Passive potentiometer design help

[ehous]

DIY's,

Need help with a design issue. I have used Passive pots for many years. My latest has me at a stand still.

4 gang Alps blue, 50k. A pc system, Juli@ sound card, balanced out into the passive (in the pc case), balanced out to amps, 68K ohm input.

With the pot in the path, the system has no response below 50hz. At 49, nothing, at 51hz it plays. I used an oscilloscope to verify my ears. With the pot out of the system connected to a frequency generator, the pot passes signal below 50hz.

I also used the PC with Fubar's software volume control and have signal below 50 hz through to the amps.

Any idea's?

Thanks in advance, Ed

[pacificblue]

Is your amplifier DC coupled? I. e. no DC blocking cap at the input?

The soundcard has an output high-pass filter with a capacitor in series followed by a resistor to ground. The amplifier's input impedance goes in parallel with that resistor and the potentiometer as well. Common values for that resistor are 100 k to 1 M.

Assume that resistor to be 100 k -> 100k||50k||68k = 22k37. The high-pass filter frequency rises by a factor of ~4,5, if you use a DC coupled amplifier. Skip the potentiometer and you get 100k||68k = 40k48. The high-pass frequency rises by only 2,5.
If the filter without anything connected to the soundcard is set to 11 Hz, that frequency will rise to ~28 Hz with only the amp connected and to ~50 Hz with amp and potentiometer connected.

Assume that resistor to be 1 M -> 1M||50k|68k = 28k -> frequency rise by a factor of 35,7 or from 1,4 Hz to ~50 Hz.
1M||68k = 63,67 -> frequency rise by 15,7 or from 1,4 Hz to ~22 Hz.

Use a buffer before the potentiometer or make the amplifier AC coupled to achieve the high-pass frequency the soundcard was designed for.

[AndrewT]

Hi,
how are you connecting a passive pot (or any pot) to a balanced output?
How does your pot give a balanced output to send on?

Balanced cabling DEMANDS very accurately matched resistances and impedances to achieve the attenuation they are designed for. Putting a 4gang pot in the route will in most probably ruin the resistive matching.