Re: Pre-Amp?


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Posted by Gus on September 01, 2001 at 12:44:37:

In Reply to: Re: Pre-Amp? posted by Bob Gollihur on August 23, 2001 at 07:08:21:

Hi,

First of all: diagram to build a high impedance preamp.

The preamp I'm using NOW.

http://www.till.com/articles/GuitarPreamp/index.html

Basically impendace is how much current comes with a signal.

The sound is "encoded" in the voltage, every electric signal comes with
a voltage and current.

A low impedance input "eats" a lot of current and a high impendance input
"eats" just a bit of current.


A piezoelectric pickup is high impedance (gives very little current) , and it
gives less current in the low frequencies than in the high ones.

When you plug a piezo in an low impendace input, the input wants to "eat"
more current than the pickup can suply,(overall in the low frecuencies) so the
voltage sinks, and the signal is now trebbly without lows, the signal has been
damaged due a lack of high impedance input.

I have a electric guitar that sounded without highs, and an acoustic guitar
that sounded without lows. (I connected them to a mic input).

Now I have a VERY HIGH IMPEDANCE PREAMP , and both guitars sound cool, the
preamp almost doesn't amplify voltage (only 2.5 decibels) , but amplifies the
current to a medium impedance 6k.


- Low impedance inputs tend to damage high impedance signals. If bass
amplifiers have 50k input impedance then the high frecuencies of the magnetic
pickup are zapped, which sure is not accidental, with a preamp like the mine,
it would sound with highs. (Magnetic pickup have higher impedance in the higher
frecuencies.)

- Low impedance input doesn't do harm to low impedance signal, these signals
often a PREAMP signal are plenty of current, so the input can eat a lot of
current.

-High impedance input doesn't do harm to low impedance signal.

Basically , the higher the input impedance the better, the lower the output
impedance the better.

This is just a personal experience.

Hope this was useful.


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