Technical data:

Type: tube/transistor class-D
Height: 1 hu
Power: 400 Watts/4 Ohms
Damping: >2000/1 kHz/4 Ohms
Tube: 12AX7
Inputs: active/passive
Tone control: 5 band active
Protection: short circuit,open output,DC-protection output, temperature protection transformer, mismatch, clip, delayed switch
Divers: mute-out,headphone output, mixable effect loop with footswitch, symmetrical line-out with pre-post switch
Output: 2x Speakon
Weight: 8,5 kgs

The Hevos 400D is an exciting professional bass amplifier with a tube preamp and a MOSFET class-D power amp. It’s feature-rich and well thought out, yet the visual styling is elegant and clean. Components are of the highest quality, including double-sided PCBs, MKP capacitors, top-drawer resistors, Neutrik connectors, and dustproof pots.

The 400D is only one rack unit high (1.75″), making it the slimmest professional bass amp. Yet a good portion of the 8.5 kg (18.7 lbs) weight is from the heavy power supply transformer, which is a breakthrough with its low-profile ultra-flat configuration. This transformer provides very stable power for the tube preamp and the power amp, and has built-in thermal protection.

The double triode 12AX7 tube in the preamp is fed with DC current to avoid hum, with a built-in startup delay on the anode current for longer tube life. The 400D is designed to reproduce your instrument cleanly and accurately with the EQ set flat, but the active 5-band EQ can compensate for difficult acoustic environments. The EQ controls all increase in effect as they are moved farther from neutral. This brilliant feature provides finer control near the center, and greater effect near the extremes.

The power amplifier is a class-D (PWM) type designed with Philips UcD technology, resulting in an efficiency of more than 90%. The amplifier produces hardly any heat, so there’s no cooling fan or even external fins. You can easily use the 400D in a studio environment because there is no fan noise. Read more on “Bassics”.

As a professional piece of equipment, the power amp in the 400D is protected against short circuit, clipping at the output, and DC. If clipping or DC at the output occurs, the amplifier turns off automatically. And there is absolutely no pop or thump when turning the unit on or off!

The 400D amp has very low total harmonic distortion of  <0,1% at 1 kHz and 400 Watt, and an very high internal damping of 2000. This makes it an extremely stable amplifier.

German Gitarre & Bass wrote about the 400D:

  ” Both a real tube preamp and 400 Watts power in an amplifier of only 1hu height: Dutch company Hevos has built the flattest professional bass amp. All the more remarkable is that the 400D has a grown up weight of 8 kgs, which points at a healthy power supply with a heavy transformer. The ultra flat transformer is again breaking a world record, and that is for shure not the last interesting detail of the Hevos bass amplifier.

The Dutch brand is known for its excellent studio quality gear, and one can expect the same of the 400D without hesitation. The tube preamp does not give a over-accentuated warm or even distort tube sound, but has completely placed itself in the service of a flawless, tight and balanced bass sound. Here studio quality is the target, which is emphasized by the practical absence of any noise. Because the amp should also produce a impeccable pure sound at sensitive studio use, the preamp tube is heated with DC current, avoiding hum. In this case it is also an advantage that the 400D lacks a cooling fan.

The sound of the amplifier stays perfect and without distortion, even at fully opened volume. The bass sound is reproduced detailed and nice transparant, though its absolutely out of the question that the sound should becomes technically “cool”. Beside the highly detailed sound there is always a warm fundamental tone. The Hevos amp manages to embed finest details in a round total sound; the result sounds with lively dynamics  completely relaxed and unconstrained.

In case of a wish to deviate from the thorough bass sound, the 5 band tone control offers very good possibilities. A greasy bass foundation, but also the right amount of pushing punch bass is made possible by the low knob. The low-mid knob seizes good and deep upon rock growls, and also the mid control supports a straight powerful sound. Presence can be dosed in a subtle way with the mid-high, when the high knob bothers about brilliance and sparkling harmonics without getting sharp. As far as the power is concerned, the class-d amplifier convinces by a superiour development of power without roughness or other peculiarities, guaranteing the characteristical sound of the intrument even at high power levels.

Beyond all doubt is the flawless and transparant sound of studio quality delivered by the hybrid amplifier, even at very high volumes. Moreover, the 400D manages to create this translucent and detailed sound reproduction by no means in a cool, but rather a well balanced warm way. These are real top qualities!”

Review of German magazine Bass Professor  about the 400D:

  ” Hevos 400D is going more for practical use and decent looks than for all sorts of frills that are useless for everyone. The quality of the construction is very high, just like the quality of the used components: a typical Hevos product. In the sober manual it says: ” The amplifier has been built in such a way that in normal acoustic circumstances there will be no need to use the tone control “.  This sentence patently obvious brings forward the philosophy of the Dutch company : design a bass amplifier which sounds well even without  adjusting the sound endlessly, and which puts forward the sound qualities of the used instrument. This claim is fulfilled by the hybrid bass amp without any pain: super tight in the low, lineair in the mid and detailed in the high registers.

The used preamp tume makes that the sound does not get too clean, although the contribution of the tube in the overall picture is only noticable in a subtle way. The Hevos has hardly anything in common with the cliché sound of all tube bass amps (Ampeg, Bassman etc.).  With the 400D it is rather more about a detailed hifi sound with a slight tube colouring. Nevertheless, if one needs the tone control some day, it offers you an effective tool to influence the sound or to attack acoustic problems. Frequencies of the vife band tone control are chosen well, but be careful: the more you turn the potmeters from the center point, the more sensible they react. When it comes to sound pressure, the 400D cuts a good figure, becaus e even at very high sound pressure levels the sound  stays tight and well defined.

Absolutely top class is the possibility to connect 2 basses at the input. On stage two basses are at ones disposal without having to insert or pull out any cable. The control of the effect loop is also interesting: the signal of the inserted effect can be mixed continuously with the ‘dry’ bass signal.

The Dutch amp maker of the 400D has put on the market a pure-blooded hifi bass amplifier. The 400D does not places its own sound in the foreground, but conversely emphasizes the sound qualities of the used instrument. The hybrid construction with the tube preamp supplies a light ‘warming up’ of the sound and prevents the sound to get sterile in an effective way.

This test proves impressively that Hevos does not only produces wonderful loudspeaker cabinets but also high end bass amplifiers. The Hevos 400D offers a practically-oriented equipment, without needless frills, and is built in an excellent way. Furthermore it offers a sound of outstanding style: devotees of a detailed bass sound will be satisfied without any question. The small dimensions and weight make this hybrid amp an uncomplicated tool for ambitious bassists, both at stage and in the studio. In spite of the fact that the price could put off some bassists, it seems to me that the price is absolutely fair. After all, one gets “a whole lotta bassamp” for its money, and the few rivals of Hevos in this top segment use similar prices.”