Courtesy of reader Bill W comes today’s scan of a much more complex tube limiter project. Herbert Hartmann penned this 1955 article for RADIO & TELEVISION NEWS.
Similar in principle to the Altec 436/438 and the Gates Sta-Level, the Hartmann unit is a Vari-Mu design. As the author points out, the release time spec’d here is too fast to allow for totally distortion-free low frequency response. If you’ve ever messed around with an Altec tube compressor, you will know that there is a real ‘limit’ to how fast you can make the release before distortion and artifacts emerge. This was one of the real advantages of those insanely complex contemporary Fairchild and GE mastering and/or broadcast compressors, btw: through some really inventive (and expensive) designs, they overcome the timing issues inherent in simpler devices like the Altecs and Gates units. Here’s the Hartman schematic:
Reader Bill W recently sent us a few interesting ancient DIY audio projects that we had not seen before. Today’s post is from RADIO-ELECTRONIC ENGINEERING and was written by one Ed C. Miller.
The article details the operation and construction of the above-illustrated broadcast-remote mixer/amp, which includes a limiter. While the overall unit is probably not something you’d want to ‘F’ with, the limiter component could be added to a DIY tube mic preamp (perhaps with a true-bypass switch). One caveat: as the article warns, there are apparently ‘thumping’ artifacts inherent to this circuit, which was acceptable in its intended operation since it was designed for voice-frequency.
Anyhow, might be a fun project for a rainy day – build a pair of these into box with cheap 1:1 input transformers and simple line output stages? Crush some room mics? Anyone?
While searching through my output transformer shelf for a suitable reverb-driver transformer for the basket-case Kalamazoo Reverb 12 I recently rehab’d, I was glad to find this ancient NOS UTC R-58. Inside the box was a near-disintegrated wiring sheet for the entire UTC ‘R’ line. If you find yourself with any of the following units, here is what you need to know to implement it in your piece. Apologies for the poor condition of the paper, this is the best we got! INCLUDES: UTC R-27, R-28, R-38, R-38A, R-59, R-60, R-58 output transformers, UTC R1, R2, R3, R4, R5, R11, R12, R13, and R54 power transformers; and UTC R23, R24, R25, R26, R27, R28, and R29 ‘varitap’ interstage and input transformers.
We’ve covered some fairly obscure + forgotten audio publications here at PS dot com, but this one takes the taart. RADIO EXPRES was a Nederlands DIY radio/audio magazine published from 1932 through 1939, and perhaps longer. Well, it was certainly published for the entire year 1939 (22) issues, since I’ve ended up with that pile of them here. As is typical of ‘radio’ publications in the pre hi-fidelity era, the emphasis is much more on RF than AF, but I still managed to find a few interesting articles of possible use to y’all audio-folk. First off: this cute lil 4-watt Grammafoon Versterker (that’s a phonograph amplifier, btw):
DOWNLOAD THE COMPLETE VERSTERKER ARTICLE (9M PDF):
The ‘Lampen,’ or ‘tubes’ used are an E99 high-gain triode input stage and an AL5 pentode output tube. Interestingly, these are both 4V filament tubes. The 4V filament is not seen in any US-manufacture audio tubes that I have ever come across.
The article, penned by one J. L. Leistra, spans two issues of Radio Expres and it is very thorough. It covers all of the theory involved in developing the circuit, and the second part gets into some pretty extensive detail regarding the feedback-based compensation filter. It wraps up with fabrication, layout, and wiring instruction. It’s all written in Dutch, btw.
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The only other really interesting audio-bit that I could discern was this 3pp exposition of the then- ‘neu’ Phillips EL5 (incorrectly indicated as an ‘FL5’ in the above image) 18-watt output pentode. The EL5 seems like a tube worth exploring; it’s a high-power, 6.3v filamant audio output tube designed to run ona pretty low plate voltage (250- 275) for such a high powered tube. Could be useful in some situations. Anyone using EL5s? Here’s the whole article for you to…. look at. (unless you read Dutch, of course).
I’ll leave off with a quick survey of the best part of this run of magazines: the incredible, world-envied graphic design that the Nederlands is still renowned for almost a century later. Enjoy – c.
It’s very hard to find vintage BRAUN hi-fi gear in the states; i’ve seen some amazing BRAUN receivers in Canada, but they either were not sold here, or poorly distributed, because they just ain’t around. Sporting a design aesthetic that would be popularized in the NAD, ADS, and PROTON lines of the 80s, BRAUN kit owed it’s beautiful, so-far-ahead-of-its-time character to Dieter Rams. Rams put his stamp on a huge range of goods for BRAUN; I once found a Rams-designed mini desk fan (!) that was shockingly valuable as a collectors item. Anyhow, here’s a great synopsis of the career of a man who’s ideal could be summed up as Less But Better: CLICK HERE.
Via Wikipedia, here are Rams’ ten principles off good design. Enjoy.
Is innovative – The possibilities for progression are not, by any means, exhausted. Technological development is always offering new opportunities for original designs. But imaginative design always develops in tandem with improving technology, and can never be an end in itself.
Makes a product useful – A product is bought to be used. It has to satisfy not only functional, but also psychological and aesthetic criteria. Good design emphasizes the usefulness of a product whilst disregarding anything that could detract from it.
Is aesthetic – The aesthetic quality of a product is integral to its usefulness because products are used every day and have an effect on people and their well-being. Only well-executed objects can be beautiful.
Makes a product understandable – It clarifies the product’s structure. Better still, it can make the product clearly express its function by making use of the user’s intuition. At best, it is self-explanatory.
Is unobtrusive – Products fulfilling a purpose are like tools. They are neither decorative objects nor works of art. Their design should therefore be both neutral and restrained, to leave room for the user’s self-expression.
Is honest – It does not make a product appear more innovative, powerful or valuable than it really is. It does not attempt to manipulate the consumer with promises that cannot be kept.
Is long-lasting – It avoids being fashionable and therefore never appears antiquated. Unlike fashionable design, it lasts many years – even in today’s throwaway society.
Is thorough down to the last detail – Nothing must be arbitrary or left to chance. Care and accuracy in the design process show respect towards the consumer.
Is environmentally friendly – Design makes an important contribution to the preservation of the environment. It conserves resources and minimizes physical and visual pollution throughout the lifecycle of the product.
Is as little design as possible – Less, but better – because it concentrates on the essential aspects, and the products are not burdened with non-essentials. Back to purity, back to simplicity.
I’d never been particularly interested in learning solid state electronics. There just didn’t seem much point; considering that you can buy a 4-channel Sytek mic preamp for $900, there just ain’t much to motivate anyone to DIY ss kit. Tube stuff is another matter – it’s a different sound, and well-made ‘real’ high-plate-voltage, transformer i/0 tube gear is super-expensive. So I learned to make the tube gear both for my own studio and as a way to make some add’l income by custom-building for other engineers.
All that being said, there is an undeniable appeal to be able to build something useful that doesn’t require a heater circuit and the attendant 60-cycle-hum battles that come from those hi-current windings. Solid state is just easier, which is prolly why it has won-out in the world of consumer electronics, if not necessarily in the pro-audio world. In my endless diggin for ancient tubes and transformers and bakelite meters I invariably come across stashes of ole germanium and silicon transistors, and I recently decided to take the plunge and try and cross this bridge once and for all. Cos I can talk tubes and tube audio circuits up+down, but frankly I don’t know shit abt solid-state and maybe it’s time I learned.
The old RCA Tube Manuals have always been my primary source of information for my tube-audio builds and experiments. The circuits that they recommend are the most solid, reliable, and practical that you will ever find. I trust them implicitly. And why not? After all, this was the company that made the tubes themselves! So when I decided to try and get into SS, I started with the RCA Solid-State Hobby Circuits Manual. In the scan above you will find a mic preamp, a line-level compressor, and a fuzz pedal. I’ll be building all three eventually and I will LYK how it goes. In the meantime, if any of y’all beat me to it, drop us a line and report back,,,
Note: I performed extensive frequency, level, and actual studio tests on the 864 clone today, and several interesting details were revealed. Text has been edited to reflect that.
From 1954 through at least 1963, the Federal Television Corporation built an audio limiter called the AM 864/u for the US Air Force and US Army. The 864 is a simple, rugged device that accepts 600 ohm balanced or unbalanced line-level signal, offers a single front-panel input-attenuator control, and compresses the output level at a 10-to-1 ratio once the threshold point is reached. The output is also 600-ohm balanced or unbalanced, and it offers a maximum 36db of gain. The rear panel of the unit displayed the threshold and ratio controls, although these are confusingly referred to as (respectively) CURRENT and THRESHOLD in the manual and schematic. Attack and release times are fixed, and the manual indicates them at .05″ and 2″ respectively.
As you can see in the diagram above, the 864 was intended to be used as what we call a ‘broadcast limiter’ – the final step in the signal chain before the broadcast transmitter.
Download the original 1963 manual for the AM864 (apologies to whomever did the epic work of scanning this 55pp document; I have long forgotten where I got this file from)
After having scratch-built an Altec 436 compressor years ago, I wanted to try building an 864. The circuits are very similar, although the 864 uses the older 1940s-era octal tubes and uses a feedback circuit from the plates of the input tubes (rather than the output tubes, as in the 436) for its compression control signal. More importantly, though, the 436 remains a bit of an oddball underdog in the vintage-compressor market while the 864 enjoys a very strong reputation. Anyhow, like the 436, the parts cost to build one of these things is negligible, so I figured what the hell.
This is going to be a very long + detailed +technical article, so I’m going to ask y’all to please click the link below if you dare to READ-ON,,,,
Products covered, with text, specs, and photos, include: Mullard EF86 (6267) pentode, Mullard ECC83, ECC81, and ECC81 twin triodes (12AX7, 12AT7, 12AU7), EL84 (6BQ5), EL34 (6CA7), and EL37 (6L6) power tubes, and EZ80, EZ81, and GZ34 rectifier tubes (6V4, 6CA4, 5AR4).
There’s plenty of info here, operating points, some basic schematics, etc… good for any tube audio-maker to keep on hand. I stopped by the shop briefly today and was pleasantly surprised that to see that E.Lyon had found this for me… Thanks again! Enjoy…
Via this eBay auction: a UTC A-12 mounted on a plate-retained octal plug, with the pins wired to correspond to the pin-out of an Altec 4722 input transformer.
It had simply never occurred to me that the mounting-diameter of a UTC A-series corresponded to the mounting-diameter of those octal-mount plates. Well done sir. Great idea if you have an Altec 1567, 1566, or ANY piece of old pro audio gear that uses octal transformers – and a great many do. RCA, Newcomb, Ampex, the list goes on….
In NYC in the mid-seventies, an electronic-based band arose amongst all the guitar punks, a band that was known as much for their confrontational post-beatnik vocals as for the strange and intense sounds that emanated from their famously homemade electronic sound equipment. A band who has become, in the decades since, one of the few acts that is truly ‘required reading’ in the lexicon of avant-garde rock n pop. Or, as James Murphy so brilliantly puts it in his apocryphal tale of musical uber-taste, “I was there, in 1974, the first Suicide practices in a loft in New York City… I was working on the organ sounds…with much patience” (skip to 2:50… or, actually, don’t… this song kinda rules).
So yeah I am talking about Suicide. If you don’t know ’em, check ’em out… it is amazing+terrifying that this record came out in 1977… truly truly AOTT. And plainly awesome too. I really love this band, and they inspired me greatly in the early 2000s, when I was performing with a punk band in Brooklyn using an analog drum-machine rig based around some old Roland beatboxes, voltage controlled filters, and a CV-generating homemade theremin to control the whole thing.
This felt fairly fresh to me in the year 2001; so that fact that Suicide was doing this same thing 25 years early was mindblowing. I had to wonder; how the hell did these guys make all the stuff? Even in the year 2000, DIY’ing synth equipment was fairly unusual for rock musicians; but in 1975? That was like black magic! Well I think I found the grimoire.
NEways… kinda a long setup to what will be… the first OUT OF PRINT BOOK REPORT we’ve had in a while. And oh boy will there be more coming. I was recently at a really fascinating estate-sale somewhere in Marin County, California, where I met an elderly engineer who sold me a library of ancient audio-tech books and wished me luck on my travels… the pick of the litter was the above-depicted “Experimenting With Electronic Music,” by Robert Brown and Mark Olsen. Published in 1974, it is TAB books catalog number 666. No joke. This just keeps getting better.
The book starts with some fairly uninteresting discussion of various commercially-available synthesizers circa ’74, but soon gets into a wealth of both schematics and ideas regarding DIY’d audio electronic circuits. Here’s the TOC:
There’s a ton of great stuff in here, and while I honestly have no idea whether or not the particular transistors spec’d in these circuits are still available, I would imagine that there are subs available… even if you never build anything from the book, I think anyone with an interest in early electronic music will find it fascinating. Here’s a few projects that I plan to do at some point:
“Experimenting with Electronic music” is available from a few sellers on Abe Books. It ain’t cheap, but I’ve been digging for these sorta books for 20 years now and this is the first copy I ever came across.
Courtesy of reader Bill W comes today’s scan of a much more complex tube limiter project. Herbert Hartmann penned this 1955 article for RADIO & TELEVISION NEWS.
Comments and suggestions are welcome,,,
