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

[Arny Krueger]

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

I tested Radio Shack's "Ground Isolator" 270-054 using
test signals that maxed out around 2.5 v RMS. ZSource = 150 ohms, ZLoad =
10K ohms.

The measured performance was truely amazing for a pair of transformers case
and cables selling for only $16.65.
All IM, THD, and noise artifacts were at least 80 dB down with most in
the -100 dB range or better. Frequency response showed a 2 dB peak at 20 Hz
and then 10 dB down at 10 Hz. There was a 3 dB peak at about 51 KHz falling
to about 10 dB down around 100 KHz. +0.5 dB at 20 KHz



I also did some experiments with varying the source
and load impedances. My recollection is that the transformer is reasonably
non-critical with normal audio production source and load impedances for
line-level equipment. It looks like it will also work fairly well with
consumer audio gear, which is of course what it was intended for.

 

[Arny Krueger]

...
Better.

Lynx doesn't have any transformers on their boards.

Welcome to the 21st century for line-level inputs. ;-)

Perhaps you have never seen a system with the slots right
next to a whomping 400W switchmode power supply
transformer.

I've seen how a LynxTWO performs in such a lotcation. Same-old, same-old.

One of the biggest problems in mixed signal design is doing quality audio on
a board that also has TTL clock signals on it. Of course that describes just
about every PCI audio card ever made. It also describes just about every
standalone DAC, ADC, and CD player. Once that problem is solved, the rest
follows.

 

[Richard Crowley]

"Jax" wrote ...

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.

Precisely define "tap on my home telephone line"? What exactly
do you mean by this. What are you doing? What does it look like?
Where did you get it? Does it have a make/model number, etc.?

In the USA, for example, it is not permitted to attach to the telephone
line except by isolation & protection circuit . At the very least, this is
a
magnetically-coupled transformer, and also includes capacitors,
voltage-limiting devices (varistors, diodes, etc.) and other components
depending on what kind of functionality you are looking for.

This is an important question and all the other discussion here
is dependent on this.

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.

If you are somehow making DIRECT connection between the
telephone line to your computer, THIS IS AN EXCEDINGLY BAD
IDEA! You MUST use some kind of device to protect your computer
from what is happening on the phone line. The ring voltage,
(something approaching 100V AC) for example is particularly
damaging to your computer audio card.

With a proper audio coupling device, you should not have any
problems connecting to the input of most any computer audio
system.

 

[Jon D]

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.

ISTR this must be a special sort of capacitor (reversible, tantulum,
or something like that).

Could you or someone else please clarify this.

 

[Rod Speed]

ISTR this must be a special sort of capacitor
(reversible, tantulum, or something like that).

Correct, but 100nF isnt going to be tantalum, just plastic.

 

[Dave Platt]

ISTR this must be a special sort of capacitor
(reversible, tantulum, or something like that).

Correct, but 100nF isnt going to be tantalum, just plastic.[/QUOTE]

I'd suggest reading the information at

http://www.tkk.fi/Misc/Electronics/circuits/teleinterface.html

You'll want a cap rated at no less than 250 WVDC - a 400-volt cap
would not be overkill. Plastic-film caps (e.g. mylar/polyester) are
probably the best type to use.

 

[Mr.T]

I tested Radio Shack's "Ground Isolator" 270-054 using
test signals that maxed out around 2.5 v RMS. ZSource = 150 ohms, ZLoad =
10K ohms.

The measured performance was truely amazing for a pair of transformers case
and cables selling for only $16.65.
All IM, THD, and noise artifacts were at least 80 dB down with most in
the -100 dB range or better. Frequency response showed a 2 dB peak at 20 Hz
and then 10 dB down at 10 Hz. There was a 3 dB peak at about 51 KHz falling
to about 10 dB down around 100 KHz. +0.5 dB at 20 KHz

Yes truly amazing, just a shame they couldn't be bothered providing ANY of
the figures that you have.

MrT.

 

[Mr.T]

Welcome to the 21st century for line-level inputs. ;-)

Or the second half of the twentieth century even

MrT.

 

[Eric Furness]

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.

I use a USB sound adaptor for coupling my amateur radio tranceiver to
my computer which is a pretty demanding application for digital modes.
This gets the sensitive circuits out of the computer, much lower
noise. Also, Radio Shack sells a small audio transformer that is
ideal for isolation, can't find part number right now but it is pretty
easy to find on the rack. That is if you are comfortable doing a
little soldering to make up cables including the transformer.

 

[Ken Maltby]

"Jax" wrote ...

Precisely define "tap on my home telephone line"? What exactly
do you mean by this. What are you doing? What does it look like?
Where did you get it? Does it have a make/model number, etc.?

In the USA, for example, it is not permitted to attach to the telephone
line except by isolation & protection circuit . At the very least, this
is a
magnetically-coupled transformer, and also includes capacitors,
voltage-limiting devices (varistors, diodes, etc.) and other components
depending on what kind of functionality you are looking for.

This is an important question and all the other discussion here
is dependent on this.


If you are somehow making DIRECT connection between the
telephone line to your computer, THIS IS AN EXCEDINGLY BAD
IDEA! You MUST use some kind of device to protect your computer
from what is happening on the phone line. The ring voltage,
(something approaching 100V AC) for example is particularly
damaging to your computer audio card.

With a proper audio coupling device, you should not have any
problems connecting to the input of most any computer audio
system.

This thread is a hoot.

Didn't they go through the dial-up modem craze in the UK?
What's a cheap V90 modem go for now a days, $12? Don't
they all have audio outputs?

Luck;
Ken

 

[kony]

ISTR this must be a special sort of capacitor (reversible, tantulum,
or something like that).

Could you or someone else please clarify this.

Tantalums typically fail, sometimes violently if connected
reversed. They are not used for this.

Use either a metal film (plastic) type or ceramic, rated for
at least 250V. Metal film is preferred.

 

[John A]

ISTR this must be a special sort of capacitor (reversible, tantulum,

or something like that).

Could you or someone else please clarify this.

The guys above know their stuff! Something with a plastic dielectric - or
even a ceramic will do. Nothing special in the way of polarisation - and
certainly nothing as exotic/expensive/unreliable as a tantalum. A 250VDC
rating is more than enough. Its there to pass the audio but to prevent the
transformer shorting the service provider's DC supply - that's all!

John A

 

[Richard Crowley]

"John A" wrote ...

The guys above know their stuff! Something with a plastic dielectric - or
even a ceramic will do. Nothing special in the way of polarisation - and
certainly nothing as exotic/expensive/unreliable as a tantalum. A 250VDC
rating is more than enough. Its there to pass the audio but to prevent the
transformer shorting the service provider's DC supply - that's all!

250V rating is actually marginal. That is barely 2x the peak ring voltage.
400V would be a safer rating. That is what I used last time I made
a telephone line connection circuit.

 

[John A]

"John A" wrote ...

250V rating is actually marginal. That is barely 2x the peak ring voltage.
400V would be a safer rating. That is what I used last time I made
a telephone line connection circuit.

You'd better tell BT to stop using 250VDC rated capacitors in this
application, then.

 

[Richard Crowley]

"John A" wrote...

...

You'd better tell BT to stop using 250VDC rated capacitors in this
application, then.

Maybe you guys use lower voltages over there on your
side of the Pond. Telephone lines are really pretty
hazardous things, especially to solid-state equipment.

 

[Rod Speed]

John A wrote...

Maybe you guys use lower voltages over there on your side of the Pond.
Nope.

Telephone lines are really pretty hazardous things, especially to solid-state equipment.

Not that hazardous to plastic caps tho.

 

[jasen]

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.

It's called a "voicemodem" it's a voice-capable modem, and the cheap
ones approximate the desired price.
There exists software to turn a PC into an answering machine etc.

Bye.
Jasen

 

[joseph2k]

"John A" wrote...

Maybe you guys use lower voltages over there on your
side of the Pond. Telephone lines are really pretty
hazardous things, especially to solid-state equipment.

Not for appropriately designed equipment. Transformers are simple, easy,
quick, and effective; about 20 years ago 100% silicon single chip solutions
started to be available outside the telco manufacturing stream. I remember
seeing all silicon SLIC (subscriber line interface circuit) about 30 years
ago.

 

[joseph2k]

"Jax" wrote ...

Precisely define "tap on my home telephone line"? What exactly
do you mean by this. What are you doing? What does it look like?
Where did you get it? Does it have a make/model number, etc.?

In the USA, for example, it is not permitted to attach to the telephone
line except by isolation & protection circuit . At the very least, this
is a
magnetically-coupled transformer, and also includes capacitors,
voltage-limiting devices (varistors, diodes, etc.) and other components
depending on what kind of functionality you are looking for.

This is an important question and all the other discussion here
is dependent on this.


If you are somehow making DIRECT connection between the
telephone line to your computer, THIS IS AN EXCEDINGLY BAD
IDEA! You MUST use some kind of device to protect your computer
from what is happening on the phone line. The ring voltage,
(something approaching 100V AC) for example is particularly
damaging to your computer audio card.

With a proper audio coupling device, you should not have any
problems connecting to the input of most any computer audio
system.

The transformer is no longer required, the requirement is performance based,
and there are all silicon, resistor, capacitor solutions. They do take
some expertise to implement. A transformer (better a store bought TIA or
DAA) is quite the better solution for OP.

 

[Tomi Holger Engdahl]

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 have never seen any normal PC sound card with isolation transformers
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.

Check the ground loop information and solving tips here:
http://www.epanorama.net/documents/groundloop/index.html

Here you can find list of some audio isolation transformer products
http://www.epanorama.net/documents/groundloop/audio_isolators.html