PCI audio mixer for hum-free line-in recording?

  • Thread starter Thread starter Jax
  • Start date Start date
J

Jax

I run XP on a old VIA266A mobo + Duron 1800 MHz processor. I am in the
UK.

---------------

I want to plug a line-input from a tap on my home telephone line or from
my audio player which runs off the mains.

I get a lot of hum plugging the line input into my mobo (Syntax SV266A
with a VT8235 chip for AC'97). Probably is a ground loop.

Earthing one end or another of the line input lead doesn't help much.
I'm told that a good way to minimise hum is to use an isolating
transformer.

If I get a modest audio mixer card which fits into a PCI slot, then is
it likely to have an isolating transformer (or some other hum isolating
device) already built in?

I am thinking of a card costing about 15 to 20 UK POUNDS (20 to 30
DOLLARS). This is not trying to be audiophile but is for high quality
speech files.
 
I run XP on a old VIA266A mobo + Duron 1800 MHz processor. I am in the
UK.

---------------

I want to plug a line-input from a tap on my home telephone line or from
my audio player which runs off the mains.

I get a lot of hum plugging the line input into my mobo (Syntax SV266A
with a VT8235 chip for AC'97). Probably is a ground loop.

Earthing one end or another of the line input lead doesn't help much.
I'm told that a good way to minimise hum is to use an isolating
transformer.

If I get a modest audio mixer card which fits into a PCI slot, then is
it likely to have an isolating transformer (or some other hum isolating
device) already built in?

I am thinking of a card costing about 15 to 20 UK POUNDS (20 to 30
DOLLARS). This is not trying to be audiophile but is for high quality
speech files.
Click to expand...
It's very unlikely that a PC audio card would be transformer isolated.
The general method of interfacing is:
http://www.tkk.fi/Misc/Electronics/circuits/teleinterface.html#audioint

I suggest that you could use a 1:1 600 transformer, almost anything
would do, I like OEP tranformers, more than adequate for the job


martin
 
Jax said:
I run XP on a old VIA266A mobo + Duron 1800 MHz processor. I am in the
UK.

---------------

I want to plug a line-input from a tap on my home telephone line or from
my audio player which runs off the mains.

I get a lot of hum plugging the line input into my mobo (Syntax SV266A
with a VT8235 chip for AC'97). Probably is a ground loop.

Earthing one end or another of the line input lead doesn't help much.
I'm told that a good way to minimise hum is to use an isolating
transformer.

If I get a modest audio mixer card which fits into a PCI slot, then is
it likely to have an isolating transformer (or some other hum isolating
device) already built in?

I am thinking of a card costing about 15 to 20 UK POUNDS (20 to 30
DOLLARS). This is not trying to be audiophile but is for high quality
speech files.
Click to expand...

A modest card will not have a transformer on it. What you might be able
to find is one with an electronically balanced input. This is done by
usually by employing a differentially connected opamp.
If you're getting hum from the telephone line then an isolation
trasformer is not going to help much if at all as the noise is likely on
the line.
 
A modest card will not have a transformer on it. What you might be able
to find is one with an electronically balanced input. This is done by
usually by employing a differentially connected opamp.
If you're getting hum from the telephone line then an isolation
trasformer is not going to help much if at all as the noise is likely on
the line.
Click to expand...


Definately Not recommended, you NEED transformer isolation.
1) to protect your equipment from HV crap on the line
2) to protect the Telco from you putting crap on the line


martin
 
I want to plug a line-input from a tap on my home telephone line or from
my audio player which runs off the mains.

I get a lot of hum plugging the line input into my mobo (Syntax SV266A
with a VT8235 chip for AC'97). Probably is a ground loop.
Probably.

Earthing one end or another of the line input lead doesn't help much.
I'm told that a good way to minimise hum is to use an isolating
transformer.

If I get a modest audio mixer card which fits into a PCI slot, then is
it likely to have an isolating transformer (or some other hum isolating
device) already built in?

I am thinking of a card costing about 15 to 20 UK POUNDS (20 to 30
DOLLARS). This is not trying to be audiophile but is for high quality
speech files.
Click to expand...

You are unlikely to find an audio card OF ANY PRICE that includes
transformer isolation of input or output. Easiest to use an external
iso transformer device.
 
...

You are unlikely to find an audio card OF ANY PRICE that includes
transformer isolation of input or output. Easiest to use an external
iso transformer device.
Click to expand...
bet Neve would make one.......


martin
 
bet Neve would make one.......
Click to expand...

bet they wouldn't....

1) Not in the business of making computer plug-in cards.
2) Wouldn't want the hassle of maintaining drivers, etc.
3) Not enough height for any decent transformer.
4) Hostile environment for high-quality audio circuitry.

Likely other reasons, besides :-)
 
Jax said:
I run XP on a old VIA266A mobo + Duron 1800 MHz processor. I am in the
UK.

---------------

I want to plug a line-input from a tap on my home telephone line or from
my audio player which runs off the mains.

I get a lot of hum plugging the line input into my mobo (Syntax SV266A
with a VT8235 chip for AC'97). Probably is a ground loop.

Earthing one end or another of the line input lead doesn't help much.
I'm told that a good way to minimise hum is to use an isolating
transformer.

If I get a modest audio mixer card which fits into a PCI slot, then is
it likely to have an isolating transformer (or some other hum isolating
device) already built in?

I am thinking of a card costing about 15 to 20 UK POUNDS (20 to 30
DOLLARS). This is not trying to be audiophile but is for high quality
speech files.
Click to expand...

What is the input level into the 'line' in?
can you put a pot in between?
 
Jax said:
I run XP on a old VIA266A mobo + Duron 1800 MHz processor. I am in the
UK.

---------------

I want to plug a line-input from a tap on my home telephone line or from
my audio player which runs off the mains.

I get a lot of hum plugging the line input into my mobo (Syntax SV266A
with a VT8235 chip for AC'97). Probably is a ground loop.

Earthing one end or another of the line input lead doesn't help much.
I'm told that a good way to minimise hum is to use an isolating
transformer.

If I get a modest audio mixer card which fits into a PCI slot, then is
it likely to have an isolating transformer (or some other hum isolating
device) already built in?

I am thinking of a card costing about 15 to 20 UK POUNDS (20 to 30
DOLLARS). This is not trying to be audiophile but is for high quality
speech files.
Click to expand...

There is a nice long web page here, on interfacing.

http://www.infomatek.com/Telephone_transmission characteristics.htm

And one sample device here:

http://www.omnicronelectronics.com/PC/LIC-1PC.htm

Search terms: "telephone audio to computer".

Paul
 
Jax said:
I run XP on a old VIA266A mobo + Duron 1800 MHz
processor. I am in the UK.

---------------

I want to plug a line-input from a tap on my home
telephone line or from my audio player which runs off the
mains.

I get a lot of hum plugging the line input into my mobo
(Syntax SV266A with a VT8235 chip for AC'97). Probably
is a ground loop.

Earthing one end or another of the line input lead
doesn't help much. I'm told that a good way to minimise
hum is to use an isolating transformer.
Click to expand...

Use this with your existing card. Some plug adaptors may be needed.

http://www.radioshack.com/product/index.jsp?productId=2062214
 
martin said:
Definately Not recommended, you NEED transformer isolation.
1) to protect your equipment from HV crap on the line
2) to protect the Telco from you putting crap on the line
Click to expand...

There is 50VDCon telephone lines.

geoff
 
martin griffith said:
It's very unlikely that a PC audio card would be transformer isolated.
The general method of interfacing is:
http://www.tkk.fi/Misc/Electronics/circuits/teleinterface.html#audioint

I suggest that you could use a 1:1 600 transformer, almost anything
would do, I like OEP tranformers, more than adequate for the job


martin
Click to expand...

What Martin has suggested is absolutely the right (and probably the cheapest)
way to eliminate the hum. The reason you're getting the hum is that your phone
line is a balanced line, whereas the inputs on your sound card is unbalanced.
When one side of your phone line is grounded (the shielded side of the sound
card input), the line is unbalanced, and tremendous hum results.
The best way to solve the problem is to keep the phone line balanced by use of a
transformer as shown in the link, and let the transformer's secondary handle the
unbalanced input on the sound card.
You can use almost any 1:1 600-ohm transformer for the job... phone line
fidelity isn't good enough to cause any worries about transformer quality.

Cheers!!!
--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Some days you're the dog, some days the hydrant.
 
martin griffith said:
Definately Not recommended, you NEED transformer isolation.
1) to protect your equipment from HV crap on the line
2) to protect the Telco from you putting crap on the line
Click to expand...

3) required by law in many countries.

MrT.
 
Richard Crowley said:
4) Hostile environment for high-quality audio circuitry.
Click to expand...

That old furphy!
Funny that companies like Lynx can obtain PCI audio performance as good as
Neve equipment then.

MrT.
 
...

That old furphy!
Funny that companies like Lynx can obtain PCI audio performance
as good as Neve equipment then.
Click to expand...

Lynx doesn't have any transformers on their boards.

Perhaps you have never seen a system with the slots right next
to a whomping 400W switchmode power supply transformer.
 
Richard Crowley said:
Lynx doesn't have any transformers on their boards.
Click to expand...

No, they don't need them.
Perhaps you have never seen a system with the slots right next
to a whomping 400W switchmode power supply transformer.
Click to expand...

In fact I've never seen one that didn't. The results still speak for
themselves though.

MrT.
 
martin said:
Definately Not recommended, you NEED transformer isolation.
1) to protect your equipment from HV crap on the line
2) to protect the Telco from you putting crap on the line
Click to expand...
Also need some kind of voltage limiting (back to back zeners?). If the
phone rings while it's hooked up to the card....

jak
 
You can see that the overwhelming, and correct, advice is to get a
one-to-one isolating transformer and connect it between the telephone line
and a PC audio input.

However, when you connect it all up, you must connect a capacitor - say 0.1
microfarad (aka 100nF) - in series with the transformer's connection to
the telephone line.
 
I run XP on a old VIA266A mobo + Duron 1800 MHz processor. I am in the
UK.

---------------

I want to plug a line-input from a tap on my home telephone line or from
my audio player which runs off the mains.

I get a lot of hum plugging the line input into my mobo (Syntax SV266A
with a VT8235 chip for AC'97). Probably is a ground loop.

Earthing one end or another of the line input lead doesn't help much.
I'm told that a good way to minimise hum is to use an isolating
transformer.

If I get a modest audio mixer card which fits into a PCI slot, then is
it likely to have an isolating transformer (or some other hum isolating
device) already built in?

I am thinking of a card costing about 15 to 20 UK POUNDS (20 to 30
DOLLARS). This is not trying to be audiophile but is for high quality
speech files.
Click to expand...

No, normal audio PCI cards have no isolation transformer.
If you can find an antique telephone you can get the transformer from that,
I also once designed some circuits with an optocoupler as isolation,
worked very well actually.
 
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