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!

Advertisements

On the bench: Zenith K731 (7M07)

This poor radio has seen better days and doesn’t quite live up to modern safety standards with regards to mains electricity.  But the look is great and there’s generous space inside for a small tube amp. Because the enclosure must be allowed to vent for the tube amp to dissipate heat, the speaker (which I also plan to modernize) will be a small challenge. This is a great candidate for a DIY tube radio restoration.

Probable features:

  • 3-5W single channel output
  • Bluetooth connectivity
  • Aux input (analog)
  • Volume control
  • EQ (either treble/bass knobs or loudness contour)
  • Safety!

 

DIY tube RIAA calculator sheet

All-In-One RIAA Calculator

Here’s a spreadsheet I built for calculating RIAA values in two stage tube phono preamps. When comparing results to other published designs using the same filter network, everything looks correct (within a few percent due to estimation of Rp). I used this sheet for El Matématico.

If you want to estimate values for something like a CCS loaded stage, you can set Rload on the appropriate stage to 1M or thereabouts. If you’re looking at using a cascode, mu follower, SRPP, etc 1st stage, you’ll need to make sure the Zout figure the sheet uses (cell I6)  reflects the Zout of the topology because it is used to calculate R1. Same thing goes for cell I3 (Miller capacitance of 2nd stage) if you use a gain stage after the filter that affects this (cascode, grounded grid, etc).

Go build a phono preamp!

Shunt regulator proposal

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.

dc-shunt-reg-v3

New Page: Shunt Regulators

shunt-reg

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.

Letters to WTF: rectification

Q:

What is your opinion on rectification?

A:

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, here, here, 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.