FM Power Amplifier 250mW

This project is a simple 2-transistor VHF power amplifier, with about 16dB gain, and requires no tuning or alignment procedures. Wideband techniques have been used in the design and the circuit is equipped with a "lowpass" filter to ensure good output spectral purity. The project has been designed for assembly on a single-sided printed circuit board. The circuit is specifically designed to amplify the output of 7mW to 10mW WBFM transmitters (wide band) to a final level of 250mW to 300mW, after the filter.

Circuit Description
The first stage (Q1) operates in Class-A. Although Class-A is the least efficient mode, it does offer more RF gain than other clases of bias, and Q1 is a low-level stage, when compared to the higher power Q2 stage. The output of this stage is around 70mW of RF power. The stage is
untuned so that it gives a very broadband characteristic. The transistor is biased by means of R5, R6 and L6, and the residual (standing) DC current is set by R4. The input signal is coupled by C9 to the Base of the transistor. Q2 is operated in Class-AB which leads to greater efficiency, but the RF gain is only about 8dB, but it amplifies the output of Q1 to typically 250mW. Q2 is
biased by means of R3, R2 and L4. The input signal from Q1 is coupled to the Base of Q2 via C7.
The voltage regulator Q3 (78L08) is used to regulate the supply voltage to Q1 and the bias votages to both Q1 and Q2 so that the output RF power is relatively constant, even with large variations of supply voltage. Q3 also removes supply ripple as well as providing power for an
FM transmitter like Kit 3018 wireless microphone with the required DC 8v power.

The output of the amplifier is filtered with a low-pass filter to reduce the output spurious and harmonic content. The output filter consists of C3, C4, L1 and L2.

COMPONENTS
Resistors 5%, 1/4W, carbon:
10R R1 brown black black
22R R7 red red black
47R R3 yellow orange black
120R R4 brown red brown
470R R2 yellow violet brown
2K2 R5 red red red
4K7 R6 yellow violet red
2N2369 Q1
2N4427 Q2 1
Ceramic caps
33p C3
47p C4
1n C5 C6 C7 C8 C9
10n C1 C11
Ecaps:
220u/16V C2
10u/25V C10
78L08 Q3
RFC L4 L5 L6
Ferrite L3
3 turn coil L2
5 turn coil L1
2 pole terminal block
HS106 heatsink ( 

A Simple Hybrid Audio Amplifier Circuit

A Simple Hybrid Audio Amplifier Circuit diagram. The debate still goes on as to which are better, valves or transistors. We don’t intend to get involved in that argument here. But if you can’t make your mind up, you should try out this simple amplifier. This amplifier uses a valve as a pre-amplifier and a MOSFET in the output stage. The strong negative feedback makes the frequency response as flat as a pancake. In the prototype of the amplifier we’ve also tried a few alternative components. For example, the BUZ11 can be replaced by an IRFZ34N and an ECC83 can be used instead of the ECC88. In that case the anode voltage should be reduced slightly to 155 V. The ECC83 (or its US equivalent the 12AX7) requires 2 x 6.3 V for the filament supply and there is no screen between the two triodes, normally connected to pin 9. This pin is now connected to the common of the two filaments.

Project Image :

The filaments are connected to ground via R5. If you’re keeping an eye on the quality, you should at least use MKT types for coupling capacitors C1, C4 and C7. Better still are MKP capacitors. For C8 you should have a look at Panasonic’s range of audio grade electrolytics. P1 is used to set the amount of negative feedback. The larger the negative feedback is, the flatter the frequency response will be, but the smaller the overall gain becomes.

Circuit diagram:

Simple Hybrid Audio Amplifier Circuit Diagram

With P2 you can set the quiescent current through T2. We have chosen a fairly high current of 1.3 A, making the output stage work in Class A mode. This does generate a relatively large amount of heat, so you should use a large heatsink for T2 with a thermal coefficient of 1 K/W or better. For L1 we connected two secondary windings in series from a 2x18V/225 VA toroidal transformer. The resulting inductance of 150 mH was quite a bit more than the recommended 50 mH. However, with an output power of 1 W the amplifier had difficulty reproducing signals below 160 Hz. The distortion rose to as much as 9% for a signal of 20 Hz at 100 mW. To properly reproduce low-frequency signals the amplifier needs a much larger coil with an iron core and an air gap. This prevents the core from saturating when a large DC current flows through the coil.

Parts layout:

Such a core may be found in obsolete equipment, such as old video recorders. A suitable core consists of welded E and I sections. These transformers can be converted to the required inductor as follows: cut through the welding, remove the windings, add 250 to 300 windings of 0.8 mm enamelled copper wire, firmly fix the E and I sections back together with a piece of paper in between as isolation. The concepts used in this circuit lend themselves very well to some experimentation. 

The number of supply voltages can be a bit of a problem to start with. For this reason we have designed a power supply especially for use with this amplifier (Quad power supply for hybrid amp). This can of course just as easily be used with other amplifiers. The supply uses a cascade stage to output an unstabilised voltage of 170 V for the SRPP (single rail push pull) stage (V1).

PCB layout:

During initial measurements we found that the ripple on this supply was responsible for a severe hum at the output of the amplifier. To get round this problem we designed a separate voltage regulator (High-voltage regulator with short circuit protection), which can cope with these high voltages. If you use a separate transformer for the filament supply you can try and see if the circuit works without R5. During the testing we used a DC voltage for the filament supply. 

Although you may not suspect it from the test measurements (see table), this amplifier doesn’t sound bad. In fact, it is easily better than many consumer amplifiers. The output power is fairly limited, but is still enough to let your neighbours enjoy the music as well. It is possible to make the amplifier more powerful, in which case we recommend that you use more than one MOSFET in the output stage. The inductor also needs to be made beefier. Since this is a Class A amplifier, the supply needs to be able to output the required current, which becomes much greater at higher output powers. The efficiency of the amplifier is a bit over 30%.

Author: Frans Janssens - Copyright: Elektor Electronics

22 Watt Car Subwoofer Amplifier

22W into 4 Ohm power amplifier, Variable Low Pass Frequency: 70 – 150Hz

This unit is intended to be connected to an existing car stereo amplifier, adding the often required extra "punch" to the music by driving a subwoofer. As very low frequencies are omnidirectional, a single amplifier is necessary to drive this dedicated loudspeaker. The power amplifier used is a good and cheap BTL (Bridge Tied Load) 13 pin IC made by Philips (now NXP Semiconductors) requiring a very low parts count and capable of delivering about 22W into a 4 Ohm load at the standard car battery voltage of 14.4V.
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Circuit diagram:

22 Watt Car Subwoofer Amplifier Circuit Diagram

Parts:

P1_____________10K Log Potentiometer
P2_____________22K Dual gang Linear Potentiometer
R1,R4___________1K 1/4W Resistors
R2,R3,R5,R6____10K 1/4W Resistors
R7,R8_________100K 1/4W Resistors
R9,R10,R13_____47K 1/4W Resistors
R11,R12________15K 1/4W Resistors
R14,R15,R17____47K 1/4W Resistors
R16_____________6K8 1/4W Resistor
R18_____________1K5 1/4W Resistor
C1,C2,C3,C6_____4µ7 25V Electrolytic Capacitors
C4,C5__________68nF 63V Polyester Capacitors
C7_____________33nF 63V Polyester Capacitor
C8,C9_________220µF 25V Electrolytic Capacitors
C10___________470nF 63V Polyester Capacitor
C11___________100nF 63V Polyester Capacitor
C12__________2200µF 25V Electrolytic Capacitor
D1______________LED any color and type
Q1,Q2_________BC547 45V 100mA NPN Transistors
IC1___________TL072 Dual BIFET Op-Amp
IC2_________TDA1516BQ 24W BTL Car Radio Power Amplifier IC
SW1____________DPDT toggle or slide Switch
SW2____________SPST toggle or slide Switch capable of withstanding a current of at least 3A
J1,J2__________RCA audio input sockets
SPKR___________4 Ohm Woofer or two 8 Ohm Woofers wired in parallel

The stereo signals coming from the line outputs of the car radio amplifier are mixed at the input and, after the Level Control, the signal enters the buffer IC1A and can be phase reversed by means of SW1. This control can be useful to allow the subwoofer to be in phase with the loudspeakers of the existing car radio. Then, a 12dB/octave variable frequency Low Pass filter built around IC1B, Q1 and related components follows, allowing to adjust precisely the low pass frequency from 70 to 150Hz. Q2, R17 and C9 form a simple dc voltage stabilizer for the input and filter circuitry, useful to avoid positive rail interaction from the power amplifier to low level sections.

Notes:

• IC2 must be mounted on a suitable finned heatsink
• Due to the long time constant set by R17 and C9 in the dc voltage stabilizer, the whole amplifier will become fully operative about 15 - 30 sec. after switch-on.

Technical data:

Output power (1KHz sinewave):
22W RMS into 4 Ohms at 14.4V supply
Sensitivity:
250mV input for full output
Frequency response:
20Hz to 70Hz -3dB with the cursor of P2 fully rotated towards R12
20Hz to 150Hz -3dB with the cursor of P2 fully rotated towards R11
Total harmonic distortion:
17W RMS: 0.5% 22W RMS: 10%

2W Amplifier Circuit

Modelsed for self-powered 8, 4 & 2 Ohm loudspeakers, Bass-boost switch

This amplifier was designed to be self-contained in a small loudspeaker box. It can be feed by Walkman, Mini-Disc, iPod and CD players, computers and similar devices fitted with line or headphone output. Of course, in most cases you will have to make two boxes to obtain stereo. The circuit was deliberately designed using no ICs and in a rather old-fashioned manner in order to obtain good harmonic distortion behavior and to avoid hard to find components. The amplifier(s) can be conveniently supplied by a 12V wall plug-in adapter.Closing SW1 a bass-boost is provided but, at the same time, volume control must be increased to compensate for power loss at higher frequencies.
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Circuit diagram :

2W Amplifier Circuit Diagram

Parts:
P1----------10K
R1----------33K
R2----------33K
R3----------33R
R4----------15K
R5----------1K
R6----------1K
R7----------680R
R8----------120R-1/2W
R9----------100R-1/2W Trimmer Cermet
C1 ----------10µF-63V
C2 ----------10µF-63V
C3-----------100µF-25V
C4-----------470µF-25V
C5-----------47pF-63V
C7-----------470µF-25V
C6-----------220nF-63V
C8-----------1000µF-25V
D1-----------1N4148
Q1-----------BC560C
Q2-----------BC337
Q3-----------TIP31A
Q4-----------TIP32A
SW1---------SPST switch
SPKR--------3-5 Watt Loudspeaker

In use, R9 should be carefully adjusted to provide minimal audible signal cross-over distortion consistent with minimal measured quiescent current consumption; a good compromise is to set the quiescent current at about 10-15 mA.  To measure this current, wire a DC current meter temporarily in series with the collector of Q3.

You have purchased construction quality Hi-Fi amplifier operating in class AB. Thanks to its small size, protected against short circuit on the output, overheating and the possibility of attachment directly to the metal box will find its use in many places.Whether it is ozvučování concerts or fitting into the estate, such as the guitar.Jet shocks to the speaker when you turn on and off the amplifiers in this circuit is minimal.Watch out, the device operates with a relatively large currents and tensions.Do not construction in both hands, it is under stress.

Wiring diagram

This is the recommended involvement of the circuit TDA7294 company SGS Thomson. All material lies in the ICs. How preamplifier, as well as end-stage DMOS, MUTE function, STBY, protection against overheating, short circuit on the output. Capacitors C6 and C8 are the filter and cover the tip of sampling, it is not necessary dimension to the exercise of the power supply, which will benefit permanently. For example, the sound of drums in the strike will cause many times higher collection of resources than the median value. These capacitors in particular avoiding the emergence of bias, resulting in the strong signals as a result of excessive voltage decreases in resources.

Capacitors C7 and C9 improve the stability of the RF amplifiers zákmitům. Resistor R3 between 2 and 14 outlets providing negative feedback as DC (stabilizes the working point, respectively. DC voltage at the output), and together with R2 alternating link. It determines the overall gain, reduces distortion and balances characteristic frequency amplifiers. Here listed are recommended by the manufacturer, to achieve the best parameters, but if necessary, within certain limits vary. For example, if you need to limit the performance of amplifiers because of a smaller cooler or have loudspeaker systems for smaller performances and to prevent their destruction, just increase R2 value, or reducing the value of R3 reduce gain. DC capacitor C1 separates from the amplifier input signal sources that had no direct working point amplifiers. Through switches and resistors R4 and R5 will bring tension to STBY (Stand By) and MUTE. C3 and C4 allow continuous activation and deactivation of these functions.

Construction

Place PCB PT002 (supplied with green nepájivou mask, printing components and drilling machine CNC), which is designed to be received from the external environment as far as possible to interference and match proudovému load. Therefore, the power and output connections as broad as possible. It is used cuprex FR4 with 35μm strong copper foil.

Order mounting: 1st capacitors, resistors 2nd MKT 5 mm 3rd terminals 4th switches 5th other capacitors 6th TDA7294

DA7294 pájejte last try, and the shortest possible time. Pájejte conscientiously so as to avoid cold couplings. Across-the-board connection through the large currents, only high-quality construction makes the minimum risk. Connection output signal is limited in one place. There is no pattern nanesena nepájivá. At this point apply a drop of tin to improve the conductivity and the possibility of a larger load without the risk of burning the journey. You have two ways to complete construction:

A: not intend to use the features STBY and MUTE. Then switch SW1 and 2 zapájíte directly into the joints and their situation will be running for the top amplifiers. In this connection, which is based directly from the diagram, you can only disable the function, rather than immediately turn on. Can be activated only by switching radio before switching on the amplifier and the spent condensers. Use: for animation, when we want to measure sleep donation but later exploit these features.
B: Do you intend to use the features. In this case, the switch will not be soldered into the joints (logically, not because of any change to open the box or combo), but lead out through the wires (included). In this engagement - see the picture, are optional features, the possibility of activation and deactivation at any time during the operation. SW2 se zapojuje stejně jako SW1. SW2 is involved as well as SW1. When the two structures is necessary in the terminals MT and SB bring positive supply voltage.

The last thing it should be in construction to make, is attached aluminum profile, which allows direct attachment to the metal skříním and easier attachment coolers. The body must be first navrtat - in the middle of a hole for attaching IO, d = 3mm, with navrtáním d = 6mm in the rear of one quarter of units for flush screw head to the back wall to enclose the entire area to chladícímu element. Then, just as necessary drill holes on the edges of the profile. And again, for example, d = 3 mm, the zavrtáním d = 6mm in one quarter (2.5 mm) of its thickness. See the real picture above. Adequate evacuation of heat from ICs provide aluminum plate on the surface 0,75 dm ^{2 and} a thickness of 5 mm. There are screws and nuts to facilitate the handling of the plate mounted amplifier in the animation. Thermal resistance of external cooler: 0.8 K / W.

Its not required but is symmetrical source of tension, there is attached recommended tested and well-functioning wiring diagram. Toroidal transformer best, at least 150W, optimally 2x25V/200W. Policies to protect the source and amplifier from destruction. LEDs indicate the tension on both branches. Once unlit, for example, LED1, you know, it is burned fuse, or other problem in a positive power. Capacitors C1 and C5 are the filter. The minimum recommended amount 5mF, but higher values can only benefit - less badboy. Then it depends on how much youre willing to finance investment in resources and its quality, because comparatively, with a capacity of capacitors is growing, as well as their price. In the greater interest we are willing to source with the PCB and all parts sold as a kit.

Revival

Equipment is not necessary to recover. When good design, sufficient resources sized works on the first engagement. In any case, do not amplifier without the cooler. Threatened with destruction or degradation of the quality of reproduction.

R1, R3, R4 R1, R3, R4	22k	C1 C1	MKT 220 - 470n MKT 220 - 470n	C6, C8 C6, C8	1000m/50V
R2 R2	680	C2, C5 C2, C5	22m/50V	C7, C9 C7, C9	MKT 100n/63V MKT 100n/63V
R5 R5	10k	C3, C4 C3, C4	10m/50V	IO1 IO1	TDA7294