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L C Audio Technology
H.C. Andersensvej 56
8230 Aabyhoej
Denmark
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info06@lcaudio.dk
or Gert Frederiksen
+45 6062 9017
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AC97 PC Sound Upgrades
AC97 is the most common standard for PC sound. The AC97 sound system in your computer can be a soundcard,
or residing on the
motherboard as a codec chip.
PC Sound i.e. MP3, WMA etc. quality has a rather weak reputation in Audio circles, because of poor
resolution
listening fatigue, poor soundstage and difficulty of placing instruments.
It doesn't have to be like that! You can upgrade your AC97 system for better
sound!
Important! during any kind of upgrading or measurement procedure on your PC, the power must be
turned off,
and you need to protect your system against static electricity, which may otherwise destroy parts of your
computer!
Keep one hand on the enclosure at all times, when you operate inside your PC. This way you ensure you
don't have
static charges stored in your body that can discharge through the circuits inside the PC. Another
helping action is
to take ALL plugs out of your PC when operating inside. This way, if a charge should spark to the PC
circuits, it has
nowhere to channel to Earth, and so is less likely to cause damage.
Disclaimer L C Audio Technology can not assume any responsibilities, in case your PC should
accidentally
take damage during the proposed upgrade procedure. On the other hand we have built or upgraded in house
dozens of computers, and never experienced even one damage from static electricity, simply because the
above mentioned
simple precautions were observed.
The Upgrade step 1.
| First of all we would recommend you change the DC blocking caps to better types. They are residing on
your motherboard
or soundcard to block DC voltages from the Audio Codec, from reaching the output. The word Codec is a
contraction
of the two words 'Coder' and 'Decoder' and tells you the same chip holds both D/A and A/D converter. (For
the microphone input).
Start by identifying the Audio Codec in your system. The most common AC97 codec's are VT1612 (2 ch.) and
VT1616 (5.1 ch) both
from VIA Technologies. If your system has a 5.1 codec, but you are only using 2 channels in your sound
system, you only
have to upgrade the two capacitors you are actually using.
And how do you find these two? The easiest way is by inserting your
audio cable in the usual output of the PC, | | and ohming (with a multimeter) from the hot wire in each
channel, to the
electrolytic cap's in the vicinity of the VT16xx chip. (Or similar type). If you are looking at the right
capacitor the ohm meter will
show less than 1 k resistance, but most likely even 0-1 Ohm connectivity. If they are not the ones you
are looking for
the reading will show 10k or more.
On several motherboards we have looked at, the value of the blocking caps was 100uF/16V. This value is
way too high!
If you turn off your PC when the amplifier is connected, large amounts of DC will cause the amplifier to
give huge
pop noises in the speakers. You can solve this by replacing the cap to a better type and more well
considered value,
depending on the impedance of your amplifier. As a thumbs rule use:
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- 100 nF (MKT or PP) if your power amplifier has an input impedance of 100 k or more (tube
amplifier).
- 1 uF (MKT or Tantalum) if your power amplifier has an input impedance of 10 - 47 k (most
common).
- 10 uF (Tantalum) if your amplifier has an input impedance of 1 - 2 k (several of our amplifiers have
this).
Do NOT use larger value than this!!!
| This way you can significantly improve the sound quality of your PC for just a dollar or two. As a
DIY'er you are
probably inclined to use expensive high-end capacitors to ensure the best possible sound quality. Don’t!
The large
mechanical size of a high-end cap will catch more of the digital noise that will always | | be flying around
inside the
PC enclosure. A smaller standard cap will catch much less of this, and subsequently give you a better
sound.
Furthermore even the most expensive caps for loudspeaker x-overs, are completely useless for small signal
transmission, cheap standard caps will almost always sound better!
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The Upgrade step 2.
| Jitter on the sampling clock will deter sound quality, especially sound stage, placement of
instruments,
and worsen listening fatigue. If you take a look on page 20 of the AC97 standard spec as published by
Intel, (link below)
you can see that this factor is also eminent in AC97 PC sound.
To remove jitter from you PC sound system, you can clock your codec with a clean signal, as from the
LClock XO3.
It is very important that the clock signal is symmetrical, and has an | | amplitude compatible with the AC97
chip. Otherwise the AC97 chip may
be destroyed. Furthermore it is also important that the feed cable is UHF type screened, as not to pick
up unwanted
digital noise from the motherboard and CPU of the computer. In some PCs depending
on the power supply you should also connect a Clock Disable signal to the main VCC 3.3 of the
motherboard, to ensure the
clock goes off when the CPU is power-downed. LClock XO3 has all these features as standard.
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Caution! As we are
aware of at this point, no clock's offered on the market, except the XO3 has features to safely support
AC97 re-clocking,
and we can NOT assume any responsibility, should our instruction be used with other clocks than proposed.
We strongly recommend
that you DO NOT attempt using other standard clocks than XO3, with this instruction, as it will most
probably lead to malfunction
or destruction of your computer!
This is how LClock XO3 should be connected to the VIA VT1616 chip. Before you connect the external clock,
you should remove the
old crystal from the board. It is found close to the VT1616 chip, and has the inscription: 24.5 (and
maybe more decimals).
You remove it by adding lots of solder to both terminals until it goes loose by itself. Do NOT use wick
or solder remover.
One of the solder pads from the crystal is also connected to pin 2 of the chip, and can be conveniently
used to connect the
LClock feed wire. Use the Ohm meter to find out which pad is the right one.
12V supply for the LClock XO3 you can find easily, on the many superfluous plugs for disc drives etc. The
yellow wire
has 12V on it. Set the output mode to 3.3V (by NOT adding solder to the 5V bubble).
Shorten the COAX cable to a suitable length, and make sure no threads from the screen connects to the
inner 'hot' wire
of the feed cable.
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