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

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

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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.

Reply to
Jax
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It's very unlikely that a PC audio card would be transformer isolated. The general method of interfacing is:

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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

Reply to
martin griffith

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.

Reply to
Pen

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

Reply to
martin griffith

Probably.

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.

Reply to
Richard Crowley

bet Neve would make one.......

martin

Reply to
martin griffith

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 :-)

Reply to
Richard Crowley

What is the input level into the 'line' in? can you put a pot in between?

Reply to
JAD

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

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And one sample device here:

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Search terms: "telephone audio to computer".

Paul

Reply to
Paul

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

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Reply to
Arny Krueger

There is 50VDCon telephone lines.

geoff

Reply to
Geoff

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 
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Reply to
DaveM

3) required by law in many countries.

MrT.

Reply to
Mr.T

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

MrT.

Reply to
Mr.T

Whilst it may be perfectly adequate, I had to laugh at the listed "Tech Specs"

Model 270-054 Product Type Isolator Enclosure Color BLACK Body Material Multi

Tells you all you need to know I guess.... it's BLACK :-)

MrT.

Reply to
Mr.T

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.

Reply to
Richard Crowley

No, they don't need them.

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

MrT.

Reply to
Mr.T

Also need some kind of voltage limiting (back to back zeners?). If the phone rings while it's hooked up to the card....

jak

Reply to
jakdedert

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.

Reply to
John A

On a sunny day (Tue, 27 Feb 2007 21:44:46 GMT) it happened Jax wrote in :

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.

Reply to
Jan Panteltje

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