(power) supply chain

Postponement is a powerful supply chain concept employed to minimize inventory/capital for a business by delaying configuration of a specialized product until as close as possible to delivery to the end-user. For great examples, see Dell’s made-to-order computer business or Edcor’s made to order transformers.

In vacuum tube land, the transformer is a critical component. Tubes come in all shapes and sizes, requiring a variety of voltages for optimal operation. This has lead to many different power transformers and filter configurations for various circuits. We even have transformer companies whose entire business strategy is founded on servicing the myriad of transformer configurations and custom options.

What if we could find one power transformer that could be used with any circuit? What would it look like? Well, if I were to build one it would probably look something like this:

2018-02-19_14-41-28

Looks pretty simple to be universal, doesn’t it. But what’s up with the 50V winding? It’s not a heater tap but it could be used for bias, I suppose. The use I have in mind is something like this:

In conjunction with the 300V winding, the 50V winding will allow you to create 250V and/or 350V outputs (all voltages AC of course). Using the extra winding and some filter math, you could easily tune in any target B+ from low max voltage tubes like 6V6/EL84 to higher max voltage tubes like EL34 or KT88. Careful attention would need to be paid to phase labeling and any power supply would use a bridge rectifier, but those are pretty small prices to pay for more flexible parts.

A transformer like this with a 250mA current rating might be the only transformer a builder would ever need for a variety of projects. Fewer parts means less money tied up in iron for users and fewer SKUs means more economies of scale for transformer manufacturers. That’s the beauty of postponement.

Bench update:

  • Aikido Headphone amp is underway
  • Push-push octal monoblocks are in design phase
  • Working on write-up for latest peanut watt SET “El Cacahuate”
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New page: The Cascode

cascode generic

A design in the works calls for more gain than can be practically squeezed out of a single grounded cathode but not nearly as much as would be got from two stages (unless I want to apply feedback, and I don’t). To the rescue comes an interesting totem pole circuit, the cascode.

I have an overview page of this circuit posted here. Details to come on the design that will use it, but you get a hint at the bottom of the page!

Class A FET power buffers

I’ve been kicking the hybrid amplifier can down the road for quite a while. In essence, I’m looking to do a bigger EL Estudiante. An amp capable of driving speakers to a dozen or two watts using a MOSFET follower output stage for current gain and a tube handling the voltage gain. While this is not especially difficult on paper, making something that is an interesting and practical alternative to tube output stages is not necessarily so straightforward.

On one hand, one should consider the target user. Most tube enthusiasts do not need so much power, so we can bias in Class A and avoid a whole lot of AB headaches and worry about bias adjustment, crossover distortion, etc. This has to be balanced against heatsinking and thermal considerations, of course. Coming from the world of tubes, our audiophile anti-bodies have already pretty well encysted any commonsensical tendencies we may once have harbored, but the smoke point of drywall hasn’t changed. That is to say solid state does not magically make Class A cool, efficient, or pragmatic, but why make something hotter, more wasteful, and more burdensome than vacuum tech? We’re hoping to make something that is more than the sum of its parts.

ccs load se

The Aikido Hybrid 16W SE by John Broskie has about as much power as most probably need. Sixteen is two to the fourth watts, so four times three decibels per doubling of power for a 12db increase over nominal speaker rating (eg your 90db speakers peak at 102db). The quiescent current is a very serious 2A. It is a single-ended MOSFET source-follower loaded by a current source. There’s little not to like other than the coupling caps (contrast this with output transformers for AC coupling in full tube amps). See also Rod Elliott and Pavel Macura’s CCS-loaded follower here.

inductor load se

We can make single-ended more efficient with an inductive load.  Just like a choke load with tubes (eg Luciernaga), an inductive load on a MOSFET lets it swing voltage past the power rails.  The MoFo is an example of a single-ended source-follower MOSFET with a simple passive choke load (50-150mH and very low DCR). This is much more efficient than an active CCS in an absolute watts dissipated sense (note the much lower supply voltage), but chokes ain’t cheap and you still need a good dose of current.

diode bias pp

If you want to lower the quiescent current needed, but stay in Class A, push-pull source-followers are the way to go. Papa Pass’s F4 power buffer does exactly this to cut the quiescent current needed in half. The need to match FETs may be unappealing, but compared to his single-ended F3, F2, or Aleph J, the power delivery into 4 ohm loads is much improved. Note Broskie suggests the same push-pull MOSFET approach (with a different biasing scheme) in the Moskido amp design.

Any of the above would probably sound pretty good: like a tube feeding a very transparent solid state amp. If the amount of power you need is modest (and it probably is if you’re a tube enthusiast), these approaches have made very nice speaker amps. Hopefully I’ll have my own design to contribute soon. MOSFET followers would also make a great multi-watt amp for low sensitivity and low impedance headphones, like the HIFIMAN HE-6.

The HE-6 are rated at a sensitivity of 83.5db at 1 mW.  With 1000x more power (1W), we’d make 103.5db (a 20db increase). Around 5W input makes it a cool 110db. Coincidentally, this is the power rating of the amp HIFIMAN recommends as a pairing.  The HE-6’s 50 ohm impedance lowers the quiescent current needed in a MOSFET output stage, though still requires a voltage rail high enough to prevent clipping. A quick approximation for voltage would be:

power = Vrms²  / impedance

5W x 50 ohms = 16 Vrms²

16 Vrms x √2 x 2 = 46 Vptp

So about a 48V power rail (or +/- 24V) gets us in the neighborhood. Doing the same for current:

power = Irms² x impedance

5W / 50 ohms = 0.32 Irms²

0.32 Irms x √2 = 0.5 Ipeak

We only need about a 48V power rail and 0.5A quiescent current per channel to get us 5W into a 50 ohm load if using a CCS loaded MOSFET. If we choke load, cut the 48V in half. If we use Class A push pull, cut the current in half. The heatsinks aren’t going to be tiny, but a desktop size amp isn’t out of the question.

 

 

 

 

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.

Ode to the ST-70

dynaco st70.png

The ST70 is a beautiful and historic amplifier (and surprisingly compact if you see one in person). It’s also the best selling power amp of all time (at least so says Wikipedia). All things Dynaco inspire much talk here around the water cooler at the WTF Amps institute of higher learning about vacuum tube stuff. Here are some loosely organized tidbits and thoughts on amplification!

Generalized Topological Design Trends in Discrete Amplification

Forget for a moment that some amps are made with tubes while others are made with transistors. Deep down in their vacuum or silicon hearts they are really both just simple three-pin devices used to accomplish the same thing (gain). Forget all the audio-speak we abuse in our efforts to approximate the many facets of circuit performance. Forget the preconceptions we file away in our minds under “T” for tube or “S” for solid state. We aren’t thinking about tubes or transistors, right? Good.

To SE or PP (tee-hee)

Beyond all other aspects, the amplifier topology choice that impacts a design the most -in performance, efficiency, and cost- is whether the amplifier will be single-ended or differential. The difference can be boiled down to whether the amplification devices handle the entire signal through to the output (single-ended) or “split” the signal phases and re-combine them at the output (differential). Differential amplifiers are sometimes also referred to as push-pull. There is no such thing as balanced amplification, but that’s another discussion.

Single-ended amplifiers tend to be more inefficient in both a power consumption and economic sense. Because they are Class A by necessity, they dissipate more heat per watt of amplification. Single-ended amplifiers need a squeaky clean power supply to achieve a respectable noise floor because they do not benefit from the same kind of ripple rejection as differential amplifiers. They tend to produce more distortion, but the distortion that they produce usually has an even-order-dominated harmonic spectrum. Studies say even-order distortion harmonics are less offensive to most listeners.

In contrast, differential amplifiers produce less distortion when designed well, but what they do produce is dominated by odd-order harmonics, which are less pleasing to most listeners. Differential amplifiers are capable of far more efficiency than single-ended amplifiers because both output phases do not need to be “on” all the time. By nature, differential amplifiers reject power supply noise because they only amplify the difference between the phases and any power noise appears equally in both.

/r/outoftheloop

The distinction between single-ended and differential is the most fundamental taxonomy that can be applied to amps. The next most important design choice with regards to the circuit and its behavior is whether the amplifier will be open-loop or closed-loop. A closed-loop amplifier injects a portion of the output back into the circuit in order to correct non-linearities created by the act of amplifying with non-imaginary devices. This requires extra gain from the amplifier to be spent on suppressing these distortions. An open-loop amplifier is able to get by with less overall gain and enjoys more polite clipping behavior at the expense of generally higher THD. We are very deliberately avoiding the term ‘negative feedback’ here, by the way.

If you’re following along, you see that less-efficient single-ended amplifiers with less-objectionable distortion spectrum might naturally gravitate towards open loop circuits. Furthermore, you can imagine that more efficient differential amplifiers, with power to spare but a less pleasing distortion spectrum, are logical candidates for closed loop circuits. Your powers of comprehension do not fail to impress, dear reader. In practice a blend of single-ended and differential, open loop and closed loop, choices are made at the stage/component level in order to balance the relevant strengths and weaknesses, but the broader structure of amplifiers is usually one or the other.

WTF were we talking about again?

I’m going to tell you a secret now. Please do not react too loudly or cause a commotion. Come closer… Single-ended, differential, open loop, and closed loop has nothing to do with whether an amp uses tubes or transistors. Yes, that’s quite something isn’t it? While it’s true that historically certain devices and topologies are strongly associated one to another, this is a question of device availability coinciding with design trends and market demands, not choices dictated purely by the devices used.

This brings us back to the topic of the Dynaco ST-70. This is a closed loop differential amplifier running in Class AB, much like the earlier Williamson or Leak tube amplifier designs. The overall topology is not much different from current Class AB transistor amplification because these solid state amps are simply a continuation of the same design trend (AB differential, closed loop). While today we associate tubes with single-ended open loop design and transistors with differential closed loop design – and all the baggage these topologies drag about – the reality is that performance has more to do with circuit choices than with the devices used.

The ST-70 was in some ways a pioneer. Though it was not the first of its kind, it was the standard bearer of the contemporary design values. Today we prize much of the ST70’s topological progeny in solid state Class AB (whether integrated on a chip or built with discrete components) but we also revere designers such as Nelson Pass who is charting his own course through both open loop single-ended transistor and Class A low feedback differential amplification. Amplifier design is not so much a timeline as it is a spectrum; the limits to what constitutes good amplification (subjective as that may be) are found not in the parts choices, but in the creativity of the designer.

TL;DR: Design, not device, makes the amp.

Here’s Dan Fraser’s write-up on the launch of the modern ST-70 series 3 (Dynaco was purchased by Radial Engineering in 2014)

Letters to WTF: why doesn’t anyone include tone controls?

Tone controls get (an undeserved) bad reputation in a lot of DIY hifi circles. They are very difficult to get close to technically perfect (eg exactly Xdb boost at all frequencies above Xhz) and they’re math heavy, so you don’t often see them fully detailed in audio DIY.  And in principal all the equipment we’re building is supposed to be perfectly flat and transparent, right?  Well that’s what the engineers say, but others might say that transparent is the enemy of fun. I would say that you don’t see tone controls because that’s just not how hifi “is done.” No, that’s not a good reason. And maybe the world needs a simple preamp design with bass & treble…

Here’s some good reading from Baxandall, the papy of modern tone control:

Here’s a good article from John Broskie on his Tilt Control board/kit:

That Tilt Control is a different take on tone controls, but I think it’s pretty elegant.  Broskie’s boards and kits are top notch too (not affiliated, I just have a engineering crush on him):

Ideally you’d sandwich this kind of tone control between two cathode followers (or one low gain stage and one follower).

New page: grounding

If you didn’t already catch it, I’ve added a page to the power supply section on grounding. This is a hard topic to do justice because there is no one approach, there are just approaches that work for individuals/projects. I try to give a rough outline of my thought process and strategy for grounding projects on the new page.

Here are a couple of other good reads on the topic:

David Davenport “Audio Component Grounding and Interconnection” on diyaudio.com

Bruce Heran “Grounding and Shielding for your DIY Audio Projects” on diyaudioprojects.com

Letters to WTF: why can’t tubes run on 12V like transistors?

This is a good question and a topic that rears its head pretty regularly in DIY. If we don’t have to work with 300V, we’d all prefer not to; however, there’s a reason that we brave high voltage in our quest for tube audio.

There are a few tubes that will operate reasonably well at low anode voltages —see PDF article here or space-charge tubes like 6GM8– but the majority of tubes are going to want 50V+ on the anode to reach respectable linearity. There is some discussion of this in the write up for the El Estudiante headphone amp.

A triode is unlike the collection of transistors in an opamp; think of it more like a single NPN. Let’s assume simple single-ended operation, too. The more supply voltage you have, the more anode/drain voltage swing you can realize with variations in the grid/gate before you run into current cut-off or out of the transistor saturation region (which is like hitting positive grid voltage on a tube). More supply voltage will allow you to bias the tube/transistor in more linear regions of the transfer curves:

supply voltage.png

Recall also that tubes pass current in mA so producing usable power (V * I) is going to require lots of voltage (typically hundreds of volts). That high voltage and low current pass through the output transformer to become the low voltage and high current that’s suitable for driving 8 ohm speaker loads.

A lot of it comes down to fundamental construction. Tubes have a vacuum gap across which current must flow, so a decent amount of voltage is needed to create enough potential difference to get electrons moving reliably from cathode to plate. Maximum potential current is limited by the anode voltage according to Child’s Law (three halves power of the anode voltage divided by the squared distance between electrodes). Transistors are a silicon sandwich and capable of much higher current, but they are voltage limited by the breakdown rating of the semiconductor region material/thickness.

Hybrids are a really cool topic, IMO. SS has an advantage in directly driving low impedance loads with high current, while tubes have advantages in small signal voltage amplification (no NFB, high Zin, simple, low noise).

So, can tubes run at low voltage? Some of them do ok for musical instrument applications (eg guitar effects), but usually we want more potential (voltage) to overcome the vacuum gap and get the lazy little electrons moving in order to reduce distortion and achieve good tube-to-tube repeatability.

Santa, the slave driver (and misc tube news)

IMG_20170825_073348746.jpg

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?

hybrids.png

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.