Balanced Microphone Preamp Project
Full circuit design for this very high quality, low-noise preamplifier.
A simple PCB-based design that may be used as a separate low noise microphone preamplifier or incorporated into a mixer. Design by Paul White and Simon Bateson.
We have had several readers' letters on the subject of mixer front ends, and most of these have requested a transformerless balanced design that could be incorporated into their own projects.
As it stands, the design presented here provides a maximum gain of 60dB and includes a 20dB pad switch, additional information being provided for those of you who wish to add a line input and/or a phantom powering facility.
This type of circuit is employed in many commercial mixers where a discrete pair of transistors, usually PNP types, feed the differential inputs of an op amp. PNP transistors are used because they generally exhibit better noise characteristics than NPN types but there is another approach.
It is widely known that the noise generated by a transistor is roughly proportional to its collector current and so, if many transistors are connected in parallel and the collector current effectively shared between them, what happens?
You could be forgiven for thinking that fifty transistors connected in parallel would each produce two percent of the noise that a single transistor carrying all the electrical current would produce, and so when the noise from all fifty transistors is added together, you end up back where you started.
Fortunately, this is one of those rare occasions when the laws of physics are on our side or, to put it another way, the jam butty lands jam side up.
The signal applied to all the transistors is the same, and so the outputs will also be the same, plus the noise generated by each individual transistor. The output signal therefore, is the sum of the output currents from all fifty transistors, but what of the noise?
Noise is random or non-correlated and so, when two or more sources of noise are added together, some components will add whilst others cancel, the result being that the sum of fifty noise sources is considerably less than fifty times the original noise level.
When compared to the noise figure product by a single pair of transistors, fifty pairs show an improvement of 17dB which is a significant and worthwhile reduction.
Unfortunately, 50 pairs of transistors are expensive, take up lots of room and are a real pain to solder in place! This is where my good friend Simon Bateson came to the rescue when he found a single IC that just happens to contain fifty pairs of transistors and so, a few soldered joints later, the design was looking a lot more appealing with relatively few modifications to the original circuit.
For all other purposes, IC1 may be considered as being just a single pair of transistors in one can, the overall gain being adjusted by means of RV1 which controls the amount of negative feedback via C4. Maximum gain occurs when RV1 is set to minimum when the theoretical gain is defined by the ratio of R12 and R7 which is 1000 or 60dB. To obtain a progressive level control, RV1 should be a reverse log pot but, as these are hard to come by, a linear one may be substituted without too much hardship.
The minimum gain of the circuit is about 6dB but this may be reduced by a further 20dB by means of the pad switch.
Figure 1 shows the full circuit diagram including the line input, phase change, and phantom powering options, the line input being attenuated by roughly 20dB in order to maintain a sensible range of gain control.
Insert all the components into the PCB, (see Figure 2a), wiring the jack socket as shown in Figure 2b with off-cuts from the resistor legs. This ensures that the jack socket and pot are the same height, making mounting much tidier.
The switch may be a pushbutton type mounted straight onto the board, or a DPDT toggle switch may be hard-wired in if preferred. As always, check the polarity of electrolytics and ICs before powering up, the optimum supply voltage being between +/-12 and +/-15 volts [see E&MM July 84 RackPack project].
House the PCB in a screened box or instrument case; the pot and jack socket nuts will support the circuit adequately and so no special mounting is required. The input socket may be an XLR or a stereo jack, but in either case, make sure that you get the polarity to agree with the rest of your system. The input labelled positive (+) undergoes no inversion throughout the circuit whilst the negative (-) input is effectively inverted, an arrangement for a phase change switch being shown in Figure 1, which may be easily incorporated if required.
Comparing the circuit performance to a conventional input stage (-124dB equivalent input noise spec), set to give the same gain, the noise performance of this circuit was significantly better. Even compared to a high-quality transformer-coupled preamp, the results were noticeably less noisy, so if you are in the business of recording quiet acoustic sounds with moving-coil microphones, a couple of these preamps will come in very handy.
A PCB for this project is available from HSR at the Editorial address for the inclusive price of £2.95. Please allow 28 days for delivery and make cheques payable to 'Music Maker Publications Ltd'.
Feature by Paul White
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