Switchable Hi-Z input impedance, how ? - Page 35 - diyAudio
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Old 3rd October 2012, 01:54 PM   #341
tinitus is offline tinitus  Europe
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ok, managed to get the supply voltage down to 14.3V

drain resistor 8.2V/1K3, 6.3ma
source res 47.2mV/7R5, 6.3ma
DC before output cap dropped 2V to now 6V

and sounds real good, best yet
feels like I lost some gain
and no doubt that all controls works much better now
will try to up the voltage to 15V

and btw, hum is gone now
but only when I shut down signal on bass guitar
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Old 3rd October 2012, 06:33 PM   #342
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by tinitus View Post
will try to up the voltage to 15V
got it to 14.9V
nothing else seems to change
and doesnt seem possible to get higher bias with this voltage
but wow, it certainly cleaned up the sound
gave it a little touch of more power
seems like endless headroom to play with
I can live with this for a week or two
ready to move into a fine home now
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Old 6th October 2012, 04:30 PM   #343
tinitus is offline tinitus  Europe
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looks like this
also built my first regulated supply
and btw, 99% scrap material
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Old 7th October 2012, 07:44 AM   #344
tinitus is offline tinitus  Europe
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backside, less pretty, but whatever, home made double sided with ground plane
and my first hardwire job
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Old 7th October 2012, 08:02 AM   #345
tinitus is offline tinitus  Europe
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but in the future ? well, when looking at this other impressive little board holding both positive and negative regulation, I might think twice
but no doubt I have learned more this way than I would just assembling a kit
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Old 10th October 2012, 08:49 PM   #346
tinitus is offline tinitus  Europe
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ok, it continues with attenuators, and hopefully buffers
I found a 'generic' simple one with a jfet and bjt
unfinished and a few parts missing
and juma posted a schematic I found interesting
and I have plenty of the BC547
lets see how it goes
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File Type: gif bass pre jfet bjt buffer.gif (3.5 KB, 15 views)
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Old 10th October 2012, 09:19 PM   #347
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by tinitus View Post
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 ?
If you look at how a vacuum tube is made the grid is literally isolated in a vacuum, not touching anything. The impedance is infinite because it is an open circuit. But grid can collect an electrostatic charge and must be connected to ground through a bleeder resister to keep the grid from being charged. The impedance is determined by this "grid leak" resister. You can get away with a 10Meg resister although "everyone" uses a 1 meg.

This does not apply only to tubes but you see this more on tube preamps, You can use up to a 10:1 audio transformer and transform the impedance any way you like. Remember that impedance ratio is the square of the turns ratio so a 10:1 turn ratio is a 100:0 impedance. In theory you can get up to a Gig-ohm but in the real world all you'd hear is noise and hum because 1Meg is already high enough to pick up noise.

Last edited by ChrisA; 10th October 2012 at 09:32 PM.
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Old 10th October 2012, 10:30 PM   #348
tinitus is offline tinitus  Europe
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thanks Chris
I know the title have become a bit 'old'
the hi-Z thing is more or less sorted
experimenting with the gain stages and buffers now

but you have a good point with having a tube as input
tho I have decided to save the tubes for alternative output
and since I have decide it should full functional with tubes pulled out, I must also have alternative SS output
and even worse, I decided to have two channels
but how it ends I dont know yet

and I really need them tone controls
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Old 11th October 2012, 07:44 AM   #349
tinitus is offline tinitus  Europe
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this schematic shows what I have in mind, so far

but Im sure juma might have a good suggestion
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Old 11th October 2012, 10:32 AM   #350
tinitus is offline tinitus  Europe
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or with a bjt on input jfet
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