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).
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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.

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.