Class AB Audio Amplifier for Small Signals

[especialista]

This is a simple amplifier for those ol' fashioned way PCB audio pickers, did by me in the past.

RV1 - Adjusts input signal
RV2 - Ajusts clipping between Q1 and Q2



(First post, sorry if I did any mistake!)

[PM3295]

If you use global feedback instead of local feedback, you should be able to get rid of the large electrolytics, improve LF response and reduce the THD% figure by a factor of about 20.

[metal]

what's the output power? Can we increase it and keep the profile as above?

[especialista]

what's the output power? Can we increase it and keep the profile as above?

Never measured, but I'd tell something around 3/4[W] at maximum.
It was not designed for "rock'n'roll". Just to amplify small signals to a small acoustic box.

[PM3295]

With a few more simple modifications, it should perform very well. Together with the higher supply voltages, it could produce about 20 W into a 4 ohm speaker load at a low THD near 0.004%. As shown all harmonics of the 1 kHz input tone is down almost 90 dB. Bear in mind that the original design without global feedback may have produced over 3% THD at full power. For good thermal stability, the diode should be mounted close to the heat-sink.

Make sure you set P1(multi-turn) for an idling current between 10-20 mA flowing in the main output transistors once the amplifier has warmed up.

Posted on: January 01, 2019, 12:33:38 12:33 - Automerged
Distortion and noise can be further cleaned up by using a better low noise opamp like the LT1028. It shows nearly a 4 times reduction in the THD going from the LF351 to a LT1028.

[Faros]

@PM3295

which software is used for those analyses ?

[dennis78]

I think this is TinaTI software

[PM3295]

I think this is TinaTI software

That is correct, although some of the features are only available on the full registered industrial version.

[mr_byte31]

I think it is better to avoid using opamp ( specially LM741) , they have bad frequency response.
I used one before and it made the sound quality very bad.


Posted on: January 16, 2019, 04:14:42 04:14 - Automerged
I also remember that you need to add a small resistance (5 Ohm) on the common of Q3 and Q4 to avoid mismatch !
at least that what I remember since long time ago.

[especialista]

I think it is better to avoid using opamp ( specially LM741) , they have bad frequency response.

You're absolutely right.
That's why I've choosed High Speed JFET LF351 for this amplifier.
You should try one of these: They are amazing for pre-amp.

[PICker]

I agree, the JFET input OPA does the difference.
There are new designs for reducing distortion and noise and increasing precision like the TI OPA3234 SoundPlus.

[metal]

I agree, those OPAs sound much better than 741 and NE5532..