Improving sound quality from computer

[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.

[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

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.

[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.

[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

[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.

[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.

[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.

[RJARRRPCGP]

Re: Improving sound quality from computer

Looks like it's dominated by AC hum!

[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.

[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

[Pinhead]

Re: Improving sound quality from computer

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

[Khron]

Re: Improving sound quality from computer

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

[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.