Improving sound quality from computer

[lti]

The sound from my computer's built-in amplifier (based on a TDA1515BQ) is really lousy. The frequency response chart is the output from the amp with no load.

I figured out how to take care of the bass boost, but I don't know where the dip at 300Hz or the peak at about 18KHz come from. My crappy speakers need a bass boost, but not at such a high frequency.

I have a partial schematic that I can post later, but it is impossible to make a full schematic on a multilayer board. For now, you can make fun of the horrible frequency response. I didn't know RightMark had a "very poor" rating.

[Khron]

Re: Improving sound quality from computer

I wonder how that response would look with, say, a 4 or 8ohm dummy-load resistor...

[Pinhead]

Re: Improving sound quality from computer

Perfect example of why I use the digital-out leading to my surround receiver.

[budm]

Re: Improving sound quality from computer

Is there tone control on the audio setup windows? It looks like the Bass and Treble are turn way up.
https://cdn.badcaps-static.com/pdfs/...e680f5e653.pdf

[lti]

Re: Improving sound quality from computer

There are no tone controls, but there is a 3D audio effect that is turned off. The headphone output has flat frequency response.

I'll work on this a little over the weekend. I'll post what I have of the schematic then. I think I'll also check the frequency response of the op-amp circuit between this amp and the audio chip. So far, I can't tell what the circuit is supposed to do. There is a big mess of resistors and caps, and it appears that the output of the 9V regulator powering the op-amp is capacitor coupled to the input of the op-amp. After trying to trace the circuit, I feel like ripping it all out and replacing it with a simple amplifier.

[RJARRRPCGP]

Re: Improving sound quality from computer

Looks like it's dominated by AC hum!

[budm]

Re: Improving sound quality from computer

What kind of software/hardware for the tool you are using to check freq. response? It looks interesting.

[lti]

Re: Improving sound quality from computer

Looks like it's dominated by AC hum!

That wouldn't be 150Hz. That is caused by the design of the feedback network for the amp, which resembles this:

Increasing the capacitance moves that big peak to a lower frequency, but that also makes the dip at 300Hz worse.

What kind of software/hardware for the tool you are using to check freq. response? It looks interesting.

RightMark Audio Analyzer
http://audio.rightmark.org/index_new.shtml
I don't have access to professional equipment, but that is good for a free program.

[RJARRRPCGP]

Re: Improving sound quality from computer

That wouldn't be 150Hz.

Looked like it was right around 125 Hz.
I can't 100 percent tell with that graph.

[budm]

Re: Improving sound quality from computer

I would like to see the graph with load (8 Ohms) connected. to get peak response like that has to two resonance happening. Simple RC filter as shown is just a low pass filter circuit, the higher the frequency, the less output you will get due to XC goes down and give more negative feedback. There must be something else in the circuits that causes the resonance to be happening. Are there ZOBEL network at the output of the amplifier?

http://sound.westhost.com/amp_design.htm

[lti]

Re: Improving sound quality from computer

Looked like it was right around 125 Hz.
I can't 100 percent tell with that graph.

It is kind of hard to tell with the logarithmic scale.

I would like to see the graph with load (8 Ohms) connected. to get peak response like that has to two resonance happening. Simple RC filter as shown is just a low pass filter circuit, the higher the frequency, the less output you will get due to XC goes down and give more negative feedback. There must be something else in the circuits that causes the resonance to be happening. Are there ZOBEL network at the output of the amplifier?

http://sound.westhost.com/amp_design.htm

It does have the Zobel network, and it appears to use the same component values as the circuit in the datasheet (4.7 ohm resistor and 0.1uF cap). The caps are surface mount ceramics, so they don't have any markings. I don't have any resistors that I can use as a load, but I could connect the speakers. It's difficult to connect everything since the output of this amp goes through a 3.5mm jack.

[lti]

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.

[budm]

Re: Improving sound quality from computer

So the signal generator output sweep signal looks like that also? Also, the signal is measured right at the + or the - input of the amp? Should measure at the left side of the resistor feeding the input.

[lti]

Re: Improving sound quality from computer

That was measured at pin 1 of the amp, which appears to be the inverting input. The test signal has perfectly flat frequency response. The op-amp circuit between the audio chip's output and the amp is the problem, and it's a huge mess. I don't know how it works at all.

[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.)

[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.

[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?

[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....

[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.

[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.