Flute Tone Investigations - Appendix 3
“Audio Lab” Control Unit Specification

This paper details the specification for a custom interface designed to facilitate audio and sound testing using readily available software.  My specific purpose is flute acoustics, but the unit would have far wider application.  The specification is not yet complete and will be updated as concepts firm and construction proceeds.

“Audio Lab” Control Unit Specification


To interface the computer to the real world to enable it to function as an efficient and comprehensive audio laboratory.  Uses and facilities will include:

  • Digital sound recording and playback

  • User-friendly connection and control of inputs, outputs and monitoring

  • Audio Equipment testing and calibration

  • Sound Level metering and analysis (FFT, etc)

  • Room acoustics measurement

  • Circuit development

  • Supply power to test rigs

  • Power control of audio equipment

  • Facilitate VOIP calls


The audio lab control unit will be custom built, and take the form of a 2U rack type box under the computer monitor.  It will be used in conjunction with an external high end computer audio interface, in my case the ProSonus FireBox.  Only the line inputs and outputs of the Firebox will be used, to give maximum flexibility and control to the control unit.  All circuitry will be designed or selected to avoid compromising the quality of the interface.  While it is expected to be used largely at 44.1kHz sampling rate (for easy consistency with other audio applications), circuits will be designed with the possibility of using the FireBox’s 48kHz or 96kHz sampling rates occasionally, should a wider frequency range be needed.

Wherever possible, readily available circuit designs (eg mic preamps, power supplies, amplifiers, millivoltmeter, oscilloscope interface, etc, kits) will be used to simplify design and construction.  Permanent connections will be via the rear panel, temporary connections via the front panel.  Incoming mains will be filtered to reduce likelihood of interference.  A Ground Lift switch will be incorporated if needed to deal with earth loop induced hum.

Input selectors

A separate input selector for each channel will provide for a convenient choice of inputs:

1.   Oscilloscope

2.   Mic

3.   Line

4.   Headset

5.   Other Channel (L->R, R->L)

6.   Computer output (“Loopback”)

7.   Calibration

8.   Spare input positions, eg for turntable (RIAA) input, Telco interface, etc

 Separate input selector switches - one for each channel – will permit situations such as:

  • routing a mono source to both channels

  • singer on mic with guitar on line input

  • monitoring output on one channel and a selected input on the other

Input Specifications

1.     Oscilloscope type inputs (BNC) that can take attenuator probes for fossicking around when faultfinding.  Connects via suitable high Z preamp.  Zin = 1M//20pF. 

2.      For sound measurement & acoustic music input.  Pair of XLR mic connectors for quality mic input to low noise and distortion mic preamp.  Phantom P48 switchable for each mic.  

3.      Line inputs, including:

  • RCA phonos for attaching devices under test, line inputs etc

  • 1/4" mono jacks for flying test leads

  • 3.5mm stereo connector for device under test or to accept input from a portable recorder, cassette deck, etc.

4.      3.5mm “stereo” connector for a headset mic for VOIP, narration etc - connects to self powering mic preamp.  Headset Mic level control.

5.      “Other” channel allows either channel to be sourced (in mono) from the other

6.      Link to computer output for diagnostic self-test (loopback) and to meter and analyse output side levels for before & after testing

7.      Calibration signals (see Calibration section below)

8.      Spare input positions will allow for future expansion, eg other specialist mics, phono preamp for disc-to-CD conversion, telco hybrid, etc)

Input Sensitivity

The various software packages offer control of level where needed, but this may not always be enough, or convenient to use, so some additional control will be offered:

  •  A switched attenuator for each channel will offer sensitivity steps of 10dB over the range +20 to – 20dB, or 10V to 100mV on the line inputs and oscilloscope input.

  • A level control will offer fine control of gain.  CCW is zero; the CW extreme of the switch is the calibrated position.


A line out side would bring the output of the sound interface box via an amp and an attenuator to ensure adequate and adjustable drive levels:

  1.  1/4" jacks for test leads

  2. RCAs for device under test

  3.  a 3.5mm stereo jack for device under test,

  4.   the CAL terminal for multimeter and oscilloscope probes. 

The attenuator would cover the range 10V to 10mV (20dBu to –40dBu)

Fine adjustment would cover the range 0 (CCW) to CAL (CW)

Calibration approach

The unit will be calibrated to the real world using the following approach:

  •   An output will be taken from some suitable software

  • The inputs will be switched to Computer Output

  •  The output will be monitored with an external AC Digital Voltmeter

  • The output will be adjusted for a convenient level, eg 1V RMS at a frequency well within the bandwidth of the AC voltmeter (eg 50-60Hz)

  • Internal calibration controls on the Audio Lab Interface will be set so that the input metering on the software reads the same level.

  • A hardware calibration signal will be available for routine calibration checks.

Calibration Selector

The Calibration is intended as a convenient check if the PC mixer gets tweaked by any of the software.  A selector switch will offer a range of convenient calibration and check sources:

  • Sine (from an oscillator).  1kHz at 1VRMS. 

  • Square (ditto).  1kHz @ 1V Pk-to-pk

  • 5% THD - a signal with 5% THD

  • 5% EHD - a signal with 5% Even Harmonic Distortion

  • Pos pulse - narrow positive-going 1V pulses to enable absolute phase checking.

  • Computer out (Internal oscillator turned off to avoid interference)

As well as appearing at the outputs, the calibrator signal will appear at a front panel terminal convenient for clipping on oscilloscope probes and a multimeter during calibration set up and checks.

Level Monitoring

Level measurement and monitoring will be provided on-screen by software emulating either the VU meter or the European PPM ballistic characteristic or choice thereof, and calibrated through the calibration routine.

Overload detection

LEDs for each channel will warn of impending overload to the preamps that would cause clipping.

Output voltage excursions will be limited to just over 0dBfs to protect Firebox input.

Sound quality monitoring

Switch speakers to monitor Input, Output or Off

Monitor Speaker level control

Headphone output with level control for the headset headphones for Skype or when narrating.

Power outputs and control

A mains socket on the rear will provide power for powered loudspeakers and the Firebox, so that these are also switched off when the Audio Lab is not in use.

On power up, output will be muted until the Firebox has had time to synch.

Consideration will be given to an automatic power down of the audio chain in the absence of sound for a set period, in the manner analogous to a screen power-saver

A separate power circuit will also control a 3pin mains socket on the back to switch off little-used devices on the desk (eg backup drives). 

A mains socket on the side provides convenient power for devices under test. 

A multipin output connector to provide 48V, +15V, -15V, 5V and 0V for circuit board and breadboard testing..

A "hum output", approximately 1V RMS of mains hum for testing Common Mode Rejection Ratio and output impedance balance of low-level inputs and outputs.


  • Stereo ¼" jacks to XLR for testing pro gear,

  • Stereo ¼" jacks to RCA for testing domestic gear,

  • Stereo ¼" jacks to croc clips for circuit testing and faultfinding

  • Oscilloscope probes for circuit testing, faultfinding

  • Mic test box – to test Level, Freq. response, CMRR, Noise of mics and mic inputs

  • Dummy mic capsules to permit testing of internal microphone electronics

  • "Soundproof box" to establish noise floor of microphones

  • Sound level meter to calibrate absolute SPL levels

  • Measurement microphone - omni pressure mic for free-field level measurements.

  • Low self-noise mic, for examining noise and clarity issues

  • Probe mics, and associated balanced adapters and calibrators.

  • Vibration sensors, for determining nature of tube vibration

  • Calibrated Lazy Susan to test microphone polar response

  • Dummy load 4/8/16 ohms for amplifier testing

  • Breadboard, for circuit development and testing

  • Cake tin, for electrostatic shielding breadboard and other test setups

  • Magnetic pickup coil for detection of strong magnetic interference fields.

  • Audio 1:1 isolation transformer for floating sources or loads

  • Artificial flute blowing machine

  • Artificial face for flute blowing machine



I intend to adapt a number of designs and PCBs conveniently available from ESP.


The Flute Tone Investigation Series so far:

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  Created 6 Dec 09