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.

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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:

  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.