Here’s another stunning build by another DIY tube hobbyist, this time of the Sofrito Preamp. Beyond the great chassis work and wiring, Joshvito has an awesome write-up on his build in his Imgur gallery here. He has a lot more progress pictures than my page, so this is going to be a great reference for others!
One day I was combing through tube characteristics using the parametric search function in the Tube Data Sheet Locator desktop app and I came across an interesting 7-pin triode. As luck would have it, I happened across a couple at a swap meet about a week later when the tube type was still fresh in my mind. I bought them, thinking they might be interesting to experiment with.
The 6AF4 is a 7-pin indirectly heated triode. I found the 6AF4 interesting because of its fairly low amplification factor (Mu of 15), decent transconductance, and high perveance. These characteristics suggest that a really simple and fairly low voltage preamp may be within reach. Check out the datasheet here (link to PDF).
I’d power this with a pair of 48V SMPS in series and wire the two 6AF4 heaters in series to be powered by a 12V SMPS. This would bring PSU costs and size down significantly. Voltage gain here would be about 10 and output impedance should be a little under 2k ohms.
Because resistor loading will still provide a lot of gain, even with a low Mu of 15, this version uses a matching transformer to step down gain and output impedance. Voltage gain here would be 2 or 3 and output impedance should be a couple hundred ohms. Edcor makes affordable candidates for output transformers in this application (WSM series). I’d also look for second hand matchers in the 2:1 to 5:1 range.
As far as a loadline and bias point, the blue blob area looks pretty good to me. Note how low the supply voltages are here. These low voltages make parts both cheaper and smaller. While I like the lower output impedance and fanciness of parafeed, the resistor load would probably sound a little tubier. It would also be simpler, cheaper, and more compact.
Whelp, I seem to have talked myself into it…
I recently spent some time in the garage workshop on a non-tube project I’ve been meaning to do for quite some time. Up to now, my chassis have all been a mitered box with a lip to inset the top and bottom plates. I cut the lip with a straight edge and a hand-held router. This works fine and produces good results, but setting it up for the cuts was a chore. The better solution for this kind of rabbet with a fixed width/depth is a router table, so that’s what I built!
My new table includes a white-board top, inset aluminum plate for the router, and t-track to adjust the fence distance. The fence has adjustable stops so I can get close to zero clearance and add shims to plane edges or remove small amounts of width from boards. This will all make cutting the same chassis I’ve been doing much quicker. It also opens up some new chassis possibilities:
Here’s a different style chassis that I’ll use in the next preamp build. The sides are still wood and include a lip to inset the top and bottom, but the front plate is 1/4″ aluminum and the back is an aluminum c channel. This will make drilling for jacks and controls much easier than using a 3/4″ wood panel. Making the same enclosure with decent precision would have been much more difficult without the new router table.
More updates to come as I begin this preamp project!
I love boxes with tubes sticking out as much as the next DIYer. Generally, the more boxes, the better in my mind. In practical domestic life however, lots of specialized chassis (beautiful as they may be) don’t always translate well to limited space or the aesthetic considerations of cohabitants (AKA: WAF).
The phono and line-level functions are a good place we can look to consolidate our pretty enclosure collection. The voltage levels are manageable, the current requirements are usually low, and the tubes used are not especially large (in most cases). The question then is how best should we integrate something like a phono preamp and a line level preamp.
The schematic above gives an idea of the approach I intend to take for this kind of phono+line level project. A phono signal travels through an RIAA section sandwiched by two gain stages. This is attached to one input of a three way switch; the other two inputs at the switch can be used with a CD player, streamer, or other source of your choice. The output of the switch feeds a volume control, which in turn feeds a transformer-loaded single ended output stage.
Using a transformer on the output allows us to set a nice low gain for the line level section. Although a CD player probably won’t need it, some vinyl recordings and cartridges benefit from a small boost (e.g. 2x voltage gain, 6db). The transformer also allows us to step down our output impedance, much like the cathode follower in the Muchedumbre project. Of course, line level output transformers that can be used in a series feed configuration are not usually cheap.
I have a pair of Lundahl 1660 AM transformers to be used in this project. These run around $500 a pair (via kandkaudio.com). They are a well-known transformer for exactly this application. I have also purchased a pair of Edcor GXSE 15k:600 transformers ($40 a pair) as a budget-minded comparison. The transformer ratios are similar (4.5 or 5 to one) and both can be used in series-feed applications. While the Lundahl datasheet is very detailed, you may have some trouble getting inductance and DCR specifications from Edcor.
This is a tale of two preamps. I intend to design and build two all-in-one preamps with the same overall topology, but different tubes and parts. One preamp will be built using NOS tubes and high-end parts, while the other preamp will be built using current-production tubes and every-man components. I’m very excited to hear how the two projects compare and to be able to publish more than one option for people looking for an all-in-one preamp project.
More to come on this topic as I work-out the circuits and parts choices!
Q: A simple question – is it possible to purchase a kit from you? If not, are there decent kits you would recommend for a first-time tube build? My husband has built a couple of solid state preamps and power amps and is intrigued by tube ware.
A: Your husband is a lucky guy to have someone encouraging his hobby!
I don’t sell any kits for my builds at this point (though I do send out prototype PCB boards to my Patreon subscribers on occasion). I am happy to provide some recommendations for beginner-friendly tube projects and/or gift ideas, though. Some of these are PCB boards that require you to select parts (or leave your husband to do so afterwards). Many builders enjoy the process of picking out and sourcing parts, so this isn’t necessarily a bad thing and you might include a ‘parts budget’ as part of the gift in that case.
Tubecad.com makes some of the best documented and flexible board kits you can find for tube hobbyists. In particular, the Aikido and CCDA designs have a great following and lots of user support on community websites like diyaudio.com:
Note the above let you add parts or order boards by themselves. Adding parts might be tricky for you to do without your husband’s input, though TubeCAD does a good job keeping the options and confusion to a minimum.
Here’s another PCB board (no kit) for a phono preamp (for turntables) that I can also recommend. The designer of this one is another well-known author on tube topics:
Bottlehead is one company that gives you everything you need in a full kit. They have a lot of tube kit options at different price points. If your husband also listens to headphones, this company is especially well-known for their headphone amp kits (two options below, but explore the site to find more).
Bottlehead’s kits are pricier, but the documentation and the all-in-one nature add a lot of value for beginners.
Lastly, Elekit is another Japanese company that does all-in-one kits. These are available through the diyAudio Store. I don’t have personal experience with Elekit kits, but I have read a lot of good things (and the manuals I’ve seen look very well-done).
Hopefully you find something in your budget in the above links. I think anything you do to show an interest in his hobby will be very well received!
Having a small stash of #26 tubes and always being curious about it as a preamp tube, I’ve embarked on some preliminary research of successful implementations. There’s a huge thread on diyaudio.com, but some choice references are Ale Moglia’s iterations and Kevin Kennedy’s classic implementation.
In general, the prototypical 26 preamp is a fairly simple single tube grounded cathode gain stage. Perhaps this topology simplicity is why there is so much experimentation in the support circuits. Gyrators, current sources, and line output transformers all make an appearance as the anode loading strategy. The B+ supply is similarly diverse: SS regulators, tube regulators, VR tubes, etc. It seems the popular consensus is for fixed bias: using the filament current drop across a resistor to set the cathode current. But there are fixed bias and traditional cathode bias implementations as well.
All of the above is fairly comfortable stuff coming from the general tube world of 9 pins and octals. The filament (AKA heater) supply, on the other hand, is something new for those used to indirectly heated tubes. In indirectly heated tubes, the cathode is a sleeve surrounding the filament heating it; this mechanical separation helps prevent heater hum (50hz or 60hz AC) from entering through the cathode. In a directly heated tube, on the other hand, the filament and the cathode are one and the same.
Depending on the circuit, AC filament power with balancing resistors and/or a ‘humdinger’ pot may be enough for an acceptable hum level. With the higher Mu (8-9) of a #26 and the need to keep front end noise/hum to a minimum (because it will only be amplified by everything following it), a DC heater solution is in order. We are looking at a requirement of about 1A at 1.5V for a #26 tube. The general approaches I have found are:
A low voltage SMPS and a dropping resistor requires no explanation if you know Ohm’s Law and can find something quiet with the right ratings (here’s a good read on this topic). Voltage regulation and current sources/sinks have been covered in principle a few times in projects and general information pages as well (see links above). Mixed strategies are what have piqued my interest the most.
Kevin Kennedy’s article suggests a 7805 followed by a LDO CCS to supply #26 filaments. From what I can gather, this is the principle also behind the Ronan Regulator (which I see mentioned frequently but I can’t seem to find the ‘official’ schematic). In these strategies the voltage regulator makes a first pass at cleaning up the raw DC and absorbs some power dissipation. The constant current source follows and sets the filament current to a fixed value (in turn setting filament voltage as per Ohm’s Law). Including a CCS to limit current has a protective side-effect as well: cold filaments are otherwise eager to soak up a lot of current, potentially stressing the power supply and filament/cathode itself.
Rod Coleman also has a very interesting approach to DHT filament regulation. You can find boards/kits for sale here (no commercial interest, just admiration for the design).
This circuit feeds the filament from the ‘positive’ end with a gyrator, also known as a cap multiplier in this configuration. The transistor Darlington pair sees a low-passed capacitor at its base and works to amplify this smoothed signal at its low impedance emitter (effectively making the cap seem much bigger than it really is). This doesn’t regulate voltage because the gyrator doesn’t have a fixed reference, but it does reduce ripple drastically.
The ‘negative’ end of the filament is connected to a constant current sink. This is a ring-of-two CCS design which will have a lower operating voltage requirement than a cascode CCS or many ICs. Because our current is relatively high, low dropout voltage is a benefit in reducing overall power dissipation.
The filament is fed a low ripple voltage with a CCS setting the current. It’s simple, but reports seem universally positive for Coleman’s regulator approach. Whether filament bias or cathode bias, the filament supply should be left floating (it finds ground through the bias resistor or grounded cathode).
There’s little reason to reinvent the wheel here as far as I can tell. Although I’m still casually reading, I’ll more than likely try one or more of the above approaches to powering the filaments in my upcoming #26 project. More to come on this project as parts arrive and the ideas ferment.
P.S. here’s a FET version of the same gyrator-CCS one-two punch:
These two buffers are very similar. In fact, the XL came about as a simple mutation of the regular version when the cost/size of an extra tube was not an issue and there was the possibility of driving a lower impedance load (long cables, solid state amps, etc). In theory, the power supply requirements are identical down to the B+ current draw. “In theory.” Although if you study the schematics carefully and read the write ups, you’ll see that my steel-trap mind is missing a spring or two.
The original Muchedumbre called for a Triad C-3X (500 ohm DCR). On the schematic for the XL, I seem to have spec’d the Traid C-7X (270 ohm DCR). The inductance rating for these two chokes is identical, but the C-7X has about half the DCR. Is that going to be a problem? In short: no. I love tubes; they make up for all my shortcomings.
Solder and wire beats paper and pen.
The regular Muchedumbre is an ultra-simple buffer with few parts and straightforward operation. It’s a great beginners circuit for high voltage tube applications.
If you’d like something with just a little more nerd sprinkled on top (and an extra 12AU7), try the following. This XL version uses a White Cathode Follower buffer for about half the output impedance of the vanilla version. It requires a few extra resistors and caps and the heater reference voltage is tweaked just slightly so I can sleep better.
For ideally symmetrical drive ability, the series resistor in the anode of the upper 12AU7 should be calculated as:
Rseries = (Rp + 2 * Rload) / Mu
If we break down the circuit into a cathode follower (upper triode) and a grounded cathode amplifier (lower triode), we can see that this creates a nifty push pull circuit. The cathode follower is non-inverting, so it’s pulling the output in the same direction as the input signal. The lower triode is a grounded cathode amplifier and so it inverts the input that it sees. But the input that it sees is from the anode of the upper triode, which is already inverted. You invert the inverted and you get non-inverted (same ‘direction’ as the upper triode). Tada! Push (lower) pull (upper).
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.
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
Power = Voltage² / Impedance
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).
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.