Passive Preamp Phase Change Question

[tschanrm]

I recently built a passive preamp using alps rk27 50k stereo pot and a 9.5k resistor in series with rca input and output based on the old H.H. design.

The wiper of the pot goes to rca output, the input and output rca is connected by the 9.5k resistor. Ground is then connected to pin 1 of the pot, with the rca ground for input and output connected to that.

Anyway, I read this

Equalized passive preamp

And wondered, with my shunt design, how much phase shift am I currently having? According to the 50k pot, I am seeing almost an 80 degree phase shift at 20khz. But this is a straight pot only, not shunt design. My interconnects are very short, only 6 inches from source to preamp and six inches form preamp to power amp.

Does anybody know how much phase shift I am experiencing in my design? Thanks for the help.

[Leolabs]

Is your passive pre sounds "unusual",esp at high frequency????

[tschanrm]

no, not that I can tell. It sounds very similar as my active pre-amp, only more detailed and a wider soundstage than my active pre-amp.

Also, my amp has a 50k and 1v input if anyone wants to know.

[analog_sa]

You seem to have a constant 9k5 resistance which will determine the timeconstant. It is similar to a worst case position (minimum volume) of a 10k pot but in your case it is not volume dependent.

In order to make any meaningful conclusions you'll need to know the total capacitance between the pot and amp.

[tschanrm]

In order to make any meaningful conclusions you'll need to know the total capacitance between the pot and amp.

Thanks for the help analog,

How would I go about doing this? From the pot to the RCA's is about 2 inches of copper wire. From the rca there is 6 inches of rca cable going to my amp. From my source to my passive preamp is also inly 6 inches. Yes, all of the compenents are tight up against each other right now, but its only temporary until I make my own cables. I don't exactly have much equipment besides a multimeter and soldering iron, so I'm quite limited.

Ok, I looked over that article. For 1 meter of cable, they rated the worst (some cheap cable) one as 476 pf, best case (expensive cable) as 67 pf. So worst case scenario, my 6 inches of RCA cable is about 72 pf, best case my 6 inches of rca cable is about 10pf. My cable is really nothing special, a pair of shielded gold plated rca "Y" cables to use temporarily that I got from radioshack until I make my own. So my cable probably falls toward the worst case scenario.

So, how much influence does 50-70 pf of capacitance make? How influence much would 10pf make, this hopefully being the case when I make my custom length cables? Is there some equation that I can use to calculate phase shift? Thanks for the help

[Steve Eddy]

Don't worry about phase shift in an of itself. Just because there's phase shift doesn't mean that the signal waveform is being altered. Any frequency independent delay will result in phase shift with frequency (with higher frequencies being shifted more in phase than lower frequencies). However the signal waveform is not altered and all frequencies arrive at the same time.

What will alter the signal is frequency dependent delays. In this case frquencies don't all arrive at the same time and the waveform will be altered.

So what you would want to be concerned with is group delay, not phase shift. You can have a significant amount of phase **** yet have a flat group delay. What group delay does is effectively ignore frequency independent delays and gives you a picture of frequency dependent delays.

se