Sound card microphone preamplifier

Overview

This article describes a simple preamplifier to enable use of a dynamic microphone on a computer sound card.

Design

Sound cards commonly have sufficient sensitivity for close speaking to an electret microphone, but often don't have sufficient sensitivity for use with a moving coil microphone which typically has about 20dB less output than common electrets.

Table 1 tabulates measured and calculated values for a Creative Vibra 128 sound card under Win2K, and the dynamic microphone to be used. The microphone output level is 28dB short of the clipping level with the mic input gain set to maximum and the optional 20dB internal preamplifier active.

A spreadsheet that performs the above calculations is available, click to download the design spreadsheet.

A ready solution to the problem is the "Pre-Champ Preamplifier" described in Silicon Chip Magazine July 1994. With minor modification it will suit, and kitsets including a PCB are readily available at low price.

The Reserve Gain quantity is included to provide for a small amount of adjustment using the sound card controls and an allowance for shortfall of actual gain over the simple prediction based on the feedback network.

Microphone sensitivity can be obtained from spec sheets or measured. Dynamic microphones intended for vocal performance will commonly have values around -55dBV/Pa, a Shure SM58 for example is specified at -54.5dBV/Pa (though there is usually a fairly wide tolerance... so quoting to a precision of a tenth of a dB seems questionable). Be suspicious of sensitivity specs of -75dB, they are probably referenced to 1 dyne / sq cm or 0.1Pa... add 20dB to calculate the sensitivity in dBV/Pa.

The amplifier is modified to reduce current so that it can operate from the DC supply on the ring of the microphone-in connector of common sound cards. If you sound card does not supply power in that way, you will need to obtain a DC supply for the amplifier separately.

Implementation

Sound cards usually incorporate a power supply for electret FET amplifiers on the ring of the microphone-in jack, and the supply is typically +5V through a resistor of several kilohms.

The collector resisistor of Q2 is increased to 4.7K to reduce current consumption so that the DC supply rail for the amplifier operates at about 3.5V. Under these conditions, the amplifier was capable of 500mVpp out before onset of clipping.

The 2.2K feedback resistor should be changed to provide the design gain figure. In the case of the design in Table 1, the required gain called for 7981 value, so 8.2K was used, being the preferred value that will deliver the required gain as calculated for Rf in Table 1. Actual gain will be a little less than predicted using just the two resistors in the feedback network, especially at high gain levels. (Excessive gain is undesirable, increases noise, reduces bandwidth,  and increases the risk of clipping in the sound card mic amp stage. The 10dB reserve gain allowed provides sufficient additional gain to allow the record gain setting to be at about 75% of maximum.

The prototype exhibited a signal to noise ratio on recordings of better than 55dB, 65dB with the mic switch in the OFF position (which shorts the mic audio out). This is much better than from a typical PC headset. The frequency response with 8.2K and 330pF in the feedback network is adequate, lower 3dB point is 78 Hz, and upper 3dB point is 44kHz (well in excess of the upper limit of typical dynamic microphones). Measured gain was 36dB.

Sourcing parts

Prices from Jaycar's web site on 12/11/2004. Total parts price is $20 to $25 depending on the choice of input connectors.

Common sound card specifications

Be aware that the microphone input to a sound card is usually mono. Do not be mislead by the tip-ring-sleeve jack, the ring is used for a DC supply for the electret amplifier.

Links

Changes

V1.04: 23 April 2008 21:10