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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.

Item Value Units
Sound card mic input required for full output at max gain 0.010 Vpp
Sound card mic input required for full output at max gain -40.0 dBV
Microphone sensitivity -56.0 dBV/Pa
Sound pressure level of speech at 300mm (wrt 20uPa) 74.0 dB
Speaking distance 0.120 m
Sound pressure level of speech at microphone 82.0 dB
Microphone output -68.0 dBV
Microphone output 0.40 mV
Reserve gain 10.0 dB
Required amplifier gain 38.0 dB
Cf (use preferred value less than this) 413 pF
Rf (use preferred value greater than this) 7981 Ohms

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.

The AmpChamp preamplifier built in a small die cast box with input and output jacks. You could build it in a plastic box, but the diecast aluminium box has shielding advantages.

The output jack is a 3.5mm tip-ring-sleeve (stereo) which is used for the audio out connection and power supply from the sound card.

The input jacks are a 6.5mm tip-sleeve (mono) jack and XLR F jack in parallel.

Modified circuit showing the 4.7K resistor in the collector of Q2.

The 10K bias resistor is only for electret microphones and should not be used for a dynamic microphone.

Note that the 2.2K feedback resistor between the collector of Q2 and the emitter of Q1 may need to be changed depending on the required gain. In my case I used a 8.2K as preferred value that will deliver at least the required gain as calculated for Rf in Table 1. Additionally, the 0.0015µF cap in parallel with Rf should be decreased in proportion to the increase in Rf to preserve frequency response. I used 330pF.

Modified board layout showing the 4.7K resistor in the collector of Q2.

The feedback components which will need modification to suit needs are the 2.2K nearest and the 0.0015µF capacitor that are next to each other between Q1 and Q2.

Sourcing parts

Item Jaycar Price
Amp Champ Pre-amp kit KC5166 7.95
Aluminium die cast box HB5062 8.65
3.5mm stereo jack PS0132 1.60
4.7K resistor    
2.7K resistor (or as needed for Rf)    
330pF capacitor (or as needed for Cf)    
DC power connector (optional) PS0522  
Miscellaneous hardware, screws, feet etc    

Choose one of more of the following input connectors

6.5mm mon jack PS0162 1.35
XLR 3 pin jack PS1024 5.00
3.5mm stereo jack PS0132 1.60

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

Item Value
Microphone-in Sensitivity: 10mV to 200mV RMS
Impedance: 500-600 Ohms
Stereo input jack: TIP Signal RING Bias (if available) SLEEVE Ground
Line-in Sensitivity: 100mV to 2V RMS
Impedance: 47K-50K Ohms
Line-out 0 to 2V RMS
Impedance: assumed 10K Ohms, some cards 600 Ohms

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

Version Date Description
1.01 11/11/2004 Initial release.
1.02 13/11/2004 Added information on frequency response.
1.03 15/11/2004 Added details of gain / frequency response. Design spreadsheet updated to calculate Cf.
1.04 16/11/2004 Added words re DC supply from ring of mic-in connector.

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