On which circuit are you talking about? Because I'm lost here.

 

Should the dc voltage between the Q7 and Q8 emitters not be 6 volts?
And how do I achieve that?

No. RB4 and RB6 have a voltage drop that causes the bottom of the waveform to be clipped if the output emitters voltage is 6V. Increase the emitters voltage or remove RB4 and RB6 then add negative feedback instead.

Add DC negative feedback like all audio amplifiers have. The DC negative feedback will probably also add AC negative feedback that reduces the gain and reduces the distortion.

 

On which circuit are you talking about? Because I'm lost here.

Hi:

I am talking about the circuit on the right side of the attached file.

David

 

I am talking about the circuit on the right side of the attached file.

But in this circuit, you have change RB6 from 1.5kΩ to 2.2kΩ and now the DC voltage at the Q7,Q8 emitters is 4.55V.

 

But in this circuit, you have change RB6 from 1.5kΩ to 2.2kΩ and now the DC voltage at the Q7,Q8 emitters is 4.55V.

Ok. Here are my two circuits. Both have the emitters at 1/2 supply. The
circuit on the left has a positive peak voltage of 374mV and a negative
peak of 441mV. Something is wrong.

I think the one on the right is okay.

 

Very strange my LTspice shows this:


As you can see the output voltage is more than 2V for the right circuit for 500mV at the input.

 

Very strange my LTspice shows this:
View attachment 163056

As you can see the output voltage is more than 2V for the right circuit for 500mV at the input.

I wonder if my spice models for the transistors are somehow messed up?

 

I doubt, it must be something else.


Here you have the models that I use.

.MODEL TIP31C NPN(IS=1.62181E-13 ISE=1.75416E-11 ISC=4.36516E-14 XTI=3 BF=83.6 BR=20.607 IKF=6.98433 IKR=0.997156 XTB=1.5301 +VAF=110.5 VAR=159.374 VJE=0.636 VJC=0.408 RE=0.56 RC=0.96 RB=164.793 RBM=0.100291 IRB=1.24287E-7 CJE=4.77E-10 CJC=7.29E-11 +XCJC=0.589205 FC=0.5 NF=0.9899 NR=0.989511 NE=1.95 NC=1.014 MJE=0.327 MJC=0.339 TF=2.3733E-8 TR=1.0000E-8 ITF=1 VTF=10 XTF=10 EG=1.1605 VCEO=100 ICRATING=3 MFG=TEXAS)

.MODEL TIP32C PNP(IS=6.77594E-13 ISE=1.31133E-11 ISC=1.31133E-11 XTI=3 BF=198.8 BR=25.4966 IKF=0.891251 IKR=0.410482 XTB=1.2648 +VAF=77.429 VAR=70.9603 VJE=0.59 VJC=0.5 RE=0.06 RC=0.16 RB=161.0 RBM=3.097 IRB=3.548134E-5 CJE=2.7E-10 CJC=1.07E-10 +XCJC=0.589205 +FC=0.5 NF=1.001 NR=1.004 NE=1.98 NC=1.12 MJE=0.319 MJC=0.352 TF=2.3733E-8 TR=1.0000E-8 ITF=1 VTF=10 XTF=10 +EG=1.0863 VCEO=100 ICRATING=3 MFG=TEXAS)

 

I doubt, it must be something else.


Here you have the models that I use.

Thanks alot these models work much better.

The output for the circuit on the left is still skewed and I am not sure how to fix it.

The one on the right looks good.

Could I please trouble you for the models for the 2N3904 and 2N3906 just in case
there is a problem with them as well.

Thanks,

David

 

Ok. Here are my two circuits. Both have the emitters at 1/2 supply. The
circuit on the left has a positive peak voltage of 374mV and a negative
peak of 441mV. Something is wrong.

I think the one on the right is okay.

Your circuit on the left side does not have enough base current (from RB3) for Q2 to pull up the output but Q1 produces plenty of current for Q3 to pull down the output.

 

Very strange my LTspice shows this:
View attachment 163056

As you can see the output voltage is more than 2V for the right circuit for 500mV at the input.

The circuit at the left side has Q1 overloaded by the output transistors. the circuit at the right side has one emitter follower feeding another emitter follower so the load on Q6 is very low resulting in more output level.

 

By the way.Please note that in the right circuit, the emitter resistor of the first stage has a large value.This reduces the gain of the first stage and reduces distortion.Although a large amount of undistorted output can be obtained using general feedback, not messenger.The right-hand circuit has an additional amplifier, which gives a greater current to the base of the npn transistor at the output.

 

I still have a comment on the size of the output capacitor.The dynamic loudspeaker has a mechanical resonance at the lower edge of the frequency range.Therefore, for better damping it is better to take a larger capacitor.Those.the capacitor at the output of the amplifier should not determine the frequency response.This should be done in the intermediate and input stages of the amplifier.