pm1000/gen'l audio questions

[subsonicsaxx]

Hey everyone. I'm new on the boards but I've been reading a bunch of pm1k posts in preparation for my racking project (parts are in the mail!) You can pretty much assume I know almost nothing about electronics besides some basic theory I've read on allaboutcircuits.com. So, I have a couple of questions:

1- I've been looking at this page:
http://www.groupdiy.com/index.php?topic=7685
I liked the idea of the caps on the line input in order to isolate it from phantom power. I ran across one thread discussing ideal values of caps to use (which I have since been unable to find.) But, I couldn't find any discussion of how specifically to wire it up. Any thoughts/links to thoughts? (btw I will be using Neutrik switching combo jacks).

2- Does anyone know of a way to add an LED indicator to the phase invert switch without screwing with the signal? Ideally using a DPDT switch?

3- Has anyone had any experience with par-metal pre-cutting the rack faceplates? Like, especially as pertains to pricing? I've been messing around with Google SketchUp trying to get a design down and I'd like to avoid excessive metal cutting (using substandard tools) in my garage-less Brooklyn apartment.

4 (may not fit into the lab thread, but oh well)- I am missing some basic conceptual knowlege on how, exactly, electrical signal carries sonic information. Does anyone know of a good site that lays down how variations in direct current from +0A to +whatever get translated into a signal wave that has both positive and negative values over time? I am pretty sure this relates to AC theory somehow, but then how can this information be relayed by a DC circuit? Can one assume that more current in the signal path translates to higher amplitude of a sound wave, or does higher voltage do so? I am also trying to figure out why it is, exactly, that a capacitor filters certain frequencies. These all seem to be ideas that people on this board take for granted, but they're stumping the hell out of me! Any help with this, or just a link to some comprehensive info would be hugely appreciated.

 

[Scodiddly]

2. You could have the front panel switch controlling a DPDT relay. Let the relay do the signal switching, and control the LED from the switch.

4. Your local library is probably a good place to start - but basically it's possible for there to be both DC and AC components in a signal. DC is basically an offset from zero volts applied to the AC component - sort of like how you can have waves (AC) on the surface of a body of water (DC).

 

[rodabod]

4. Maybe start by just considering Voltage which is analagous to pressure in water. You need to be aware of current, but since we are dealing mainly with voltage amplifiers, think about that first. Audio is AC and we tend to "de-couple" (remove) any DC in the signal path as it is unwanted. Consider capacitors as just being like resistors except their resistance value is very high for low frequencies, but very low for high frequencies. This characteristic varies depending on the stated capacitance of a given capacitor. DC has a frequency of 0Hz.

Regarding having DC de-coupling caps on the line input, you would just put them in series with the hot (and cold if it exists) lines. The value of capacitance needs to be chosen so that they do not cause any bass roll-off. This depends on the series resistance which they are connected to as it forms a potential divider (more specifically an RC filter).

Do you understand what a potential divider is and its uses? If not, have a look. Then look for a simple series RC filter which is similar except you have a capacitor in series with a resistor instead of just two resistors in series.

Good luck,

Roddy

 

[subsonicsaxx]

thanks guys, I think that helps some..so in the case of the pm1k is the voltage for the audio signal added to and subtracted from the 44vDC that powers the circuits? or am I way off?

[quote author="Scodiddly"]2. You could have the front panel switch controlling a DPDT relay. Let the relay do the signal switching, and control the LED from the switch.[/quote]

so for the control voltage for the relay can I just run off the +44v bus and step it down with the appropriate valued resistor? that won't create some sort of crazy coupled inductance through the relay, bypassing the input trafo, will it?

[quote author="rodabod"]Consider capacitors as just being like resistors except their resistance value is very high for low frequencies, but very low for high frequencies....
The value of capacitance needs to be chosen so that they do not cause any bass roll-off. This depends on the series resistance which they are connected to as it forms a potential divider (more specifically an RC filter).[/quote]

I guess this has to do with the amount of current stored and discharged, right? Like, the larger the cap value, the slower it is to respond to (in other words to even out) changes in voltage, so it only affects lower frequencies. So if I have it right, I just have to figure the circuit out so that the cap is responding to around 20Hz or lower.

Guess it's time to read up on my wavefunction algebra. Oof. High school calculus was such a long time ago...