Tag Archives: compressors

Audio & Design (Recording) ltd. Compressors c. 1979

ADR_Gemini_CompressorI was recently given hundreds of pages of Audio & Design (Recording) ltd. (hf. ADR) literature from the 1970s, and I’ll be uploading it over the course of… my lifetime, I suppose.  Anyhow, here’s a package of original data sheets ETC on their rack-mount compressors of the era:

DOWNLOAD: ADR_comps_1979

AND – BONUS – a period essay regarding compressor-usage by founder M. Beville: Beville_ADR_Comps_77

Products covered, with text, specs, and images, include: ADR Gemini Compact (their ‘prosumer’ unit), Gemini East Rider, F690 ducking limiter, Compex compressor / expander, F600 broadcast limiter, FM Stereo Ex-press limiter, and Transdynamic processor.

ADR_Gemini_compact ADR_FM_stereo_Ex-Press ADR_F690_compressor ADR_F600_compressor ADR_CompexWe previously featured ADR’s ‘Scamp’ series of modular processors from the same era.. click here for that jazz…

CBS Volumax and Audimax circa 1964

CBS_Audimax_1963 CBS_Volumax_1964 CBS_Limiters_1965CT!  CBS labs, Stamford, 1963 – 1965.   These CBS units still seem like the ugly ducklings of the vintage limiter market.  I have had 3 of the later solid-state audimax 4440’s in the basement for the past 3 years.  They work fine.  No one wants em!  Eventually these will be re-discovered by some hotshot mixer dude and prices will rise.  The 4440 is so goddamn complicated inside, its like a fkkn analog computer.  Anyone using the Audimax and/or Volumax lately?  Drop us a line and let us know…

UPDATED: Compressor Roundup c. 1963

Compressors_1963_1Today on PS dot come: a short but v v informative piece from BROADCAST ENGINEERING , July 1963, which gives specs for nearly all of the broadcast compressors that were available that year.  Models covered include: Collins 26J Auto-level, Collins 356E, Fairchild 666A, 666, and 663; Gates M-5167 Sta-Level, GE BA-9 Uni-levele, ITA AGC-1A, Langevin AM-5301 Leveline, Quindar QCA-2, and the RCA BA-25A

DOWNLOAD: Compressors1963

UPDATE: T. Fine was so kind as to provide the entire 3-part article as a compact PDF.  click here to download it: BrdctEngnrgAudioLeveling_1963


Novel Vacuum Tube / Diode Compressor: 1950

Download a short piece from RADIO-ELECTRONICS, 1950, on the subject of a DIY audio-compressor:

DOWNLOAD: DiodeCompressor1950

This promises to be an interesting unit.  It’s designed (like a Shure Level-Loc) for Mic-Level-In, Mic-Level-Out, but that could pretty easily be changed for balanced-line operation by omitting the first 12AT7 stage and using a modest step-up transformer (maybe 1:3 or 1:5) going into the input level pot, and then adding another output stage and output transformer.  (Since 1/2 of the 2nd AT7 is unused, I am thinking: eliminate one of the 12AT7s entirely, build this circuit minus the first stage using one 12AT7, and then add the output stage from the Altec 1566).  The unit promises to add distortion, and there is some sort of low-pass network before the 3rd grid that will also need some sort of variable components added in order to control the quality of that distortion.  My biggest question, though: will plain ‘ole IN4007 Diodes work in the circuit?  I plan to build this thing soon and all questions will be answered… always looking for new (old) sources of novel grit+crunch….

Vacuum-Tube Output Stage for Shure Level Loc

Many of my regular readers will be familiar with the Shure Level-Loc.  For those unfamiliar, the basics:  the Level-Loc is a brickwall limiter made by the Shure microphone company for public-address-system use (podium mics, specifically) in the 1960s.   It uses discrete transistors and transformers in the signal path; it offers balanced mic-level i/o and an unbalanced consumer-level 10K ohm impedance output as well.  There is an input-level control (simply a pot that follows that secondary of the input transformer) and a switch marked ‘distance selector,’ which seems to me to be a threshold control.  That’s it for control.  It is fairly noisy (full-bandwidth noise), even after a recap,  and the transformers are not especially well-shielded.  It runs off of a 9V battery.  For more information on the Level-Loc, you might want to start here.

Anyhow…seems like a toy/piece-of-junk and maybe it is, but these things have become highly coveted for use in recording rock drum performances.  How much so? Well, how many other prosumer PA-system pieces are currently available as a plug-in, an API-500 series module, and a boutique re-build? (image source for above)

I recently picked up a clean Shure Level-Loc for a few dollars at a yard sale; after the aforementioned re-cap (and we’re talking about 20 capacitors here…), it was sounding like it was probably operating within its original design parameters.  I was intrigued, and figured it might be worth getting it into the racks at Gold Coast Recorders to see what it could do.   GCR is a big, live-sounding room, so there’s plenty of sound to get out of it with a squashed compressor.  The only potential problem: the Level-Loc offers only mic-level or low-level medium-impedance output.  I like to run my mic preamps directly into the Lynx convertors; so for the most direct signal, I would need a bridging amp to bring up the level and lower the impedance of the Level-Loc.  It would be nice to have an output level control too, and I wanted the piece to be as physically small as possible so that it could sit directly next to the Level-Loc on a 2RU rack shelf.  Here’s what I did to solve all of these problems and fold the Level Loc into the studio alongside all the other outboard mic preamps.


Most of the RCA Receiving Tube manuals have a schematic for some sort of ‘audio input amplifier’; I wanted one that would provide about 20db of gain as well as a very low-impedance output so that I could drive a 15K:600 output transformer easily (I used an NOS UTC Ouncer to save space).  Based on this i selected the circuit above RCA manual # RC-24.  One 12AU7 tube.  Simple, easy.  The physically smallest plate/filament transformer that I had on hand was an NOS Stancor 120v:120V/6.3V, so I used the voltage-doubler B+ supply circuit as-found in the Altec 1566 and 438:  here is that schematic via Tangible Technology:

power_2b  For once i actually did not bother wiring up a DC filament supply, since the gain of the unit is pretty low.  This was the right choice, as i can hear no hum at all in the finished unit.  I added a 500K pot at the input jack and voila.  The whole thing fit inside one of those aluminum Bud Boxes that some folks use for DIY’ing guitar effects pedals.  I left the power transformer bolted outside the unit on the rear; it’s always a good idea to keep power transformers away from audio transformers if you can.   Here’s the interior of the unit:

…and below you can see the finished piece.  Not my finest piece of industrial design but it does the job.  I put the 1/4″ TS input on the front of the unit so that I can use it as a DI input for Keyboards ETC if the need arises.  The circular grill on the top surface of the piece is a heat-vent positioned directly above the 12AU7 tube.

The unit performs well, especially considering that there are only two stages of filtering in the power supply.  I always use four stages of filtering in the equipment that I build for customers, with at least one choke; I was curious this time to see how the basic Altec B+ scheme worked, though, and it seems just fine!  People love their 1566s and 438s so fukk it.  Good enough is sometimes good enough…

Above, the two units side-by side.  So how does it sound?  TW and I were putting down live drums on a track at GCR and here’s the result we got.

First, the drumbeat: close-mics only: CloseOnly

Here’s the same mix, but with the Level Loc signal added: in this case, the Level Loc was amplifying a figure-8 ribbon mic 20 feet from the kit, with the null of the mic facing the kit; the waveform was then re-aligned to eliminate some of the delay: withLevelLoc

And finally, the Level-loc signal only: LevelLocOnly

The Level-Loc is aptly named.  Regardless of what you put in – a baby’s breath or an atomic blast – you get the saaaaame level out.  Zero dynamics.  It’s pretty uncanny.  And a great sound for heavy rock drum beats.  This is the 2nd track that I have used it on in a week and I think it will continue to get a lot of use at the studio.   The output of the balancing amp is a little low – even with the input attenuator all the way open it cannot quite get to full level via the Lynx convertors.  It’s good enough, but it could stand to put out a few more DB.  If you build one of these devices for use with your Level-loc you might consider using a 15K:60K interstage transformer at the input to get a little bit more level out of it; or re-bias the two stages in order to use a 12AT7 instead of the 12AU7.

Altec 436 Compressor: Taming the output level: part 2

In a previous post, we looked at the Altec 436 vari-mu tube compressor.  I built one of these a few years ago, and it never really got used all that much because the output level is so hot.  The 436 is a very primitive compressor design, and it sounds awesome- but it was also built primarily for service is installed sound systems: industrial paging use, etc.  The stock 436 circuit adds a lot of level to your signal if you have the input level set high enough to actually cause significant compression.  I built an external attenuator box using a 600-ohm Daven T-pad attenuator and a UTC transformer to re-balance the signal, but this was not really an ideal solution.  The box was pretty big and heavy and I generally could not be bothered with taking it out and setting it up.

The solution came to me when I was examining the circuit of the Gates sta-level.   Here’s the schematic if you want to take a look.   Now, this may look a lot different than the Altec 436 schematic, but the differences are not too significant – other than the fact that the gates has a regulated power supply, the circuits function in the same way; the main difference is the particular types of tubes that are used.  Both are fully-balanced vari-mu compressors which are staged as (input transfo)-(attenuator pot)-(vari mu input amp)-(driver stage in Gates only)-(output amp)-(output signal rectified, timed,  and sent to grid of input stage to regulate input stage amplification)- (output transfo).

The Sta-level, however, has an output level control, whereas the Altc 436 does not.  So how do they implement this?

Pretty simply.  5 resistors and a normal linear taper pot give us an output loss that we can vary between 10 and 16db, while still maintaining a safe operating impedance.  In all fairness: the BEST way to do this would be to use a balanced H-pad variable attenuator, which would give us the ability to vary the output from NO loss to, say 20db or so; but balanced H-attenuators are crazy expensive and very large physically; too large to fit inside an altec 436 chassis, certainly.   Another option would be to use a variable T-pad after the output transformer, and then add an additional 600/600 transformer after the T-pad in order to re-balance the signal (there is a certain vintage vari-mu compressor that works this way, but i can’t seem to recall which; anyone?).  This solution is also not ideal from a cost and size perspective, although it would certainly be less expensive than the balanced H attenuator.

Anyhow, the major downsides to the ‘Gates-solution’ are: 10db loss is inevitable; output impedance will vary slightly with use of control; variation range is limited to 6 db span.  Well; i used my output modded 436 in a session yesterday, and for what its worth, here’s what I can tell you:  the minimum 10db loss is welcome – it put the 436 into the same basic operating range as my 1176 and Distressor; the impedance mismatch (into a Lynx Aurora) did not cause any audible problems that i could detect when used on guitars and drum machines; and the 6db control range was fine as well – i was dialing in levels and getting sounds into Pro Tools with no fuss.


Before I drilled a fresh hole into the face of my 436 clone, I built the intended circuit into a tiny outboard box.  When the design confirmed itself, I added the circuit into the 436 itself.  But about this little test-circuit box:  it’s lightweight enough that it can simply hang off the patchbay, supported by the patch cables themselves.  Now every vintage mic preamp that I have can be given variable output control quickly and easily.  This will allow me to dial in extra-gritty sounds using the preamp gain control (which is generally interstage rather than input or output), and then use this little device to get the level back down to an appropriate level to hit the convertor.

If you own a vintage Altec 436 or 438 and you find that you have to battle the high output level, I highly suggest that you give this modification a shot.  It’s very easy to simply build it into an outboard project box at first in order to see how you like it before you drill a hole in yr Altec.  And if you use any vintage tube gear in the studio: try making one of the little boxed-versions of the circuit.  It will really open up some new creative and sonic possibilities for the gear you already have.

Pro Audio hardware of the early 1950s

The General Electric (GE) BA-5-A Limiter

Continuing our review of the first two years of AUDIO magazine, today we will look at some of the more interesting bits of pro audio kit in evidence during 1954/1955.  AUDIO magazine had just made the transition to its new moniker in the wake of the introduction of the AES Journal (Audio Engineering Society), and for the moment, AUDIO sill covered a bit of the pro audio equipment that would soon largely leave its pages.

The GE BA-5 pictured above is, AFAIK, the largest and most complicated analog audio compressor ever made.  Although it has much less tubes, it’s kinda even more sophisticated than the Fairchild 660/670.    Here’s the schematic if you are interested.  From what i recall,  the BA-5 works by creating an ultra high frequency sidechain to obtain the control voltage value for the compression; I can’t recall the details at the moment but the basic concept was to allow the unit to have huge amounts of compression with very fast timings, but without any pumping or dipping artifacts.  Which was also the intent of the 660/670 design.  If anyone out there has a better explanation of this monster, please chime in.

The General Electric BA-6-B remote amplifer/mixer

The General Electric BA-9-A compressor, a much more basic pro audio compressor.  Circuit is essentially the same as the Gates Sta-Level.  The BA-9 is also known as the the uni-level; schematics are readily available online.

The General Electric BA-1-F plug-in preamp and BA-12-C plug-in power amp

The Hycor 4201 equalizer.  Similar to a Pultec program EQ but without the makeup gain amp; the Hycor is a fully passive device.

Langevin 5116 modular preamp

The full Langevin modular line of 1954: 5116 preamp, 5117 power amp, and 5206, 5208 power supplies

Another remote amp from 1954 – the Magnasync G-924.  Looks very cool.  Magnasync would soon be merged with the Moviola corporation and become a brand name for sound-for-film equipment.  See this previous post for an experiment with the Magnsync URS device.

An early ad (1955) for the Altec 604 duplex loudspeaker.  The 604 would remain a studio-standard recording/mixing monitor speaker well into the 1970s.

Okay this is getting pretty tech-y but here’s an advert announcing some new-ish tubes you might want to consider: the Tung-sol 12AX7 and the 5881 (AKA ruggedized 6L6).   Transistors were on the market at this point (1955) but were a ways off from reaching the performance and reliability that these great tubes offered.

Altec 436 Compressor: Taming the Output Level

Ah the classic Altec 436C compressor (see here for previous coverage on PS dot com).

Here’s a fresh high-res scan of the original product-sheet (2pp):

DOWNLOAD:  Altec_436C

I built one of these some years ago and it really sounds great.  I used UTC ouncer -series transformers.  Don’t be fooled by the tiny size – these are very good units.  In fact, Ouncers are used in the early Urei 1176 as well as UA-175 and 176 compressors.

You can see how super-simple the circuit is. Aside from using a conventional power-supply circuit (rather than the voltage doubler that the original unit uses), i built mine pretty much exactly the same as the schematic.

Now, if you look at the schem, you can see that there is no provision for an output control.  This is a problem because these things add a lot (like 20 db or more) level to whatever you put into them if you have the input control high enough such that the unit is actually compressing.  In the past I have gotten around this by using an outboard Daven H-pad attenuator that I mounted in a little box.  This is not ideal for ergonomic reasons.  Anyhow…  while studying the (very similar) Gates Sta-Level schematic the other day, I was intrigued by the very simple, very inexpensive variable balanced output pad that the Sta-Level uses.

It’s like $5 of parts. Five 1/2 watt resistors and one pot.   Based on information in the Sta-Level manual, adding this circuit after the output transformer of the 436C will provide a minimum 10db and a maximum 16db attenuation.  Perfect.  Now, it’s true that using this control will vary the effective output impedance of the unit slightly; but according to Gates, “This pad has been carefully tested to assure that the small impedance mismatch resulting from this range adjust-ment will not affect frequency response or other characteristics. ”   And Gates was writing this back in the day when the Sat-Level would almost certainly be seeing a 600ohm load.  Considering that nowadays it will more likely see a bridging load of 1500 – 10k ohms, I think it’s safe so say that this circuit should be (at least as) sonically-transparent (as a vacuum-tube vari-mu limiter can be).

Gonna dig up some matched 160 ohm resistors and give it a shot…  more to come…


Orban Audio Processors of the 1980s

Download the four-page circa 1984 Orban ‘condensed catalog’:

DOWNLOAD: OrbanCondCatalog

Photos and descriptions (no specs) of the following models: Orban 111B reverberation; 245F stereo synthesizer; 418A stereo compressor/limiter; 424A Gated compressor/limiter/De-Esser (i.e.,  ‘make-it-as-loud-as-the-FCC-will-allow’ box); 516EC 3-channel De-Esser; 526A De-esser; 622B 2-channel parametric equalizer; 672A equalizer; 674A stereo equalizer.

I have been using a 111B reverb for many years in the studio to augment the usual reverb plug-ins.  It is the best spring reverb unit I have come across.  I have found that it absolutely needs a little pre-delay applied, or else weird phase cancellation occurs when I sum the reverb output back into the board along with the direct signal.  There are may easy ways to do this, but it is worth noting.

Orban was founded three decades ago by Bob Orban.  They have always made audio equipment aimed at the radio broadcast market.  Prices for their classic 80’s gear were reasonable until recently.  Plenty of these out there, tho, so if yr patient and you will get a deal.

Since Orban is in the business of supplying broadcasters, their website have excellent technical support.  You can download the original manuals for all their products at this link on their site.