Santa, the slave driver (and misc tube news)

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Since May, the sun has risen and set on my beautiful baby girl. Daddy does not resent any of it for a second, but babies and the holidays make for slow progress on tube projects. I think my New Year’s resolution will be weekly posts, even if they aren’t all in-depth technical posts or finished designs.

I’m starting early because something that is [sadly] unusual has just occurred. Someone released a new tube audio kit/board:

I’ve used Boozhound Lab’s products in the past, but this is the first kit Jason has released for tubes. It’s a push-pull 6C45Pi amplifier that puts out about 6W. With just a pair of triodes sandwiched between input and output transformers, it’s also a minimalist’s wet dream (and similar to what I did with the Bad Hombre Mk 1 for headphones). I love it already and I hope it encourages people to pick up their soldering iron and bite the Edcor lead time bullet.

Jason has a great discussion of the design and building the amp here.

WTF Updates:

Chassis work for a TubeCAD headphone amp build is done: this will be a review and test of a circuit hack JB suggested (see SRCFPP), pretty paduak wood

Chassis work for a small SET amp is nearly done: this will be a published design, kind of a study in traditional cap-coupled single-ended amplifier design, goal of making this write-up very beginner friendly with a focus on applying fundamental concepts

What’s the deal with hybrid amps?

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All books on audio design that stoop to cover the archaic and backwards idea of vacuum tube amplification begrudgingly admit tubes are wonderful open-loop voltage amplification devices. They’re very linear (much more so than transistors without feedback), tolerant of high voltage, and forgiving of approximated parts values. Tubes do not make great current gain devices though. Therein lies the problem for us glow bulb fanatics.  To make power, we need both voltage and current. We usually side-step the current-handling weakness of tubes by developing large voltage signals with multiple stages and then using an output transformer to turn the big voltage at modest current into modest voltage at big current.

Let’s look at an example.

A somewhat classic single-ended triode uses two halves of a 6SN7 and a 300B in cascaded stages followed by a 3.5k to 8 ohm output transformer:

6SN7-300B-Single-Ended-Tube-Amp-Schematic

We know that the voltage gain of a grounded cathode with a bypassed cathode resistor is the Mu multiplied by the plate load divided by the sum of the plate load and the plate resistance. Accordingly, the amp above develops voltage gain of about 18x in the first stage, 16x in the second stage, and 3.2x in the final stage. This is an overall voltage gain of about 900x, meaning a 1V signal at the input becomes a 900V signal at the output. In reality, the 300B runs into grid or current cutoff before it gets anywhere near that much voltage swing at its plate and a more likely figure is about half this or 450V peak to peak.

This 450V peak to peak is still quite a lot of voltage. If you could directly drive an 8 ohm load with it [narrator: you can’t] you’d produce thousands of watts. To produce the thousands of watts, you’d use dozens of amps. You have about 0.06 amps [sad trombone]. We use an output transformer to step down the voltage and step up the current. We know that the voltage ratio of an output transformer is the square root of the impedance ratio. In the case of a 3.5k to 8 ohm transformer, that is the square root of 3,500/8 or about 21. Divide 450V by 21 and we get the voltage swing that the 8 ohm speaker is seeing. It’s about 22V peak to peak (seven and a half watts).

We created a hell of a lot of voltage just to step it down to a measly 22V peak to peak. This is where hybrids might come in. Solid state is quite happy driving amps of current into an 8 ohm load and only need a supply voltage of a couple dozen volts. They do away with the multiple voltage gain stages and output transformer. If you can create 22V of signal with a single tube stage, a transistor doesn’t need to make it any bigger; it just needs to provide enough current to drive a low impedance load like a speaker or headphone. Let the tube do what it does best (voltage gain) and let the transistor do what it does best (source lots of current).

So why don’t we see more hybrid designs? For one thing, the power supplies get complicated. You often want a bipolar (plus and minus) supply for the solid state section, a low voltage heater supply, and a high voltage supply for the tube’s plate. Although you rid yourself of an output transformer, you probably added a power transformer (and rectification, filter, etc). Another reason we don’t see more hybrid designs is that many designs which do exist don’t use the devices to their strengths and so cast doubt on the concept. When you see a single tube in an integrated amp, it’s often there as a simple cathode follower. I’ve got nothing against cathode followers, but that implementation is about as much a hybrid design as a burger with lettuce and tomato is a salad.

But by far the most likely reason we don’t see more hybrids (in my opinion) is that devotees of the objective/subjective, transistor/tube, modern/traditional design school are too human. If modern politics hasn’t sufficiently convinced you, the state of the audio market should. We’re kind of a bunch of tribal-minded, technocentric, get-off-my-lawn jerks. If you build a hybrid, you piss off both sides.

So yeah. Screw that. This was the long way of saying I’m building a hybrid amp.

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.

New preamplifier write-up: Sofrito

So*fri*to

noun

an aromatic base used to add subtle flavor to cuisine (esp. Puerto Rican cooking)

Click here for the write up for a 6V6 preamplifier with extra big bottles. Although biased for linearity, this preamp is designed to impart a little triode flavor to your system. Fundamentally, it’s a classic resistor-loaded grounded cathode amplifier stage, with some extra power supply and VU meter shenanigans.

Voltages here are high due to the power transformer used, but it could be adapted easily for lower voltage secondaries. Just shoot for about 350V before the feed-forward hum-bucker tube in the power supply.

Listening on someone else’s system

Like many audio enthusiasts, I have a general philosophy for audio that guides me when designing (or shopping for) new gear. In a nutshell, I value an objective and empirical approach to design, but this is tempered by the notion that music is art. At its core, art appreciation is a subjective, and often situational, experience. Objective design for subjective ends reads like a paradox; designers have egos too and so maybe conflict between engineering and ‘the feels’ is inescapable. If you’ve been on audio forums or blogs long enough, you know that objectivity and subjectivity do not usually mix in the audiophile hobby. I’ll steer clear of that morass, save to add one recently encountered perspective.

Last weekend I delivered a preamp (design write-up on the way) to its new owner, J. We spent a couple of hours listening to his system with and without the new piece. J’s system is different than mine and the music it makes sounds different, too (including recordings I know). We both had fun going through albums and cranking up the tunes. In a way, it was a little like seeing a favorite group perform live. You know the music but can appreciate fresh nuance all the same. That we got to do so together, on a social level, only added to the enjoyment.

Now what if we were all uncompromising in our objectivity? What if all systems and designers pursued the same goal and weighed compromises equally? Worse yet, what if compromises did not have to be made and all playback was “perfect?” While I know this is ostensibly what many of us seek in the audiophile hobby, where would it leave the hobby on a social and experiential level? I would visit J and hear the same songs in the same way that I always do.

The art in music is not a one-way street. The lenses and filters we use to experience and share art enrich both the art itself and culture as a whole. The process of internalization, expression, and rebirth keeps music relevant and vital. I’m off into abstraction, but there is a kernel of truth for audio here, too: how terrible the tyranny of ‘exactly as the artist intended’ could be if we took it too literally.

You are the artist of your listening, the world is your mixing console, seek out new stereos, and all that jazz.

 

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.

An Estudiante Build in the Wild

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XRK971, whose pocket amp I recently reviewed for Audio Primate, shares his Estudiante build at diyaudio.com here.

He had some issues with the LM317 CCSs and so I’ve decided to update the BoM to use the LM317HV (which was on the schematic but not noted specifically in the write-up). He’s currently listening with resistor loads, which will also work fine if you run out of heatsinks or space for the extra TO-220 packages.

Looks great, XRK!

Reference: WTF Amps El Estudiante write up.

DIY amplifier top plate easel

I wanted a better way to wire my amps and had been lusting after Decware’s amazing assembly room for a while.  While I can’t afford the custom extrusions and equipment that Decware has, I can get creative with common materials.  I decided to try a simple easel based on a couple of rails of t track and some standard angle and rod extrusion.

The basis of my easel is two 2ft sections of t track. Finding a 4ft section with bolts and knobs on sale at Rockler was what pushed me over the edge to build this daydream. The t track is 3/4″ wide by 3/8″ deep. Rather than trying to route a channel in a thick board, I sandwiched the t track in scrap with 3/4″ thick scrap under the track and 3/8″ to form the outside. These were glued and clamped face down so that I could be sure the t track would be flush with the top edge. 

My bench is cantilevered from basement joists with 2×6 vertical supports. I ran a 1/2 aluminum rod between the supports to provide lateral movement and adjustment to the t track rails. The rails simply have a 1/2″ hole through which the rod passes. By mounting the rod 12″ from the surface, the 24″ rails give a 30 degree angle. This leaves plenty of clearance for transformers and is comfortable to work on while standing or sitting). I purchased some 1/2″ washers and collars, but they may not really be necessary (I’ll find a use eventually). 

My horizontal ledges are 1/2″ by 3/4″ aluminum angle (3/4″ side is flat against the t track rails). I used self adhesive cork sheet to protect plates in the easel from scratching on the support. Depending on your knobs or wing nuts, you may have to trim some of the horizontal rails so that they can be completely tightened. I cut my rails to fit a 18″ wide top plate, but aluminum extrusion is cheap if I ever want to build something wider. 

All in all, this is a handy and relatively simple addition to my tube amp building bench. And it is a lot cheaper than custom extrusions or lab fixtures!

 

Letters to WTF: What kind of rectification am I supposed to use with this power transformer?

I was helping someone with an amp build over Telegram (chat app) yesterday when this question came up. He had in fact been trying to use a diode bridge with a center tapped transformer with both the center tap and the bridge grounded. He released the magic smoke from his transformer, though there were a couple of other issues that may have contributed to this.

When I was starting out, I had some confusion with power transformers and rectifiers, too. Probably like many others, I started with small solid state circuits, where center tapped transformers are rare. Once I started building with tubes, the secondary ratings of center tap transformers were another source of confusion. So here’s a by no means complete rundown of transformer configurations.

1. My transformer doesn’t have a center tap and I want a full-wave rectified DC output.

You want to use a diode bridge (figure 4-8). This is four diodes arranged to rectify both positive and negative phases of the power transformer’s AC output. Your ground will be taken from the bridge, NOT THE TRANSFORMER. This ground at the junction of the diodes creates a return path for current that ‘switches’ with the changing phase of the secondary’s AC output.

2. My transformer has a center tap and I want a full-wave rectified DC output.

You want to use a “conventional” full-wave rectifier (figure 4-5A). This requires only two diodes (solid state or a rectifier tube). Your ground is taken from the center tap of the transformer (which is then the return path for current). Many center tapped transformers are rated as the full end-to-end secondary voltage. For example, a 300VAC center tapped secondary would actually provide 150VAC into a conventional full-wave rectifier. You’ll sometimes see the same transformer listed as 150V-0-150V.

Here’s a great clarification of what’s going on with full-wave bridges and conventional full-wave rectification.

How much voltage do I get?

With either of the above, the unloaded DC output into a capacitor-input filter is approximately the AC output from the secondary times the square root of two, minus the voltage drop across the diodes (minimal for solid-state, can be considerable for tube rectifiers). Into a choke-input filter (unloaded, ignoring diode drop), the output will be approximately two times the square root of two divided by pi (about 0.9) of the AC output of the transformer secondary.

3. My transformer secondary has a center tap, but I want a bipolar power supply.

Here you can combine the center-tapped transformer and the aforementioned bridge style rectifier. See figure 5.1c here. This creates two separate full-wave rectified voltages, one positive and the other negative with respect to the center tap. If you read a lot of TubeCAD, you see bipolar tube circuits pretty regularly.

4. My power transformer is 300VAC (150V-0-150V) center tapped, but I want 400VDC!

Another way to combine the center tapped transformer and bridge rectifier is to ignore the center tap altogether. Do not connect it to ground; just SAFELY tape it off and tuck it away. Now you have basically a non-center tapped transformer and you can treat it like number 1 above. Note that current capacity in this configuration is typically half of what the transformer was originally rated for.

5. My power transformer is 120VAC without a center tap and I want 300VDC!

To achieve this, you can use a voltage doubler (see figure 4 “Delon circuit). This requires two diodes and two capacitors. Because the capacitors will see large pulses from the diodes and will be supplying the rest of the circuit continuously, they need to be a fairly large value. But because each only sees half of the supply voltage, their voltage ratings are a little more relaxed in comparison to what is required in a filter. The unloaded DC output into a capacitor-input filter is approximately twice the AC voltage from the transformer secondary times the square root of two. Current capacity must be de-rated at the output voltage by a factor of at least two.

6. My power transformer has dual matching secondaries and no center tap. What do I do?

This is common with toroidal power transformers in particular. You can wire the two secondaries in parallel (making sure the polarities are matching) and use a bridge rectifier like number 1 above. The AC output of the transformer will be the same as either secondary by itself (and current capacity will be doubled). You can also wire the secondaries in series by connecting a positive and negative from each secondary (not the positive and negative from the same secondary!). This creates a center tap at the junction. The AC output end-to-end will be twice the AC output of a single secondary if the secondary is not grounded (see number 4 above). If you ground the secondary you created, you can use a rectifier like number 2 above.