[sdiy] impedence stuff???

[media at mail1.nai.net]

At 8:42 PM -0400 09/15/01, ultrahype wrote:
>
>    HI   is there a simple way to calculate what needs to be  done in the
>>following scenerios:  

The input impedance of the pre-amp or pedal often effects the sound of an
instrument or microphone, yet unless it is clipping or horribly noisy it's
often a matter of taste.  Generally, for unbalanced connections, you would
want the input impedance at least five times the source.

Off the top of my head hear are some typical values:

Guitar output         10 - 100K
Consumer input        47K
Line in               10K - 50K (20K is very common)
Line out              50 - 200 Ohm
Synth out             1K  (varies widely)
Speaker input         2-16 Ohm (8 Ohm is very common)


Then again, I've rack mounted effects boxes that had output impedances as
high as 10K.  Low impedance mics are usually 30-50 Ohms, but can be as high
as 1K and still have an XLR connector.  Mic pre-amps vary widely, lets say
between 150 and 600 Ohms.  Transformers don't have fixed impedances, they
only impart a ratio between devices, but for some reason, output
transformers are considered 75 Ohm.

I have no idea about effects pedals.

They sell two kinds of passive devices to be used like cable adapters, pads
(attenuators) and matching transformers, that can be used to solve these
kinds of problems -- so you can use a high-Z mic with a low-Z input, for
example.

>line level to gtr pedal

That often works, but sometimes line level is too hot.  It depends on the
pedal.

>gtr pedal to line level

That almost always works.

>speaker level to line level

A speaker is an inductive load often capable of drawing massive amounts of
current -- the exact opposite of a line input.  The output of an amp can
easily be way too hot, as many speakers can handle very high voltages.  So
I'd avoid doing this if I were you.  Nevermind the 100V law, the output of
a wall socket isn't that different than the output of a PA amp playing a
sustained B-flat :)

Using headphone outputs is safe, but might sound bad.

>line level to instrument ( amp ) input  

That is black art.  They sell devices for "re-amping" the outputs of line
level effects.  As you can see, line out is both a lower impedance and a
higher voltage than guitar.  Regardless, it often still works fine.

>i have a bunch of pedals i want to use with my  console and my results
>have not been to hot yet  

In my experience, I haven't had any trouble doing that.

>also i want to add line outs yo a bunch of ( ciruit  bent) stuff i have and

In that case I would add an op-amp.  Op-amps have very high input
impedances (we're talking megaohms) so they will draw very little current
when tapping the signal off some hacked-up gizmo, and provide the gain and
low output impedance to drive line-level.