Monday, March 25, 2024




I was dead wrong about this piece of gear. When I first saw pictures of the Warm Audio WA-MPX preamplifier I was put off by some of the obvious marketing moves. In this review we'll be looking at the single channel model of this vacuum tube powered microphone and instrument preamplifier.


The big VU meter on the front is actually the same budget meter they use in the WA-76 compressor with a black plastic shroud to make it look like an ancient Ampex component. It has a switch controlled function called “Tape Saturation” but no recording tape is involved (so why call it tape?). 

If these things bothered you at all, the only thing I have to say after buying and trying the WA-MPX is put any preconceived ideas away because this piece of hardware rocks!

The WA-MPX has a big, bold and authoritative “clean” sound when you set it for low-distortion. I did A-B comparisons with preamps by Neve, Grace, Locomotive Audio and the Warm Audio WA-73. I was mainly using it on vocals and it proved to be a good source of preampfification with BIG tone. 

The Weight Tank WT-72 by Locomotive Audio is a favorite vacuum tube microphone preamp here. Time will tell but at this point, I feel like the WA-MPX is capable of the big tube preamp tone I associate with the Weight Tank and I'm a fan of the output meter on the MPX.

Honestly, I expected the WA-MPX to sound a bit kazoo-like but I was very wrong; the WA-MPX preamp is powerful, easy to use and flexible; fully capable of clean reproduction with low distortion. 


Like all Warm Audio electronics the WA-MPX has a sturdy-feeling, formed steel body and chassis. The aluminum front panel has a brilliant polished chrome look, two big knobs are the primary user controls for INPUT and OUTPUT level. Eight chunky two-position toggle switches provide additional control options and a ninth toggle switch turns the AC power on and off. A small three position rotary switch selects from instrument, microphone or line input. A front panel mounted 1/4” phone jack is provided for unbalanced instrument input.

MPX rear inputs/outputs, XLR & parallel TRS connectors

The signal path through the WA-MPX features a Mu-metal shielded, transformer-coupled input stage, three tubes (two 12AX7, one 12AU7) and a hefty transformer-coupled output stage. Both of the transformers are made in the USA by Cinemag. The internal power supply transformer is also quite beefy and offers selection between 115 and 230 volt operation. All the front panel mounted toggle switches have a nice positive click and feel very solid. The input and output level control knobs are smooth in operation.


After unboxing the WA-MPX I rack mounted it and gave it a full day powered ON to allow the three internal vacuum tubes plenty of time to “burn in”. For voice testing I went right to my favorite tube microphone, an Oliver Archut U47 clone. When I brought up the output level control on the WA-MPX and heard the clean output character for the first time I got a great big smile on my face. The output sound is BIG, bold, solid, full and without the slightest hint of distortion or fuzz. 

After getting fully acclimated to the clean sound of the WA-MPX I engaged the extra gain switch which boosts gain to 90dB (that's a lot of gain). Even with the Gain knob up at 3 o'clock position and the output level about the same position the tone remained robust and clear. Activating the “Tape Sat” switch finally brought the serious distortion effect I had expected since I first saw this product. 


With the newly found “crunch” tone of the MPX I had to break out an SE ribbon mic and try some vocals with that. This was also a good opportunity to test the high-pass and low-pass filters in the MPX. With a little crunch dialed in and the low-pass filter engaged (high frequency rolled off above 2kHz) the resulting sound was a very ruff vocal, reminded me immediately of listening to old Robert Johnson era tracks. 

Truth be told I'm not a fan of the distorted vocal effect but if I ever need it I know where in my kit to turn. Fortunately the WA-MPX delivers Big clear reproduction of which I'm a huge fan. 


  • Great clean audio reproduction
  • Excellent crunchy vocal effects
  • Easy to use
  • Controls are easy to read and set
  • Process microphone and line-level signals through preamp tubes
  • Solid construction with in-built power supply
  • Big VU meter reads output level
  • Uses modern and available vacuum tubes
  • Attractive front panel in the Style of Vintage Ampex 350/351


  • Why “Tape Sat”? Why not just “Saturation”? There is NO TAPE!

I am tired of gadgets being marketed as some form of “Tape” sound when in fact they have nothing to do with any actual recording tape. I am a child and student of the “recording tape era” and I'm not lusting or questing after a return to slow rewind/fast-forward waits, head bump, demagnitizing/cleaning heads and all the other problems and maintenance tasks associated with tape. If my tape recorder sounded like the crunchy “tape sat” sound of the MPX, I'd take it in for repair. 

TAPE SOUND HINT: If you have a recording which you know, in-your-heart, will definitely improve with a pass through real tape recording process, contact “Welcome to 1979” in Nashville about their service in which they actually process your tracks on well maintained, MCI tape recorders. HoT has no affiliation with Welcome to 1979.


In spite of my griping I've actually grown quite fond of the WA-MPX since I started using it. This IS a fun and useful box of highly adjustable recording tone which continues to impress me every time I fire it up. The WA-MPX complements FET based microphones, dynamic microphones, ribbon mics and even tube-driven large-diaphragm condenser models. Depending on what microphone you plug into the WA-MPX be sure to try both of the transformer-impedance tone selections because every now and then you'll find a mic that benefits greatly from one position or the other (like my vintage Shure ribbon mic).

Whatever YOU DO, don't do what I did, which was to jump to a false conclusion about the tonal characteristics and useful processing of the WA-MPX before trying it and hearing it in action. All distortion effects aside, this preamplifier can deliver a wonderful big, full range clean sound. The WA-MPX is not inherently noisy and it has many unique tonal modifiers available to the adventurous sonic explorer. I'm not sure I need the stereo model of the WA-MPX for processing full mixes through....but I might ;-)

Thanks for reading High on Technology, Good Music To You!

©March 2024 by Mark King, it's not ok to copy or quote without written permission.


I've been recording since the 1950's but the old Ampex tape deck preamp sections were never on my radar (too expensive & too rare). I learned to repair television sets when I was 12 and that experience fueled my distaste for working on old tube gear. Various people sent me photos so I could remind everyone of what the “good ole days” looked like. Enjoy this vintage Ampex walk down memory lane.

With the WA-MPX Warm Audio has delivered an affordable and reliable taste of old tube preamp technology without all the hassle of chasing down discontinued vacuum tubes, odd connectors and weird rectifier parts. 

It's only a matter of time before someone is willing to pay enough and scarf up the Ampex brand so they can market more pro audio stuff to mimic the good ole days of "tape".