Switchable Hi-Z input impedance, how ?

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impedance%20switching%20-%20SPDT.GIF
 
Ex-Moderator R.I.P.
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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
 
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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.
 

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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.
 
Ex-Moderator R.I.P.
Joined 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
 
Ex-Moderator R.I.P.
Joined 2005
Hundred's of M fairly easily, low G a bit harder? Is that enough for a guitar?

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

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 :mad:

:)

seems a fet input is the obvious choise
 
What sorts of signal sources are you planning to accommodate with your bass guitar amp? Or put another way, what problem would this be used to solve? COnsider that commercial bass amps don;t usually have input impedance selectors, just signal pads. And yet there is seldom any problem relating to impedance of signal source.
 
Ex-Moderator R.I.P.
Joined 2005
What sorts of signal sources are you planning to accommodate with your bass guitar amp?

commercial bass amps don;t usually have input impedance selectors, just signal pads. And yet there is seldom any problem relating to impedance of signal source.

usually there will be a passive/active input, by a switch, or with two jacks
I'm quite sure the active input is optimal
but knowing that 1Mohm is optimal fore passive, I doubt that the passive input is optimal

but its tricky
I'm no expert
but seems to me like some may be using active pickup systems fore passive drive as well
might be different with their lower output impedance
this may indicate that it won't need a change in load impedance, but 'only' a change in gain on amp input
but I'm really not sure about any of this, obviously

as fore the signal sources
well, DIY is sweet

I can imagine buiding a multi-convertible
but only imagine ;)

it could have modules added
making it into a sub with stereo top
could be used for additional mic/PA
a one in all system
and with a USB input module, it could even be a party monster
 
usually there will be a passive/active input, by a switch, or with two jacks
I'm quite sure the active input is optimal
but knowing that 1Mohm is optimal fore passive, I doubt that the passive input is optimal

but its tricky
I'm no expert
but seems to me like some may be using active pickup systems fore passive drive as well
might be different with their lower output impedance
this may indicate that it won't need a change in load impedance, but 'only' a change in gain on amp input
but I'm really not sure about any of this, obviously

The main point is WHY do you want variable input impedance?, which essentially is only of any use for passive instruments.

You can happily plug an active guitar in a passive input, the signal levels are not that much different, and feeding low into high is what you're looking for (but you don't actually 'need' that big a ratio).

The reasons for using a lower impedance with a passive guitar are basically because the higher the impedance the more noise the input can pickup, and the more effect the cable has on it.

If you want a variable input impedance, make an FET preamp with a 2M input impedance, and simple add a 2M variable resistor across it's input - this will allow you to vary the impedance from 1M to zero ohms. By putting a resistor in series with the pot, you can set the minimum impedance you can adjust to (that resistor in parallel with the 2M of the FET preamp). It will only have an effect on passive guitars though, where reducing the impedance will alter the sound somewhat (good or bad?, who knows).
 
unless you also want a piezo pickup input
needs a 4-10Mohm input

Then increase the input resistor on the FET - and switch out any variable impedance. But piezo's are pretty obscure anyway.

active picups are around 4.7Kohm
and passive 10Kohm, or more
the bigger humbuckers can be up to 47Kohm

Active pickups are probably even lower than that, a few hundred ohms I would imagine - it depends if they add a series resistor or not to increase the output impedance.

Passive guitars are much higher than 47K, they usually use 250K pots in the guitar, so you need at least 1M to stop it loading them excessively.
 
I still don't understand the problem. As anything from 0 to 10M input impedance is easily achieved using valves, what is the issue? Anyway, here is my solution:

Have an input stage which is basically grounded cathode but with switchable cathode degeneration. Two resistors in the cathode. The one next to the cathode sets the bias, and may be bypassed. The other, bigger, resistor to ground may be bypassed (high gain, low Z) or not (low gain, high Z). The grid bias resistor goes to the junction of the two cathode resistors, so in the low gain high Z position it gets bootstrapped so its effective value is raised.
 
Ex-Moderator R.I.P.
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Nigel, your numbers are including active curcuits, pots, etc
my numbers are raw output from the pickup

please note that this is all about trying to get an understanding how things work

like, what do you reckon is max possible input impedance of a 2sk170 buffer ?
why? because I have plenty

btw, found this interesting balanced 'discrete opamp'
http://www.forsselltech.com/media/attachments/Class_A_JFet_Opamp.PDF
 
Ex-Moderator R.I.P.
Joined 2005
I still don't understand the problem. As anything from 0 to 10M input impedance is easily achieved using valves, what is the issue? Anyway, here is my solution:

Have an input stage which is basically grounded cathode but with switchable cathode degeneration. Two resistors in the cathode. The one next to the cathode sets the bias, and may be bypassed. The other, bigger, resistor to ground may be bypassed (high gain, low Z) or not (low gain, high Z). The grid bias resistor goes to the junction of the two cathode resistors, so in the low gain high Z position it gets bootstrapped so its effective value is raised.



cool, that is what I wanted, thanks

:)
 
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