Tuesday, November 19, 2013

The EMP-351 - an Ampex 351 inspired dual microphone preamp scratch build

I realized today that it's been nearly a year since this blog has been updated, but that doesn't mean that Wombat Amps hasn't been busy!  On the contrary, we've now launched an improved web site, I've established a formal Wombat Amps workspace in a building that also houses dozens of bands -- as a consequence I've been doing a fair amount of repair work -- more than I had really intended actually.  Nevertheless, I have been working on some interesting projects, including a cool custom amp that was recently completed and a solid state equalizer.. But details of these just haven't made it to the blog for want of time.

However, I'm working on a new microphone preamplifier project that I'm really excited about, and I hope to document a fairly complete build log here.  This mic preamp was commissioned by Andrew Everding, perhaps best known as the keyboard player for the band Thursday. However, with Thursday currently on hiatus, Andrew has moved on to audio engineering, and is currently located in Auckland, New Zealand, where is an audio engineer working directly with Neil Finn (of Crowded House fame) at Neil's well-known Roundhead Studios.
In any case, Andrew is a big fan of tube microphone preamps based on modification of old Ampex reel-to-reel players.  As a tube aficionado, this makes perfect sense to me - in a world now flooded with sterile-sounding ultra-low distortion preamp choices, more and more musicians are realizing that a bit of distortion, of the right kind, can be just the ticket for producing a warm, full sound.

One of the preferred platforms for the mod community has been the amplifier from Ampex 351 reel-to-reel player, as shown above.  In fact, there's a fair amount of documentation online (especially at the Electric Audio forum, courtesy of Greg Norman), describing how to mod one of these units into a preamp.  So why not just mod one of these, and be done?  Well, it turns out that there are a number of good reasons.  First, the Ampex 351 is now highly sought after, and prices for these units have skyrocketed. You can easily pay $1300 or more for one on fleaBay, and what you get will be in uncertain condition. And this is before the mod process, which turns out to be not at all straightforward.  These were constructed on now crufty old first-generation PCBs that are prone to traces lifting from accumulated moisture, and are not particularly amenable to modification.  Also, there is a ton of other circuitry in there that gets in the way and is unneeded after the mod, you're for sure going to have to replace all the power supply capacitors anyway, etc., etc..  Even after you go through all the trouble, you're still going to be left with just a single preamp, not a dual. So you'd need to do this all twice to get a dual setup!  And this single would not really be optimised in terms of layout, ground scheme, noise, etc.  It also wouldn't provide modern amenities such as phantom power and fully regulated DC tube heaters, at least not without having to do a bunch of extra work.  But aside from all that, the Ampex is still a pretty damn good reel-to-reel player, and I just hate seeing beautiful old gear being canabilized in that way.   So the decision was made early on in the process to scratch-build from new components a dual preamp that stays true to its heritage but incorporates just the components necessary for a preamp.  In addition, it should incorporate features such as phantom power and regulated DC tube heater supply, and do it all in an attractive 3U rack-mountable package.  Here's the design brief:

  • Dual microphone preamp inspired by the Ampex 351 circuit;
  • Each section to combine elements of the Record and Repro boards of the original in a manner similar to the Electric Audio preamp mod;
  • Tube complement of each section will be 2 x 12AX7 and 1 x 12AU7;
  • Tubes provided will be New Old Stock (NOS) selected for low noise, gain, and balance;
  • Custom wound transformers audio transformers from Sowter UK;
  • Custom wound power transformer from Heyboer USA;
  • 3U rack-mountable enclosure with laser-engraved faceplate;
  • Black anodized aluminum contrasting with large high-quality brushed aluminum control knobs;
  • Regulated 12.6VDC filament heat for all tubes;
  • Tube rectifier replaced with silicon rectifier;
  • Switchable between 120V/60Hz and 230Hz/50Hz power;
  • Regulated 48VDC phantom power;
  • Switchable -20 dB input attenuation on the XLR mic input;
  • A separate 1/4" stereo jack line level input with fixed -30 dB attenuation;
  • A front panel Mic/Line selector switch;
  • A mono 1/4" instrument input directly to the input tube grids;
  • Phase switch;
  • Variable output attenuation using a  Bourns 600Ω T-Pad Attenuator;
  • Bypass switch for disengaging output attenuation;
  • Primary gain control and gain trim adjustment;
  • Ruggedized turret board construction for main amplifier boards;
  • Custom hand-etched PCB for phantom and filament power supply;
  • The unit will be equipped with an internal EMI/RF line power filter;
  •  Power supply features 105deg-rated low Z electrolytic capacitors for heat resistance and longevity;
  • Signal capacitors are high-quality film types in the nF range, and silver mica in the pF range;
  • Audio path wiring is aircraft-grade solid core 20AWG with PTFE insulation, shielded where necessary;
  • Controls for gain and gain trim will utilize sealed 2W milspec potentiometers;
  • 750V rated 2W metal film resistors will be used for low noise and durability;
  • Where accurate balance is required for best CMRR, 0.1% precision resistors will be used;
  • The ground scheme will be optimised for low noise, and will adhere to the AES48 standard to avoid the "Pin 1 problem";
We'll discuss the circuit itself in another post, coming soon! Please follow along as this project comes together.

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