the chassis though).

linestage estudiante

In addition to lowering the idle current in the MOSFETs, we can change the big nasty electrolytic cap found in the headphone amplifier to a higher quality film cap. Electrolytics are great where you need a large capacitance in a small and affordable package, like the output coupling cap in a headphone amplifier, but electrolytic capacitors have been shown to create distortion at low frequencies (see Douglas Self’s Small Signal Audio Design) and exhibit leakage current that creates a thump on power down (which may just be annoying on headphones, but potentially damaging on a high power speaker amplifier).

For an input impedance of 10,000 ohms and a -3db point of 5 hertz, Our new cap size in microfarads (uF) is calculated as:

1,000,000 / (2 x Pi x 10,000 ohms x 5 hz) = ~ 3uF

A film cap of this size at a rating of only 63V+ is not hard to come by. I’d probably buy an assortment just to see if I could hear a difference. We should also increase the size of the loading resistor on the output from the 1k in the headphone amplifier to something like 100k or 1M so that we aren’t rolling off the bass or unnecessarily loading down the MOSFET output stage.

Finally, because we’re reducing the current in the output stage, our power supply requirement is relaxed, maybe opening up more wall-wart options to power the project. So if you’re looking for a simple, low-voltage, and cheap tube preamp option, modifying a headphone amplifier like the El Estudiante may be a good option. I’ve even used the headphone amp to feed power amplifiers in a pinch and it sounds surprisingly good.

Simple high current VR tube regulator

VR and transistor regulator

If you’ve looked through many of the designs on this website, you’ll see I have a love of glowing things. A current project of mine requires a ~150V supply and my mind immediately went to the beautiful purple glow and sultry curves of the 0D3 VR tube.

close up 0d3

The problem was that I wanted around 40mA from the supply. In the usual VR tube shunt regulator configuration, we’d size the ballast resistor based on the load current and the current we want through the VR:

vr resistor calc.png

With a large load current, the ballast resistor (Rb) will be small. But at start up with a tube amp/preamp, the load current will be zero until the heaters are warm. This will force the VR to pass the entire load current (in addition to its own quiescent current) until the rest of the circuit is warmed up. VR tubes are generally specified for only 5-40mA. Too much current at start up will stress the VR, leading to a shorter lifespan and potentially arcing.

Transistors to the rescue. The simple schematic above uses a VR tube as a voltage reference on the base/gate of a BJT/MOSFET. The emitter/source provides a very low output impedance to the load.  The output voltage is the VR tube reference voltage (150V for 0D3) less the Vbe of the transistor (approximately 0.7V for BJTs and 4-5V for MOSFETs). The current limit in this configuration is limited by the pass transistor and heatsinking, rather than the VR tube.

I’ll be building and testing this supply in the near future with an 0D3, but I don’t see any reason it shouldn’t work with 0A3, 0B3, 0C3, and/or series combinations of these voltage references. Just be wary of the transistor max voltage.

Guest Post at Audio Primate: JDS Labs CMOYBB Review

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One part market research and two parts DIY hobby service: click here for another review of a small solid state headphone kit/board at Audio Primate. JDS Labs has done an excellent job with this kit. Everything is clearly labeled, the board is good quality, and the documentation is excellent.

If you want a place to start with DIY amps and line-level gear, look no further than the classic CMOY.