![]() ![]() I have used solid 'tag' tantalum caps in this circuit, but they are (or were) revoltingly unreliable and I stopped using them after it was necessary to change every tantalum cap in a batch of about 20 small 8 channel stage mixers. Use of Low Leakage electros may be worth the effort, but I have not experimented with this option. The requirement for electrolytic capacitors does not compromise anything, and for the impedances involved relatively large value caps are a must. Needless to say, the use of metal film resistors is a must to get the best possible noise performance. Even then, lower noise was certainly possible, but the circuit complexity and cost increased very quickly. These days it's easy to get lower noise, but when these were made in quantity (in the 1970s) it wasn't a bad effort for such a simple circuit. This is completely unattainable in practice, because of the noise from the microphone itself as well as other extraneous noises which cannot be eliminated. This translates to an equivalent input noise of about 3.2nV√Hz, or 446nV for a 20kHz bandwidth. Measurements taken when I was building lots of these show that the equivalent input noise was about -127dBm, so with a gain of 40dB, signal to noise ratio should be about 86dB relative to an output of 0dBu (775mV RMS). (Note that the value of R5 has been revised from its previous value of 560 ohms.) The alternate values (4.7k and 180 ohms) can be used if you need to drive a lower than normal impedance. The value of 390 ohms is optimum when driving higher impedances. ![]() A value of 330 ohms biases the emitter of Q2 to about 19V, and this allows an output swing of ☑0V into a 22k ohm load. If the circuit is driving a following stage with an input impedance of around 22k, it is beneficial to reduce the value of R5 if the maximum possible output swing is needed. That means that Q1 is operated from close to a perfectly constant current, which raises the gain and makes the transistor much more linear. All this from a single amplifying transistor ! This is obtained due to the bootstrap circuit (R2, R3 and C2), which forces the voltage at both ends of R2 to be (almost) the same. The open-loop gain of this little circuit is about 3,400 - this is obtained by disconnecting the feedback resistor, and bypassing R5 with a suitably large value capacitor. For this reason, making the preamp with variable gain is almost an essential requirement. The output level from a well known brand of vocal mic has been measured at over 1 Volt peak-to-peak with loud singers, so the 'conventional wisdom' of mics having low output is clearly wrong. Note that this is far too high to use with any microphone used for close-miked vocals or instrument amplifiers, but is suitable for normal speech.īy making the 'Set Gain' resistor a 50k linear pot, the gain can be varied from virtually 0 up to a maximum of 40 (32dB), with low noise and distortion at all settings. With the component values shown, impedance matching is correct for a 600 Ohm mic, and the gain is about 40. It requires a well regulated (or extremely well smoothed) supply voltage of 30V, and will typically be able to supply a maximum output level of around 7V RMS allowing for typical component spreads. I have used this circuit in Front-of-House, foldback (monitor) and studio mixers, and managed to obtain extremely good results - I still have a little 6 channel mixer (which I use only occasionally now) using this circuit, and have never been even slightly tempted to replace it with even the best of opamps (or Figure 3 for that matter). If you happen to have a suitable one in your junk box, give it a try with this circuit - I doubt that you will be disappointed with the result. It is most regrettable that good mic transformers are rather hard to come by, and are expensive. The shield of the balanced cable must be earthed of course, but in my experience with live music and studio work, less noise is picked up if the internal wiring is floating. In most cases, a good mic transformer will actually outperform active balancing circuits, because there is (or should be) no ground reference. One limitation is that it is not balanced, which is not a problem in a home recording environment, but may allow the mic lead (and case) to pick up noise with long cable runs or in a hostile environment.Īs shown, it is not really suitable for professional work (although it has been used in a stage mixer with excellent results), but the addition of a 1:1 microphone transformer on the input will convert it into a balanced preamp with very high performance. This is a design for a low noise microphone preamplifier, which is ideally suited to low impedance (600 Ohm nominal) microphones. ![]()
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