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Presonus Eureka

Recording Channel By Hugh Robjohns
Published May 2004

Presonus EurekaPhoto: Mike Cameron

This smart new unit offers EQ and compression, plus a transformer-coupled Class-A preamp and an unusual Saturation control.

Despite the rise of the virtual studio, some hardware devices will always remain necessary. Microphones and loudspeakers are clearly indispensable, but so too are decent microphone preamps. The 'channel in a box' units — combining preamp, compression, and equalisation — are consistently popular solutions and, despite the advancing quality of plug-ins, in many cases analogue compression and EQ offer a sound quality or character that most plug-ins can't match.

Photos: Mike Cameron

The American manufacturer Presonus have produced a wide variety of cost-effective but always well-designed preamps and channel units. Many have been valve-based designs, but there have also been several solid-state devices. One such product was the VXP reviewed by Paul White in SOS March 2001, and the Eureka under review here bears some striking similarities to that unit (although at a significantly lower UK price). Most notable is the porting across of several key technologies, including the transformer-coupled Class-A preamp and the unusual and adjustable saturation distortion control. This bestows the Eureka with a sound character between a traditional valve-based preamp and a solid-state one.

Facilities Overview

The facilities provided by the Eureka are fairly conventional for a mono channel strip. There's a switchable mic, line and DI input, a flexible solid-state compressor, and a three-band, fully-parametric EQ. There is also a balanced insert point immediately after the preamp section, and the equaliser can be switched into circuit either before or after the compressor.

As already mentioned, the mic input is transformer balanced and features a discrete Class-A FET buffer followed by a 'dual servo' gain stage using industry-standard NE5534 op amps. The FET buffer circuitry incorporates the unusual Saturation facility (created by varying the FET drain current) which generates an adjustable degree of even-order harmonic distortion. This provides a variable quasi-valve warmth, yet with the benefit of solid-state precision and reliability. Another facility included in this preamp design — and one which is becoming increasingly popular — is a variable input impedance, spanning 50-2500Ω in this case. A simple three-LED input meter is provided, showing -20dBu, 0dBu, and clipping (+22dBu). Although better than nothing, I felt that this input metering was a little too simple to be very useful.

The compressor section is based around the familiar THAT 4301 VCA chip, and includes a complete set of user controls rather than the preset modes offered in the VXP. A variable high-pass filter is also included in the side-chain to enable simple frequency-dependent dynamic processing — such as crude de-essing — and the compression characteristic can be switched from the default hard knee to a soft-knee mode, enabling both dramatic and more subtle compression effects.

The three-band parametric equaliser section is versatile, and avoids the dual-concentric controls that Paul found frustrating in the VXP, although the close proximity of controls still makes it fiddly to adjust for anyone with fingers thicker than a supermodel's. The band gain controls provide a slightly smaller boost and cut gain range than most designs, with just 10dB on offer, but there's usually something seriously wrong if you need more than that from a general-purpose analogue equaliser! The side effect of a restricted control range is that the precision of gain setting has been usefully enhanced.

The bandwidth of all three sections can be varied from a narrow two thirds of an octave (Q value of two), to a broad two octaves (Q value of 0.4), and with the widest bandwidth settings and extreme frequency positions the top and bottom bands can be made to serve as reasonable shelf filters too. The frequency ranges of all three bands overlap well to provide seamless coverage of the entire spectrum.

The electronically balanced main output is provided on both XLR and TRS sockets, and the nominal level can be controlled between +10dBu and about -80dBu. A small moving-coil VU meter in the centre of the front panel can be switched to show either the output level or the compressor's gain reduction. The insert point is fully balanced, with separate TRS sockets for send and return.

An optional 24-bit A-D module can also be installed via the rear panel if required, operating at all the standard sample rates from 44.1kHz to 192kHz, and providing both AES-EBU and S/PDIF outputs. Since these are both dual-channel interfaces, an auxiliary balanced TRS socket accepts an external analogue line-level signal to access the second A-D channel — the Eureka's own output signal occupying the first digital channel. Unfortunately, this A-D converter module was not provided for the review, so I am unable to comment on its performance.

Presonus EurekaPhoto: Mike Cameron

Internal Circuitry

The Eureka is certainly well built. The majority of electronics are contained on a main PCB which covers most of the available floor area of the box, with mic preamp circuitry to the left, dynamics and EQ in the centre, and an integral linear power supply to the right. Three power-rail regulator chips are mounted on a metal strip running from front to back inside the unit, and this acts as the only heat sink — but the power consumption is very small so heat isn't really an issue. The main circuit board has a rectangular section cut away in the centre at the rear to accommodate the optional A-D converter module. This can be fitted by the user very easily, as it is simply bolted in place and connected to the main unit via a small multi-pin plug.

Most components on the circuit board are surfacemount devices, although there are a few conventional-sized components too, including a pair of socketed NE5534 op amps in the input stage. Two small daughterboards run just below the lid to carry and the top row of front-panel switches and controls, while the toroidal mains transformer and mu-metal screened mic transformer are located at opposite sides of the box. One point worth noting is that the mains operating voltage is factory configured and not user adjustable. The IEC mains inlet incorporates a fuse holder accessible from the outside, and the mains power switch is also located here on the rear panel — which may be a blessing or a hindrance depending on your point of view!

Front-panel Layout

The Eureka looks very clean and tidy, with lots of small blue knurled knobs on a brushed-steel front panel. The panel markings are small but clearly legible, and all of the continuous rotary controls have a light detented action traversing about 40 'clicks', which gives good tactile feedback. Unfortunately, though, this detented action feels rather more precise than it really is, and doesn't help in locating the unity positions of the gain controls, for example. So, to set the output level to the calibrated unity gain mark, or to cancel the gain of an EQ section, you are forced to look closely at the panel markings, instead of just feeling for a centre detent. It's a small point, but one which some users might find frustrating after using the unit for a while — I know I did!

Here you can see the printed circuit board which runs the entire width of the rack casing. The toroidal mains transformer is at the right-hand side, and the cutout for the optional rear-panel A-D converter board is at the top.Here you can see the printed circuit board which runs the entire width of the rack casing. The toroidal mains transformer is at the right-hand side, and the cutout for the optional rear-panel A-D converter board is at the top.Photo: Mike CameronThe input stage is easy and logical to use, with three rotary controls along the bottom and five illuminated buttons running in a row above. Microphone input gain is continuously adjustable from +10dB to +54dB, but this can be supplemented by an additional 10dB of make-up gain in the compressor and a further 10dB at the output level control, giving a maximum overall gain of 74dB. The microphone preamp is specified with an EIN value of -127dB (weighting not given) and distortion is quoted as below 0.005 percent with no saturation (rising to 0.5 percent with full saturation).

The buttons provide phantom power, a 20dB pad, an 80Hz high-pass filter, and polarity reversal. One of the other two rotary controls adjusts the Saturation from zero to 100 percent, and increasing the Saturation certainly adds a warmer character to the sound, making it sound thicker and richer. However, this is pretty subtle most of the time, and it's not quite of the same character as the distortion you get in a typical valve preamp. Then again, it isn't as unpredictable as valve preamps, and won't wear out either! I'm sure many users will find it a very useful facility, but don't audition the Eureka with the expectation of it sounding like a traditional valve preamp.

The third rotary control is actually a switch to change the input impedance in five steps from 50Ω to 2500Ω. Many preamps offering variable input impedance provide a higher maximum figure than this — typically 5kΩ or so — but the range offered here is sufficient to match a wide selection of mics, including most ribbons, and affords useful tone-shaping characteristics which are far more subtle than most EQs can manage.

The fifth and final button selects the fixed-level line input, which bypasses the mic input stage altogether and so is unaffected by the Gain, Saturation, and Impedance controls. The front-panel unbalanced instrument input presents a 1MΩ input impedance and is selected automatically when a plug is inserted. The DI input is processed through the microphone gain stage (with a lower overall gain spanning 0-44dB), and is therefore able to take advantage of the Saturation effect, which is a nice facility. In fact, the DI input sounds particularly good, with a full rich character — I find many preamp DI inputs tend to sound rather too clean, but not the Eureka. I'd still recommend recording guitars and basses through a decent simulation system like the Pod XT, but if you want a simple clean input then this is a good one.

EQ & Dynamics

The front panel places the dynamics control section before the equaliser, and the default signal path follows the same order, but an illuminated button allows that sequence to be reversed should you need to equalise before compressing. The compressor is fully featured, with six rotary controls and a pair of illuminated buttons. The Threshold control spans +20dBu to -40dBu, enabling the compressor to be used effectively as a peak limiter at high threshold values and ratios. The Ratio control is scaled rather oddly from zero to 10, but actually offers a range of 1:1 up to 2:1, and has a hard-knee response by default. A soft-knee curve can be switched in when a more subtle effect is required. The Gain control offers a ±10dB range, and the Attack and Release controls are both marked simply with Fast and Slow at their control extremes. However, the attack-time range is detailed in the specifications as being adjustable from 0.1ms to 200ms, while the release-time covers 0.05s to 3s.

The last rotary control determines the turnover frequency of the side-chain's high-pass filter. This is adjustable from 10Hz to 10kHz, and is useful for reducing the compressor's sensitivity to bass instruments at the lower end of the scale, and for helping the compressor react only to sibilance at the higher end of the scale. The entire compressor can be bypassed by another button, and the amount of gain reduction can be displayed on the central VU meter.

Presonus EurekaPhoto: Mike CameronThe equaliser occupies most of the panel space to the right of the VU window, with three sets of three rotary controls plus another pair of illuminated buttons. The first button places the EQ before the compressor, as described above, and the second bypasses the entire equaliser circuit. The controls in all three bands follow the same order left to right — Q (bandwidth) followed by Gain then Frequency — but the middle section is reversed vertically compared with the outside two. In other words, the Gain control is above the Q and Frequency controls for the centre section, but below for the other two. This caused some operational confusion initially, but once familiar with the unit I found it fairly easy to navigate the controls. I have already mentioned the ranges afforded by the Gain and Q controls, so to complete the picture the frequencies covered by each band are 20-300Hz, 200-3000Hz, and 2-20kHz respectively.

As you would expect, the equaliser section is versatile and sounded pretty good to my ears — far better than the average budget mixing console's EQ, and capable of better high-frequency correction than most software plug-ins. The ability to vary the Q over a reasonably wide range is certainly helpful when trying to tune in to a specific part of the spectrum, allowing each EQ section to affect a broad range for gentle tonal shaping, or a narrow range for more delicate corrective surgery.

The final control panel section is the output stage, with the output fader and a button to switch the VU meter between output and gain-reduction levels. The output can sustain signals up to +22dBu from a low 51Ω source impedance, which is the same as the balanced insert send point. The balanced insert return and line input both share a 10kΩ impedance and can accommodate signals to +22dBu.


The Presonus Eureka doesn't have the larger-than-life low-frequency quality associated with true high-end preamps, but it remains well controlled at the bottom with a detailed top end and a neutral overall balance. I would liken the Eureka's preamp stage to that of a good-quality, traditional mixing console — there's something about a transformer input stage that just sounds right. The Eureka offers a worthwhile step up in quality from the relatively simple mic input stages of budget mixers and computer interfaces.

The Saturation control can be used to smooth any hard edges from the top end, while adding an extra richness and density through the mid-range. With most sources — such as vocals and some instrumental sources like electric or electronic keyboard parts — the subtle distortion sounds musically flattering. However, it's not an effect to overuse: the addition of those extra harmonics not only thickens the sound, but also tends to obscure transient detail, so it can be damaging on complex and delicate sources — such as acoustic guitars (especially 12-strings) or crisp percussion parts. Overall, the Eureka is a very good-sounding unit — especially given the UK price — and easily stands up to comparison with higher-priced channel strips from the other major players in this field.


  • High-quality transformer-coupled Class-A preamp.
  • Adjustable Saturation control.
  • Flexible three-band fully parametric EQ.
  • Highly controllable VCA compressor with side-chain filter.


  • Control knob size and density makes operation fiddly.
  • No clear centre detents on gain controls.
  • Input metering too simplified.


The Eureka is a well-designed voice channel with the benefit of a transformer-coupled Class-A mic preamp, a useful Saturation feature, and effective and controllable compressor and EQ facilities. The optional high-sample-rate A-D board should enable easy integration with digital recording systems when it becomes available.


£499 including VAT.

Hand In Hand +44 (0)1579 326155.

+44 (0)1579 326157.