Ok, so what we often call a gyrator is not technically a gyrator. This page is named after the circuit popularly referred to as a gyrator, not an actual gyrator.
I’m developing a little PCB for a simple gyrator circuit to be used in an upcoming integrated preamp project (2nd stage of phono circuit: needs gain and low enough output impedance to drive a volume pot). The thing that’s most intriguing to me at this point is how a gyrator lets you set an anode voltage rather than anode current (but still maintains a high impedance for AC). On paper, this looks more flexible in rolling compatible pinout tubes than setting a current. And what the heck, it’s a new circuit to try!
I love boxes with tubes sticking out as much as the next DIYer. Generally, the more boxes, the better in my mind. In practical domestic life however, lots of specialized chassis (beautiful as they may be) don’t always translate well to limited space or the aesthetic considerations of cohabitants (AKA: WAF).
The phono and line-level functions are a good place we can look to consolidate our pretty enclosure collection. The voltage levels are manageable, the current requirements are usually low, and the tubes used are not especially large (in most cases). The question then is how best should we integrate something like a phono preamp and a line level preamp.
The schematic above gives an idea of the approach I intend to take for this kind of phono+line level project. A phono signal travels through an RIAA section sandwiched by two gain stages. This is attached to one input of a three way switch; the other two inputs at the switch can be used with a CD player, streamer, or other source of your choice. The output of the switch feeds a volume control, which in turn feeds a transformer-loaded single ended output stage.
Using a transformer on the output allows us to set a nice low gain for the line level section. Although a CD player probably won’t need it, some vinyl recordings and cartridges benefit from a small boost (e.g. 2x voltage gain, 6db). The transformer also allows us to step down our output impedance, much like the cathode follower in the Muchedumbre project. Of course, line level output transformers that can be used in a series feed configuration are not usually cheap.
I have a pair of Lundahl 1660 AM transformers to be used in this project. These run around $500 a pair (via kandkaudio.com). They are a well-known transformer for exactly this application. I have also purchased a pair of Edcor GXSE 15k:600 transformers ($40 a pair) as a budget-minded comparison. The transformer ratios are similar (4.5 or 5 to one) and both can be used in series-feed applications. While the Lundahl datasheet is very detailed, you may have some trouble getting inductance and DCR specifications from Edcor.
This is a tale of two preamps. I intend to design and build two all-in-one preamps with the same overall topology, but different tubes and parts. One preamp will be built using NOS tubes and high-end parts, while the other preamp will be built using current-production tubes and every-man components. I’m very excited to hear how the two projects compare and to be able to publish more than one option for people looking for an all-in-one preamp project.
More to come on this topic as I work-out the circuits and parts choices!
Here’s a sneak peak of the long-term push-pull project. The second monoblock is one cathode bypass capacitor away from being ready for playback. A bad tester tube took out the cap on one side during testing with a bang, but I’ll have replacements soon. Until then here’s the intro of the project write up.
The monkey on your back
Everything should be made as simple as possible, but not simpler.
There comes a time in every DIY builder’s life where he or she gets the urge to stretch beyond single-digit output power and single-ended amplification. There is no shortage of worthwhile projects to choose from: variations on Williamson, Mullard, or Dynaco push-pull topologies are easy to find discussed in forums and tweaked to compensate for modern parts. You can even find kits for something like the Dynaco ST-70.
When the double-digit power bug bit me I could not bring myself to abandon my usual no-feedback, triode output, class A comfort zone. This is the simplest (but not the only) path to good sound and my speakers are efficient enough. I’m also too lazy to do feedback math but that doesn’t mean open-loop, class A triode designs aren’t an engaging challenge. This build faced the following complications (which are common to many push-pull amplifiers):
Class A requires healthy current in the output stage: this needs to be balanced in the output transformer to preserve inductance
Two cascaded grounded cathode input stages is too much gain, but one stage is generally not enough
The input stage must have low enough output impedance to drive the triode output tubes
For the most part, my solutions to the challenges strive for simplicity. As is often the case in tubes and life, simplicity in some areas is traded for complexity elsewhere. This push-pull amp has only two stages, the outputs are cathode biased, and it requires only three tubes per channel. To make this seeming simplicity possible, I used solid state helper circuits on PCBs. While these helper circuits are not technically complex, they drive up the parts count and require some measurement and adjustment.
Here is the conceptual topology for Los Monos:
Pictured is a two-stage triode output push-pull amplifier. The output stage is garter biased and the voltage gain and phase splitter stages are combined in a folded cascode long tail pair. This is all described below with a full schematic (showing lots more parts).
More of this write-up is on the way as soon as I’ve got both channels playing and glamour shots are taken!
Student “D” sent me some pictures of his Mighty Cacahuate project with a twist and it’s too unique not to share. D developed a PCB for his build and mounted all the amplifier parts to a top plate as would usually be done. Instead of a boring wooden box, D dropped this into a boombox enclosure for all-in-one listening. I love to see the creative use of a basic schematic I posted here on my little website.
We initially troubleshooted some wiring over email, mostly due to my omission of the details of heater wiring and pin numbers on the original schematic. Once sorted though, D says the amp started playing and sounding great without a hitch.
It was an amazing feeling the first time they powered on.
Careful there, D, that feeling is habit forming!
Update on the mono-blocks: left channel is done and right channel is coming together quickly. Write-up for the project is also underway. Looking forward to playing in stereo!
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…
Today I came across a website project called American Radio History and it reminded me that the internet is an amazing resource for education and sharing/archiving information. American Radio History is an online depository for endangered antique electronics knowledge captured in the form of hobbyist magazines from the early 20th century (back when tubes were the dominant active device).
The site’s FAQ provides a glimpse into how it came about:
There is so much printed material about radio and television that is becoming harder and harder to find. Libraries are discarding (often to the recycle bin) many titles. Other collections are very limited in access so “the rest of us” can’t find information we want.
The site began over a decade ago when I found I was often being asked questions I could answer from my own library. So I went digital for all to see!
Just one person does most of the work. I have off-site backups at several locations under the custody of well-respected radio historians. I also have several hearing-impaired persons who help with the major flatbed and Atiz Bookscan digitizing projects.
Here’s a short list of some of the dozens of relevant magazines for DIY tube reading:
It is not difficult to find schematics in our hobby, but it is somewhat more challenging to find schematics accompanied by articles that reveal their inner workings. That is one advantage still held by professional publications like the audio magazines cataloged by American Radio History.
There is a new post at Hackaday detailing a very cool 3D printed resistor organizer. Resistors are an indispensable component for DIY tube projects and we typically collect a large variety of both resistance values and power ratings. If you’re into boutique parts and Holco/Riken/Caddock flavors, it is even more overwhelming. On the upside, resistors are pretty cheap to buy in bulk; but if you buy in bulk and don’t have a way to organize things, you end up with a tangled mess of kinked-up leads and unmarked parts.
That was me a couple years ago: always buying extras and just tossing them into a parts organizer with too few cubbies and not enough labels. Eventually I got sick of repeatedly buying the same values because I was too lazy to sift through my inventory. Doing a little reading on hobby forums, I came across a really cheap and useful solution: trading card binder pages.
Trading card pouches are just the right length to store resistors without having to bend up the legs. This is best for 1W and smaller sizes, so I still have some storage for power resistors. But I also don’t buy power resistors in bulk very often (because I’m cheap), so there’s less to store. Go for either tape and reel (see above) or keep your loose resistors in baggies:
The parts baggies you get from Mouser/Digikey are already labeled, so you just need to fold it with the value facing up and slide it into the card slot. Easy peasy. Best of all, the trading card pages and the binders themselves are easy to find. The only downside to this resistor storage solution that I’ve found so far is the high likelihood of dumping everything on the floor if you pick the binder up upside-down (true story, twice). I’m currently on the lookout for a Hello Kitty trapper keeper with a zipper to solve this. For science.
Bench update: the push pull mono-block project made its first music through a speaker yesterday. I’m looking forward to cranking out the second amp and posting the project for others.
A recent discussion on diyaudio.com reminded me of the opamp-based RIAA idea I shared last summer. It turns out that someone else has done something similar and reports very good results. Koifarm was after a more integrated build with phono, streaming, and line level all in a box, but the basic idea of using an opamp to perform the RIAA corrections and a tube to provide some/all of the voltage gain is the same. We differ just in how it’s integrated: I’m after a simple RIAA module with outboard tubes while Koi was going for an all-in-one.
Here’s where I’ve landed so far on a board to contain the opamp bits and bobs:
The opamp runs from the 6.3Vac heater winding that would be included on any tube-centric power transformer, meaning there are no special windings or an extra transformer to power the solid state section. The output of the RIAA module would be fed to a simple tube gain stage of your choice. Here’s a grounded cathode application, but keeping the tube off the board means there’s tons of flexibility.
So, will it work? Koifarm thinks so and he’s a pretty prolific phono preamp builder. I’ve also already tested the same RIAA correction scheme in the battery powered phono project. I’m saving up a few designs to place a board order, but I’m hoping this RIAA module would make for a relatively simple and fun build this year.