What do these capacitors do?

[reddish75]

I'm trying to understand what these capacitors do in this circuit. They are the ones circled.

Thanks

[srb1954]

C103 is a power supply decoupling capacitor to reduce noise on the positive supply rail feeding into the input stage of the amplifier.

C105 is a combination DC block/high-pass filter that sets the DC feedback factor of the amplifier to unity to stabilise the DC operating point of the amplifier. It also determines the lower frequency point where the amplifier mid-band gain starts rolling off reducing to unity at DC.

C108 is a "bootstrap capacitor". It provides a boosted voltage, above the positive supply rail, to the base circuit of Q105 to maximise the positive going output swing of the amplifier to be as close as possible to the positive supply rail. It is labelled a bootstrap capacitor because the circuit works in a equivalent way to the impossible task of lifting yourself up off the ground by pulling on your own bootstraps.

It is not clear what role C110 has in the circuit. It doesn't appear to do much and it may just be a drawing error around that part of the circuit.  Knowing the resistor value that is partly obscured by the marker pen would perhaps help understand the circuit function.

[David Hess]

Even if R117 was lower than the apparent 1.X kilohms, C110 is too large in value to make a Zobel network on the output.  I agree with srb1954 that there is likely a mistake in the schematic around C110.

[reddish75]

Thanks guys, I'll try and digest your replies, here's hopefully a readable full circuit

[srb1954]

Thanks guys, I'll try and digest your replies, here's hopefully a readable full circuit

Still not clear what C110 does, assuming the circuit diagram is accurate. However whatever effect it does produce will be quite minor as the low amplifier load resistance will be dominant with the higher value resistors R117 and R118 connected to C110 isolating its affects from the output.

My best guess is that connection of R117 and R118 slightly modifies the DC bias position of the upper and lower and lower halves of the push-pull output stage to overcome some residual distortion problems. Since the value of C110 is quite large it comes into play for frequencies above 1Hz further modifying the load on the output stage to perhaps control distortion problems. With the low frequency where C110 has an effect it suggests that there may have been thermally induced distortion problems in the output stage and this was an attempt to alleviate those problems.

Of course all of the above is just speculation. Probably the only way you will find out for sure is to ask the original designer.

[Jwillis]

For C110
Maybe I'm looking at this wrong  but its in parallel with R118 ( I think its R118)  . Wouldn't it have something to do with impedance and phase angle?  The impedance would change with frequency but the phase angle looks to remain the same.

[David Hess]

For C110
Maybe I'm looking at this wrong  but its in parallel with R118 ( I think its R118)  . Wouldn't it have something to do with impedance and phase angle?  The impedance would change with frequency but the phase angle looks to remain the same.

The resistance of R117 in series with the C110 network is so high compared to the low output resistance at the output transistor emitters, that C110 cannot have any effect.