2ChPre

Description: A dual-channel audio preamplifier designed to be an optimal preamplifier for the high-quality Audio Analog to Digital Converter AD24QS. The aim was not to create a smaller or cheaper preamplifier with similar functionality, but rather to develop a superior product. A modified version of the AD24QS is mounted on the preamplifier's board, fitting into a 200 x 100 x 50 mm enclosure (Fischerelektronik SG210). The design prioritizes quality over cost, resulting in a more expensive kit. The fully balanced design offers advantages such as reduced ground loop issues, minimized crosstalk, and improved dynamic range by increasing the maximum internal signal level by 6 dB. It is important to note that a single-ended design does not significantly reduce the required circuitry. The 2ChPre is a high-quality dual-channel preamplifier intended for use with the Audio Analog to Digital Converter AD24QS and is available as a DIY kit. It features a microphone input via an XLR receptacle and a line input through a 1/4" TRS jack, both integrated into an XLR/TRS hybrid chassis connector. When the TRS connector is used, the microphone preamplifier is automatically disconnected. The nominal output level for the analog, unbalanced Cinch (RCA) on-board monitor output is -2 dBu (615 mVRMS), with optional balanced (XLR) outputs at a nominal level of +4 dBu (1.23 mVRMS). Both digital audio outputs—optical and unbalanced Cinch—can operate simultaneously, with the Cinch output being galvanically decoupled by a digital audio transformer. An additional balanced (XLR or AES-3) digital audio output can be connected externally. Enclosures are not provided due to high costs in small quantities, but they can be purchased from a designated service. The microphone preamplifier employs a standard balanced amplifier IC (INA103) from Texas Instruments (formerly Burr Brown), known for its balanced outputs and excellent technical specifications. It offers three selectable gains of +10 dB, +30 dB, and +50 dB. Low gains are suitable for high-output sources but introduce more noise than higher gain stages. The combined microphone gain range is +7 dB to +90 dB, which is sufficient without being excessive for microphone inputs. An input pad is not included, as overdriving the preamplifier is unlikely, and pads tend to add noise. Input protection is implemented using a diode network connected to lower, zener diode-stabilized voltages to avoid excessive voltage drops that could diminish protective effects.

The dual-channel audio preamplifier is engineered for high fidelity, ensuring that audio signals are amplified with minimal distortion and noise. Each channel operates independently, utilizing a balanced configuration to enhance performance metrics, such as signal-to-noise ratio (SNR) and total harmonic distortion (THD). The INA103 integrated circuit is pivotal in achieving these specifications, providing a robust and low-noise amplification solution.

The preamplifier's design incorporates careful consideration of grounding and shielding to mitigate electromagnetic interference (EMI) and radio frequency interference (RFI). The use of high-quality components and precise layout techniques further contributes to the overall performance.

The enclosure dimensions (200 x 100 x 50 mm) allow for adequate space for heat dissipation, which is crucial for maintaining the integrity of audio signals during operation. The hybrid connector design simplifies the connection process for users, ensuring compatibility with various audio equipment while maintaining signal integrity.

The output stage is designed to handle both unbalanced and balanced signals, providing versatility for different audio applications. The galvanic isolation of the Cinch output through a digital audio transformer prevents ground loops, ensuring clean audio transmission to connected devices. The option for an external balanced digital audio output expands the preamplifier's capabilities, accommodating professional audio setups.

In summary, this dual-channel audio preamplifier is a high-quality solution for audio professionals and enthusiasts seeking superior sound reproduction and reliability in their audio systems. The thoughtful design choices and high-grade components position it as an excellent choice for integration with the AD24QS, making it a valuable addition to any audio production environment.A dual channel audio preamplifier as an appropriate preamplifier for the high quality Audio Analog to Digital Converter AD24QS. It is, of course, easy to build a much smaller and much cheaper preamplifier with comparable functionality, but making it cheap and small was not my goal.

A slightly modified version of the AD24QS is piggyba ck mounted on the preamplifier`s board and together they fit into a 200 x 100 x 50mm ³ enclosure ( Fischerelektronik SG210 ). The design goal was definitively not a cheaper preamplifier than commercially available ones - it is designed to be a better one.

As a consequence, even as a kit, it is expensive! The fully balanced design is more expensive than a single ended one, of course, but it has advantages: No (or 40dB less) hassle with ground loops, crosstalk by common grounds, hum or switching noise caught by ground etc. Also the maximum internal signal level can be increased by 6dB which may help to increase the dynamic range under certain circumstances.

Note that for a single ended design by far not half of the circuitry can be saved: In order to de-balance input signals and re-balance them for the outputs again, actually 2 op-amps (one dual op-amp) per channel could be omitted only(!). The 2ChPre is a high quality dual channel preamplifier designed to operate in conjunction with the Audio Analog to Digital Converter AD24QS.

It is available as kit for DIY (Do It Yourself). It features: The microphone input is an XLR receptacle, the line input a 1/4" TRS jack, both combined in one XLR / TRS hybrid chassis connector. When the TRS connector is plugged, the microphone preamplifier is disconnected automatically. The nominal output level of the analog, unbalanced Cinch- (or RCA. ) on-board monitor output is -2dBu (615mVRMS). As options balanced (XLR-) outputs with a nominal level of (+4dBu = 1. 23mVRMS) can be connected. See the DIY section for details. (Note: These outputs are part of the add-on A/D-converter, my AD24QS. ) Both digital audio outputs, the optical and the unbalanced Cinch output, can be used simultaneously.

The Cinch output is galvanically decoupled by a digital audio transformer. Also as an option a third, e. g. balanced (XLR- or AES-3-) digital audio output with its own digital audio transformer can be connected externally. I do not provide enclosures. Particularly the front and the rear panel are, in these small quantities, quite expensive. I cannot stock and sell them under these circumstances. But you can directly buy them from a special service I prepared them for. Have a look in the DIY section for more information. The microphone preamplifier uses a standard balanced amplifier IC ( INA103 ) from TI (formerly Burr Brown).

This is the only mic-preamplifier which provides balanced outputs (as far as I know), and it features real good technical data. I provided three different gains of +10dB, +30dB and +50dB. Low gains are needed for sources with high output levels, but they produce significantly more noise than high gain stages.

Together with the subsequent line amplifier the total microphone gain range is +7 up to +90 dB, by far enough, but not too much for a microphone input. I did not provide an input pad as it is unlikely that the preamplifier will ever be overdriven and, moreover, I do not appreciate pads as they introduce quite a lot of noise.

An input protection is provided with a usual diode network, but not, as usual, with the diodes connected to the supply rails, but connected to lower, zener diode stabilized voltages. This is because the protective series resistances are so low (15 ©, in order to avoid resistance noise) that quite high currents can flow through the diodes and might cause too high voltage drops across them, so that the protective effect might be reduced or lost in case the diodes would be connected to the power supply rails.

Originally I wanted to design a very low noise microph