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Chip Amps Amplifiers based on integrated circuits

LM1875 in parallel configuration and used in a composite amplifier.
LM1875 in parallel configuration and used in a composite amplifier.
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Old 20th February 2020, 12:40 PM   #491
FauxFrench is online now FauxFrench  France
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Join Date: Dec 2017
Location: The mountains, calm and quiet.
I finally implemented 100pF (foil) and 6K2 in series as feed-back for the OP-AMP in my composite amplifier. Good stability but an important over-shoot at the output of the OP-AMP with a fast square-wave at the input due to the OP-AMP being faster than the power stage. For frequencies in the audio-band it is not a problem. I will try to clamp the output amplitude of the OP-AMP.
I measured an output off-set of 10mV (0.5mV at the OP-AMP input) and noticed that the "pop" at start-up and shut-down now appeared. It is most likely due to OP-AMP reaching its operational equilibrium slower that the power stage. Still no hiss or hum.
Tested the amplifier on the speakers with different kind of music inclusive Turtles "Happy together". I found no artefacts and the bass is powerful. With Turtles, the choir (and some trumpets I believe) was still transparent.
I calculated the loop-gain at 1KHz to around 120. That should bring the overall THD to the level 0.000X% as measured by palstanturhin. Exactly how much, my ears cannot tell and my present sound-card does not allow tests below 0.00X%.
I will experiment with OP-AMP output clamping and my better regulated power supply before constructing the other phase-inverted half.

Last edited by FauxFrench; 20th February 2020 at 12:58 PM.
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Old 20th February 2020, 01:18 PM   #492
chermann is offline chermann  Austria
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Join Date: Jan 2017
Location: Vienna
yes.yes.yes....very good FF
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Old 20th February 2020, 10:37 PM   #493
FauxFrench is online now FauxFrench  France
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Join Date: Dec 2017
Location: The mountains, calm and quiet.
I got my regulated symmetrical power supply on it tonight. Very low dynamic output impedance. A quick test with +/-24V supply and 4 Ohm indicates much less "woolly" saturation tops than with my unregulated supply. I will try to take photos tomorrow.
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Old Yesterday, 02:14 PM   #494
FauxFrench is online now FauxFrench  France
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Join Date: Dec 2017
Location: The mountains, calm and quiet.
With my low-impedance regulated power supply adjusted to +/-22Vdc and with 1KHz at the input, I got the following results:

Pict.1: Clipping at 36Vpp in 2.7 Ohm. Vertical part due to recovery of OP-AMP output.
Pict.2: Square-wave at 15Vpp in 2.7 Ohm. Spikes at start difficult to see.

With my regulated power supply, the clipped flat tops are much less "woolly" than with the unregulated power supply.
I will try to remove the SQW start spikes by clamping the OP-AMP output.
Attached Images
File Type: jpg CompAmp1875_36Vpp_2.7Ohm.jpg (214.7 KB, 25 views)
File Type: jpg CompAmp1875_15Vpp_2.7Ohm.jpg (235.5 KB, 26 views)

Last edited by FauxFrench; Yesterday at 02:17 PM.
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