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!
For the current project (a line stage with added phono) I needed more than one B+ value. The difference between the two voltages I wanted and current drawn was too large for a single high voltage rail and a filter or regulator to drop the lower rail to the correct value. So I looked for ways to add a voltage doubler to a standard bridge rectifier. Turns out, there’s more than one way to skin a cat:
The “Millet Doubler” is detailed here and uses a single center-tapped winding. The entire secondary is rectified via a bridge rectifier, rather than the usual approach of grounding the CT and using a full-wave rectifier. The center tap voltage is then rectified and feeds the lower half of a stack of capacitors.
The “TubeLab Doubler” is something I found posted on diyaudio.com; it is also discussed in depth here. This one uses a single winding without a center-tap. The doubled voltage rail is somewhat lower than what you’d get in the Millet Doubler, but still potentially useful especially with inexpensive isolation transformers.
I can only find a schematic of the “TubeCAD Doubler” (no discussion), but if you’re familiar with TubeCAD’s blog, it doesn’t look too unfamiliar. See a good article on multiple power supply voltages here. This one looks a little bit like a combination of the other two variations.
In the end I went with the second version because it allows me to use an isolation transformer (and because I found it before seeing the TubeCAD one). Of course a couple of wiring oopsies are being worked out before I can report back on the power supply or preamp it is intended to feed…
This was a busy week, so all I have to post is a couple of quick pics of people I chatted with at the NAMM show in Anaheim (which I attend for my day job).
Kevin from K&K Audio was hanging out in the Lundahl booth on Saturday morning before the show. We chatted about the state of DIY, local building groups, and the NAMM show. I had never met Kevin before but we had an immediate connection over the DIY hobby. K&K is the USA distributor for Sweden-based Lundahl Transformers. Their C-core construction, multiple winding configurations, and super-detailed datasheets are unique in the tube building hobby. Lundahl products are also sought after for studio and pro line-level applications and microphones (hence the NAMM booth).
I also found a booth for Triad Magnetics at the show this year! This was the first time I’ve seen Triad at NAMM and I had to say hello and thank them for manufacturing such affordable and easy to source power supply chokes (which I use in almost everything I build). The Triad team said the show was going well and that they’d probably be back next year.
I had a whirlwind NAMM schedule this year and didn’t get to spend any real time at tube booths like JJ Electronics or Electro Harmonix. I did take some time to get scanned for CIEMs at Ultimate Ears, though. There may be a portable amp project somewhere in the future…
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:
In the past I’ve resorted to matching transformers in a parafeed arrangement or speaker transformers with low impedance headphones. I look forward to trying some of these transformers in future builds!