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Author Topic: Class AB Audio Amplifier for Small Signals  (Read 1865 times)
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especialista
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« on: December 29, 2018, 11:19:06 11:19 »

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!)
« Last Edit: January 01, 2019, 10:31:45 10:31 by especialista » Logged
PM3295
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« Reply #1 on: December 31, 2018, 08:43:30 08:43 »

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.
« Last Edit: December 31, 2018, 09:23:59 09:23 by PM3295 » Logged
metal
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« Reply #2 on: December 31, 2018, 11:41:31 11:41 »

what's the output power? Can we increase it and keep the profile as above?
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especialista
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« Reply #3 on: December 31, 2018, 02:59:23 14:59 »

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.


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PM3295
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« Reply #4 on: January 07, 2019, 12:14:14 12:14 »

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.
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Faros
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« Reply #5 on: January 10, 2019, 04:13:05 04:13 »

@PM3295

which software is used for those analyses ?
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dennis78
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« Reply #6 on: January 10, 2019, 07:41:53 07:41 »

I think this is TinaTI software
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PM3295
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« Reply #7 on: January 10, 2019, 02:47:34 14:47 »

I think this is TinaTI software

That is correct, although some of the features are only available on the full registered industrial version.
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mr_byte31
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« Reply #8 on: Yesterday at 03:17:17 AM »

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.
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especialista
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« Reply #9 on: Yesterday at 02:52:44 PM »

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.
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PICker
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« Reply #10 on: Today at 01:12:06 AM »

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.
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