Hello
Anybody did try the suggestion show in the image, in a class AB amp ?
About it, from the book "Self- amps-Douglas Self, fifth edition-2009", chapter 7.
"If we use an unregulated power supply (and there are almost overwhelming reasons for using such
a supply, detailed in Chapter 9) comprising transformer, bridge rectifi er, and reservoir capacitors, then these rails have a non-zero AC impedance and their voltage variations will be due to amplifier load currents as well as 100 Hz ripple. In Class-B, the supply-rail currents are half-wave-rectified
sine pulses with strong harmonic content, and if they contaminate the signal then distortion is
badly degraded; a common route for interaction is via decoupling grounds shared with input or
feedback networks, and a separate decoupler ground is usually a complete cure. This point is easy
to overlook, and attempts to improve amplifi er linearity by laboring on the input pair, VAS, etc.
are doomed to failure unless this distortion mechanism is eliminated fi rst. As a rule it is simply
necessary to take the decoupling ground separately back to the ground star-point, as shown in
Figure 7.4 . (Note that the star-point A is defi ned on a short spur from the heavy connection joining the reservoirs; trying to use B as the star-point."
Any opinions ?
Thank
Bye
Gaetan
Anybody did try the suggestion show in the image, in a class AB amp ?
About it, from the book "Self- amps-Douglas Self, fifth edition-2009", chapter 7.
"If we use an unregulated power supply (and there are almost overwhelming reasons for using such
a supply, detailed in Chapter 9) comprising transformer, bridge rectifi er, and reservoir capacitors, then these rails have a non-zero AC impedance and their voltage variations will be due to amplifier load currents as well as 100 Hz ripple. In Class-B, the supply-rail currents are half-wave-rectified
sine pulses with strong harmonic content, and if they contaminate the signal then distortion is
badly degraded; a common route for interaction is via decoupling grounds shared with input or
feedback networks, and a separate decoupler ground is usually a complete cure. This point is easy
to overlook, and attempts to improve amplifi er linearity by laboring on the input pair, VAS, etc.
are doomed to failure unless this distortion mechanism is eliminated fi rst. As a rule it is simply
necessary to take the decoupling ground separately back to the ground star-point, as shown in
Figure 7.4 . (Note that the star-point A is defi ned on a short spur from the heavy connection joining the reservoirs; trying to use B as the star-point."
Any opinions ?
Thank
Bye
Gaetan
Attachments
Yes they do have a non zero AC impedance and decoupling the grounds is correct,also the heavy current/noisy grounds to the smoothing capacitors should be very short but after that I depart from Douglas,s opinion and join John,s (JLH) in his truer decoupling by fitting a 10 ohm resistor separating the impedance's and thereby the noise/distortion injected by the outputs much heavier currents .
This has stood me in good stead helping to much reduce the output ripple that would otherwise be injected and amplified by the circuit and appear as noise at the output.
But I have to admit I am a follower of JLH and while I think DS is a genius it doesn't mean I agree totally with all he says and I am not alone in this thinking if you peruse EW over the years when Audio was a large part of EW/WW .
JLH,s designs worked for me but I accept some have the opposite opinion and both DS and JLH agreed to disagree in print in EW although JLH was too much of a gentleman to continue this argument in print but called himself an "Expert Witness " in an article printed in EW and every right he had to say that.
We are talking design philosophy here and both differed on opinion here having different audio circuit designs and sticking to them .
This engendered design engineers taking sides but read the articles for yourself over many years even decades and make up your own mind.
This has stood me in good stead helping to much reduce the output ripple that would otherwise be injected and amplified by the circuit and appear as noise at the output.
But I have to admit I am a follower of JLH and while I think DS is a genius it doesn't mean I agree totally with all he says and I am not alone in this thinking if you peruse EW over the years when Audio was a large part of EW/WW .
JLH,s designs worked for me but I accept some have the opposite opinion and both DS and JLH agreed to disagree in print in EW although JLH was too much of a gentleman to continue this argument in print but called himself an "Expert Witness " in an article printed in EW and every right he had to say that.
We are talking design philosophy here and both differed on opinion here having different audio circuit designs and sticking to them .
This engendered design engineers taking sides but read the articles for yourself over many years even decades and make up your own mind.
Hello
I'm not sure to understand about fitting a 10 ohm resistor separating the impedance's ?
Can you show me a simple schematic to illustrate that ?
Do you have or know any articles from JLH about decoupling by fitting a 10 ohm resistor separating the impedance's ?
Thank
Bye
Gaetan
I'm not sure to understand about fitting a 10 ohm resistor separating the impedance's ?
Can you show me a simple schematic to illustrate that ?
Do you have or know any articles from JLH about decoupling by fitting a 10 ohm resistor separating the impedance's ?
Thank
Bye
Gaetan
Last edited:
I use single rail supplies with opamps (which are classAB amps, albeit low powered ones) as one way to resolve this issue. Then keep 0V_power separate from 0V_signal by star earthing.
Hello
I have found the JLH original ideas and I did made my version of
decoupling by fitting a 10 ohm resistor.
I included the JLH original circuit and my version of decoupling by
fitting a 10 ohm resistor on a simple amp simulation. The D2 diode will do quite same result than a separated supply for the front and vas section.
Do I am ok in my version of decoupling by fitting a 10 ohm resistor ?
Thank
Bye
Gaetan
I have found the JLH original ideas and I did made my version of
decoupling by fitting a 10 ohm resistor.
I included the JLH original circuit and my version of decoupling by
fitting a 10 ohm resistor on a simple amp simulation. The D2 diode will do quite same result than a separated supply for the front and vas section.
Do I am ok in my version of decoupling by fitting a 10 ohm resistor ?
Thank
Bye
Gaetan
Attachments
Last edited:
