Electronics > Projects, Designs, and Technical Stuff
weird opamp configuration
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xavier60:

--- Quote from: stojke on January 18, 2019, 11:10:28 pm ---I am no electronics expert, all I know is from practice, but this amplifier should sound a lot louder and at a lot lesser turn of the volume pot. The owner confirmed this, some reseller, by owning multiple units where the others had a greater amplification than this one.
Still , it makes no sense for an amplifier not to amplify the input signal , whats the deal with that?
It only made sense that due to failed capacitors the voltage should be higher, and also an parallel from an Akai amplifier that amplifies the input signals via an opamp setup to almost 15x on maximum volume. (In Akai case volume pot being before the opamp)

--- End quote ---
CMOS  analog selector switches  tend to have non-linear channel resistance with signal swing.
It's common for them to feed into high input impedance buffers to minimize loading and therefore distortion.
Show us the power amplifier section so that we can calculate its voltage gain.
Doctorandus_P:
Trust me:
You've shown the classical circuit diagram for an opamp in voltage follower configuration.
Whenever there is a direct connection between the output and the inverting input of an opamp you have a voltage follower.
(With some exceptions, for example there must be a proper power supply and common mode voltages have to be met, and if the opamp is not unity gain stable, you might have an oscillator)

The rest of the schematics is utter bollocks and irrelevant, so it does not matter that it is not shown.

If you want to find out why there is no 30V on the output of the circuit of which you don't show the schematics is a complete mistery to the rest of the world, and probably your favourite secret.
stojke:
Sorry for the delay, I didn“t expect to see many people interested in my problem.
I cant upload the entire schematic due to file limits, and the scan is completely broken, I have edited the images so it fits on one sheet.

xavier60:
The gain of the power amp is calculated in the same way as a non-inverting op-amp based amplifier.
The resistors that set the gain for one of the channels are R174, R20 and R16. If I'm reading the values correctly, the gain calculates to 161.
The is unusually high by my experience.
If the power amplifier sections are responsible for the loss of gain, C16 and the 1uF input capacitor would be highly suspect.
stojke:
I dont really understand but what I am getting is the following:

CD input : 440Hz test tone @ 0.207mV AC > Passes trough the Tone board : 0.207mV AC at maximum volume on output ( @ CN1 )
If I insert my (home made tm) Op Amp modification with applied gain before the volume pot I get around ~10V AC ( @ CN1 ) at max volume and the output sounds nice and loud as it should but with distortion.

Because I have* no practical nor theoretical knowledge of precise operation of amplifier circuits I had no other choice but to modify the area that seemed the most logical to be the origin of the problem.
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