Switchable Hi-Z input impedance, how ? - Page 4 - diyAudio
Go Back   Home > Forums > Live Sound > Instruments and Amps

Instruments and Amps Everything that makes music, Especially including instrument amps.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 12th September 2011, 11:48 PM   #31
tinitus is offline tinitus  Europe
diyAudio Moderator R.I.P.
 
Join Date: Nov 2005
Quote:
Originally Posted by leadbelly View Post
But what is the point of all this? ............ Where are you trying to go?
if at all using a tube for this, I would like it to be easy and cheap to service

it will be driving a 100watt SS power amp

how about a single ecc82 with one half as cathode follower output ?
ups, and back again
  Reply With Quote
Old 13th September 2011, 12:24 AM   #32
tinitus is offline tinitus  Europe
diyAudio Moderator R.I.P.
 
Join Date: Nov 2005
how about something like in this direction ?
Attached Images
File Type: gif C3g - 6C19n preamp.GIF (7.4 KB, 162 views)
  Reply With Quote
Old 13th September 2011, 04:21 AM   #33
diyAudio Member
 
leadbelly's Avatar
 
Join Date: Dec 2002
Location: Calgary, Alberta
Well, I still don't quite get it, don't know why you would go to a transformer coupled cathode follower, so I'll just throw out recommendations until you like one.

If you just want to make a simple high-impedance front end for a SS amp, just use a single cap coupled JFET.

If your SS amp needs a balanced input and you like tubes, just make a long tailed pair with a dual triode.

If your SS amp has a lowish input impedance, you can try a dual triode with 1/2 as common cathode and 1/2 as cathode follower.

Or why not just copy an existing design, like an Alembic F2B with the tone stack left out?
__________________
Aerodynamics are for people who can't build engines. Enzo Ferrari
  Reply With Quote
Old 13th September 2011, 11:00 AM   #34
tinitus is offline tinitus  Europe
diyAudio Moderator R.I.P.
 
Join Date: Nov 2005
Quote:
Originally Posted by leadbelly View Post
....... so I'll just throw out recommendations until you like one.

Or why not just copy an existing design, like an Alembic F2B with the tone stack left out?


Alembic looks nice

hmm, ok, lets try
if I said it doesn't matter how
I need gain
I have enough of 2sk170 (non matched)
and I might use balanced classD power amp

how would you do it ?
  Reply With Quote
Old 13th September 2011, 12:27 PM   #35
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
Another way to look at the input is that impedance is all relative.

What is the output impedance of the driving device.

A general "Rule of Thumb" is for Hi-Z keep the input impedance greater than 10X the driver Output impedance. This gives a 9.09% reduction in the input signal over the open circuit output voltage.

If the input impedance is 100X the output impedance you will have 0.99% reduction, etc.

The figures I've found for guitar output impedance indicate it is generally between 20K and 40K ohms.

IF this is so, then 1M/20K=50 and 1M/40K=25 and the typical ratio for a 1Meg input impedance is 25 to 50X the output impedance which is light loading.

Once you switch to Low-Z (in your circuit it is approximatly 136K because the two 68K resistors are in series) your ratio drops to 136/20=5.4 to 136/40=3.4 which is significantly greater loading and will reduce the signal level as well as effecting frequency response. Furtheremore the two 68K resistors are in series dividing the voltage by 2 which is the main gain reduction.

Rough answer is greater than 400K is high-Z for guitar amp, Once you get down to 150K you are loading the output of the guitar and in the Lo-Z area. Somewhere in between is the transition range.
  Reply With Quote
Old 13th September 2011, 01:24 PM   #36
tinitus is offline tinitus  Europe
diyAudio Moderator R.I.P.
 
Join Date: Nov 2005
Quote:
Originally Posted by TheGimp View Post
Another way to look at the input is that impedance is all relative.

The figures I've found for guitar output impedance indicate it is generally between 20K and 40K ohms.
that is great info
I'm learning

hmm, if using passive pickups, and if I left out the passive volume pot on the guitar, I would have lower impedance, right ?
actually, with passive pickups, controls on guitar may be the worst of the impedance 'problem' (?)

edit, actually, I have considered using a chip/volume 'pot', with a buffer
but with needed gain, balanced output etc etc, it seems complicated
  Reply With Quote
Old 13th September 2011, 01:45 PM   #37
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
You could, but you have to realize there will probably be a rather extreme peak in the response of the guitar pickup. Part of the reason for the tone section is to achieve critical dampening of the response by placing a low pass filter in the appropriate place frequency wise in the output. You could add such a filter with a simple RC combo after measuring the response to locate the resonance and dampen it with a pot and cap to find the best values.

There is a good explaination here:

BuildYourGuitar.com :: The Secrets of Electric Guitar Pickups

I'm not sure how the chip pot would work, as I've not used any. Theoretically they should be fine as they are used in A/D circuits for calibration and many of the A/D circuits are up to 16 bits or more.

My understanding is the employ a MOS-FET as the pass device. Varying the gate to source voltage results in a varying drain to source resistance. Using it with a buffer would make sense, but as you say it becomes complicated quickly.
  Reply With Quote
Old 13th September 2011, 10:16 PM   #38
tinitus is offline tinitus  Europe
diyAudio Moderator R.I.P.
 
Join Date: Nov 2005
veey good info on pickups nice
  Reply With Quote
Old 15th September 2011, 10:43 PM   #39
tinitus is offline tinitus  Europe
diyAudio Moderator R.I.P.
 
Join Date: Nov 2005
Quote:
Originally Posted by TheGimp View Post
You could, but you have to realize there will probably be a rather extreme peak in the response of the guitar pickup.
even more tricky than I was aware of, yes
as I understand, the peak isn't removed, but moved in frequency
critical Q may change too, yes

since last time I looked at bass pickups a few thing seem to have changed
passive are really no longer passive, as such
but are also sold with 'active curcuit'
seems like others are learning too

also, that paper indicate that a load into lower impedance will dampen the peak
or maybe it was in the link at bottom, to commercial product

it needs further consideration
balanced or se power input
I'm still indecisive on that
like say if I used big power amp, but without needing it
maybe I wouldn't need that much preamp gain

but balanced power amp input is low impedance
'single ended' usually higher impedance
but still not quite enough
and so it goes round and round
  Reply With Quote
Old 17th September 2011, 12:14 PM   #40
tinitus is offline tinitus  Europe
diyAudio Moderator R.I.P.
 
Join Date: Nov 2005
wireless , another idea
using aportable PC as preamp/EQ
I have one, not in use

and, I can get a very cheap transmitter/receiver thing
its with both mic and line connections

but I'm not sure how much is possible with a soundcard

any ideas?
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Switchable balanced / unbalanced input buffer nigeljking Analog Line Level 4 12th September 2011 12:03 PM
How to design input stage with specific input impedance/pot value mahleriana Tubes / Valves 0 16th November 2009 09:37 PM
Input Impedance Bengali Analog Line Level 7 15th August 2007 10:24 PM
Input impedance nhuwar Tubes / Valves 24 23rd July 2007 11:11 PM
Input pot and input impedance Bricolo Parts 1 19th March 2003 01:59 PM


New To Site? Need Help?

All times are GMT. The time now is 01:59 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2