User manual AEA THE RIBBON PRE

[. . . ] 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. [. . . ] 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. When the audio is softer or louder than the transformer's linear operating range 5. If DC through a winding magnetizes the core material and it becomes less linear These variables can alter a pre-amp's overall sound in quite complex ways, both in the 20-20kHz audio-band frequency response, and the frequency response above and below these points. Interestingly enough, changes in the out-of-band response will impact a pre-amp's sound, especially with regards to transient response. As with any signal processing, the judgment is yours whether this is a change for the better or worse. To summarize everything above, the most relevant aspects of a mic pre-amp's input impedance are: 1. How does it change the useable output level and linearity of a mic?How does it alter the frequency and transient response of its mic? [. . . ] An extra series resistance in the supply will cause the supply voltage to sag unacceptably as more or higher current mics are connected. Until around 1979 the P48 standard only required 2 mA to be available per mic. This was enough for the earliest phantom powered mics as they typically only used a single field effect transistor. However this means that an old phantom power supply might not supply enough current to operate all P48 microphones. [. . . ]