The PREQ-73 is a one-channel vintage style microphone-, line- and instrument preamplifier with a two band shelving equalizer offering a smooth and effective sound shaping.
The signal path uses only discrete components like resistors, capacitors and transistors. The line and microphone input and the line output are transformer balanced, using three different transformers, each one optimized for its purpose. The fully floating output can drive a level of at least +26 dBu into a load as low as 600 Ohms. This is the way audio components were built before integrated circuits became available. The subjective sound
quality delivered by vintage equipment is often prefered over the one delivered by modern units, a situation that is even more obvious now when music is recorded with clean sounding digital audio equipment.
The circuit used in the PREQ-73 is similar to the preamp and part of the equalizer section in the classical 1073 module with a corresponding sound character that is warm, punchy, sweet and musical. These classic characteristics have been heard on countless recordings through the years and it is a versatile sound that works very well on most sound sources and in most genres. The essence of this sound is now available at a surprisingly low cost, making it available to nearly everyone.
Circuit Description
The signal first enters an input transformer - one for the mic input and another one for the line input. The primary of the mic input transformer has two windings that are either connected in series or in parallell which results in an input impedance of either
1200 or 300 Ohms. The transformers are followed by two input gain stages. For gains up to 50 dB, only one of them is being used. For gains above 50 dB, the second gain stage is inserted in the signal path. Both gain stages uses only three transistors each. After the gain stages, the signal goes to the equalizer section and then to the output level potentiometer and from there to the output stage. This stage again only uses three transistors, the last one in the chain is a hefty 2N3055 power transistor run in class-A mode, driving the output transformer. All in all, the complete signal chain contains only a maximum of 12 active elements including the three transistors in the eq section.
Compare that to the big number of transistors that are usually used in one single integrated circuit!
Modern Vs. Old
It is true that there are some great IC´s available today that achieves very low levels of static and dynamic distortion. The simple circuits that the PREQ-73 uses, and even more so the transformers, cannot match the low distortion specifications of modern
IC's. It is the distortion components that imparts a sound character to the audio signal and, if the distortion components are of the right sort, this is a good thing since it makes the recorded voice or instrument sound “better”, more musical, more pleasing to the ear. This is one reason why vintage style units are so popular today. Modern, transparent sounding audio circuits is surely not a bad thing, sometimes they are prefered over colored ones. It´s all about taste and it depends on the genre. For most modern music styles, color and character is usually a good thing.
