Switchable Hi-Z input impedance, how ?
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 9th September 2011, 12:44 PM #1 diyAudio Moderator   Join Date: Nov 2005 Switchable Hi-Z input impedance, how ? Bass preamp is there an easy way around this ?
 9th September 2011, 01:09 PM #2 diyAudio Member     Join Date: Sep 2003 Location: Midland, Michigan Switchable? Please explain a little more. What do you want to switch? Input sources or impedance? __________________ Frank
 9th September 2011, 01:32 PM #3 diyAudio Member   Join Date: May 2002 Location: Kiev, Ukraine
diyAudio Moderator

Join Date: Nov 2005
Quote:
 Originally Posted by Frank Berry Switchable? Please explain a little more. What do you want to switch? Input sources or impedance?
impedance, and something like TG suggests

probably simple, but I still don't understand how it works

and I thought there should always be a resistor to ground, on every input

I imagine something like five or six different input impedances, either switchable, or with a pot
one or two could be with added jfet buffer

edit, but with a jfet buffer I read that going too exstreme with high input impedance could cause oscillation (?)
so I reckon there are certain limits

diyAudio Moderator

Join Date: Nov 2005
found this

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AMPLIFIER INPUT/OUTPUT IMPEDANCE AND GAIN

You should remember that the gain of a stage is calculated by using the input and output signals.

Voltage gain is calculated using input and output voltage; current gain uses input and output current; and power gain uses input and output power. For the purposes of our discussion, we will only be concerned with voltage gain.

Figure 2-11 shows a simple amplifier circuit with the input- and output-signal-developing impedances represented by variable resistors. In this circuit, C1 and C2 are the input and output coupling capacitors. R1 represents the impedance of the input circuit. R2 represents the input-signal-developing impedance, and R3 represents the output impedance.

Figure 2-11.—Variable input and output impedances.

R1 and R2 form a voltage-divider network for the input signal. When R2 is increased in value, the input signal to the transistor (Q1) increases. This causes a larger output signal, and the gain of the stage increases.

Now look at the output resistor, R3. As R3 is increased in value, the output signal increases. This also increases the gain of the stage.

As you can see, increasing the input-signal-developing impedance, the output impedance, or both will increase the gain of the stage. Of course there are limits to this process. The transistor must not be overdriven with too high an input signal or distortion will result.
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 9th September 2011, 08:19 PM #6 diyAudio Moderator   Join Date: Nov 2005 is that not only half way true, and half rubbish forgetting natural input impedance of active device ?
diyAudio Member

Join Date: Sep 2003
Location: Midland, Michigan
Quote:
 Originally Posted by tinitus impedance, and something like TG suggests probably simple, but I still don't understand how it works and I thought there should always be a resistor to ground, on every input I imagine something like five or six different input impedances, either switchable, or with a pot one or two could be with added jfet buffer edit, but with a jfet buffer I read that going too exstreme with high input impedance could cause oscillation (?) so I reckon there are certain limits
If you look carefully at the schematic, you will see that there is a resistor (or a combination of resistors) to ground in each switch position.
This method uses resistors to simulate a low input impedance.
The downside is that the gain is reduced in the 'low-Z' switch position.
__________________
Frank

 9th September 2011, 09:18 PM #8 diyAudio Moderator   Join Date: Nov 2005 yep, it's slowly getting clearer well, I probably won't need any 'attenuation' of signal at input att pot will be at power amp input(SS) jfets are supposed to be the masters of hi-Z input how high input impedance can we achieve using tube only ? edit, I know it might be 'easier' to have multiple buffers here and there, but I'm trying do a 'simple' thing I could buy a studio TLAudio mic tubepre, but thats less fun hell, even a plugandplay tube mic amp for less than 200USD is possible but I have no idea how its built I think thats important, to know a little about how it works
 9th September 2011, 09:27 PM #9 diyAudio Member   Join Date: May 2007 You can get fairly high input impedance using electrometer valves and careful guarding of inputs. Hundred's of M fairly easily, low G a bit harder? Is that enough for a guitar?
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Join Date: Nov 2005
Quote:
 Originally Posted by DF96 Hundred's of M fairly easily, low G a bit harder? Is that enough for a guitar?
more than enough

1Mohm seems to become standard Hi-Z input on mic amps with optional instrument input
but with guitar onboard buffer, less would do

whatever
I just found its frequency depending
mr miller related, etc etc
things seem to get more and more complicated

seems a fet input is the obvious choise

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