[1000mp] Low audio to sound card

[Jim Smith]

Hi Pete,

Maybe you already know all this, but here goes.

The first thing we need to know is the impedance characteristics of the 
xfmr.  i.e. is it  8:600 ohms, 600:600 ohms or what?

In the meantime, let's suppose it's an 8:600.  If you apply a voltage to 
the 8 ohm side it will be stepped up by a factor equal to the square 
root of the impedance ratio.  In this case, the square root of 600/8 
which is 8.66.  i.e. 0.1V at the input (8 ohm side) will result in 
0.866V at the output (600 ohm side).  If you apply a voltage to the 600 
ohm side it will be stepped down by the same ratio.  Thus it really 
matters which way around you connect the transformer.

When you use a transformer in this application, you aren't really 
matching impedances.  The headphone amplifier is designed to drive, say, 
an 8 ohm load.  However, the source impedance of the amplifier isn't 8 
ohms.  It's probably a fraction of an ohm.  i.e. it can be modelled as 
an ideal voltage source with negligible internal impedance.  Similarly, 
I doubt that the impedance of the Line In jack on the sound card is 600 
ohms.  If I had to guess my guess would be 10 Kohm.  If someone actually 
knows what it is I would like to know.

If you really wanted to operate the transformer in a matched impedance 
state (but it may not be necessary) you would put an 8 ohm resistor in 
series with the headphone output and 8 ohm winding and a 600 ohm 
resistor in parallel with the 600 ohm winding.  (This assumes that the 
headphone amp source impedance is zero and the Line In input impedance 
is infinite.  One of these days I'll measure them and find out what they 
actually are.)

Operating a transformer at impedance levels different than those for 
which it is designed will affect its frequency response and, no doubt, 
its power handling capability.  Power handling isn't an issue here but 
frequency response might be.

Cascading an 8:600 with a 600:8 will, as you can see from the above, 
give you a 1:1 voltage ratio, less the losses in the transformers.  It 
will also degrade the frequency response..... perhaps not enough to notice.

So, if you are using an 8:600, maybe you wired it in backwards.  If so, 
when you reverse it, your signal level into the Line In may now be too 
high!  If it is, you might as well put in the 8 ohm series R and the 600 
ohm parallel R as described above  (Values aren't fussy, 7.5 ohms and 
620 or 680 ohms are standard resistor values.)  This will cut the output 
voltage in half (6 dB) and will allow the transformer to give you the 
best frequency response it is capable of.  If the voltage is still too 
high replace the 680 ohm with 2 330 ohms in series and take the output 
from their junction (i.e. a voltage divider).  This will cut the voltage 
by another 6 dB and the transformer will still be matched.

Let us know whether or not this info enables you to solve the problem.

73 de Jim    VE7FO  

Pete Smith wrote:

> Recently I set about preparing to record contests this season, and to 
> clean up my configuration for using SBDVP with TRLog anfd My Mark 5.  
> I had some fairly serious ground loop hum last year, and so I wired a 
> Radio Shack isolation transformer into the audio line between my 
> TopTen DXD's headphone jack and the sound card.  I also put a couple 
> of low value disc ceramic caps between the two audio lines and the 
> common cable shield ground, which is grounded to the TopTen box 
> chassis through the headphone plug.
>
> To my surprise, when I finished and tried it out, the audio level 
> through the transformer to the sound card input was so low that I was 
> effectively unable to record.