What is the use of Resistors in between stages

PRS

Joined Aug 24, 2008
989
R3, together with C5 decouple this part of the circuit from the rest of the supply so that signals in the supply line are not amplified by the first stage's gain.

The same is true for R7 and C2 for your second stage.

The 3dB points are at f=1/(2*pi*R*C). Stray signals get attenuated.
 

Thread Starter

setsunaseiei

Joined Mar 25, 2009
12
R3, together with C5 decouple this part of the circuit from the rest of the supply so that signals in the supply line are not amplified by the first stage's gain.
Pls. tell me how it decouples the first from the second and the second from the last. How do I find the practical values for resistance and capacitance for each?
 

PRS

Joined Aug 24, 2008
989
R3-C5 de-couple the base of Q1 so that it will not amplify very low frequencies. Since you're using a battery there is no ripple voltage present and so I can't see why this decoupling is even there.

R7-C2 do the same. I can only think that both decoupling circuits are meant to attenuate higher frequency signals to prevent the whole circuit from oscillating.
 

Audioguru

Joined Dec 20, 2007
11,251
Decoupling is especially needed with a battery (a little 9V battery) because the battery has a fairly high internal resistance that keeps rising as the battery runs down.
The internal resistance causes the battery voltage to jump up and down with the load current.
Without decoupling then the jumping supply voltage feeds into the input stages then the amplifier amplifies it, causing putt, putt, putt motorboating oscillation.
 

Thread Starter

setsunaseiei

Joined Mar 25, 2009
12
what are the best values for the R and C to use when designing, like any rule of thumb for this? thanks to all who answered.
 
RC values: You want the -3dB rolloff frequency way below the lowest signal frequency. Here R3 and C5 give a rolloff at ≈ 13 mHz, which is plenty low enough for audio.
 
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