Category Archives: Technical

Lost to time: Check out the Fairchild 627 Equalizer circa 195nvr

fairchild_627_equalizerApologies for the poor image quiality; these came from the research department of a long-gone British broadcast-gear-manufacturer via some dampish papers that I purchased recently from a rare-books-dealer in London.  I had never heard of the Fairchild 627 ‘Variable Equalizer,’ even though I recently sold an example of its (even-more-rare) predecessor the model 540, which in retrospect seems to be the ‘boost’ sections of the 627 plus a mic preamp plus a 20-watt cutter-head power amp.  Well shit.  Now i wish I had measured the values of the triple-ganged frequency-select pots.  Why?  We will get to that in a minute.

fairchild_627_equalizer_internals

vintage-unobtanium-fairchild-627-tube_1_99dc4f93df9c200f11996b06da8735f3image source

A quick google search suggests that there are thought to be only 5 Fairchild 627 equalizers in existence.  One seems to have sold on eBay recently in the $7000 region.  The 627 is a line-level EQ with continuously variable low and high bell boost sections as well as high and low roll offs. Unlike a Pultec, though, it is an active EQ, and it uses some pretty unusual circuitry – especially the high-cut section, which I don’t comprehend at all.

fairchild_627_block_diagram

fairchild_627_equalizer_schematicOne point to note: T2 should be wired to reverse phase.  I was v excited to find this schematic, because it seems like a pretty easy piece to build – there is no mention of the proper B+ voltage, but 250 is generally a safe bet; there are no weird inductors (the UTC S-23 plate-load choke is an off-the shelf part, so we know its specs  -5000ohm DC, 300 henries); the audio transformers don’t need to handle DC and the turns ratios are spec’d:  BUT:  But.  Those triple-ganged pots.  WTF do we do about them?

Can anyone crack this case? Any insights, pls let us know in the comments section.

On the plus-side:  at very minimum, this schematic does reveal an excellent and easy way to implement a 6E5 seeing-eye tube simply paralleled to a grid in the audio path.  So at very least, now i know how to use those v excellent things in my mic preamps.  Way more evil than a VU meter.

Also: the 6SN7 output section is a neat little self-contained module; def worth trying it with a Hammond 156C choke and an Edcor 15K:600 1/2 watt.   The 156C is the closest modern part to the S23 spec; it is rated 150h and 3.7K ohm, seems close enough?  I used onna these on a hunch in my OP6-semi-clone and it worked just fine in that application, so I am guessing it may work fine here as well.  fairchild_627_boost_curvesfairchild_627_high_low_frequency_rolloffsfairchild_627_curves_2

 

Argentine Hi-Fi DIY Circa 1958

Argentine_Williamnson_1958Many years ago I published this article abt digging for ancient audio ephemera in Buenos Aires.   Reader N. Dinapoli Farina uncovered some related materials and has shared them with us here. I believe that the magazine may have been called “Radio Chassis Television” and the scans below are all from the late 1950s.  Click on the images for hi-res.   Enjoy!

Williamson_Schem_1958Williamson_PS_1958Above: Williamson style amp and power supply.

AltaFidelidad_15wattAmp_1957Fifteen-watt “Alta Fidelidad” amplifier.

Preamp_1958Full-featured mono preamp

MicMixerSuper basic hi-z mic mixer

SystemSpeaker

The Fairchild Model 605 Stereo Tube Phono Preamp c. 1959

Fairchild_605_606_preHow y’all  doing on this frigid day in March,,,  so listen, srry abt not posting much new material this past year.  I’ll be frank: as phones keep getting better and better, and online content keeps getting more and more tailored TO the phone as a consequence thereof, many of us are spending less and less recreational time in front of the laptop (although I am still planted in front of some sort of Mac, invariably, for my production and composing work,,,) and more of that ‘leisure’ time with the phone.  Instagram rather than ‘scoping blogs’ seems better tailored to how most folks are spending their recreational internet time these days.  So we’ve been keeping an active+vigorous presence up there.  This blog isn’t going away, but do check out the IG account if you have not yet.

Fairchild_605_606_PS_outputOk NEways,,, I was diggin thru the archive for something today and I came across the schematic for a phono preamp that Fairchild offered around 1959 – their model 605.  Strangely enough, their prototype (image at head) was labeled ‘606.’  Production examples do bear the mark 605, though, as this example from a Russian website indicates:

Fairchild_605_606(image source)

There is very little information on the web about this unit – in fact, a google search offers,,, good ‘ole Preservation Sound Dot Com as its first result when queried.  And not much else of relevance.  We apparently ran an advert for this very unit some years back (click here for that earlier post).  So I was very excited to see that this unit, which is VERY buildable using off-the-shelf components, had not yet ‘migrated’ onto the web.  The Fairchild 606 offers both MC and MM input stages, 600ohm balanced outputs, and selectable EQ curves and stereo or true mono LP operation.  Now, I’ve built many Marantz and RCA-style tube phono preamps to great success, but this Fairchild is simultaneously exotic AND obtainable enough to be quite intriguing.   So, DIYers of the world, here ya go:  knock yrself out:

Fairchild_605_phonoPre_Schematic(click on image for a (huge) full size image)

There’s nothing exotic in it: no custom inductors, weird-taper pots, or un-source-able transformers.  The toughest thing to find might be the 4P/6T switch, but you could always sub in a pair of 2P/6T switches and just use two hands.  In fact, the input transformers, which I can confirm are 1:20 from the 600 ohm tap, appear to be garden-variety Beyers:

Fairchild_605_inputTrans(image source)

Regardless, though, you can use any hi-fidelity input transformer with a roughly 1:20 ratio and an input impedance approx.  10X whatever the output impedance of your moving-coil cartridge is.  And if you only use a moving-magnet cartridge, you can skip that part of the circuit entirely and just build the 47K ohm grid-input stage (and all that follows).  As the schematic indicates and the images confirm, this product was built in two chassis:  (preamps+EQs) and (power supply+output stages).  Other details: R109 and R209 are level controls, basic voltage dividers with a 20db range.  EQ offered is flat, RIAA, or RIAA plus add’l roll-off, and the ‘Lateral’ switching positions offered cancel out the vertical tracking information, resulting in the cleanest possible sound from mono records.  The output stage is fairly conventional, but interestingly enough requires single-ended transformers, so you will be rather limited in your options here (a 15K:600 that can handle 8ma DC unbalanced, such as a UTC A25, should work fine).

This is a fairly advanced project, which I have personally not built (yet). I cannot offer any technical support or help with this.  If you have never built a vacuum-tube phono preamp or mic preamp from scratch before, I would not advise undertaking this project.  Good luck, and if you build one, send us some pics!

The 1959 Ikegami-Tushin Limiter Inspired by the RCA BA6A

NHK_D_1Reader S. Komiya recently contacted us with some information regarding the Ikegami-Tushin limiting amplifier, an RCA BA-6A inspired piece that was built for Japanese broadcaster NHK in 1959.

SK has been so kind as to provide the schematic for this obscure device, as well as some background information.  I am posting the schematic full-size, so you can control-click it and download it for detailed viewing.

00_schematics_ikegami-limiter1959NHK_E1The photos in this post come from this Japanese auction website; the device pictured here recently sold for just Y30,000 ($300 USD).  And in working condition. 

600x450-2015011900002Here’s what SK has to say about the Ikegami-Tushin Limiter:

‘(It is) very much inspired by the RCA BA6A for sure.  It even looks a bit like it.  The tube format is very similar:  just change 6sk7 to 6ba6, 6j7 to 6au6 those goes into 6v6 PP and transformers between 6au6.  6ba6 were popular and cheap in japan because we made those a lot in japan in the 1950s and 60s.

6sk7 and 6j7 were never made in japan.  This unit also has an extra gain stage before first stage, which is pretty neat.  The components seem very high-end and some are custom made for this.  When i was gathering info about the ba6a in old tube shop, an older ham radio guy told me that he DIY’d one of these a long time ago…’NHKH_1NHKF_1*************

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20141230_123551SK also recently built his own BA6A from scratch.  It is depicted above, and you can hear audio samples of it at his soundcloud page.  SK has also scratch-built the Federal AM864 tube limiter, and he has this to say about the projects:

“The sound you will hear in soundcloud is a good comparison with the fed864.   The fed has good high open but compressed sound,  the ba6a has low mid, ton of low mid. I love them both.  The first time I used them was at a studio in Chicago back in 90s when I was living in US.  At that time I was using LA-2A mainly, but that studio had a Fed864 and BA6A.  They blew my mind, and since then I wanted them so bad!’

SK also provided some build-notes on his BA6A project; if you are planning on building you own BA6A, you might find these useful:  S_Komiya_RCA_Ba6a_DIY_notes

An DIY Tube Mixer Project c.1954

TubeMixer_DIY_1954Today’s article scan, again via reader Bill W., was written by one John S. Carroll and originally appeared in RADIO & TELEVISION news in 1954.  It describes a four-into-one mixer based on a design spec’d by UTC.

DOWNLOAD THE COMPLETE ARTICLE: 1954 Audio Mixer

1954_tube_mixer_rear1954_tube_mixer_bttmThere’s nothing terribly interesting about this particular circuit other than the output stage, which is push-pull and and is driven by a 3:1 interstage transformer which functions as both a phase inverter and provides a bit of additional voltage gain.

Output_stageI’ve never made a mic pre with a transformer phase inverter, but it could be very cool.  At Gold Coast Recorders one of the engineers’ favorite mic pre’s is a custom unit that I built based around an RCA 9362 cinema line amp (see earlier article here).  The 9362 is a push-pull output module, and it’s definitely a different sound – almost every tube mic pre made is single-ended.  Anyhow, at some point I will def build the circuit above into a mic pre.  Probably as soon as I find a pair of appropriate hi-fidelity (and shielded!) transformers for a buck.

1954_mixer_schematic

A DIY Broadcast Limiter Project circa 1955

1955_DIY_Limiter 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.

DOWNLOAD THE COMPLETE ARTICLE: 1955 Limiting Amplifier

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:

1955_limiter_SchemComments and suggestions are welcome,,,

A Simple One-Tube Compressor Circuit c.1953

1954_DIY_limiterReader 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.

DOWNLOAD THE COMPLETE ARTICLE:Constant Output Broadcast Amplifier

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.

1954_limiter_SchemAnyhow, 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?

UTC “R” Series universal replacement transformers of the 1940s/50s

UTC_r58_transformer_boxWhile 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.

DOWNLOAD A PDF: UTC R-SERIES TRANSFORMERS

IMG IMG_0001

DIY Grammafoon Versterker circa 1939

Radio_expres_magazine_NetherlandsWe’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):

Versterker_SchematicDOWNLOAD THE COMPLETE VERSTERKER ARTICLE (9M PDF):

Download:GrammofoonVersterker

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.

Versterker_partsListThe 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.

Versterker_Tagboard Versterker_Chassis*************

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EL5_schemThe 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).

DOWNLOAD: EL5_amp

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.

MastheadStoetsRadio Frequenta Bell_Telephone_Holland Sondisko Kristal_Microfoon ThermionLampen