Warm Audio WA12 Microphone Preamp Review

Warm Audio

 “I have to commend Bryce and Warm for bringing such a great sounding pre to market for such a modest price.  I have to wonder how they can make much doing it though, and suggest you pick up a pair before they realize they could easily sell these for more.”


With the significant increase in quality of on-board mic preamps in mid-range audio interfaces (thanks to a new breed of ultra clean mic preamp IC) there has been much discussion about more colored pres that might complement them for flavor.  The usual suspects are of course the pedigree preamps that are commonly mentioned on gearslutz and similar boards, but that almost always comes with a price tag that’s out of reach for most hobbyists and small studios on a budget.  Pres like the GAP73, which is based on the highly revered Neve 1272 circuit are a popular alternative, and lend themselves to several popular mods that can bring the sound quality in line with the big boys.  When I heard about the WA12 from a new company called Warm, I naturally assumed this was the equivalent of the GAP, but based around the API 312 circuit instead.  After getting my hands on a pair I am prepared to amend that notion.

At first glance, the WA12 looks exactly like I had imagined.  A low cost alternative based on the API 312 design.  There’s much more to it than that however.  It’s clear that Warm is trying to tell us something, not just with the company name but with the bright orange faceplate the WA12 is sporting.  This is a pre designed to warm your DAW tracks up.  “straight wire” preamp designs like those found on boxes like the Ensemble, Apollo, and UFX are great for capturing the nuances of a great instrument and performance, but left alone they can end up sounding clinical, even un-exciting.   Warming up your tracks is the precise reason you’d reach for an alternative like the WA12.  As is my nature, I decided to pop the top of one of the WA12s and see what’s going on inside these boxes.

A few simple screws were all that separated my prying eyes from the inside of a very robust steel chassis, which yielded quite a bit of unused space inside.  I am guessing that Warm saved a few bucks by going with an off-the-shelf chassis.  No problem there.  Inside I found three cleanly designed circuit boards, and a pair of Cinemag transformers quite neatly wired together.  A main control circuit board contains the high-z input, the preamp’s mode selectors, and a gain pot.  I would have preferred to see a gain switch here, but the stepped pot is smooth and makes repeating gain settings nearly as easy (another great cost saver).  There are 6 selector buttons, each which has its own LED to make assessing the selected mode idiot proof (a nice touch).  They control the input (mic or instrument), 48V phantom power, 20db pad, output polarity (nice to see in a budget box), tone selection, and the unit’s power.  The layout is clean and ergonomic.  The rear XLR mic input connects directly to this board, and is routed through the input transformer.  Apparently the high-z input gets routed through the transformer too, so this makes the WA12 a great guitar or bass DI when you want to fatten things up a bit.  The op-amp is based on the Melcor 1731, which is a vintage (pre 2520) gain block.  That was my first hint that this pre might sound different than most other API-style pres on the market.  It is laid out neatly on its own circuit board, and is not potted like the interchangeable gain blocks I’m used to seeing.  No problem with that.  Not only will this allow the transistors to keep cool, it also makes the gain block serviceable.  The output transformer, and balanced output jacks (xlr and trs) are connected directly to the gain board and everything is securely mounted.  Lastly, there is a neat power supply for converting the 24V AC input into +/- 18V for the gain section, and 48V phantom power for your active mics.  I would have preferred to see Panasonic or Nichicon filter caps instead of the Chinese ones used here, but the ones Warm chose are rated to 105c, are of adequate size and (best of all) easily accessible should they fail and need to be replaced.  That shouldn’t happen for a long time anyway.

The 312 mic pre design is popular because of its simplicity.  It is essentially a high-quality discrete gain block coupled to an input and output transformer with a power supply.  This means that what you hear is highly influenced by those components.  That’s why I’m glad Warm chose to use Cinemag transformers, unlike many other companies who choose to have theirs wound by Chinese contractors.  What you’ll be guaranteed of is both consistency and quality.  Cinemag is one of the best audio transformer makers in business today, and it is certainly a surprise that the WA12 ships as equipped.  This might make modders unhappy.  There’s not a lot to upgrade here.  While certainly compromises had to be made to hit the price point the WA12 ships at, it seems Warm have chosen them well and while you may notice a few you certainly won’t hear them.  Attention to detail is excellent, and high-grade components have been used where they make a difference.  Already the WA12 is looking like more than an API styled GAP (no offense to the GAP73, which is a great product at its price point).

I already have a 4-pack of API style pres, equipped with original transformers and a pair of both new and old genuine 2520 gain blocks.  I was expecting the WA12 to sound very familiar, and somewhat redundant given that circumstance, if not quite as rich.  After all, why do I need a budget 312 style mic amp when I have 4 high dollar ones that sound killer right here?  This is where things get interesting.  The WA12 shares its design heritage with those pres, but in use its sonic identity is as unique as its bright orange faceplate.  While my high-dollar 312-style pres are very aggressive and tight, the WA12 is much creamier and smooth.  I’d still categorize it as a mid forward pre, but its much less obnoxious about it.  A quick email to Bryce, the preamp’s creator and owner of Warm, shed a bit of light on this.  He deliberately decided not to have his transformers wound according to API’s original specifications.  Both the input and output transformer are his custom design.  He tells me he wanted the pre to be a little more colored that the rest of its 312 based cousins out there, and in fact Cinemag warned him that this would be at the expense of linearity.  After all, the point of this box is for warming up tracks before they hit your DAW.  Color is what the WA12 is all about.  Well, mission accomplished Bryce.  The WA12 is indeed very colored (in a glorious way) and gently imparts a very light rounding and subtle euphonic distortion of transients that’s instant music to my ears.  There is certainly an overlap of sources that would sound good through both the WA12 and my other 312 pres, but there are quite a few instances where the WA12 would be my first choice and the others would not.  I never liked my 312s for vocals, but the WA12 works great for this task.  The sound almost feels like some compression has been added, without an appreciable reduction in dynamics.  I would put its character somewhere between a traditional API style preamp, and a Neve 1272.  It’s really quite polished and quite frankly is much better sounding than I expected it to be, especially at this price point.  I don’t think I’ll be sending mine back!  The WA12 wouldn’t be my choice for fast, transient sources like acoustic guitar or mandolin, but that’s a task for my interface pres.  The WA12 sounds expensive.

There is a ton of gain on this pre so rest assured you can use your low output dynamic mics without fear, and while the design is quite vintage its noise floor is not.  That should make your SM7 happy!  There is a tone button on the front, which switches the wiring of the input transformer primaries from series to parallel.  This gives you another 6 dB of gain from the increased voltage step-up while dropping the input impedance from 600 ohms to 150 ohms.  This is likely to have a more profound difference on low output dynamic and ribbon mics, but essentially this works like a gentle high frequency roll-off in most cases.  This is highly dependent on the mic used, and may not be a great match for ribbon mics that have a higher output impedance but in many cases it may be just the thing.  Particularly harsh or shrill sources might find the lower impedance tames the highs more gracefully than EQ can.  Plus, with the button engaged, there is a whopping 71 dB of gain from this box!  You won’t be running out of gain here.  Also, +/- 18V rails ensure you will likely be limited by the next device in the chain rather than the headroom of the WA12.  There is more than enough usable headroom here.

There are a few things I wish the WA12 had, like built in metering, a high pass filter, and an output trim but it’s likely that these features would be redundant in a DAW environment and would have added to the cost more than they would have added to the usefulness of the pre.  I was also bummed to see a wall-wart included, but coupled with the adequate power supply filtering and regulation it’s more of an ergonomics complaint than a performance one.  A toroid would have added cost and complication to a well thought out design.

It turns out that this little box’s biggest secret isn’t what it is so much as what it isn’t.  It is absolutely a high-quality transformer-coupled discrete mic pre that sells for the price of a budget IC based design.  What it isn’t, however, is another API 312 clone in a sea of such mic pres, limited to the same specialized tasks the 312 design has become famous for.  The WA12s will sit proudly along side Neves, APIs, Hardy’s, and more, and even in such fine company these little guys will get used.  I have to commend Bryce and Warm for bringing such a great sounding pre to market for such a modest price.  I have to wonder how they can make much doing it though, and suggest you pick up a pair before they realize they could easily sell these for more.

The Melcor 1731 op-amp predates the well-known API 2520 and offers a similar but more vintage vibe than its later cousin. WARM AUDIO, WARM WA-12, WARM WA12

The Melcor 1731 op-amp predates the well-known API 2520 and offers a similar but more vintage vibe than its later cousin.

High-quality US-made Cinemag transformers are one of the highlights of this box. WARM AUDIO, WARM WA-12, WARM WA12

High-quality US-made Cinemag transformers are one of the highlights of this box.

Although there is an external wall-wart transformer, the WA12's internally-regulated power supply is more than adequate. WARM AUDIO, WARM WA-12, WARM WA12

Although there is an external wall-wart transformer, the WA12’s internally-regulated power supply is more than adequate.

This main board provides routing of mic/line input as well as gain and all of the switchable functions of the WA12. WARM AUDIO, WARM WA-12, WARM WA12

This main board provides routing of mic/line input as well as gain and all of the switchable functions of the WA12.








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  1. Martin Butler

    Yo Bro, great to see you somewhere other than Gearslutz ! This is a wonderful piece, and I so appreciate your technical insights. I’m almost finished doing something similar, but completely from another angle to do with this preamp. I’m recording tracks, with and without the WA12, and will post results next weekend, with an accompanying video. I think it’ll make a great companion piece to your very well written article.

    What a great beginning for what I hope becomes a regular stop during my online musical adventures.

    All the best,
    Martin John Butler
    ( MJB )

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