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Dynamic Microphone Preamplifier






Dynamic microphones have less voltage output compared to electret condenser mics (with internal FET) and tend to be less sensitive as well, so they generally need to be closer to the sound source to give of their best. Although this makes them less affected by room acoustics (which can be an advantage) it does mean they will need a bit more preamplification to achieve the same output as an electret. The preamp described here is based on the NE5534 low noise opamp which is still readily available, not too expensive and can run quite happily on a fairly low voltage which was a consideration for this project as I wanted it to run from a standard 5 volt USB port or phone charger




Schematic


Running the opamp at just 5 volts is right on it's limit. More would be better, but seeing that 5V is now available everywhere, this was my requirement. The 5 volts obtained from a standard type A USB port found on most PCs is usually swamped by electrical noise, so the 56 ohm resistor and 100u capacitor were added to form an RC filter which completely eliminates this interference without dropping too much voltage to the IC. 56R was chosen as the 'sweet spot' as anything much less would be ineffective as a filter and higher values would drop too much voltage. 5 volts isn't much to play with as it is without reducing it further! As it stands, the opamp receives about 4.8 volts which is still enough to work OK. The 47K resistors and 100u capacitor also perform a filtering function as well as providing half supply biasing for the IC. The circuit is constructed on a standard 9 x 25 hole strip board with JST XH type connectors for the terminations. The type of enclosure and audio/power sockets used is down to personal preference




Veroboard layout


The schematic is 'text book standard' showing the opamp configured as an inverting amplifier with the gain being set to about 142 by the 3K3 and 470K resistors (470,000 3300 = 142.4). The 3K3 resistor also sets the input impedance which was chosen to best suit a low Z dynamic mic which can range from approx 50 to 600 ohms (typically 150 to 300 ohms). It's not quite as easy as just choosing a resistor with the same value as the mic. This line of thinking goes back to the early days of telephone cables which ran between towns and cities, where they were interested in power transfer. Here we are more interested in voltage transfer for which there is a rule of thumb that says the input impedence of a mic preamp should be at least 10 times that of the microphone feeding it, so I chose 3300 ohms as it should be a good match for common microphones such as the Shure SM57/58 and many others

Construction Notes

When using small sized stripboard with ICs there are some tricks that can be used to make more efficient use of the space available. Here, the unused pins 1, 5 and 8 of the IC are disconnected by snipping them off on the IC socket (not on the actual IC) shown greyed out on the diagram above. This, along with the 'solder blob' link between pins 7 and 8 allows the positive and ground rails to become available either side of the IC giving a more streamlined layout. Sometimes going around an IC is unavoidable, hence the short wire link from pin 6 to the lower track for the 470K feedback resistor... you can't win 'em all, though I did hide the link under the IC. Note also that some tracks need to be cut which are indicated by an X