Switchable Hi-Z input impedance, how ?

[tinitus]

ok
closing conclusion

switchable input impedance here means

1. Hi-Z input with jfet
2. input options below that, bypassing the jfet

[Enzo]

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.

[tinitus]

Originally Posted by Enzo

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

[Nigel Goodwin]

Originally Posted by tinitus

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).

[tinitus]

unless you also want a piezo pickup input
needs a 4-10Mohm input

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

everything would be easily solved by 'onboard' pre/buffer
but man, I just hate batteries

[Nigel Goodwin]

Originally Posted by tinitus

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.

[DF96]

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.

[tinitus]

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/at...JFet_Opamp.PDF

[tinitus]

Originally Posted by DF96

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

[tinitus]

so far I understand that high grid resistance in a tube input may cause high frequency loss
but might also block out some over drive distortion

and for a bass amp, who cares
could even have positive effect