EE3305 DESIGN PROJECT
OBJECTIVE: To design an Op Amp based Audio-headphone amplifier.
EQUIPMENT REQUIREMENTS
Oscilloscope
Burr- Brown OPA 134 op amp (2 Nos)
Film Type Capacitors
0.1uF, 100 V Capacitor (2 No.)
220uF, 25V Capacitor (1No.)
1/4W Metal Film Resistor Assortment
100k Ohms Resistor (4Nos)
470k Ohms Resistor (2 Nos)
4.7k Ohms Resistor (2 Nos)
1k Ohm Resistor (2 Nos)
10k Ohm Resistor (3 Nos)
50 Ohms Resistor (2 Nos)
30 Ohms Resistor (2 Nos)
100 Ohms Resistor (2 Nos)
Portable CD player and a pair of Headphones (for Input and Output respectively)
Micro-mini Switch
DPDT micro-mini switch (2 Nos) (Radio Shack 275-626)
SPST micro-mini switch (1No.) (Radio Shack 275-624)
9V Battery (1No.)
2mA (max) low current type LED
LED Holder (1 No.) (Radio Shack 276-079)
Headphone Jacks (2 Nos) (Radio Shack 274-249)
PC Board
About (2" x 1.75") dimension vector circ-board from Mouser Electronics (1 No.)
(Stock No. 574-3677-6)
Enclosure
2.75" x 4.6" x 1" PacTec enclosure from Radio Shack (RS 270-211) (1 No.)
DESIGN GUIDANCE - A QUICK RECAP OF OPAMPS
Operational amplifiers are high-gain amplifiers with a similar general description typified by the most famous example, the LM741. The LM741 is used for many amplifier varieties such as Inverting, Non-inverting, differential, voltage follower and summing amplifier. In addition to amplifiers, op amps are used as switches and even in some digital applications as comparators or A/D converters. Op amps make use of what is called open loop gain. This open loop gain is used to for the purposes of negative feedback. Negative feedback is when the output signal is feed back to the input terminals and the gain of the op amp can be controlled. This is done because the properties of the op amp become more predicable. Negative feedback also creates a more customizable frequency response for the desired amplifier. In turn there is also and increase in the input impedance of the amplifier is negative feedback is used.
Operational amplifiers are simple to use, inexpensive and offer a very large amount of gain. These IC's come in a variety of packages with 1, 2 or 4 complete Op amps in a single IC. Op Amps have two inputs called the non-inverting and the inverting designated by the plus and minus sign, respectively. Op Amps are actually differential amplifiers because they amplify the difference between the inverting and the non-inverting inputs. Op amps will normally operate from any supply voltage in the 6 to 15 volt range. In our design of the IC based preamp we will be using an audio application specific op amp OPA134. The pin diagram for Burr-Brown OPA 134 op amp is shown below in fig 1.
FIG 1: Burr-Brown OPA 134 op amp pin diagram
Burr-Brown OPA 134 op amp has been selected for its excellent specs: FET inputs for high input impedance and low offset current, 8 MHz bandwidth, ultra low noise, ultra low distortion, etc. It has fine PSRR (power supply rejection) numbers, can run on as little as ±2.5V (very important in a portable design) and includes built-in current limiting.
DETAILED DESIGN PROCEDURE
One OPA134 Op amp, Five Metal Film resistors(values ranging from 50 Ohms to 470 K Ohms), one 0.1uF capacitor, and a DPDT micro mini switch is required for each channel (Left(L) and Right(R)) of the headphone amplifier.
STEP 1: Setting the Gain of the Op Amp
The gain of the non inverting op amp shown in fig 2 is determined by resistors R3 and R4 and is calculated by the following equation:
Voltage Gain = (R3 + R4)/ R3
FIG 2: Headphone Amplifier (One Channel)
By choosing 1K and 10K Ohms resistors for R3 and R4 respectively, we set a Voltage Gain of 11 for our OPA134 op amp. At this gain, the output impedance of the amplifier (which depends on the type of op amp) is less than 0.2 ohms throughout the audio range and a gain of 11 provides more than sufficient amplification to power a pair of audio headphones. As a rule of thumb, if a headphone amplifier has a voltage gain stage, the gain is typically set between 2 and 10.
STEP 2: Setting the Input Resistance of the Op amp and filtering Input DC Current
A) Setting the Input Resistance
In a non-inverting amplifier it is necessary to balance the source impedances on both the non-inverting and inverting terminals of the op amp. We do this by choosing R1 = R4||R3. However, in a headphone amplifier, R1 is likely to be variable, in the form of a volume control, and so we set R1 between 100K to 470K values and tune it in this range to eliminate any noise that might be associated with the output.
B) Filtering Input DC Current
In the circuit of fig 2 above, we have included a high-pass filter C1-R2 at the input. This simple filter blocks DC current at the input terminal of the op amp (e.g., from CD player) and has a corner frequency of about 15Hz.
C) Necessity for Load Resistor R5
Another notable resistor in the circuit of fig 2 is the resistor R5 at the output terminal of the op amp. R5 is an optional load resistor, which reduces residual hiss when the amp is driving low impedance headphones (e.g. 32 Ohms). Because the voltage drop across R5 reduces the maximum output of the amplifier, it is recommended trying a 50 ohm resistor first (or as low as 30 ohms). If there is still residual hiss with low impedance headphones, then increase the resistor value to 100 ohms.
D) Significance of Micro-Mini Switch
The switch SW1 at the input is used to reduce the input level as required to avoid overloading the amplifier (for example, some portable stereos have very high output voltage levels even when the volume control is set near 0). With R1 = 100K ohms, the LEVEL switch (SW1) drops the input voltage by 50%.
We have now, completed designing one channel of the headphone amplifier. Fig 2 depicts the circuit diagram for the same. An identical should be built for the second channel. Remember, there are two audio channels; Left (L) and Right(R) channel for any stereo system/headphone.
STEP 3: Designing a Portable Power Supply for the Amplifier
We are required to generate ±4.5V dual supply from a 9V battery. Fig 3 depicts a simple circuit dig to do this.
FIG 3: Battery-Based Dual Power Supply
The power supply circuit shown converts the 9V battery into a ±4.5V dual supply. Although the OPA134 could run from a single supply, it is designed for dual supplies, and a dual supply is required for direct-coupling the output. This virtual ground sits at 4.5V, but works because op amps only care about relative power supply voltages.
The left and right channels of the amplifier should be connected in parallel to the power supply. If the amplifier will be driving high impedance headphones (e.g., greater than 200 ohms) or if one simply would like more volume than what the standard design can provide, we might need to increase the power supply.
STEP 4: Putting it together
Fig 4 (below) illustrates the wiring connections for amplifier (both L-R channels along with the battery supply).
FIG 4: Headphone Amplifier Wiring Diagram (Both Left and Right Audio Channels)
A) Assembly
Using the basic wiring diagram shown above (fig 4), assemble the complete circuit on a printed circuit board while socketing the ICs using machined-contact sockets which work with low insertion force.
B) Enclosure
Finally, a 2.75" x 4.6" x 1" Pac Tec enclosure from Radio Shack (RS 270-211) with a built-in 9V battery compartment can be used to box the headphone amplifier. It comes with both opaque and red plastic front panels. Mount the headphone jacks on the plastic panel. The headphone jacks are enclosed units for 1/8" stereo plugs.
C) Good Grounding Practices
If the amplifier is housed in a plastic enclosure like the one recommended above, the LEVEL switch must be grounded or the amplifier will hum when the switch is touched. To ground the switch, strip about 1.5" of insulation from a 5" length of 22 Ga. solid wire, Tin the exposed end if necessary, and tightly wrap the exposed end around the groove at the rear of the metal mounting flange of the switch, twisting the end to form a secure, closed loop. Trim the other end of the wire to a suitable length and solder it to the circuit ground.
TROUBLESHOOTING
Built as recommended above, this amplifier is known to be a quiet performer with virtually no background noise. It is more immune to EM and RF interference than some other amplifiers. Nevertheless there may still be problems with noise.
A)
The first step in troubleshooting noise is to make sure it is coming from the amplifier itself, and not from the audio source. Disconnect the audio source and listen to the pocket amp for any background hiss, static, RF (radio frequency) or EM (electromagnetic) interference. If the headphones are low impedance types, the amplifier will function better with the load resistor R5 at higher values (between 50K and 100K Ohms).
B)
If the noise is primarily RF or EM interference and is not coming from the audio source, it is probably due to long interconnects and headphone cords, which can act as antennas that channel RF signals into the headphone amplifier. One way to resolve this problem is by placing one or more clip-on ferrite noise suppressors on the audio cables. They should be located on the end of a cable as close as possible to the input or output of the headphone amplifier. The clip-ons can be removed if the interference is temporary and subsides.
CONCLUSION
A) Project Timeline: Four Weeks
Week 1:
a) Understanding Design requirements and procedures.
b) Designing and constructing headphone amplifier for one audio channel on
Breadboard.
c) Testing the amplifier circuit for designed “noise-free” amplification (use
Oscilloscope).
Week 2:
a) Constructing on breadboard and testing portable dual power supply for the
headphone amplifier. Construct dual power supply circuit according to the design
described in STEP 3. Use an oscilloscope to test for exact +/- 4.5 VDC at the power
supply output terminals.
b) Begin constructing complete headphone amplifier circuit on PC Vector board.
Week 3:
a)
Continue constructing headphone amplifier circuit on PC vector board. Construct on single vector board amplifiers for channels L and R, parallel to each other (refer the wiring diagram in fig 4). Leave enough space on the same PC vector board to construct the dual power supply.
NOTE: When cutting the board, make sure to include at least 3 foil "buses" for the power supply and ground.
b) Construct the dual power supply and connect it to the amplifier circuits as shown in the wiring diagram in fig 4.
Week 4:
a)
Complete all circuit constructions and test your headphone amplifier. Enclose the headphone amplifier in the recommended enclosures and conclude your design project.