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100% Discrete 500-series mic preamp with transformer coupled output

By Phil O'Keefe

By now there should be no doubt in anyone's mind that Radial Engineering has made a major commitment to the 500-series rack format. Not only do they make the "bread and butter" type products you might expect, such as mic preamps, compressors and equalizers, they also make several extremely handy and useful "problem solver" modules, as well as a full line of rack units for powering and housing your 500-series modules. Today we're taking a look at another one of their preamp modules - the PowerPre.

What You Need To Know

  • The PowerPre is a 100% discrete, solid state microphone preamp that features a transformer balanced output, and is designed for use in 500-series modular racks.
  • I tested the PowerPre with a Radial Engineering SixPack 500-series rack serving as the host, but it conforms to all 500-series standards and should work fine with any well-designed 500-series rack. Maximum power draw is a reasonable 130mA, so it isn't going to place too much of a burden on your power supply.
  • The build quality is typical of Radial Engineering's products, which is another way of saying it's built like the proverbial brick outhouse, with high quality, hand selected discrete components, Mil-Spec double-sided PCB, a Hammond broadcast output transformer, and a tough all-steel housing that encloses the entire unit and provides shielding.
  • Most I/O connectivity for 500-series mic preamps (and 500-series modules in general) is provided by the host rack, but the PowerPre also includes an XLR input on the front panel. This is extremely convenient for those who work alone, since it allows you to easily plug in a mic without having to reach around to the back of your rack. Of course, the host rack's XLR input for the module is still available - the two jacks are wired in parallel.
  • The AccuState (™) input control is really quite clever. Instead of just lowering input sensitivity like many other preamp designs do (which leads to increased noise levels as you increase the input attenuation), it links two pots that proportionately adjust input sensitivity and gain simultaneously so that the signal to noise ratio of the preamp remains optimized, no matter how hot the input signal is or where you have the knob set. It works wonderfully, and keeps the noise levels nice and low at any point on the dial, with up to 55dB of gain available.
  • A ten segment LED meter keeps you appraised of what your levels are doing. The meter is designed to replicate VU meter ballistics, and it has three different LED colors (green, orange and red) so it's easy to see at a glance if things are getting too hot.
  • If that should happen, you can engage the PowerPre's -15dB pad switch.
  • A polarity invert switch is also included on the PowerPre's front panel.
  • The 48V phantom power switch is recessed and requires a toothpick or similar tool for activation. A red LED next to it illuminates when it is active.
  • However, it is possible for the LED to be illuminated but no power to reach the microphone, as Radial points out in the PowerPre's user's guide. This can occur if the internal ground lift switch (located near the card-edge connector) is switched on, in which case the phantom power LED will illuminate when the phantom power switch is engaged, but since the ground lift is also engaged, no phantom power will reach the mic.
  • Why include the ground lift? For use with keyboards and other AC-powered unbalanced sources, which may occasionally suffer from ground loop issues.
  • When using the PowerPre with a Radial Engineering 500-series rack host, the Omniport feature becomes available.  The PowerPre's Omniport is configured as a unbalanced 1/4" instrument input. Connecting to the Omniport jack disconnects the PowerPre's front panel XLR input. Input impedance is 150k Ohms, and there is up to 55dB of gain available on the Omniport instrument input.
  • The PowerPre also includes a high pass filter switch. The filter begins kicking in at about 150Hz, and has a fairly gentle slope of about 6dB per octave, making it very useful for rolling off unwanted noise and room rumble below 100Hz.
  • While there isn't any fully-variable EQ controls on the PowerPre, it does have a three position "Vox" switch. This provides you with the option of using one of three pre-set EQ curves. The middle position on this switch is labeled "Linear", and gives you just that - a flat signal with no EQ adjustment. The other two settings are labeled Breath and Punch. When the Vox switch is set to Breath, a three or four decibel boost is applied to the upper mids and high frequencies.
  • When you select the Punch setting on the VOX switch, two or three decibels of boost is applied to the lows. This gives you the ability to add girth and weight to signals. The curve is once again fairly broad, bell curve shaped, extending from about 30Hz to nearly 200Hz, and is centered at about 60Hz or so. 



  • In order to change the ground lift switch setting, the unit needs to be removed from the host rack.
  • 55dB of gain, while ample for most everyday recording tasks with condenser and moving coil dynamic microphones, may be insufficient when using lower-output microphones, such as many ribbon mikes.



Once again, Radial Engineering has delivered an outstanding product for users of 500-series racks. The PowerPre's sound is far from clinical or sterile, and it is definitely a preamp that colors the sound in a musically pleasant way; one that brings to mind such adjectives as "warm" and "vintage-sounding." The discrete electronic design and output transformer obviously contribute to this, and you can shade things further still with the Vox switch if desired. Warm and yet detailed, this is a great general purpose preamp that sounds sweet on a wide variety of sound sources and with just about any type of mic feeding it, with the possible exception of ribbon microphones being used at a distance, or on low-volume sound sources. In those circumstances, you may long for a bit more gain. Still, the versatility of the design can not be denied. The AccuState (™) input is a brilliant idea and one that does a great job at keeping the noise levels down while simultaneously making the process of dialing up the level settings simplicity itself. The Hammond output transformer sounds great when driven hard, giving you the option of running clean or adding in a little grit when appropriate. I suspect Radial is going to sell a bunch of these - including this one. I plan on purchasing the review unit.


Musician's Friend Radial Engineering PowerPre online catalog page ($600.00 MSRP, $499.99 "street")

Radial Engineering's PowerPre product web page


Phil O'Keefe is a multi-instrumentalist, recording engineer / producer and the Senior Editor of Harmony Central. He has engineered, produced and performed on countless recording sessions in a diverse range of styles, with artists such as Alien Ant Farm, Jules Day, Voodoo Glow Skulls, John McGill, Michael Knott and Alexa's Wish. He is a former featured monthly columnist for EQ magazine, and his articles and product reviews have also appeared in Keyboard, Electronic Musician and Guitar Player magazines. 
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