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Audio Line Driver
Notes:
This preamplifier has a low output impedance, and is designed to drive long cables, allowing you to listen to a remote music source without having to buy expensive screened
cables. The very low output impedance of around 16 ohms at 1KHz, makes it possible to use ordinary bell wire,loudspeaker or alarm cable for connection. The preamplifier must be
placed near the remote music source, for example a CD player. The cable is then run to a remote location where you want to listen. The output of this preamp has a gain of
slightly less than one, so an external amplifier must be used to drive loudspeakers.
Hi-Fi Preamplifier
Design: Graham Maynard
Email graham.maynard1@virgin.net
Notes:
This circuit was submitted by Graham Maynard from Newtownabbey, Northern Ireland. It has an exceptionally fast high frequency response, as demonstrated by applying an 100kHz
squarewave to the input. All graphs were produced using Tina Pro.
The Preamp's Bode Response
Squarewave Response with 100kHz Input Signal Applied
Total Noise at Output Measured with 600R Load
Signal to Noise Ratio at Output
2 Watt Audio Amplifier
Circuit : Andy Collinson
Email : anc@mitedu.freeserve.co.uk
Description:
A 2 Watt audio amplifier made from discrete components.
Notes:
This was one of the earliest circuits that I ever designed and built, in Spring 1982. At that time I had only an analogue meter and a calculator to work with. Although not
perfect, this amplifier does have a wide frequency response, low harmonic distortion about 1.5%, and is capable of driving an 8 ohm speaker to output levels of around 5 watts
with slightly higher distortion. Any power supply in the range 12 to 18 Volts DC may be used.
Circuit Description
The amplifier operates in Class AB mode; the single 470R preset resistor, PR1 controls the quiescent current flowing through the BD139/140 complimentary output transistors.
Adjustment here, is a trade-off between low distortion and low quiescent current. Typically, under quiescent conditions, current is about 15 mA rising to 150 mA with a 50 mV
input signal. The frequency response is shown below and is flat from 20Hz to 100kHz:
The circuit is DC biased so that the emitters of the BD139 and BD140 are at approximately half supply voltage, to allow for a maximum output voltage swing. R9 and R10
provide a degree of temperature stabilisation which works as follows. Ifthe output transistors are warm, the emitter currents will increase. This causes a greater voltage drop
across R9 and R10 reducing the available bias current. All four transistors are direct coupled which ensures:-
(i) A good low frequency response
(ii) Temperature and bias change stability.
The BC109C and 2N3906 operate in common emitter. This alone will provide a very high open loop gain. The output BD139/140 pair operate in emitter follower, allowing the
amplifier to drive low impedance speakers. The signal to noise ration is shown below:
This amplifier has a S/n ratio of 115dB at 1kHz. Overall gain is provided by the ratio of the 22k and 1k resistor. A heat sink on the BD139/140 pair is recommended but not
essential, though the transistors will run "hot" to the touch.
Original March 1999
Last Revision 26-6-06 by ANC
Headphone Amplifier
Circuit : Andy Collinson
Email: anc@mitedu.freeserve.co.uk
Description:
An amplifier to drive low to medium impedance headphones built using discrete components.
Notes:
Both halves of the circuit are identical. Both inputs have a dc path to ground via the input 47k control which should be a dual log type potentiometer. The balance control
is a single 47k linear potentiometer, which at center adjustment prevents even attenuation to both left and right input signals. If the balance control is moved towards the left
side, the left input track has less resistance than the right track and the left channel is reduced more than the right side and vice versa. The preceding 10k resitors ensure
that neither input can be "shorted" to earth. Amplification of the audio signal is provided by a single stage common emitter amplifier and then via a direct coupled emitter
follower. Overall gain is less than 10 but the final emitter follower stage will directly drive 8 ohm headphones. Higher impedance headphones will work equally well. Note the
final 2k2 resistor at each output. This removes the dc potential from the 2200u coupling capacitors and prevents any "thump" being heard when headphones are plugged in. The
circuit is self biasing and designed to work with any power supply from 6 to 20 Volts DC.
Low Voltage Preamplifier
Circuit :Andy Collinson
Email :anc@mitedu.freeserve.co.uk
Description:
A low voltage preamplifier optimised for 3 Volt operation.
Notes:
This is a special low voltage version of my audio preamp. T1's emitter voltage is biased close to half supply voltage (1.5V) to allow for maximum output voltage swing. Both
transistors are direct coupled and have closed loop feedback to aid temperature stability.
T2 realises the amplifiers full voltage gain, and for low noise operation, T2 collector current is about 70uA. T1 merely buffers T2 and operates in emitter follower mode
providing a good low output impedance. The overall S/N ratio measured at the output is shown below:
Signal to Noise Ratio
Capacitor C3 decouples the emitter resistor of T2. Without C3 the gain of T2 would be approximately R1 / R4. With C3 the gain of T2 is now R4 in paralell with the input
impedance of T1 / the small signal emitter resistance of T2. The overall voltage gain of the circuit is around 28 times as shown below. Harmonic distortion is about 8%.
Harmonic Distortion
Frequency response is flat from 50Hz to around 200KHz. Phase change across this range varies little, see bode plot below.
One drawback of this circuit is that the input impedance is fairly low. This is because the input of T2 is low and has C3 decoupling the emitter. The inclusion of R7 in
series with the base of T2 raises input impedance slightly. A plot of input imedance versus freqency response is shown below.
Input Impedance
The output impedance is very low, around 66 ohms as shown in the plot below. Note that this is a preamp and therefore designed to feed into a power amplifier not drive
direct loads directly.
Output Impedance
24 Watt Class A Amplifier
Circuit :Marc Klynhans
Email : Marc Klynhans
Description:
A 24 Watt Class A Amplifier made from discrete semiconductors, built and tested by Marc Klynhans from South Africa.
Specifications:
Here are the specifications:
24W Class A into 8 Ohm
100mHz - 100kHz flat
305mV input for 24W into 8 Ohm (33dB gain)
THD is very low, although I have not been able to measure it properly
Notes:
The supply voltage can be between 34V and 46V and the quiescent current should be set to 1.7A measured through R25 (a voltage of 0.75V must be measured over R25 for a
quiescent current of just under 1.7A). R23 is a trimmer and must be set to maximum resistance (10kOhm) when powering up. Then the resistance of R23 must be decreased until the
the quiescent current is achieved. If the amplifier is mounted on a big enough heatsink ( 0.6K/W at most) then the amplifier is very safe from thermal runaway. Intelligence must
be used when choosing power and voltage ratings of resistors and capacitors.
I have been using this amplifier with a pair of Celestion F20's and the sound is unbelievable!
Marc's website may be found at: http://mrcshobbies.blogspot.com |
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