Dear Friends
I am having trouble with the amplifier mentioned in the subject, I am unable to find out the cause of the oscillation shown in the video below.
I have replaced most of the electrolytic and film capacitors, including the ceramic capacitors. With the exception of one capacitor, the replaced components have no effect on the oscillation, except for the 10pF ceramic capacitor connected to the collector-base of transistor Q5 / Q10 (MPSA42). Increasing this changes the oscillation frequency and decreases it's amplitude, but even at 100pF the excitation is not eliminated (although the amplitude is an order of magnitude lower).
I have read elsewhere that it is not advisable to increase this capacitance too much, as this will increase the distortion. Also here on the forum I read that the collector-base capacitance (Ccb) of the transistor in the voltage amplifier stage is important, although it is not very high for the MPSA42 (~3 pF). Someone suggested the KSC3503/KSA1381 or the 2SC2682/2SA1142 pair.
Please help me where to look further? Thank you very much!
I am having trouble with the amplifier mentioned in the subject, I am unable to find out the cause of the oscillation shown in the video below.
I have replaced most of the electrolytic and film capacitors, including the ceramic capacitors. With the exception of one capacitor, the replaced components have no effect on the oscillation, except for the 10pF ceramic capacitor connected to the collector-base of transistor Q5 / Q10 (MPSA42). Increasing this changes the oscillation frequency and decreases it's amplitude, but even at 100pF the excitation is not eliminated (although the amplitude is an order of magnitude lower).
I have read elsewhere that it is not advisable to increase this capacitance too much, as this will increase the distortion. Also here on the forum I read that the collector-base capacitance (Ccb) of the transistor in the voltage amplifier stage is important, although it is not very high for the MPSA42 (~3 pF). Someone suggested the KSC3503/KSA1381 or the 2SC2682/2SA1142 pair.
Please help me where to look further? Thank you very much!
Apart from the capacitors has anything else been changed/altered , ie original output devices ? all base stopper resistors still present and 1K0 ?
Have you tried removing C6? It doesn't seem to offer any use at audio frequencies, but removes the common-mode operation of the differential pair at higher frequencies.
valgamaa:
Thanks! Removed C6: the oscillation amplitude reduced to 1.02Vpp (from 1.6Vpp).
As I understand it, that 100nF with the 180 ohm resistor creates a break-up frequency of around 8.8 kHz, so it also has a bit of an effect on the audio frequency band.
Thanks! Removed C6: the oscillation amplitude reduced to 1.02Vpp (from 1.6Vpp).
As I understand it, that 100nF with the 180 ohm resistor creates a break-up frequency of around 8.8 kHz, so it also has a bit of an effect on the audio frequency band.
May books claim that the break frequency is the RC product where R is the emitter degeneration resistance. In reality it is the intrinsic emitter resistance (Ic/Vt), which in this case will be higher.
The more I look at the schematic, the more I think the designer didn't know what they were doing. C5 doesn't do anything at all (current source into a (nominally) virtual earth). I would be cautious of the capacitor in parallel with R14 too.
I think the problem is lack of phase margin for the closed-loop. The cap you circled is the only component that limits open-loop bandwidth, so you are right to try changing that. Before doing so though, I would bypass VR1 to eliminate the possibility that the feedback is through the supply rails, and remove the R14 bypass capacitor.
I think the problem is lack of phase margin for the closed-loop. The cap you circled is the only component that limits open-loop bandwidth, so you are right to try changing that. Before doing so though, I would bypass VR1 to eliminate the possibility that the feedback is through the supply rails, and remove the R14 bypass capacitor.
You may be right, it looks like the 22nF capacitance in parallel with R14 is a later "development" 🙂
There is no room for the cap on the PCB.
There is no room for the cap on the PCB.
In my experience, C4 is too high. Try 10p.
Then, Miller capacitor on Q4 and Q5 with 33p.
Leave out C6.
Greetings Pat 🙂
Then, Miller capacitor on Q4 and Q5 with 33p.
Leave out C6.
Greetings Pat 🙂
I would place the capacitor you removed from across R14, and put it across the ends of VR1, not to the wiper.
valgamaa:
After those modifications (bypass VR1 and removed 22nF), the osc. freq increased to 1.8MHz, and the amplitude also increased a little:
After those modifications (bypass VR1 and removed 22nF), the osc. freq increased to 1.8MHz, and the amplitude also increased a little: