Scheme
Considering that the LM317 can operate with a maximum current of 1.5 A, the output is relatively low. Fortunately, this limitation can be overcome by connecting several LM317s in parallel, as shown in the diagram:
Click to enlarge
The maximum input voltage for LM317 is 40 V, so it would seem that the amplifier can be powered from a bipolar source with a voltage of no more than ±20 V. However, the operational amplifier allows operation with a maximum supply voltage of ±18 V. Therefore, in the author’s opinion, the work circuits from a power source with a voltage of ±15V would be a completely reasonable and safe solution.
Having decided on the supply voltage, we can calculate the required quiescent current. For an 8 Ohm load it will be 15 V/8Ω=1.875 A. The theoretical maximum power will be about 14 W, although in practice it turned out to be 12 W with a purely resistive load. Since the acoustic system is far from being a resistive load in its properties, the quiescent current should be taken somewhat higher, for example, 2.2A. In this case, the value of the current-setting resistor will be 1.25/2.2=0.56 Ω.
In this case, the resistor will dissipate slightly less than 3 W, so it is recommended to use a resistor with a power of at least 5 W. With these parameters, the power consumption of one amplifier channel will be 30 × 2.2 = 66 W.
What do you want? Class "A" !
Basic circuitry of Grimmi
- Basic circuit of the Grimmi amplifier
- Vacuum Triode Bias Voltage
- MOSFET transistors for amplifier
- Bipolar NPN transistors for amplifier
- Single-ended or push-pull amplifier
- Tubes or transistors - impact on sound
- Power repeater and power amplifier class: A and A/B
- Cascode transistor amplifiers and their circuits
Section: Modes and principle of operation of an amplifier using transistors class: A, B, A/B, C, D
- Transistor and amplifier operating mode class A
- Transistor and amplifier operating mode class B, A/B
- Transistor and amplifier operating mode class C, D - PWM
↑ Sound
I auditioned my amp with a 12AU7 tube preamp as it provides the clearest sound.
I have no idea about this amp's distortion coefficients or other numbers, other than to say that it has accurate sound reproduction and a delicately textured tonal color. To operate the amplifier, highly sensitive, efficient acoustics are required, as it produces approx. 5 Watts RMS (and up to 15 Watts peak, which I clearly observed on the oscilloscope screen). Bass reproduction turned out to be much better than one would expect from such a solution. The amplifier easily drives my 12-inch three-way speakers.
