Switchable Hi-Z input impedance, how ?

[tinitus]

ah, ok
my laptop has mic/line input and head phone out
and I can get some kind of EQ software

cost nothing to try it

anyone know if I can convert a stereo headphone out to a balanced mono signal ?

[tinitus]

my, this fet amp looks just like tube amp

[ChrisA]

Originally Posted by tinitus

jfets are supposed to be the masters of hi-Z input

how high input impedance can we achieve using tube only ?

The control grid of a triode tube has very high impenence. It's basically an open circuit. But you can't use it that way. You must provide a path from grid to ground and the minimum size grid leak" resister will be found in the tube data sheet. For most preamp tubes I thinkit's about 2M but "everyone uses a 1M grid leak resistor and most of these get soldered to the input jack. The input impedance is the same as this resistor.

But ask yourself, do you really want more than 1M? Why? I think you will get even more noise pickup from the environment

[tinitus]

[QUOTE=ChrisA;2718397
But ask yourself, do you really want more than 1M? Why? I think you will get even more noise pickup from the environment[/QUOTE]

yeah
I now also think anything above 100K ought to work

[ChrisA]

Originally Posted by tinitus

yeah
I now also think anything above 100K ought to work

No, you really don't want to plug your guitar into an amp with 100K input impedance. the pots in the guitar across the cable are 250K They are in effect in parallel with the amp and you know has parallel resistors add. The amp needs to be high relative to the tone pot.

[tinitus]

Originally Posted by ChrisA

You must provide a path from grid to ground and the minimum size grid leak" resister will be found in the tube data sheet.

ahh, I often go back and read again, in case I missed something

looking at ecc81
it says 1M limiting value
I suppose this means 1M is max recommended value 'grid leak resistor'
but if thats also the resulting imput impedance, it would be just fine

so far so good

[tinitus]

just remembered I have some OPA627, OPA637 and OPA2107

any suggestions ?

btw, my power amp might have convenient additional +/-15 out(200ma)

[tinitus]

a few links

Articles and Schematics

Guitar Effects Projects - Richardo's Pages

Francis J. Deck

http://www.national.com/an/AN/AN-32.pdf

[tinitus]

this 'thing' has instrument hi-Z input, and connects directly to USB port
even has headphone out

not really cheap, but what else do you need
plug it into your home stereo media streamer
or via PC with all kinds of software
and thats it

very tempting

[tinitus]

good stereo amp coming to me, in a few days, I hope
actually I built it, some years ago

might be a bit early days yet, but I will need a preamp
and this is what I have in mind