The SC-1 Mic Preamp is a THAT 1510/1512/SSM2017/SSM2019/INA217 based Mic preamp with phantom power, DC servo and Balanced line output driver.

Rev.1.00 PCB Design for the SC-1 Preamp

Introduction

The THAT 1510 is a high performance audio preamplifier suitable for use in microphone preamp application. It is pin compatible with the now discontinued, Analog Devices SSM2019 and SSM2017 IC chips, and the Texas Instruments INA217 and INA163 mic preamp chips.

Designed from the ground up in THAT’s complementary dielectric isolation process and including laser-trimmed Si-Chrome thin film resistors, the THAT 1510 improve on existing integrated microphone preamps by offering lower noise at low gains, wider bandwidth, higher slew rate, lower distortion, and lower supply current. The parts feature internal ESD overload protection on all critical pins.

In short, the THAT 1510 provide superior performance in a popular format at an affordable price.

Features

* Low Noise:
1 nV/ ÖHz input noise (60dB gain)
34 nV/ ÖHz input noise (0dB gain) (1512)
* Low THD+N (full audio bandwidth):
0.0005% < 40dB gain
0.005% @ 60dB gain
* Low Current: 6 mA
* Wide Bandwidth: 7MHz @ G=100
* High Slew Rate: 19 V/µs
* Wide Output Swing: ±13.3V on ±15V supplies
* Gain adjustable from 0 to >60dB with
one external resistor
* Industry-standard pinouts

Comparison between THAT, SSM and INA mic preamp chips

 

Design Goals

The goal is to provide a great performing and nice sounding mic preamp that's reasonable or cheaper in cost compared to commercial ready-made, preamp units.

It will be in kit form.  This gives the user the chance to build the preamp themselves and use their imagination and creativity in housing this preamp in it's own custom case.

- Simple, Great Performing, Quiet, High-Quality Mic Preamp
- with 48V phantom power
- LED indicator for phantom power
- powered by 18-0-18 Volts supply, with +48VDC for phantom power
- balanced output
- 12-position gain selector switch
- hook-up points for optional Trim potentiometer
- hook-up points for use of a potentiometer for gain control instead of the 12-position gain selector switch
- 0 to 66dB gain in 6dB increments
- input clamping protection
- output surge protection circuit
- output RFI protection

Future Planned Features:
- -10dB pad switch
- phase switch
- Trim potentiometer, PCB mounted
- VU meter

Block Diagram

The SC-1 Mic Preamp has a straightforward design. The approach is to use the minimum number of components in the signal path, but at the same time provide flexibility to the end-user to add in-line signal processing options.

A DC servo approach was used instead of AC coupling. The Balanced line driver is connected directly at the output of the DC Servo.

In case the user wants to use the optional trim level, "tap points" are provided on the circuit board to insert this potentiometer in the signal path. This will function as the volume control and provide more control over the 6dB per step of the gain selector switch.

These "tap points" also provide a way in the future to add additional in-line signal processors. Some things that come to mind are VCA compressors, FET-based compressors, Passive EQ, Active EQ modules, Low-cut, High-cut filters, exciters, etc...

This gives the user customization capabilities in creating his own unique mic preamp channel strip!

DC Servo

The DC servo is using an OPA134 opamp chip.

- SUPERIOR SOUND QUALITY
- ULTRA LOW DISTORTION: 0.00008%
- LOW NOISE: 8nV/sqrt(Hz)
- TRUE FET-INPUT: IB = 5pA
- HIGH SPEED:
- SLEW RATE: 20V/µs
- BANDWIDTH: 8MHz
- HIGH OPEN-LOOP GAIN: 120dB (600ohm)
- WIDE SUPPLY RANGE: ±2.5V to ±18V
- SINGLE, DUAL, AND QUAD VERSIONS

The OPA134 series are ultra-low distortion, low noise operational amplifiers fully specified for audio applications. A true FET input stage was incorporated to provide superior sound quality and speed for exceptional audio performance. This in combination with high output drive capability and excellent dc performance allows use in a wide variety of demanding applications. In addition, the OPA134's wide output swing, to within 1V of the rails, allows increased headroom making it ideal for use in any audio circuit.

OPA134 op amps are easy to use and free from phase inversion and overload problems often found in common FET-input op amps. They can be operated from ±2.5V to ±18V power supplies. Input cascode circuitry provides excellent common-mode rejection and maintains low input bias current over its wide input voltage range, minimizing distortion. OPA134 series op amps are unity-gain stable and provide excellent dynamic behavior over a wide range of load conditions, including high load capacitance. The dual and quad version features completely independent circuitry for lowest crosstalk and freedom from interaction, even when overdriven or overloaded.

Applications:

- PROFESSIONAL AUDIO AND MUSIC
- LINE DRIVERS
- LINE RECEIVERS
- MULTIMEDIA AUDIO
- ACTIVE FILTERS
- PREAMPLIFIERS
- INTEGRATORS
- CROSSOVER NETWORKS

Balanced Line Driver Ouput

The THAT 1646 is a new generation of audio differential line drivers offering improved performance over conventional cross-coupled designs. Based on a high-performance, fully differential opamp and laser-trimmed thin-film resistors, the 1646 exhibit low noise and distortion, high slew rate, and wide output swing. The parts are stable when driving difficult loads, and have short-circuit protected outputs.

This is a dual feedback-loop design that prevents the excessive ground currents typical of cross-coupled output stages (CCOS) when clipping into single-ended loads. OutSmarts uses two individual negative-feedback loops to separately control the differential output voltage and common mode output currents, making the designs inherently more stable and less sensitive to component tolerances than common CCOSes. As a result, THAT's topology prevents the loss of common-mode feedback that plagues common CCOS designs when clipping into single-ended loads. This avoids excessive ground currents that would otherwise upset power supplies and create additional distortion, even in adjacent channels.

The 1646 is pin-compatible with the TI DRV134 and DRV135, as well as the Analog Devices SSM2142.

Features

- Balanced, transformer-like floating output
- OutSmarts technology improves clipping into
single-ended loads
- Stable driving long cables and capacitive loads
- High output: 18Vrms into 600
- W Low noise: -101 dBu
- Low distortion: 0.0007% @ 1kHz
- Industry-standard pinout

Applications

- Differential Line Drivers
- Audio Mixing Consoles
- Distribution Amplifiers
- Hi-Fi Equipment
- Audio Equalizers
- Dynamic Range Processors
- Digital Effects Processors
- Telecommunications Systems
- Instrumentation

 

More information coming soon...