User manual AEA RCA 77-DX

[. . . ] 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. [. . . ] 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. 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. Such mic pre-amps can alter the load a mic drives in a variety of ways. These changes range from simply varying the resistive loading a mic must drive, to the complexity of changing the turns ratio (impedance transformation) of an input transformer, and / or its secondary loading. Changing the turns ratio and secondary loading of a mic input transformer, can change a mic's sound on transients from dull, to open, to ringing. Mic pre-amp designers typically aim for transients that are neither over-damped, i. e. However these designers have no control over which mics their pre-amps will be used with. Thus the same mic, used with different pre-amps, can yield a quite different sound. Transformers, like ribbon mics, look simple, but are deceptively complicated. Here are five common and uncommon events that will change the sound of any transformer. This "invisible" power source was designed to not affect any balanced mics that might accidentally be plugged into phantom power. For convenience here I will refer to the microphone's balanced modulation output pins as two and three, and the ground pin as one. We will ignore earlier microphone connectors and their pin numbering schemes. [. . . ] Some electronics such as the Neumann TLM 170 initially need quite a bit of current during their turn on period to achieve operating stability, and then pull less current during normal operation. The voltage source for a phantom power supply is supposed to be stable and have negligible internal resistance. Many phantom power supplies on the market though, including some from well-known manufacturers, add additional "current limiting" resistors of several thousand Ohms. This is safer than with old Stewart 48 volt phantom supplies, which used a non-standard lower resistance value. [. . . ]