Description.
Here is the circuit diagram of an opamp based subwoofer filter. Audio frequencies below 200Hz are considered to be in the subwoofer range. So a subwoofer filter will be essentially a low pass filter with a cut off frequency of 200Hz.The working of this straight forward circuit is as follows.

The left channel of the audio source is connected to the non-inverting input of opamp U1 which is wired as a buffer. Opamps U2 and U3 forms a high pass filter. Output of U1 is coupled the input of this high pass filter .The high pass filtered audio signal available at the output of U3 represents left channel output. Similarly the right channel of the audio source is connected to the non-inverting input of U8 which wired as a buffer amplifier. The output of U8 is connected to the input of the high pass filter formed by opamps U9 and U10. The filtered audio signal available at the output of U10 represents the right channel audio output.

Output of U1 and U8 are coupled to the inverting input of the opamp U4.U4 performs the job of mixing the two signals. Output of U4 is coupled to the input of the low pass filter comprising of opamps U5 and U6.The low pass filter has a cut off frequency of 200Hz.The output of the filter is coupled to the inverting input of opamp U7 through the POT R22. U7 works as an output amplifier and POT R22 can be used for adjusting the gain. The audio signal available at the output of U7 represents the subwoofer output.

Circuit diagram.

Notes.

• The circuit must be assembled on a good quality PCB.
• The ICs U1 to U 10 are all opamps and you can use any opamp with a split power supply.
• I used two TL074(quad opamp) and one TL072(dual opamp).
• I used +9/-9 V DC dual supply for powering the circuit.
• The power supply connection is not shown in the circuit .Positive and negative supplies must be connected to the appropriate pins of the IC by yourself.
• Pin configuration of your opamp can be understood from its datasheet.
• It is better to mount the ICs on a holder.
• LED D1 is just a power ON indicator and it is optional.
• Some capacitance values shown in the circuit are hard to find in the market. The best solution is to combine available capacitors serially or parallely to obtain the required value.

Description.
This circuit is designed in response to a request made by Mr Vignesh. What he requested was a circuit for recording and playing voice. I think this circuit is enough for the purpose. This circuit is based on the IC ISD1020A from ISD. The ISD1020A is a CMOS single chip record/playback device. The chip can record voice for 20seconds and has many features like automatic gain control, anti aliasing filter, built in audio amplifier and smoothing filter. The IC is fully compatible to microprocessors and can be used for a myriad of applications. The voice is stored in their natural form in the non-volatile memory cells inside the IC which enables a high quality reproduction. The recorder voice can be retained in the chip for as long as 100 years and the chip is capable to have a 100000 read/write cycles.

The circuit is designed as per the application diagram in the datasheet.Pin23(CE) is the chip enable pin and it has to be held low using switch S1 in order to perform a record or playback cycle. Pin 27 (P/R) is the playback/record pin and a high level on it selects a playback cycle while a low level on it selects a record cycle. The selection can be done using the switch S3.The pin 24 (PD) is the power down pin. It has to be held high using switch S2 in order to pull the device to a extremely low power mode while there is no recording or playback (idle state).The resistor R1 and capacitor C1 determines the release time of the internal AGC circuit. The resistor R5 and capacitor C5 connected across the pin 20 and pin 21 provides an additional cut-off to the low frequency end of the voice pass band.
Circuit diagram.

Notes.

• The circuit must be assembled on a good quality PCB.
• The circuit can be powered from a 5V DC source.
• Switches S1 to S3 must be miniature SPDT switches.
• Speaker K2 can be a 16 Ω loud speaker.
• K1 can be an electret microphone.

Description.
Here I present a very simple and powerful headphone amplifier using OPA134. In addition to the IC OPA134, the circuit uses only few passive components and can easily generate a lot of sound from even the most inefficient headphones and there will be no compromise for the quality.
OPA134 is low noise, low distortion operational amplifier from BURR-BROWN semiconductors and it is exclusively meant for audio applications. The FET based input stage provides the IC with high input impedance and it makes the circuit very flexible in terms of the audio source. You can plug almost all types of sound sources like, mp3 players, iPods, mobile phones etc to the circuit.
In the circuit IC OPA134 is wired as a non-inverting amplifier. The +/-4.5V split power supply required for the IC is obtained from a 9V PP3 battery using the circuit comprising of components D1. R6, R7.R8, C2 and C3. D1 is an LED which indicates power ON. Switch S1 can be used as a ON/OFF switch .Resistor R2 and capacitor C1 forms a high pass filter with corner frequency around 15KHz.POT R1 can be used as a volume controller. The load resistor R5 will stabilize the virtual ground and prevents any noise or distortion in the output, but the output will be DC coupled.
Circuit diagram.

Notes.

• Assemble the circuit a good PCB.
• The circuit can be powered using a 9V PP3 battery.
• POT R1 can be used as a volume controller.

Description.
This is a very versatile voice modulator circuit using IC HT8950A from Holtek Semiconductors. The IC is capable of creating 7 upward or downward steps on the frequency of the input voice at a rate of 8Hz. There is also two special variation effects namely Vibrato mode and Robot mode. This circuit finds a lot of application in systems like telephone, speech processors, toys, mixers etc. A microphone is used to pick up the input voice. Push button switches S2 and S3 can be used for the upward and downward frequency stepping .Push button switch S1 can be used to activate Vibrato mode and push button switch S4 can be used to activate the Robot mode. IC HT82V733 (also from Holtek) is used to amplify the output of the voice modulator.LED D1 indicates the voice level.

Circuit diagram.

Notes.

• The circuit can be assembled on a Vero board.
• Do not give more than 4.5v to the circuit.
• Switches S1 to S4 can be miniature push button switches.
• S5 can be a miniature ON/OFF switch.
• K1 can be an 8 ohm speaker.
• IC1 and IC2 must be mounted on holders.

Description.
Here is the circuit diagram of a 6W amplifier using the TA7222AP from Toshiba.TA7222AP is an excellent integrated audio amplifier which can deliver 5.8W to a 4 Ohms load at 12V supply voltage. The IC has very good features like, muting function, low distortion, high ripple rejection, short circuit protection, thermal shut down etc. This amplifier can be operated from 8 to 12V and this makes it ideal for car radio applications.

Circuit diagram.

Notes.

• The circuit can be assembled on a Vero board.
• Use 12V DC for powering the circuit.
• The IC must be heatsinked.
• Speaker can be a 4 ohms one.
• For optimum performance input and output must be separately grounded.