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Pre Amplifiers
(pre-Amps) Page Two (2)
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PreAmps do not have to be transistor devices. Modern Electronics has introduced the "Integrated Circuit". Now whole pre amps and multiple preamp stages can be contained in tiny little packages. This really simplifies the design of a mixer circuit and allows for more devices and features.
In my "early" days of Mixer design, I contracted with a company in Phoenix, Arizona (I grew up and lived in Middle Tennessee) to build my mixer IC's so that they couldn't be copied. Essentially, I designed and laid out the individual stages, and Del's people at Certronics put them into a single SIP package. A few of my pre amp stages and my driver amp stages were also encapsulated into DIP ICs. My favorite has always been the 28-Pin and 40-Pin DIP. But that was back in the late 1960's and early 1970's - Very Small Large Integrated Circuits (VSLICs) were not even thought of. One of my Remote Broadcast Mixers only had five ICs in the whole mixer. It incorporated three stereo lines, two microphones (with phantom power) and featured both a powered balanced stereo line (in the late 60s through the seventies broadcast services used powered telephone lines to carry remote signals back to the Transmitter/studio site), and a Stereo balanced Program Output to allow for the use of a UHF Studio Transmitter Link (STL). This was somewhat innovative, because many small stations never realized that they could easily set up an STL and not have to pay the phone company the $1,240 that they were getting just to set up the Broadcast Remote quad (two pairs to get right and left channels). The arrangement provided for a talk back circuit utilizing a "phantom pair" that allowed the transmitter/studio to talk back to the remote operator without affecting the program feed (after all - telephone circuits were a two-way circuit - it was just a matter of using a hybrid and balancing the two Pairs to allow for TX on pair one and two, and Talk-Back/RX on The phantom pair. Things were reversed at the studio/transmitter site). It was the use of these "custom" ICs that allowed me to incorporate these nifty features and sell my remote mixers.
Here we are going to discuss and lay out a few of the IC PreAmp circuits. There are many OpAmps available for use in a preamp circuit. Some are really noisy (such as the LM709 and LM741). While popular - these are not the quality that we strive for in a viable mixer circuit. We need low noise, and often high gain - but these parameters do not "always" go hand in hand..
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