Converting Speaker Out to Line Out

Discussion about old sound cards, MIDI devices and sound related accessories.

Converting Speaker Out to Line Out

Postby dosgamer » 2018-4-15 @ 21:32

Anyone ever done this? I had to on this card. It wouldn't output any sound. On closer inspection, I found that the SMD resistors on the Zobel network on the LM386 outputs had their tops blown off, and the SMD capacitors looked overheated. :neutral: I don't even. I desoldered the the two LM386 amplifiers and they were dead as well. Luckily, the ES1688 chip itself was fine. I jumpered the AOUT_L and AOUT_R pins directly to the output jack via two 10uF coupling capacitors as per the datasheet. Now it's a proper line out. It should improve the sound quality as the cheap amplifiers used on sound cards are usually crappy and noisy.

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Re: Converting Speaker Out to Line Out

Postby Tiido » 2018-4-15 @ 22:59

It would be a good idea to stick some decent opamp like 4580 as a buffer between AV out and the line outs of the sound chip. Outputs of the chip are fragile and it is possible to cause damage. I usually put 100ohm current limit resistors in series with the lines if there's no buffer involved.
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Re: Converting Speaker Out to Line Out

Postby dosgamer » 2018-4-15 @ 23:10

It depends on the chip. The ES1688F can directly drive a 10kOhm load. That's the the standard input impedance of a line input. If you look at page 106 of this datasheet:

http://www.philscomputerlab.com/uploads ... manual.pdf

All that's between the chip and line out is the coupling capacitors.
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Re: Converting Speaker Out to Line Out

Postby Tiido » 2018-4-16 @ 00:09

My comment was more about the momentary shorts that happen when plugging/unplugging a TRS connector as well as occasions when devices with big potential difference are connected i.e one grounded device and another ungrounded device (that uses an SMPS which puts its ground to half mains voltage level due to certain parts in them).
Another reason 100ohm resistor is good to have is preventing oscillations on output of the buffer inside the chip due to cable capacitances. Those are often not in audible ranges but will cause excess distortion due to eating away bandwidth of the buffer.
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Re: Converting Speaker Out to Line Out

Postby dosgamer » 2018-4-16 @ 09:58

So you mean just two 100 Ohm resistors just after the coupling caps? Maybe I'll give it a try. Problem is, I've put the card back in and it works, so now I have no motivation to take it back out. :P

Come to think of it, back in the day I had a laptop that had a very noisy sound when using headphones. Buzzing, crackles, etc. I made an adapter lead with two 150 Ohm resistors in series and that completely cured it, the sound was perfect after that.
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Re: Converting Speaker Out to Line Out

Postby Tiido » 2018-4-16 @ 16:05

As long as the resistors are placed between chip and the AV out connector all is fine, there's no strict requirement for them to be before or after DC blocking caps, as long as they're there
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