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.
Although I love me VR tubes something fierce, they aren’t in current production and I’d like a simple (or well as simple as possible, I guess) shunt regulator alternative for things like phono preamps or line stages (20mA of load current or thereabouts).
This shunt regulator uses an EL84 for the heavy shunt lifting and a 12AX7 differential amplifier (using non-inverting output) to amplify any ripple on the output (which the EL84 ‘cancels’ across Rs). Quick calculations look like about 100V of headroom would be nice to have so rectified 250Vac with the shunt should be good for about 250Vdc regulated output.
Update: Putting the differential amplifier before the shunt will unload it a bit and provide more B+ headroom for higher gain.
Why not build an all-tube MC phono preamp?
I haven’t built an all-tube MC phono preamp. I build MM stages with the assumption that MC users will use step up transformers. Generally, its much easier to keep the whole thing quiet that way. This is a simplified explanation, but I hope it gets the general idea (SNR
Tubes impart noise in a few ways but they’re all usually tiny. MC carts need around 60-70db of gain to bring them up close to line level. This is multiplying the MC signal from the cart by 1000-3000x before it gets to your amp. In contrast, a MM cart usually needs 40-50db, which is an amplification of 100-300x.**
The more tubes used, the more very tiny sources of noise get introduced. Although the noises are tiny, they are amplified by the preamp, just like the signal. So the more gain required to get the signal to a desired listening level, the closer the tiny noises get to being audible as well. Several stages of tubes for a very large amount of gain can lead to unwanted noise for this reason (guitar amps and phono preamps are both good examples of this).
The step up transformer cannot practically do more than about 20db-25db of gain without having some undesired consequences, but they don’t have ‘moving parts’ and are very good at rejecting noise/hum. The 25db of gain from a transformer is enough to lower the tube gain needed from 3000x (70db) to 175x (45db), meaning any noise from the tubes is amplified much less. It’s kind of like giving the tube part of the preamp a head start in the race against noise.
So that’s why I haven’t built a MC phono preamplifier. If I were to build one, I’d most likely add step up transformers to a MM preamp like the El Matematico
or similar. To me, this is the most practical approach with the highest likelihood of success.
**At the risk of muddying the waters, phono preamps actually need about 20db more than the numbers mentioned here but this is then attenuated by the RIAA correction filter to result in ~40db or ~60db net gain, MM and MC respectively.
I’ve been doing some reading on tubes in shunt regulator power supplies lately (lots of great articles on TubeCAD including this one). I’m planning to incorporate one in an upcoming build. In operation, this isn’t too different from the VR regulator power supply in my Matemático Phono Preamp, but a shunt regulator with a triode would have an adjustable output and might afford even better ripple rejection.
My recent series regulator project is another example of power supply regulation.
Click here to see the new page on shunt regulators.
Pete Millett’s Starving Student was one of the first amps I ever built completely from scratch. Unfortunately, the 19J6 tubes have become rare (or at least no longer dirt cheap) due to all the bright eyed DIYers scooping them up to build amps. I think the world needs another <50V tube amp for beginners, so I’m designing one. Like the original, it’s an oddball tube with a MOSFET buffer and an off-the-shelf power brick (same brick, in fact).
Millett is one of my personal tube heroes. This is a tribute. Full write up coming soon (and parts values subject to change once tested).
What is your opinion on rectification?
Tube vs SS rectification is a hotly contested topic in the audio world and what I tell you will be opinion. That said, I’ve used both and never had a problem (eg here
). When designing from scratch, my choice is dictated by my power transformer, space available on the chassis, and whether a tube rectifier is in the budget. Sound quality is not really a consideration.
Of all the parts in a Class A single-ended amp that contribute to the sound, I think rectification is one of the least important. If you end up with the right B+ voltage and you filter it so that there is no ripple, you’re 99.9% of the way there. The debate about tube vs SS rectification usually focuses on theoretical advantages or disadvantages without respect to whether a supply is well-designed and implemented.
All things being equal, and space and budget allowing, I tend towards tubes because I like the way they look and I like knowing that the high voltage will start up just a little bit slower. All things being equal, others would go for solid state because it has a lower output impedance and allows for larger filter capacitors.
There’s more emotion than measurement in either decision.
274B borrowed from thetubestore.com and 1N5408 borrowed from PartsExpress.com. I claim Fair Use here and anyways they can get bent if they don’t like me linking to them in exchange for the pics.