Improving sound quality from computer

[lti]

Re: Improving sound quality from computer

I finally made the preamp with the resistor values in my previous post. It works, but the output is still a little too quiet. The headphone output is slightly louder (with the headphones and speakers I have), but only by one step on my keyboard's volume control. That will be good enough for now.

It does sound a lot better than the original circuit, but I still need to find the right cap values for the bass boost. There is still some missing midbass, but that's a lot better than the Walmart mini-stereo frequency response I was getting before.

[lti]

Re: Improving sound quality from computer

I haven't felt like messing with this yet, but I noticed that the output volume varies a lot more than it used to. Some sound files and songs are only slightly quieter than they were before, while others are so quiet that I have to have all the Windows volume controls turned all the way up. I didn't have to adjust the volume that much or that often before.

I did eventually find enough resistors for this. I've ended up with 61.9KΩ and 15KΩ resistors for the op-amp (if that amount of gain is even going to work with the wide variations in output volume) and 33KΩ resistors for the headphone amp. Those were just the values I had on junk boards (the 61.9KΩ resistors came from the original op-amp mess circuit - they're marked 77C).

[lti]

Re: Improving sound quality from computer

All of the resistors in the headphone amp circuit are 10K ohm. I guess I need to change those to 20K ohm. For this preamp, I was going to use 20K ohm resistors in series with the inputs and 82K ohm feedback resistors for a gain of 4.1. It will be an inverting amplifier. If my calculations are correct, this will give a gain of 12.26dB.

[budm]

Re: Improving sound quality from computer

Learn more here:

[Th3_uN1Qu3]

Re: Improving sound quality from computer

How do I find the input impedance of the final circuit? The op-amp has infinite input impedance (ideally), but some other part of the circuit creates some input impedance. The sound chip is designed for a 10K ohm load on its output, and I don't want to go lower than that with this amp and the headphone amp.

In an opamp circuit, the lower feedback resistor defines the input impedance. With an inverting amplifier circuit, that would be the one that goes between the source of the signal you want to amplify and the inverting input of the opamp.

[lti]

Re: Improving sound quality from computer

This is going to be a simple inverting amplifier.

According to my calculations, I will need a gain of 4. Since the op-amp is a TL064 powered by a 78L09 regulator, the output will be clipped at high volumes, but the original mess clipped too. I found that out when Windows decided to play the startup sound at full volume.

How do I find the input impedance of the final circuit? The op-amp has infinite input impedance (ideally), but some other part of the circuit creates some input impedance. The sound chip is designed for a 10K ohm load on its output, and I don't want to go lower than that with this amp and the headphone amp.

[SeanB]

Re: Improving sound quality from computer

Feedback resistors need to be simple, no capacitors across them and typically about 47k between output and inverting input and between 4k7 to 10k to analogue ground ( not PC ground but likely a voltage of around 2.5V. Will give the desired gain and a reasonably flat response, you just have to remove the capacitors across the existing network ( not the 10uF to ground if present, it has no affect on frequency just removes the need for a Dc bias) to simplify it.

[lti]

Re: Improving sound quality from computer

This is what I get with only the left channel of the op-amp mess bypassed. I did this earlier yesterday to see what would happen before I posted last. I need to increase the gain to get the same output level I was getting before. Is there a way to calculate the gain from the output level in dB? It looks like I need 12dB gain, and I'm going to reuse half of the TL064 op-amp to increase the gain so the amp itself won't amplify the noise from inside the computer.

[lti]

Re: Improving sound quality from computer

Here is what I get after bypassing the op-amp mess. The amp is now connected directly to the sound chip output. I also added 0.047uF caps in parallel with the caps in the feedback circuit. There really was nothing wrong with the amp or audio chip (even though this is an ESS chip).

Unfortunately, the output is quieter now. I might modify the op-amp circuit so it works as a simple amplifier instead of the pointless mess it currently is. Actually, it might be a tone control circuit to get the best sound quality from the Compaq/JBL monitor-mounted speakers. I think I also increased the value of those caps too much. There is a little midbass missing.

[lti]

Re: Improving sound quality from computer

I don't know the exact impedance of the speakers, but it's less than 4 ohms.

The problem with this amp is the op-amp mess on its input and the low-frequency peak in the frequency response. If I connected a good audio source to the input of the amp, it would have a more normal frequency response. The high-frequency peak seems to be caused by something in between the amp and the op-amp circuit. There's nothing wrong with the audio output from the sound chip either since the headphone output sounds good (aside from needing bigger input coupling caps).

[Khron]

Re: Improving sound quality from computer

I think it is. However, I can't disable the onboard audio, this computer doesn't want to play any audio out of the sound card I have, and the speakers have a really low impedance that no other amp I have can handle.

Somehow, i'm not buying that What OS are you running on that computer? Even with Winamp you should be able to select what output device you're using.

Low impedance, eh? And just how low might that be? Cobbling together an amp with a couple TDA20x0's or LM1875's shouldn't quite be "rocket science", if you're willing and able to change components on the mainboard

Then again, there's more than a few "car audio" linear amplifier chips (some TDA____'s that i can't recall right now) that have two or four amplifiers in one chip, specifically made for single-supply (and/or bridged) operation

[lti]

Re: Improving sound quality from computer

I can reduce the high frequency peak by putting a cap in parallel with the feedback network for the TDA1515BQ. It looks like a 2.2nF cap will be the best for that. I think I should try a cap in the feedback for the op-amp next.

Also, putting a larger cap in place of the one that is already in the feedback for the TDA1515BQ reduces the low frequency peak, but the peak doesn't move like it did earlier. I don't know what happened.

[lti]

Re: Improving sound quality from computer

You might hate me for saying this, but to me, this sounds terribly similar to "polishing a t*rd" And discounting the Mythbusters' achievements in this field, it just ain't gonna happen

I think it is. However, I can't disable the onboard audio, this computer doesn't want to play any audio out of the sound card I have, and the speakers have a really low impedance that no other amp I have can handle.

I have found a few things that I can change. The coupling caps on the outputs of the sound chip are only 0.1uF, and my calculations with a 10K ohm load give me exactly the same result I'm seeing directly from the output of the op-amp. Next, I can remove the caps in the feedback of the op-amps and increase the value of the caps in the feedback for the speaker amp to get the bass boost where I want it. It's more of a midrange boost now. Finally, there is another RC filter between the op-amp and the TDA1515BQ that causes the big dip at 300Hz. I'm going to find some of those surface mount caps and resistors to take care of that stuff.

[Khron]

Re: Improving sound quality from computer

You might hate me for saying this, but to me, this sounds terribly similar to "polishing a t*rd" And discounting the Mythbusters' achievements in this field, it just ain't gonna happen

If i were you, i'd just pick up something like an older M-Audio sound card from fleabay or whatever - the Envy24HT-equipped ones had AKM converters, if memory serves, and quite highly regarded back in their day

Or if you don't mind software-resampling in your music player (and provided you can find drivers), how about a Creative SB Live 24bit? I had one for several years, and i was real happy with it

[lti]

Re: Improving sound quality from computer

Here are some more charts.

On the first one, the white line is the output measured directly at the sound chip. It jumps around a lot because it was taken at a really low volume and contains a lot of noise. The green line is measured directly at the output pin of the op-amp leading to the TDA1515BQ amp. The second image is the frequency response of the headphone output with no load. This is driven by a TDA1308 clone.

There is a big string of crap between the op-amp and the speaker amp, and the circuit for each channel is completely different. I think I just need to start over and design a new audio distribution circuit.

[lti]

Re: Improving sound quality from computer

"The original circuit design had pins 2, 3, and 12 of the chip shorted together, and everything that should connect to pin 2 was connected to pin 12 instead." Are you talking about the power amp chip?

Yes, I am talking about the TDA1515BQ.

My thoughts would be to use a better DAC as the next step.

There is no external DAC that I can replace. The audio chip outputs analog audio. Also, the headphone output sounds good. I wonder what the frequency response directly from the sound chip or from the output pin of the op-amp would look like.

As long as you don't distort the amplifier, then it will sound better. I think you probably know that though xD
That, however, might not provide enough volume, so you might need a bigger amplifier. Also, the amp might be put in overdrive, like the typical LM326 chip circuits with a gain of 200....

http://www.youtube.com/watch?v=_Dvwd8Hyh-Y
Making the frequency response look a little more normal will improve the sound quality. I never noticed any of the weird stuff except the huge peak at low frequencies.

[ben7]

Re: Improving sound quality from computer

With a load connected, the frequency response doesn't change much. Also, the input to the amp (the second graph - only the left channel is actually shown here) looks similar.

I'm guessing that the circuit was designed for the computer's optional internal speakers or the monitor-mounted speakers that were available for Compaq's monitors at the time. However, the amp circuit has already been modified because the original circuit had almost no output.

My thoughts would be to use a better DAC as the next step.

As long as you don't distort the amplifier, then it will sound better. I think you probably know that though xD
That, however, might not provide enough volume, so you might need a bigger amplifier. Also, the amp might be put in overdrive, like the typical LM326 chip circuits with a gain of 200....

[budm]

Re: Improving sound quality from computer

"The original circuit design had pins 2, 3, and 12 of the chip shorted together, and everything that should connect to pin 2 was connected to pin 12 instead." Are you talking about the power amp chip?

[lti]

Re: Improving sound quality from computer

The speakers aren't that bad, but the bass is weak for their size. This project started as an attempt to move the amp's bass boost to a lower frequency where it would help. That is really all I wanted, but that dip in frequency response at 300Hz gets worse if I do that. That dip isn't actually audible to me, but it might become audible if I try to modify the circuit.

I have actually found a mistake in the circuit. The original circuit design had pins 2, 3, and 12 of the chip shorted together, and everything that should connect to pin 2 was connected to pin 12 instead. I had to add the 100K ohm resistors shown in the datasheet circuit between those three pins to get any audio out of it at all, but I still needed pins 2 and 12 shorted. I don't think that will cause any problems, but I might try fixing that.

[Logistics]

Re: Improving sound quality from computer

So you're blaming your on-board audio hardware for the lousy sound, but you have admittedly "crappy" speakers that need compensations to their frequency response?

Perhaps your speakers could be blamed for a multitude of sins that could be rectified by replacing them or improving them. (I'd op for improving them, but I l'm a glutton for punishment. read that DIY'er.)