Switchable Hi-Z input impedance, how ?

[tinitus]

and why the noise almost goes away with the stereo hifi preamp driving the output

must be something to do with going from 0/+ supply to symmetric +/- supply

looking at Nelson's B1 buffer, could this be the reason why he 'constructed' a pseudo midpoint for the jfet's grid to ground resistors

 

[tinitus]

tried adjusting drain and source resistors
15ohm source res stays in, no doubt
not sure what kind it is
old and beige colored, so I suspect its a carbon film, 1/2W or something
tried a metal film, but didnt sound so nice
(signal resistors are metal films)

Drain resistor is lowered a bit, from 1K5 to about 1K2 or 1K3

measured +7.18V over drain res, and -82mv over the source res
dont know if those are the best values, but sounds good

 

[tinitus]

15ohm source res stays in

Drain resistor is lowered a bit, from 1K5 to about 1K2 or 1K3

nope, drain res is still 1K5
well, actually 1K6, since its 2 paralel 3K2
I removed series resistors from the power supply
and thats it
at input, instead of 1M to ground I have 850K
will try increasing input/gate series resistor

 

[shanx]

Is it a hum? or more like white noise? I have a couple of suggestions, but depends on the type of problem. You may have a bit of ground loop on the input. Do you have an insulated input jack?

 

[tinitus]

ups, been busy

yes its snerring hum
my present setup have variable attenuation on final output, and takes away most of it(its a stereo preamp buffer)
at 12 o'clock nothing, but above that it begins be audible, and gets really aggressive when turned up fully
almost no hiss, and in general very silent right now
weird setup btw

bass guitar--compressor--jfet boz
then devides into two channels
one channel, jfet boz--headamp--stereo pre--poweramp--mid/tweeter
second channel, jfet boz--stereo pre--poweramp--woofer

inserting the headamp gives me a lot more control, and a picture of how it ought to work
I think it will work very well when Im able to control every gainstage in both channels
its getting closer

 

[tinitus]

btw
I have tried almost all possible configurations with the effect units I have, and some sounded pretty weird
one acted like a synth, another like pure distortion, and others sounded just muffled and phased
tells it not so simple at all with effect 'pedals
I only use compressor pedal unit right now
would like to do without, but right now its not possible
but maybe later

 

[shanx]

For the hum situation, I would try an isolated input jack, input signal wire shielded from jack to jfet input, but shield terminated only at the signal ground directly at your jfet's source resistor ground. That is the most direct route for return path to your instrument. Adding a bypass cap across the source resistor may help. Check all other signal grounding points in your preamp are star grounded or bussed onto a heavy gauge copper grounding wire in your cabinet. You may need to add shielding metal panels in future if you are picking up ambient EMF. Your outputs are splitting to two different stereo preamps? You may have a ground loop issue there too. let me know if they are separate preamps powered on AC, DC etc. one output may need to be isolated to break ground loops on outputs.

 

[tinitus]

hey, will try bypass cap on source res

note, its not the channel splitting
hums also with only one channel
but its a proto built on a piece of plywood

the other evening I could hear faint spanish radio
but was gone again just by a simple off/on switching
seems to be a one time only

but I might have a faulty plug on my bass
its gets noisy when not touching the strings

 

[tinitus]

hey, will try bypass cap on source res

tried 22uf el cap
sounded ok, but a bit muffled, or maybe more like phased

then a 1.0uf film cap
sounded more open, but nervous

then 2.5uf
bang, sounds just right, very smooth
great 'egalite', responsive and tuneful, tight bass
I would say spot on

 

[tinitus]

2.2uf works ok too
actually, in some way even better
compressor sounds ok again

for some reason my compressor didnt work so well with 2.5uf
could be because it was a different type
also tried a 3.3uf orange colored cap
didnt last one minute

should I have a selector option, it would switch between 2.2uf and 2.5uf
worth the trouble ? yeah, its enough to make that little important difference
might be it actually affects the tuning pitch just a bit, but enough for 'certain occations'
anyway, a switch would at least make it easier to test different caps

well, enough of that
working on a simple regulated supply for the jfet

 

[shanx]

Hi Tinitus, glad you tried out the bypass caps. It may help also with hum and radio interference too.
The different cap types can affect the subtle differences in sound. Also tolerance on caps is wide so was could to try different types to get it sounding right. Was the 2.5 uF a film cap?

 

[tinitus]

Was the 2.5 uF a film cap?

yes, the 2.5uf was poly type
a bipolar smooth foil might be worth a try

still happy with the 2.2uf
30year old and weird Wima MKC4 type

my regulator is finished
only needs power, and a check

 

[tinitus]

my regulator is finished
only needs power, and a check

and its working, 16.6V
sounds more precise, and plays faster

 

[shanx]

Nice work! I put two 10uF in parallel (electro type)on my first stage fet bypass. One with a switch to play with frequency response/gain, so will see what happens.

 

[tinitus]

I have removed bypass cap again
with the regulated supply something changed
sounded out of phase, like loosing hearing on one ear
I did try to increase cap from 2.2uf to 3.3uf
and it did help, but still better without
seem I got my sound back, in the most simple way
small simple regulated supply, and nothing else

wonder if it works a bit like a ccs
then there should be no bypass cap, right ?
maybe thats what it needs, a ccs
will look at inserting a LED in the source res

 

[shanx]

The bypass cap is increasing normally the gain on the stage, and you could be hitting saturation. Ids, gate bias and bypass cap will all play a part in whether you are at a signal operating point that is pleasing on the ears. For guitar there is more reasons to play with saturation and cutoff, but you may want a clean signal for bass at least at the first stage, with plenty of headroom.

 

[tinitus]

Ids, gate bias and bypass cap will all play a part in whether you are at a signal operating point that is pleasing on the ears.

I think you have a good point with that
regulator drops about 2V
heck, right now Im thinking maybe 2V is too small
anyway, I still measure about the same 80mv over the source resistor

well, that said, I found the sound nice softy round
but I prefer a more dry sound in the long run
fells kind of less 'picky'

so I threw in a couple of 3300uf, seperated by a 1ohm resistor, and the bleeder/LED
and that actually gave a much more dry sound

I will try to go down to 15V
experiments not over yet

 

[tinitus]

measured voltage before and after regulator drops only 1V from 17.8V to 16.7V

and thought I would see what kind of DC I have before output cap
close to 8V(or -8V), and about equal to drain voltage
sounds like a lot

 

[tinitus]

trying to figure out that LED thing
a found interesting stuff
well, I have heard it before, and not had a clew what
lets see how it works

plate voltage divided by plate resistor gives bias current
ups, sorry, drain and not plate
I have about 8V and 1K5
and that gives 5ma
wow, I think 5ma have been mentioned by Nelson
and to check source resistor, I have 80mv and 15R
funny, but thats also 5ma...wow, it works

so, to calculate the LED current flow I would need to know its inner resistance, I guess
and ups again, forgetting about jfet type and idss

 

[tinitus]

ahh, appears that 5ma is close to where a led turns off
does not sound very nice
but whatever
if I have undestood any of it, the LED bias is meant to lower source(cathode) resistance to achieve better or more stable gain
but a 15R resistor is already low, so whats the point anyway