LM1875 Input noise

Thread Starter

Xerxsea

Joined Feb 23, 2022
8
(1) Op Amp to Push Pull Emitter Follower | All About Circuits

Hello, so thanks to help from members of this forum I was able to build a class ab amplifier. I ended up using a lm1875 class ab ic, as it was less components and easier for me to heatsink. I have a few questions and they seemed beyond the scope of the original post as it is a different design, so I decided to make a new one. I hope that is okay.

the circuit I mostly copied from the typical application circuit diagram in the datasheet:
1709237485965.png

only modifications I made was changing c6 with a 100uf, as I did not need such low frequency response, R5 with a 470 ohm resistor, and changing R6 with a 500k potentiometer ( I wanted to test a very wide range of gains)


With my built design I had an issue with oscillation, in my original post I was told a capacitor parallel with r6 in this diagram would help with oscillation, I tried using a 47pf capacitor here to fix this but it didn't seem to change anything. what I ended up finding worked for removing the oscillation was increasing the value of C5 to about 0.7uf. problem solved but I would like to know why the first solution did not work here. I understand my solution increased the flow of high frequency current through R7-C5.

But main issue I am not trying to resolve is noise, I have a large amount of noise on the input, its audible on my speaker with no connection to input, and being on the input is increased in amplitude with the gain. I dont know if it's from poor prototype board design layout, i have also read resistors can give off something called thermal noise. There just seems to be many sources for the noise and I'm not sure where or how to begin troubleshooting it.

some points on the noise(measured at input of amplifier with nothing connected to it and amplifier powered):
about 13us between noise spikes
10mv peak, which does NOT change depending on vcc voltage or gain.



any help is greatly appreciated!
 
Last edited:

Audioguru again

Joined Oct 21, 2019
6,694
The LM1875 is a power amplifier. Its original gain is only 20 times then you do not near noise. For more gain use a low noise opamp as a preamp.
Your gain is much too high at 500k/470= 1064 times.
 

Thread Starter

Xerxsea

Joined Feb 23, 2022
8
The LM1875 is a power amplifier. Its original gain is only 20 times then you do not near noise. For more gain use a low noise opamp as a preamp.
Your gain is much too high at 500k/470= 1064 times.
So a 10mv peak input gain is normal across the input of this amplifier? I just need higher signal level going in so the noise insignificant in comparison to my signal?

I will try to attach a pre amp stage to this then!
 

Audioguru again

Joined Oct 21, 2019
6,694
10mV RMS is the very low output signal from a microphone. Peak is simply the real RMS voltage number enlarged.

An LM1875 power amplifier powered from +25V/-25V produces 20W into 8 ohms with low distortion. and its gain is 20 times, then the amplifier output voltage is 14.14V RMS. The amplifier input is 14.14V RMS/20= 0.71V RMS.

With an the output from a preamp of 14.14/20= 0.71V RMS then the preamp needs a gain of 0.71V/10mV= 71 times.
 

Thread Starter

Xerxsea

Joined Feb 23, 2022
8
okay so I attempted to make an pre amp stage using one of my op07 op I had laying around... my amplifier is in a much worse state now, and I cant figure out why, it seems to have a huge amount of noise or maybe oscillations, after trying to build it and failing I went to the simulator and everything looks right so I cant figure out why this isn't working.

with no input signal it produces a high pitch noise on the speaker and consumes a lot of power. 8W input because I current limit it to potentially protect the circuit ( my voltage drops to 8v from 12v). The audio does indeed get amplified but nowhere near as much as my previous circuit and with way more noise plus a high pitch whine it seems.

oscilloscope picture is it current limited further from my psu. output watt on the psu is at 5.5w. 1/vdiv

it wont let me upload the .lib file here but there is the spice simulation anyways. I am not sure why this is the case

1709267941206.png
 

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Audioguru again

Joined Oct 21, 2019
6,694
The OP07 opamp is not low noise, it has a poor high frequency response and a poor slew rate and its manufacturers say, "Not recommended for new designs" maybe because of its age and poor specs. Maybe your OP07 and/or LM1875 are fake or defective.

I have made MANY high gain audio amplifiers and none oscillated. Use a shielded input cable and keep the output wiring away from the input wiring.

EDIT: Can your mic hear the speaker?
 
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