1969M Small Class A Amplifier

1969M Small Class A Amplifier
 
In 1969, the British John Linsley Hood designed a push-pull output transistor amplifier HOOD1969.
The amplifier based on this design is called the 1969 Class A amplifier. The circuit
 
diagram of the 1969 Class A amplifier
is as follows:
   
 
           John Linsley Hood published this circuit in 1969. It is a 10W pure Class A amplifier. The circuit is very simple. Each channel is composed of 4 transistors. Although the power is not large, the sound quality is beautiful, which attracts many DIY enthusiasts.
 
          John Linsley Hood's 1969 circuit is simple, easy to make, and the sound quality is also good, so many versions of 1969 have been derived.
 
1969M is one of them.
 
   The 1969M (1969MOS) circuit designed according to 1969 is as follows. Because the final stage is changed to a field effect tube, it is referred to as 1969M. This version can work in Class AB, which means that the working current is not so large and the power is greater than 1969. .
The original 1969 can only work in pure Class A, with low efficiency and only 10W output. The current is large, and a heat sink of considerable size is required.
 
 
The circuit diagram of actual application
 
is analyzed. The circuit improvement plan
 
 
explains the red circled part in the figure. R1, the bias part adopts the structure of the original 1969. If this resistor is too large, the midpoint voltage is not half of the power supply. If it is too small, the startup impact sound is loud. I use 30K, and there is a "bang" sound when the power is turned on, but it is acceptable. R2, the value of this resistor will affect the working point and the sound. If it is too small, the sound is thin, and if it is too large, the distortion is large. I use 8.2K here, and the low frequency feels fuller and sounds quite comfortable.
R3, this resistor adjusts the final current. When I powered on for the first time, I used 1K. Hey, don't say it's just right, but the heat sink is still hot after a long time of use. However, if the current is adjusted down, the sound quality will be worse, so if the heat sink is sufficient, the current should be above 300mA.
Improvement plan I used a 10K adjustable potentiometer to control the current. The current is best at 1.8A, and the sound quality is excellent, provided that heat dissipation is done well. R4, don't underestimate this resistor. When I used 1K, I felt that the sound speed was very good, but it can't be too small. After all, it has loss and will increase the burden on the power amplifier. If it is 10K, the low-frequency speed will be slower. It depends on your choice. . .
The 47K resistor in the figure is used to adjust the midpoint voltage. Using a fixed value will limit the playability of the board. The improvement uses a 100K potentiometer, which can adjust the midpoint voltage at any time to match the problem that the input voltage is low and cannot be used. 12V to 48V input can be used.
 
 
 
 
   I have to say that the output capacitor is much better than a single capacitor in parallel. I have tried it and the effect is obvious.
 
 
Secondly, the vibration-damping capacitor from the base to the collector of TIP41 was cancelled by me. There were no problems in use. This is all due to reasonable wiring. Finally,
the source resistor of IRF260N with a value of a few tenths of an ohm was also cancelled by me.
The above is the actual situation in my production. I dare not guarantee that you will not have any problems if you follow my way. The key is to make the layout reasonable.
 
 
 
 
 
 
Preliminary listening experience:
the sound source is Bluetooth 5.0, and the volume control is directly used on the mobile phone without going through the pre-stage. . The speaker is a channel of the multimedia speaker without a built-in amplifier, and a mono test is performed. As soon as the sound is turned on, I feel that the sound quality is not bad, it is relatively full, the low frequency is loose, and the low frequency volume is more than the amplifier of the multimedia speaker itself, and it has a faint taste of the original 1969 version. The feedback grounding resistor used in the test machine is 562 ohms, so the level is low, so it is adjusted to 1000 ohms. The ear is close to the speaker, and there is no obvious hum and noise. Change to another channel for testing, the basic situation is the same as above, and the test machine is basically successful.  The sound source of
 
 
 
the test machine
is ROON player.
The music is a master tape burned file .
If you need the file, you can send it to private chat.
This speaker has been used for many years, the rubber edge of the speaker has been replaced, and the sound quality is not as good as before. It is just right to use 1969M to drive it.
 
 
There is also a second version of the file
in progress
 
 
and it is on the way to run away.