Revisiting the X20

I have long been been interested in the work of Clive Sinclair and the products that sinclair radionics introduced. They where the first company to introduce a commercial class D amplifier called the X10 in 1965, by all reports it

Sinclair X10 Amplifier Ad

was ahead of the available transistor technology and was shortly replaced by the X20 design, which also had a short shelf life. In the days before EMI was a problem this amplifier drove the loudspeaker directly. After this Class D amplifiers disappeared from the mainstream market for a long time in favor of linear designs. This got me thinking how would a slight modification of these old designs work with modern transistors and a more conventional output amplifier.

The X21 Amplifier

As usual with Sinclair designs its a masterclass in how do some complex electronics down to a consumer cost point, the transistors Q1 and Q2 form a multi vibrator circuit working at about 40KHz

X21 Schematic Page 1

The ratios of the charging circuits lead to a waveform with a short pulse high as shown on the scope plot below which is looking at the junction of R4 and R5.

Transistor Q3 then acts as a integrator, the scope shot below shows the connection of R7 & R8 forming a saw tooth waveform.

The original X20 design was designed for pickup from a record player, this per-amplifier is formed by transistors Q4 to Q6. As I only had a phone for a music source instead I used Q6 as a simpler pre-amp shown below this gives a typical 200mV signal coming from a phone about a gain of 5.


The audio signal and the triangle wave are then summed together(Q7). RV1 is used to adjust the duty cycle of the PWM signal so the amplifier can be set to mid range. The combined signal is next put through the discrete schmitt trigger formed by Q8 &Q9

X21 Schematic Page 2


Q10 forms a level translator for the power transistors Q11 and Q12, this short Output PWM video clip below shows the PWM output before the filter.

Looking after the filter the Audio signal is recovered

Considering the oscillator is at 40KHz the sound quality is very good as it gets to the higher frequencies there is some noticeable breakup.

The Bill of Materials for the project is below

QTY  PART-REFS           VALUE               
---  ---------           -----               
2    R1,R14              10k                 
2    R2,R11              100k                
1    R3                  27k                 
3    R4,R12,R21          1k                  
1    R5                  15k                 
1    R6                  270k                
1    R7                  2.2k                
1    R8                  4.7k                
1    R9                  6.8K                
1    R13                 220                 
2    R15,R17             3.3k                
1    R16                 39k                 
5    R18,R20,R23-R25     470                 
1    R19                 100                 
1    R22                 1M                  
1    R26                 22k                 

4    C1,C8,C9,C17        100u                
2    C2,C5               470p                
1    C3                  270p                
2    C4,C12              1u                  
1    C6                  1nF                 
1    C11                 10n                 
1    C13                 5u                  
1    C14                 690n                
1    C15                 680u                
1    C16                 1000u               
6    C18-C23             1uF                 

7    Q1-Q3,Q6-Q9         BC557               
1    Q10                 BD139               
1    Q11                 TIP147              
1    Q12                 TIP142              

4    D1-D4               DIODE               

4    J1-J3,J6            CONN-SIL2           
2    J4,J5               TBLOCK-I2           
1    L1                  22uH                
1    RV1                 125k