Engineers Needed How do you reduce THD in audio, how to get more gain, and more

In summary, the preamp with the Class AB power amplifier has a theoretical frequency response of 20 Hz to 20 kHz, but in reality it only produces a 30 dB difference at 50 Hz and 2 kHz. It needs less than 0.1% THD at 1Khz for the preamp, but is getting upwards of 2% and 3% at the load. There is no way that 324 is going to be able to drive a speaker. Replacing the LM324 with a uA741 reduces the THD in the preamp.
  • #1
ski
8
0
Greetings!

I posted a few weeks ago about the preamp for the tape head I'm working on. I solved for all of my resistors and capacitor to get the right frequency response, and below is the theoretical frequency response, followed by what I actually get:
http://www.littleboyinc.com/uploader/uploads/graph3.JPG
http://www.littleboyinc.com/uploader/uploads/graph1.JPG

Here is the schematic of the circuit:

http://www.littleboyinc.com/uploader/uploads/preamp2.JPG

And the preamp WITH the Class AB power amp. I need 400mA at 1kHz (no DC) at the output to drive the 8 Ohm speaker so I can get at least 1 Watt, but I'm in the microamps range... crapola :(

http://www.littleboyinc.com/uploader/uploads/ampbig.JPG

Questions (if you can help me with even one, I will be very grateful!)

1. I'm only getting 30 dB relative difference on my preamp at 50Hz and 2kHz. I have tried different values for R1, R2, R3, and C1. Is there something I can add to get better gain, or other specific values I should try?

2. I need less than 0.1% THD at 1Khz for the preamp. I'm getting upwards or 2% and 3%! Yikes. What are some ways to reduce THD in the preamp. Also, I'm getting 2000% THD at the load (end of the power amp), what type of feedback should I try to add to reduce this?

3. Simple one -- I need to split the 13.6V car battery into +6.8 and -6.8. I did that in the circuit above, but when I connect it to the class AB power amp, the top voltage line drops from +6.8 to +1.5, and the bottom line goes from -6.8 to -12.1. I'd like to run this off of just the car battery, so is there any way I can rig this so I can hook up the voltage lines to the power amp without them changing? You can see I just put separate DC sources in for now.

Thanks... I know this is long, but hopefully I have described everything okay! :)
 
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  • #2
What sort of load are you trying to drive with the 324?
 
  • #3
It's a standard 8 Ohm speaker... I added a coupling capacitor in line with the speaker so no DC current would go through it.

Anyone want to take a stab?
 
  • #4
There is no way that 324 is going to be able to drive a speaker. Really all that the power supply splitter does is set a bias point for the input. Consider it 'virtual ground'.
 
  • #5
I changed the LM324 to a uA741 on the preamp (first opamp) to reduce the THD in the preamp.

Should I try a uA741 for the second opamp?
 
  • #6
This is a VERY elementary question, but I'll ask anyway to make sure:

If you have voltages VDC and VAC (a sine wave offset by a DC voltage), how can you get rid of the DC offset to make the sine wave (AC) centered about zero?
 
  • #7
You can try connecting a 10 kohm pot between the offset null pins (1 and 5) of the op amp. The wiper arm of the pot. connects to the negative supply voltage. Then adjust the pot. to zero the offset. It should be noted that this only cancels the output offset at one particular operating point. With temperature changes or simply over time, the circuit might need to be readjusted.
 
  • #8
Take a closer look at the data sheets for the op amps you are using. It takes a different op amp than a 324 or 741 to be able to drive a speaker. The 741 won't do it either. I'll commen more later. Sorry, kinda busy.
 
  • #9
Ok. My apologies. I didn’t realize that you were driving the speaker with a transistor output. I failed to look at the second schematic. But, I can give you some advice on how to make the circuit work better and simplify it all at the same time. I also have questions for you.

In the schematic you have drawn 3 batteries. Are you actually using 3 separate batteries or isolated power supplies?

In the output stage you have the emitters connected directly to each other. Bad design move. You need to use a low value resistor on each emitter and have the resistors tie together and feed the output. This prevents a thermal runaway condition and may help your crappy output.

Pin 5 of U2B is floating. Bad design move. This pin should be tied to ground through a resistor to prevent the voltage on it from floating around. Since it is connected only through a capacitor its DC voltage can float anywhere it wants which in turn causes the output to directly follow it.

You have the source showing a 0 VDC output. Is this really the case? Is it wired directly to the tape head? If so, you don’t need to use the capacitor C4. For testing purposes you may want it, but when wired to the tape head it’s DC voltage should be zero. However, keep the capacitor C2. Its job is to prevent the DC voltage gain from being the same value as the AC voltage gain. You may think that since the input is 0 VDC it shouldn’t matter. But since no op-amp is perfect there is a slight input offset and the C2 keeps that offset from being amplified. Now, let’s take a step back to pin 5 of U2B. I mentioned it should not be left floating. Since we can assume that the output of the first stage has VERY little DC offset on the output why can’t we just drive the next stage (pin 5) directly without the cap? The truth is that you can. Go ahead and eliminate it. Actually, try it both ways. Experimentation is how we learn.

Now for your THD and gain problems. Where are you measuring the THD at? And what are the gains throughout the whole thing? I think that you should split the circuit up and test preamp with the output stage totally disconnected. Even from the power supplies. I am considering the second part of the op-amp part of the output stage. Reducing THD from 10% to 9% is easy. It’s an easy place to gain 1%. But reducing it from 2% to 1% is another story. There are MANY more subtle things that can keep that THD up there where you don’t want it. Any noise on the power supply can cause it. I don’t see any bypass capacitors on the power supplies. These are a must. I’ll leave it at this for now. There are a few other things I want to address but let’s get a few things nailed down first. It shouldn’t be a hard project for you to get off the ground with a guiding hand.
 
  • #10
* B U M P ! *
 

1. How do you reduce THD in audio?

To reduce THD (Total Harmonic Distortion) in audio, engineers often use techniques such as negative feedback, harmonic cancellation, and distortion masking. Negative feedback is when a portion of the output signal is fed back into the input to cancel out any distortion. Harmonic cancellation involves using a circuit to generate a signal that is equal and opposite to the distortion, canceling it out. Distortion masking is a technique where engineers add a small amount of noise to the audio signal, which can mask the distortion and make it less noticeable to listeners.

2. How do you get more gain?

To get more gain in an audio system, engineers can use techniques such as cascading amplifiers, using higher gain components, or employing feedback loops. Cascading amplifiers involves connecting multiple amplifiers together, with each one providing a small amount of gain, resulting in a higher overall gain. Using higher gain components, such as transistors or operational amplifiers, can also increase the gain. Feedback loops can be used to boost the gain of an amplifier by sending a portion of the output signal back to the input, amplifying it again.

3. How do you design an audio system for low noise?

To design an audio system for low noise, engineers must pay attention to the signal path and minimize the introduction of noise at each stage. Proper grounding and shielding techniques can also help to reduce noise. Careful component selection, such as using low-noise resistors and capacitors, can also contribute to a low-noise audio system. Finally, implementing proper gain-staging techniques, such as using the appropriate gain level for each stage, can also help to reduce noise in an audio system.

4. What is the difference between analog and digital audio?

Analog audio is a continuous signal that represents sound waves in their original form, while digital audio uses a series of discrete numbers to represent the sound. Analog audio is susceptible to noise and degradation, while digital audio is more resistant to these issues. Digital audio also allows for more precise manipulation and editing of the sound compared to analog audio. However, some argue that analog audio has a more natural and warm sound compared to digital audio.

5. How do engineers ensure audio equipment meets industry standards?

To ensure audio equipment meets industry standards, engineers must carefully test and calibrate the equipment to meet specific criteria. This can involve using specialized testing equipment and following industry protocols for measurements such as frequency response, distortion levels, and signal-to-noise ratio. Engineers may also need to make adjustments and improvements to the design of the equipment to meet these standards. Additionally, regular maintenance and quality control checks are necessary to ensure that the equipment continues to meet industry standards over time.

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