You are here

Warm Audio WA-MPX & WA-2MPX

Valve Microphone Preamplifiers By Bob Thomas
Published April 2023

Warm Audio WA-MPX & WA-2MPX

Countless hits were recorded using Ampex tape recorders and their characterful tube input stages. These preamps promise that legendary sound in a convenient format.

Warm Audio’s latest releases, the WA‑MPX single‑channel tube microphone preamplifier and its two‑channel companion the WA‑2MPX, are inspired by an Ampex tube‑based microphone preamplifier that, in its various incarnations, contributed to the sonic signature of a significant number of iconic 1950s and 1960s studio recordings, in particular from the United States.

From the late 1940s into the early 1960s, the California‑based Ampex Electric and Manufacturing Company designed and manufactured professional tape recorders, with tube‑based audio electronics. In 1950, Ampex introduced the Model 300 and continued to develop its tube‑based preamplifiers over the years, introducing the 350 in 1953 and the 351 in 1958. The 351, whose influence is evident in the external appearance and internal circuitry of Warm Audio’s MPX and 2MPX, was a very different beast to its predecessor, in that PCBs replaced point to point wiring and 12AX7 and 12AT7 tubes displaced the older unit’s metal envelope tubes. In addition, the 351 was fitted with an internal power supply, as opposed to the large external version that powered the 350, saving both weight and cost.

As these tube‑based Ampex audio recorders were scrapped in favour of their solid‑state replacements, a number of studio engineers began to use the 300, 350 and 351 preamplifiers as standalone mic preamps — a trend that was repeated later with, for example, microphone preamplifiers from the Neve desks that had lost out to SSL consoles. However, unlike Neve preamps, which are relatively easy to reconfigure or reproduce, reconfiguring the Ampex preamps requires a more than passing knowledge of tube electronics and a healthy respect for potentially lethal voltages. Thus, these units have remained scarce and expensive, and have flown under the RADAR of many musicians and engineers.

Outside & In

With their 2U silver front panels and black graphics, black control knobs and black VU meter housings, both the MPX and the 2MPX certainly look the Ampex part. However, the presence of the MPX’s bank of eight chunky toggle switches (seven for control, plus power on/off) and the 2MPX’s line of 15 (two sets of seven control switches, separated by power on/off in the centre), all of which have individual LEDs that illuminate when a function is active, indicate that these units are much more than mere recreations. Across the two units, the channel controls, I/O connectors and metering are identical. On the MPX, there is plenty of room on the front panel between controls. And, despite the doubling of population density on the 2MPX, reducing the size of both meters and control knobs has allowed Warm Audio to retain a similar feeling of space on that unit too.

There are lots of toggle switches on the front panels, and most control their associated functions using relays on the inside.There are lots of toggle switches on the front panels, and most control their associated functions using relays on the inside.

Controlling the signal path through the channel, a three‑position rotary switch selects between the XLR balanced microphone, TRS jack balanced line and the unbalanced front‑panel instrument inputs. A Preamplifier Gain control, scaled from 0‑90, sets the amount of tube gain applied to the input signal, and an unscaled Output Level control, acting as a 600Ω output impedance attenuator, adjusts the level of the signal leaving the channel and gives you the ability to increase preamp gain to saturate the signal without overloading the input of the next device in your signal chain.

All other front‑panel channel functions are switched, with the actual switching being carried out by relays on the channel PCB. 48V phantom power and polarity are present as you’d expect, with gently‑sloping high‑pass and low‑pass filters, whose corner frequencies at 80Hz and 2kHz respectively are welcome, and unexpected, additions. The remaining three switches deliver facilities that I don’t recall having come across previously on a studio microphone preamp. The first of these is the High Gain switch that brings in 20dB of clean gain, increasing the maximum gain available for low‑output microphones and low‑level sources from 70 to 90 dB. Then there’s the Tone switch, which reconfigures the input step‑up transformer from a 1:9 turns ratio to a higher 1:18 ratio, decreasing the channel input impedance from 600Ω to 150Ω in the process. Finally, there’s Tape Sat, which inserts an emulation of magnetic tape saturation.

The PCBs, components, layout and construction of the hand‑assembled MPX and 2MPX are all of a very high standard with not an integrated circuit in sight. The channel cards (one in the MPX and two in the 2MPX) are all identical, their major landmarks being the seven 12V relays that are activated by the front‑panel switches, the three tubes (each in individual screening cans), and the input and output Cinemag transformers, which have been custom‑wound to the original vintage specifications. Because of where the input transformer sits on the PCB I couldn’t make out its model number, but the output transformer is a CM‑351BPC, the 351 denoting the fact that this transformer has the extra winding, first seen on the Ampex 351, which feeds the negative feedback loop that helps to reduce distortion in the 12AU7 push‑pull output stage. On both the MPX and the 2MPX, toroidal mains transformers feed chunky analogue power supplies, whose main task is to deliver a 300V B+ supply to the three tubes.

The rear‑panel audio I/O are provided on both XLR and TRS, to make connection to your studio easy using your existing cables.The rear‑panel audio I/O are provided on both XLR and TRS, to make connection to your studio easy using your existing cables.

Transformers, Tubes And... Tape?

The skeleton of the channel signal path is a familiar one: from input transformer to preamplifier, then phase inverter to push‑pull amplifier to output transformer. The step‑up microphone input transformer is a little unusual in that activating the Tone function switches its input impedance and secondary winding ratio from 600Ω/1:9 to 150Ω/1:18. The 150Ω/1:18 setting approximates to the microphone input transformer setup in the original 351 (150Ω/1:15). Activating the Tone function also increases the step‑up gain through the input transformer from 17dB at the 1:9 ratio to 23dB at the 1:18 setting.

As in the 351, the balanced line input signal runs through a resistor network that pads it down to microphone level, before it reaches the microphone input transformer. This arrangement, which routes the line signal through the microphone preamplifier and its high‑ratio input transformer was, apparently, not always considered optimum in a sonic sense by recording engineers back in the day. This appears to have led to many major studios either bypassing the 351’s mic preamp when using the balanced line input or using the 351’s unbalanced line input, which bypassed the microphone transformer. The front‑panel instrument input, which is situated where the original 351’s headphone jack sat, is the MPX’s equivalent to that unbalanced input.

All the tubes in the signal path (two 12AX7s and one 12AU7) are twin triodes, which indicates that they are each made up of two identical triodes sitting in the same glass envelope. The first 12AX7 acts as the preamplifier stage, the second is the first tube in the output stage. The latter’s two triodes make up the phase inverter that drives the 12AU7, whose triodes are configured as the push‑pull output stage. In normal operation, only one triode in the first 12AX7 is active. Engaging High Gain brings that tube’s second triode into circuit, resulting in an extra 20dB of clean gain, which increases the maximum gain available through the MPX for low‑output microphones and low‑level sources from 70 to 90 dB.

I must confess to a wry smile when I first saw the Tape Sat switch on the front panel. But the more that I think about it, the more I like having that function in a unit based on a 1950s original, since the effect of recording audio on magnetic tape is a major component of the sound of tracks from that era. The all‑discrete, all‑analogue combination of transistors, diodes and resistors that delivers this effect has been designed to emulate the slight compression, increased harmonic distortion and the filtering out of the lower sub‑bass and upper treble frequencies that you’d get from magnetic tape saturation in a tape recorder running at 15ips (inches per second), plus a few extra dB of gain.

The 2MPX added a sense of warmth, weight and density to produce a clean yet characterful performance that gave my B&O BM5 an additional dimension in its sound that I hadn’t heard before.

In Use

What you’ll really notice when you switch on an MPX (or a 2MPX), other than the VU meters’ comforting yellow‑orange glow, is the very low noise floor, even with gain and output levels at maximum. This obviously reflects a very careful approach to design and build. To start with, I went old‑school, plugging my Bang & Olufsen BM5 stereo ribbon microphone straight into both channels of the 2MPX, with High Gain engaged to give me the 150Ω/1:15 combination that would have been used back in the day with ribbon mics. The result was, to my ears, absolutely stunning. The 2MPX is so intuitively controllable, and it added a sense of warmth, weight and density (thanks to those transformers and tubes) to produce a clean yet characterful performance that gave my B&O BM5 an additional dimension in its sound that I hadn’t heard before.

Increasing the preamplifier gain and reducing the output level allowed me to experiment with driving the BM5‑2MPX combination into distortion. Fun though that was, it didn’t sound that great, and I much preferred the character of the distortion that became available when I replaced the BM5 with my rather ancient dynamic microphones, a Sennheiser MD421 and a Shure SM57. Backing off the gain while keeping the High Gain in circuit cleaned up the distortion and coaxed great‑sounding, characterful performances from those two microphones, and allowed me to add in all the grit, girth and density that I could wish for.

Switching out the High Gain on the 2MPX to go back to the default 600Ω/1:9, I brought my AKG C414‑ULS pair into play. As with the stereo BM5, I often partner these microphones with a Pendulum SPS1 dual‑channel microphone preamplifier for stereo and Mid‑Sides recording, when I want their linearity and neutrality to shine through unsullied. Using the 2MPX and aiming for maximum clarity and fidelity, those innate characteristics are still all there, although the colour and character that the MPX and 2MPX can add to their overall response can become rather addictive.

It was a similar story with the line inputs, which I tried using the 600Ω/1:9 transformer setting. The sense of warmth and weight that the 2MPX could add to the stereo stems that I experimented with really spoke to me. Having cut my engineering teeth on an early Neve console and a Studer A80, I’d have absolutely no qualms about using an MPX or a 2MPX, with the Tape Sat engaged across a track or bus just to add a touch (or more) of vintage colour.

The instrument input, which is actually switched via a relay, was a slightly different story, since the only front‑panel function that affects it is Tape Sat. However, the onboard preamp on my Godin was also perfectly capable of driving the balanced line input unbalanced, so I could access all the MPX’s functionality — and I can report that the MPX offers rather splendid distortion opportunities to the discerning musician.

However, all this is only scratching the surface of what the MPX circuitry can do. Once you get a feel for the different input transformer impedance/turn ratios affect your microphones, line‑level signals and instruments, and also how to manipulate the gain inside the MPX, you’ll find yourself with a vast, intuitive palette of character and colour at your disposal. One slight drawback, which will be of interest to some more than others, is that the rotary controls are not detented and their scaling is rather coarse, so recalling a particular setting is going to require a recall sheet, a photograph or a black chinagraph pencil.


The Warm Audio MPX and 2MPX are, to my mind, superb and rather special microphone preamplifiers. Unless you’re really into the sonic signature of old‑school recording, neither are likely to be your ‘one and only’ mic/line preamp. But of course most of us already have capable ‘vanilla’ preamps on our interfaces, and when you’re looking for a way to enhance the sound of a source, either as you’re recording it or after the fact, be it drums, bass, vocals, keyboards, synths, brass or whatever, the MPX and 2MPX both offer immediately accessible, intuitive and very controllable means of adding character, warmth and weight. You can drive their input transformers hard, slam an extra 20dB into their phase inverters to ensure that the push‑pull 12AU7 output tubes saturate their output transformers, and still keep the signal going into the next stage of your chain at the level it needs to be. What you do and what you get is down to your taste, creativity and ears.

While the circuits are the same, I’d personally like to have both of these units in my studio! If I had to choose between them, my heart would take a pair of the single‑channel MPX over the 2MPX, largely because they look fabulous together! But my head would point out that I don’t have that much free rack space, which makes the 2MPX perhaps a more realistic choice. Given the prices of restored original 351 preamps these days, I reckon the MPX and the 2MPX both offer exceptional value for money. If you’re searching for a preamp with a vintage vibe or for a preamp that can add colour and character to any signal passing through it, then you absolutely have to audition Warm Audio’s MPX and 2MPX. At least one of these review units won’t be making its way back to Warm Audio!  


Other than tracking down a rebuilt original 351, the only hardware alternative to the MPX I could find is the EC3, from Austin‑based Electric & Company. That device appears to be, essentially, a 351 clone. However, neither an original 351 nor the EC3 have tone‑shaping abilities that come close to those offered by Warm Audio’s MPX and 2MPX.


  • Capable of adding a wide range of character and colour to any source.
  • Getting great results is simple and intuitive.
  • Exceptional value for money.
  • Very cool cosmetics.


  • None.


Superb recreations and expansions of the signature sound of an iconic 1950s microphone preamplifier — the MPX and 2MPX deliver so much more than just a ‘vintage tube sound’.


WA‑MPX £759. WA‑2MPX £1329. Prices include VAT.

WA‑MPX $749. WA‑2MPX $1399.

Sweetwater Affiliate logo 14px