New page on headphone output stages


I posted a new page on power output stages with a headphone focus (though it can be applied to speakers as well). My aim is to eventually try all the variants.  Between the Estudiante MOSFET hybrid, Papa Rusa parafeed SE, Bad Hombre differential, and Luciernaga series-feed SE, I’m part way there. Been working on ideas for an all-tube OTL lately.  Hopefully I’ll tick that box soon!

I love kits, too

Especially when they are high quality kits.  Here are the contents of a TubeCAD Aikido kit that just arrived. John Broskie’s boards are top notch, the parts are bagged and labelled logically, and the included manual is excellent.  I’ll be building this kit up in a unique way (see TubeCAD’s article on the SRCFPP) and will post a build and my impressions in the coming weeks.

In the meantime, if you aren’t subscribed to and reading The TubeCAD Journal, you should be. Also consider contributing to John Broskie’s Patreon: for less than the cost of a Netflix subscription, you’ll support excellent vacuum tube DIY content and resources for everyone in the hobby.

New 300B kit by Elekit (via

Note I have no affiliation with Elekit other than being an admirer of what they do for DIY audio hobbyists.

Elekit’s kits seem to hit attractive price points for what’s included (tubes, transformers, components, and a chassis) and the quality of the documentation. VK Music (Canadian importer of Elekit) just announced a new 300B amplifier on here. Although details are sparse, we can maybe glean some ideas from the specs and the previous incarnation (Elekit TU-8300).

Specs for TU-8600:

• Tube Set : 12AU7 X 2 + 12AX7 X 1 + 300B X 2
• Now compatible with low to high impedance headphones
• Frequency response (-3dB) : 10Hz – 80kHz
• Max. output (THD 10%) : 8.3W + 8.3W (Input voltage : 250mV r.m.s)
• Residual noise : 42uV rms (IHF-A)
• Power consumption : 80W when no signal; 80W at max. output

The previous TU-8300 used MOSFET regulation for a B+ of 375V and because the new amp is rated for the same power, I think we can assume the new version operates at about the same voltage with a similar bias (-60V). Rather than two stages of 12AT7 on the input, the new design uses two 12AU7 and one 12AX7 triode per channel. So how are these arranged?

Let’s reverse engineer the numbers for an educated guess. We get full output of 8W from a -60V biased tube with an input of 250mVrms. If the 250mVrms (0.7V peak to peak) fully drives the output tube’s -60V bias (120V peak to peak), we have a gain of about 160x. That could be two successive stages of 12AU7, but then what is the 12AX7 doing? We appear to be using one half of the 12AX7 dual triode per channel. It would be an odd choice for a buffer stage, but it’s a plentiful tube and would make sense in that regard.

The other likely explanation is that we have some feedback at work and the 12AX7s are used for voltage gain. Maybe this is a grounded cathode 12AX7 into a 12AU7 SRPP. In terms of driving the Miller Capacitance of a 300B, this seems like a plausible arrangement. Gain would be in the 400-500x neighborhood, but feedback would knock this back down to the 160x overall and lower the output impedance.

Lastly, this amp includes a headphone output. Maybe the extra triodes are employed in some kind of follower specifically for the headphone section. A 12AU7 white cathode follower seems like a potential candidate. Whatever it is, I’ll be anxiously awaiting the manual and schematics to see what Mr. Fujita has come up with!

If you’re headed to the LA Audio Show (June 2nd-4th), the amp will be on display in the VK Music booth.

New Project: La Luciérnaga (PSU)

This write-up will have two parts.  The first (the PSU) is posted and hopefully I’ll have the amp write-up done shortly.  This project is the most ambitious one I’ve written up for the site so please excuse the omission of some of the finer calculations and details.

Although the power supply is rather complicated, the amplifier will be pretty straightforward (pinky swear). The supply can be used with other amps and the amp can be used with other supplies, which is one of the reasons I decided to split it into two pages.

Click here to descend into the madness!

Coming soon to a workbench near you…

In a world where near-field and headphone listening has become an unstoppable force. Where every DIY builder is bored to death of rational and safe two-stage, single-ended triode designs. Where power supplies have become an afterthought and parts values are just plugged and chugged. Prepare your butt for a new madness. Prepare it for La Luciérnaga…


Note db scale for the predicted frequency response graph: +/- 1 dB from 40-20khz. If that’s not good enough, I’ll show you how to make it even better. There’s mucho tube math coming your way, amigo.

Here’s the worksheet I used to create the above: