Showing posts with label preamplifier. Show all posts
Showing posts with label preamplifier. Show all posts
Friday, January 10, 2014
Electric Guitar Violin Preamplifier
Magnetic pick-ups in musical instruments have a relatively high output impedance. This can result in a reduction in treble response when connected via a long cable run or to equipment with a low input impedance. This preamplifier provides a high input impedance and a low impedance output, solving both issues. It has adjustable voltage gain and can run off a battery or DC plugpack. The input signal is AC-coupled to the non-inverting input of IC1a, part of a TL074 quad op amp.
This has JFET input transistors and the input impedance is set by a 330kΩ bias resistor which also sets the DC level at this input to half supply (Vcc/2). This is generated by a voltage divider comprising two 10kΩ resistors and bypassed by a 47µF capacitor to reject noise and hum.
IC1a is configured as a non-inverting amplifier with a gain of between 2 and 20, depending on the setting of VR1. IC1a’s output is fed to VR2 via a 22µF capacitor, allowing the output volume to be set. The audio then passes to the non-inverting inputs of the remaining three op amps (IC1b-IC1d) which are connected in parallel to provide a low output impedance; it will drive a load impedance as low as 600Ω. The 100Ω resistors in series with the outputs provide short-circuit protection for the op amps and also prevent large currents from flowing between the outputs in case they have slightly different offset voltages.
The buffered signal is then AC-coupled to two output connectors using 47µF electrolytic capacitors. For Output 1, a 47kΩ resistor sets the output DC level to ground and a 220Ω series resistor provides further short-circuit protection. Output 2 is similar but includes another potentiometer (VR3) to allow its level to be set individually. Note that this means the impedance of Output 2 can be high (up to 2.5kΩ depending on the position of VR3’s wiper).
The total harmonic distortion of this circuit is typically less than 0.01% with the gain set to six. If a TL064 is used instead of a TL074, the current drain will decrease but there will be more noise at the output. Finally, the input impedance can be increased by increasing the value of the 330kΩ resistor to suit high-impedance pick-ups.
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This has JFET input transistors and the input impedance is set by a 330kΩ bias resistor which also sets the DC level at this input to half supply (Vcc/2). This is generated by a voltage divider comprising two 10kΩ resistors and bypassed by a 47µF capacitor to reject noise and hum.
IC1a is configured as a non-inverting amplifier with a gain of between 2 and 20, depending on the setting of VR1. IC1a’s output is fed to VR2 via a 22µF capacitor, allowing the output volume to be set. The audio then passes to the non-inverting inputs of the remaining three op amps (IC1b-IC1d) which are connected in parallel to provide a low output impedance; it will drive a load impedance as low as 600Ω. The 100Ω resistors in series with the outputs provide short-circuit protection for the op amps and also prevent large currents from flowing between the outputs in case they have slightly different offset voltages.
The buffered signal is then AC-coupled to two output connectors using 47µF electrolytic capacitors. For Output 1, a 47kΩ resistor sets the output DC level to ground and a 220Ω series resistor provides further short-circuit protection. Output 2 is similar but includes another potentiometer (VR3) to allow its level to be set individually. Note that this means the impedance of Output 2 can be high (up to 2.5kΩ depending on the position of VR3’s wiper).
The total harmonic distortion of this circuit is typically less than 0.01% with the gain set to six. If a TL064 is used instead of a TL074, the current drain will decrease but there will be more noise at the output. Finally, the input impedance can be increased by increasing the value of the 330kΩ resistor to suit high-impedance pick-ups.
Saturday, March 23, 2013
Preamplifier For Soundcard
This circuit can be used for inductive pick-up elements and dynamic microphones Most soundcards have a ‘line’ input and one for an electret (condenser) microphone. To be able to connect an inductive tape-recorder head or a dynamic microphone, an add-on preamplifier is needed. Even in this day and age of integrated microelectronics, a transistorised circuit built from discrete part has a right of existence. The preamplifier described in this short article goes to show that it will be some time before discrete transistors are part of the silicon heritage. The preamplifier is suitable for use with a soundcard or the microphone input of a modem. As you will probably know, most sound-cards have input sockets for signals at line level (stereo), as well as one for a (mono) electret microphone.
For the applications we have in mind, connecting-up an inductive pick-up element or a dynamic microphone, both inputs are in principle suitable, provided the source signal is amplified as required. The author eventually chose the microphone input on the soundcard. Firstly, because the line inputs are usually occupied, and secondly, because the bias voltage supplied by the micro-phone input eliminates a separate power supply for the preamplifier. The microphone input of a soundcard will typically consist of a 3.5-mm jack socket in stereo version, although only one channel is available. The free contact is used by the soundcard to supply a bias voltage to the mono electret microphone. This voltage is accepted with thanks by the present preamplifier, and conveniently obviates an external (mains adaptor) power supply.
Preamplifier For Soundcard Circuit diagram:
A classic design:
In true transistor-design fashion, the preamplifier consists of three stages. Capacitor C1 decouples the signal received from the microphone or pick-up element, and feeds it to the input of the first stage, a transistor in emitter configuration, biased to provide a current amplification of about 300 times. Together with the source impedance of the microphone or pick-up element, capacitors C2 and C3 form a low-pass filter which lightly reduces the bandwidth. In addition, the output low-pass, R2-C3, reduces the dynamic collector resistance at higher frequencies. In this way, the filter reduces the gain in the higher part of the frequency spectrum and so helps to eliminate any oscillation tendencies.
The first, high-gain, stage is terminated by T2. Unlike T1, this transistor does not add to the overall gain, because the output signal is taken from the emitter (common-collector circuit). T2 thus acts as an impedance converter, with C4 reducing any tendency to oscillation. The output stage around T3 is a common-emitter circuit again. In it, preset P1 determines the voltage amplification. T3 is biased by means of a direct-current feedback circuit based on components R7 and C5. To this is added an ‘overruling’ dc feedback path back to the input transistor, via R6. This measure guarantees good dc stability in the preamplifier. The circuit is small enough to be built on a piece of veroboard or stripboard, and yet remain reasonably compact.
To prevent interference from external sources, the completed board should be mounted in a properly screened (metal) enclosure, with the connections to the input source and the sound card made in screened cable. The preamplifier provides a frequency-linear response. In case the source signal is marked by frequency correction (e.g., RIAA), then a matching linearization circuit should be used if the relevant signals are used by the computer.
Source : http://www.ecircuitslab.com/2011/07/preamplifier-for-soundcard-circuit.html
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