I just received some pictures from builder MG of his Papa Rusa amplifier. He’s used some beautifully figured wood to build a striking enclosure for his Papa Rusa 6S45Pi parafeed amplifier. It has a cool instrumentation vibe to it with the subtly angled front panel and exposed transformer and heatsinks.
I recently received some pics from W of a Bad Hombre Mk2 build (12AU7 + ECC99). This one uses Lundahl LL1676 input transformers and LL2765 output transformers, and features a regulated B+ and a loaded front panel of controls. Very nice!
The LL2765 is a fine transformer with ECC99 output tubes in this circuit. It has a 5k primary and multiple output taps for 32, 150, 600 ohm headphones. The 5k primary will result in more power than the 8k in the original design, but it can also be used to reflect a higher impedance depending on what taps and headphone load one chooses (e.g. 300 ohm headphones on the 150 ohm tap for 10k load).
On a side note, I see Lundahl also now has a LL2774 3k primary headphone output transformer (16, 64, 300 ohm secondary taps). This is a very similar turns ratio but it looks like the LL2774 is available with a bigger gap in single-ended configuration for higher current output tubes. Great to see new options for headphone output transformers!
I just finished an article for Headphonesty on headphone amplifier basic terms. It focuses on building block concepts and aims to make technical marketing copy more comprehensible by users without an electronics background.
It seems to me that there are three fundamental obstacles for beginners in the DIY tube hobby:
Layout and connection of component parts for best hum/noise performance
Choice of parts for correct and safe ratings/types/etc
Chassis fabrication and layout
Complete kits with chassis, parts, PCBs, and the whole ball of wax hit all of the points, but they are a daunting investment in both time and parts. See great examples from Bottlehead or Elekit. In a baby-steps approach, I’ve begun experimenting with putting entire circuits on a PCB design (image shows the El Estudiante). This addresses the first point.
I have ideas on ways to tackle the other challenges that minimize capital requirements and keep the hypothetical business idea agile and scalable (brushin off the old business and supply chain lingo). It might even be enough to turn into a respectable side-hustle. Hopefully I’ll be posting more on what I’m calling “quarter kits” in the near future.
Headphonesty is a digital magazine dedicated to high-fidelity headphone audio culture. Outside of designing and building a few heapdhone amps, I’m personally a regular high fidelity headphone user, especially now that I have a toddler in the house. I recently connected with the chief editor at Headphonesty on Reddit and agreed to lend a hand with an article on headphone and amplifier impedance.
Unlike speakers, which are really rated at 4 or 8 ohms 99% of the time, headphones have a wide spectrum of impedance ratings. From 16 ohm in-ear-monitors to 100k+ ohm electrostatic headphones, matching sources and loads is a confusing aspect to the hobby that’s often done through trial and error or hearsay. This article written by Trav Wilson and edited by yours truly seeks to explain some of the concepts in an easy-to-read headphone-centric way.
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.
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:
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!
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?
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: