Switchable Hi-Z input impedance, how ?

[tinitus]

Bass preamp

is there an easy way around this ?

[Frank Berry]

Switchable? Please explain a little more. What do you want to switch? Input sources or impedance?

[TG]

[tinitus]

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

[tinitus]

found this

---------------------------------------------------------------------------------

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.

[tinitus]

is that not only half way true, and half rubbish

forgetting natural input impedance of active device ?

[Frank Berry]

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.

[tinitus]

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

[DF96]

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?

[tinitus]

Originally Posted by DF96

Hundred's of M fairly easily, low G a bit harder? Is that enough for a guitar?

more than enough
funny to find myself stting reading about medical equipment

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