lm1875 audio amplifier help, poor voice quality

what is wrong in my circuit, i am getting poor voice quality on 40watt speaker, if i play any music i only get distorted sound.
i am using 12-0-12 transformer with bridge diode that outputs 24v DC. i am feeding sound into lm1875 from lm386.
lm386 sound output is good which i have checked with 0.5Watt speakers.

It might be worth checking your supply rails, can they supply several amps? What capacitor value do you have to get rid of the ripple after the bridge rectifier?
Also, is the circuit on a breadboard, or stripboard, or a PCB?
What heatsink have you got connected?
As a side note, it may eventually be worth replacing the LM386 with an op-amp based preamp, for higher quality (the 386 is very average quality).

I see many schematics that omit R2-R3-R4.

I believe the LM1875 is one of those audio IC's that provides its own internal bias network, to lift the non-inverting input to supply_V/2.

Also is C8 oriented at the proper polarity?

BradtheRad said:

I believe the LM1875 is one of those audio IC's that provides its own internal bias network, to lift the non-inverting input to supply_V/2.

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No, it's not. If he's using a single-ended supply, he needs R2, R3 and R4. C8 is the wrong way around on the schematic though.

Is the DC at the amplifier output 1/2 of Vcc ? I suspect that the amplifier is oscillating at at supersonic frequency.
Frank

thank you all for the advice.



i have rebuilt the above circuit. sound quality is ok but when i am playing any song which has bass, sound gets distorted. how can i suppress bass component? with a band stop filter?

You could suppress the bass, but that may sound pretty awful too.
The real issue could be the power supply, or your speaker (it is in an enclosure, and is it a single loudspeaker or with a woofer and tweeter)?
You may need to read up on audio and speakers, to decide how you wish to improve it.
Or, just keep the volume low.
If you want to suppress the bass, google for baxandall tone control for a circuit.
Or just adjust it on the source equipment (e.g. ipod has a sound adjustment in the menu).

when i am playing any song which has bass, sound gets distorted. how can i suppress bass component? with a band stop filter?

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Your first schematic shows the output capacitor is 2200 uF. Although this would normally be sufficient, there's a chance your speaker is drawing enough power at low frequencies, to cause the capacitor to distort all sounds.

At first we might think the capacitor would only attenuate lows, as a low-pass filter. However in reality it clips the peaks, causing a type of distortion known as shatter. It's the same effect as if the supply voltage is too low.

The capacitor must be large enough to pass the most intense waveforms your system puts out. Usually this turns out to be the bass frequencies, which is why it generally helps to reduce the bass.

i added a 1000uF capacitor at output and sound have improved. still sometime i feel sound gets clipped but its ok.
how can i control volume? i am playing songs through PC, from PC i can adjust the volume but it would be nice if i can add volume control in amplifier itself.

devil6600 said:

i added a 1000uF capacitor at output and sound have improved. still sometime i feel sound gets clipped but its ok.
how can i control volume? i am playing songs through PC, from PC i can adjust the volume but it would be nice if i can add volume control in amplifier itself.

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Your second drawing doesnt show any cap in the output to the speaker... why did you take it out ?

You can put a volume control at the start if the input circuit, 10k would do
one side to GND other side to input signal and the wiper to the junction of C1, R5

cheers
Dave

how can i control volume? i am playing songs through PC, from PC i can adjust the volume but it would be nice if i can add volume control in amplifier itself.

Click to expand...

For a mono signal it's a potentiometer...



It looks simple, however a problem which may develop is oscillations at odd times. This was the case when I made an add-on sound system for my computer. I had to try various ways, by bridging across contacts with different resistors, or inserting a series resistance somewhere, before I got rid of the oscillations.

For stereo a dual-gang potentiometer has always been traditional. These may not be easy to get nowadays. I have not figured out an easy way to merge two ordinary pots into a ganged pot.

Otherwise it might work to shine a single LED on two photocells, one each for left and right channels. Vary the LED brightness to change resistance in the photocells.

What would be great is if we could raise or lower gain according to what volt level we apply to a terminal on the IC.

how can i make it more loud?
i am using 12-0-12 center tapped transformer with bridge diode rectifier, if i replace it with 24-0-24 transformer will it make output more loud?

24-0-24 is a bit too high. 20-0-20 is OK.
(Maximum supply voltage is 60V DC or +-30V DC).

devil6600 said:

how can i make it more loud?
i am using 12-0-12 center tapped transformer with bridge diode rectifier, if i replace it with 24-0-24 transformer will it make output more loud?

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The LM1875 is rated to put out 20W. This suggests it can put 12.5 V across an 8 ohm speaker. (Or 9 V across a 4 ohm.)

You are already giving it 24 VDC. It should be able to give you loud sound as it is now.

However it is dissipating heat when you run it at loud volume. Did you attach a heatsink? If the IC gets overheated then it probably goes into shutdown mode until it cools.

Try putting a finger on it to detect its temperature. If it's too hot to touch then it needs more heatsink.

Did you configure it for a certain amount of voltage gain? Say your computer puts out 1/2 VAC amplitude (a typical amount). You need over 20X voltage gain, so that the IC will deliver several volts to the speaker.

Does the supply V hold up at high volume? Or does it drop a few volts during loud passages?

At loud volume your supply needs to be able to provide over 1 amp continuous. (Or over 2 amps if your speaker is 4 ohms.) Is your transformer rated for this amount?

This thread is a little old, but I think I can contribute. If TS have figured out the problem, maybe someone else has the same one.

12-0-12V transformer will only give out +-17V, 15.6V after a full wave rectifier. This will give an output power of less than 10W, maybe 6-7W if your decoupling cap's aren't that big. You need a 18-0-18V transformer to get the most out if the LM1875.

Another thing, if the sound is bad at all sound levels it could be oscillating. The 0.1uF caps should be as close to the power pins as possible.

Also, there is no need for an output cap when running it on a dual sided supply. I'm not sure if it's any point using an input stage on this amp, is there? Isn't the input impedance all ready quite high?