Audio Engineering Associates By Wes Dooley Basic Description · STUDIO RIBBON MICS · MIC POSITIONERS · TALL STANDS · ACCESSORIES TRP - "The Ribbon Preamp" Operating Manual © May 2006 The AEA TRP is a low-noise, high-gain, two-channel preamplifier for ribbon, moving coil, and tube microphones that do not use or want phantom power. With all of the features that a ribbon mic user ever needs, its minimum-path JFET design features ultra-high gain, and an 18,000 ohm input impedance. This brings out the best in all ribbon mics, especially those with narrow ribbons such as the RCA 77 and Royer Stereo, with their 1600 to 1800 ohm bass resonance peaks. When you plug a microphone into "The Ribbon Pre" you are taking a step back in time, to before phantom powering became ubiquitous. As you will discover in this manual and Appendixes A and B, it is not a good idea to use phantom power with mics that do not require it -- not even with moving-coil mics, such as a Shure SM57. The TRP first stage electronics are minimal path, so there are no phantom power blocking capacitors. The pre-amp is protected, but without the extra coloration that results from using capacitors. Zener diodes are used instead to clamp down on external phantom power voltages while staying out of circuit for microphone level signals. First stage gain is set from + 6 to + 63 dB by a 12 position Grayhill switch. Another 21 dB of gain is available after the output level potentiometer. Internal audio switching for polarity reversal and highpass filters is handled by gold-contact Aeromat relays. These high-end components help keep the audio patch short and simple for best sound. Each channel features a three-LED level display. Green and red LEDs snap on to full brightness at -5 and +20 levels respectively. The amber LED increases in brightness between 0 and +20. Designed by Fred Forssell to handle extreme dynamics, the TRP recovers instantaneously from unexpected "solid red" overloads. Professional Neutrik 3 pin XLR and ¼ inch TRS phone connectors provide simultaneous balanced and unbalanced outputs. The balanced outputs deliver more than +28 dBu, and the unbalanced outputs exceed +22 dBu. All outputs will drive a 600 ohm load, but normal bridging loads of 5,000 ohms or more are recommended for maximum headroom. All inputs and outputs maintain proper polarity with each other and international standards. The 3 pin balanced mic input and line output connectors are wired Pin-2 + or high, and Pin-3 ­ or low. The unbalanced outputs have the tip as the high or + side, with the ring and barrel connected as the ­ or low side. 1029 N. Allen Ave., Pasadena, CA 91104 www.wesdooley.com Phone: (626) 798-9128 Fax: (626) 798-2378 An external desktop power supply with a grounded AC plug is used with the TRP, so hum fields can be kept away from the high-gain electronics. The power supply uses a seven pin circular DIN connector for its output, which is the same as the Symetrix 300 series. Additional Details A Grayhill rotary switch and high precision resistors set first stage gain to ensure exceptional gain accuracy. The match between channels is typically within 1/100 of a dB. 21 dB of additional second and third stage gain is provided after the 5,000 ohm level control pot. A typical operating position for this potentiometer would be around 2 o'clock. Except for fade outs, this control should not be used below its center position at 12 o'clock. The high-pass filter is intended to moderate the bass boost "proximity effect" inherent with a ribbon microphone when used up close. With very close talkers, this bass lift can become excessive to the point that speech intelligibility suffers. The balanced output stage drivers allow either Pin-3 or Pin-2 to be tied to pin one, with a reduction of headroom being the major performance penalty. To preserve maximum headroom however, we recommend only using the XLR outputs to drive balanced, floating, or differential inputs. The unbalanced phone jack outputs can drive either balanced or unbalanced loads and compared with the balanced outputs, provide 6 dB less gain. To minimize weight and preserve the TRP's good looks, the half-rack wide by one rack unit high chassis is aluminum with a tough gray and black powder coat finish. A single line schematic and the front and rear panel markings are permanently laser engraved on the chassis. The power supply transformer is not designed to be used outdoors or in a wet environment. It has two 17.25 volt AC secondary windings, which are used within the TRP to provide three regulated DC voltages: bi-polar 18 volt currents are provided for the electronics and +12 volt currents for the Aeromat relays. Physical Specifications Electronics Specifications 83.5 dB of gain -129 EIN noise figure for 150 source -0.5 dB at 160 kHz No phantom power, 18,000 ohm input impedance Green and Red LEDs indicates channel level Balanced XLR, and Unbalanced ¼ inch TRS outputs External Features Switched Gain and Continuous Level Controls Phase reverse and high-pass switches Finish: Gray and Black powder coated aluminum User Interface: Laser engraved legends and single line schematic on chassis External power supply for minimum noise i. Front view ii. Top view Audio Engineering Associates www.ribbonmics.com iii. Rear view Power B Mic In A Mic In No Phantom Dimensions Half-rack wide, one rack unit high (8.5" x 8.5" x 1.7") Weight: Two pounds, power supply is 1.5 lbs "The Ribbon Pre" User Comments Allan Meyerson used The Ribbon Pre with AEA R84 mics while recording trombones for X Men III. Allan set up his pre-amps out on the stage with the mics and musicians. His only negative comment about using with the TRP was that he had to go into the studio to trim the gain. Shawn Murphy used the AEA TRP to record acoustic guitars for Disney's Invincible at Sony Scoring. He used it with R84 mics and reported The Ribbon Pre was quiet, and sounded good, even at its highest gain settings. This is an important detail for Shawn, as he knows some pre-amp's sound changes at higher gains. He ran the GAIN switch at maximum and the LEVEL pot at 12 o'clock, which provided 65.5 dB of channel gain with another 18.5 dB in reserve. Note from the Author: Much of what I know is because others have shared their experiences with me. Thank you for the opportunity to pass along that knowledge to you. A glossary that covers many of the technical audio terms used throughout this manual can be found at www.stereosoundbook.com. Appendix A Ribbon Mics and Mic Pre-amps © 2004, 2006 by Wes Dooley A RibbonMics.com Publication Studio ribbon microphones offer smooth and versatile performance that sounds good on a wide variety of music. They've proven themselves in daily use for more than 70 years. However, to achieve this great sound quality, using them with the right mic preamplifier is essential. This presupposes an understanding of how mic pre-amps interact with mics, in this case, with an emphasis on ribbon and other dynamic mics. Which mic pre-amps are best? Although this appendix will present some criteria for you to evaluate them, you should trust your ears and start listening carefully to mic pre-amps you own, borrow, or are considering purchasing. As you gain experience, you'll discover that a quiet, distant, solo acoustic guitar, for example, will need 70 dB or more of clean, quiet gain. Older classics such as the `class A' Neve units were designed to provide this gain. Likewise, many contemporary, stand-alone mic pre-amps sound good and can be ordered with extra gain. High performance, specialty pre-amps start at around $700, and choices multiply amazingly as you get to $1,000 per channel. Ribbon mics deliver extraordinary dynamic range, especially with percussion. Your experiences will tell you whether a specialty pre-amp might help you best capture the sounds you value. Mic pre-amps with transformers tend to have a sound of their own and lower input impedance. Transformerless designs tend to be more neutral and open sounding, generally are quieter, and have higher input impedance. The input impedance should be at least 1,000 ohms for ribbon mics, although 1,500 ohms is better. The lower range of impedances influences wider ribbons less, such as those found in AEA and Coles mics. Narrower ribbons, such as in the RCA 77 and the Speiden/Royer stereo mics, are more susceptible. Ribbon microphones with integrated, DC-powered active electronics, such as the old Cambridge or the new Royer P48 powered ribbon mics, reduce the influence of a mic pre-amp's input impedance. Transformers are still used between the ribbon and the internal electronics, but these internal electronics keep the recording electronics from being affected by a ribbon's low frequency resonances. As always with P48 powering, the 6,800 ohm current limiting resistors place a limit on the maximum current or voltage available. Ribbon mics have the potential, especially on percussion, for handling extreme transients and dynamic ranges. Thus in many cases it is better to use a ribbon mic with usercontrolled external electronics. Completely transformer-less ribbon mic signal chains are possible, but their current requirements would be high enough that P48 powering would be impractical. Mic output loading is another term for the interaction between a mic's output impedance and a mic pre-amp's input impedance. Richard Werner of RCA published an AES paper in 1955 showing that the overall frequency response is altered by this interaction. He measured an RCA 77's impedance variation with frequency, and an RCA mic pre-amp's input impedance at the same frequencies. His paper demonstrates how this changes the overall frequency response. The nominal source impedance of a balanced output professional mic can be as low as 20 ohms, is typically from 150 to 300 ohms, and occasionally is as high as 600 ohms. A balanced low impedance design minimizes hum pickup, and reduces high frequency loss and slew rate limiting caused by the higher capacitance of longer mic cables. Mics are not designed to drive a matching load. If your mic's output impedance is 300 ohms and the pre-amp's input load is 300 ohms, you`re attenuating the mic's signal by six dB. Raising the input impedance to 2700 ohms, reduces that attenuation to one dB, and increases the system's signal to noise factor by five dB. Mic output and pre-amp input impedances can vary with frequency, sometimes significantly. Richard Werner's AES paper documented a worse case scenario: He paired a 250 ohm output RCA 77D, narrow ribbon mic, with the 1,500 ohm input of a transformer-coupled tube mic pre-amp. The mic's nominal 250 ohm output impedance soared to 1300 ohms at its 50 Hz resonance. At this frequency the pre-amp's nominal 1500 ohm input impedance sagged to around 600 ohms. The result was a deep hole in the bass response of this combination below 100 Hz. To hear a mic as its designer intended, the pre-amp's input impedance should be at least 1,500 ohms and consistent across the audio band. Even in the ranges above and below the 20-20 kHz audio band, the input impedance should remain high, as out-of-band load changes can affect in-band sonics. A rule of thumb is that a mic's load impedance should be five or more times the mic's source impedance. For a 300 ohm mic, such as the Coles 4038, this would translate to a 1,500 ohm load. For a 150 ohm source mic, such as most Neumann mics, this would mean a minimum load of 750 ohms, however, higher is even better. Thus, microphone impedances and pre-amp loadings seem to be a perpetual source of confusion. Ideally the mic's source impedance should be as low as practical, and the load impedance it feeds should be as high as possible. Microphone pre-amplifier input impedances have been at 1,000 ohms or above now for decades. (Although 19th century voice-powered carbon telephone transmitters had to be loaded into matching 600 receivers for maximum power transfer, even the BBC abandoned that approach well before the 20th century ended.) As mentioned above, with such low impedance loads, P48 phantom powered electronics run out of current at high sound pressure levels. With such mics, the sound can improve significantly if loaded into a higher input impedance. The lower pre-amp input impedance a mic must drive, the harder it has to work. For condenser mics or others with built in electronics, driving too low of a load impedance is heard as decreased headroom, increased distortion, and especially when driving long mic cables, as high frequency limiting at high SPL. With the recent resurgence in popularity of "vintage" equipment, some contemporary mic pre-amp manufacturers provide selectable mic input impedances, as if this is a useful feature or a wonderful effect. Unless you are working with antique 30 or 50 ohm microphones, however, it is neither. Impedance matching is bad for signal to noise, increases di ...