Need a 1 meg input pre stage-?! - diyAudio
Go Back   Home > Forums > Source & Line > Analog Line Level

Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

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 10th March 2011, 06:39 PM   #1
diyAudio Member
 
colwaiting's Avatar
 
Join Date: Dec 2002
Location: boston Ma.
Angry Need a 1 meg input pre stage-?!

I am new to posting here ('cept for selling stuff) but am ready to take the plunge!
been lurking for ages.

I have scoped out the OPA 2134 for my first stage. total gain (over several stages) must be ? 80? There seems to be quite a range of problems when working at 1 meg input. (I got, allegedly, 2.0 mV @ 1 megohm to amplify)

The OPA spec sheet says to correlate your input imp with the resistance in your feedback loop, but that might mean 1.5 meg//250K (to ground) which seems a bit out of whack.
Also it would appear difficult to get much gain in this stage, also. that i could live with, but 9 or 10 sure would be nice.


Do i need a Jfet or complementary Jfet input buffer here?
I have no design skills to speak of --just cookbook competence.

Advice, please. I'll happily use a 6DJ8 if I have to. Would i know what Im doing? No.

I am also correct that the inverting input is out of the question?
  Reply With Quote
Old 10th March 2011, 09:00 PM   #2
juma is offline juma  Germany
diyAudio Member
 
juma's Avatar
 
Join Date: Nov 2005
Location: Berlin
You can use SE JFET buffer (see attachment). Input impedance is 1M and you can load this buffer with 5k or more. P1 (multiturn pot) sets DC offset.
Mouser sells BF862 JFETs.
Attached Images
File Type: gif bf862buffer.gif (8.3 KB, 157 views)

Last edited by juma; 10th March 2011 at 09:08 PM.
  Reply With Quote
Old 11th March 2011, 12:36 AM   #3
diyAudio Member
 
colwaiting's Avatar
 
Join Date: Dec 2002
Location: boston Ma.
that is very useful AND I see that there are no boutiqe parts!
It would be good to expand my knowledge of fet's

I have an excellent text (Horowitz & Hill) but nothing clicks
until I put it into wire and bits.

Doug
  Reply With Quote
Old 11th March 2011, 09:47 AM   #4
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
a gain of 80 times (+39dB) for an opamp is not too much to ask.
A FET input opamp can easily achieve 1M for Zin.

The opa2134 meets both requirements and it is unity gain stable.
__________________
regards Andrew T.
  Reply With Quote
Old 15th March 2011, 12:24 AM   #5
diyAudio Member
 
colwaiting's Avatar
 
Join Date: Dec 2002
Location: boston Ma.
Smile Low level input--Low noise-- OPA 2134

Oh good I was waiting to hear that.

I will try setting that up. I would have to guess why Burr Brown warns against excess noise & distortion. Would you have a suggestion for R1//R2? something like 500K/ 62K?
  Reply With Quote
Old 15th March 2011, 12:28 AM   #6
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Quote:
a suggestion for R1
Is that a Yamaha?
__________________
regards Andrew T.
  Reply With Quote
Old 15th March 2011, 03:25 AM   #7
diyAudio Member
 
colwaiting's Avatar
 
Join Date: Dec 2002
Location: boston Ma.
Default yamaha ?

the 2 mV 1 meg I connect to comes from the MPX board of a McIntosh 113 tuner. Also MR73
  Reply With Quote
Old 12th April 2011, 01:44 PM   #8
diyAudio Member
 
colwaiting's Avatar
 
Join Date: Dec 2002
Location: boston Ma.
Long delayed thanks to Juma! I see that Q8 is set up as a current source??
Done some more reading in Horowitz et al. I had previously no real notion of Jfets.

Am I to assume that distortion is not an issue? Normally i think of any amplifier stage as being enclosed in a feedback loop--

My text sez "you can build discrete Jfet stages, but normally one looks at Op-amps that use Jfets in the input stage." Hello TL072?
  Reply With Quote
Old 13th April 2011, 12:27 PM   #9
juma is offline juma  Germany
diyAudio Member
 
juma's Avatar
 
Join Date: Nov 2005
Location: Berlin
Quote:
Originally Posted by colwaiting View Post
... Q8 is set up as a current source??
Yes, Q8 represents a working load for Source Follower Q7.

Quote:
Originally Posted by colwaiting View Post
... distortion is not an issue?
Not at all.
  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
Help with input stage driver for EL84 output stage AudioGeek Tubes / Valves 2 20th March 2010 01:38 AM
How to design input stage with specific input impedance/pot value mahleriana Tubes / Valves 0 16th November 2009 09:37 PM
1 meg audio pot needed danielm Vendor's Bazaar 1 12th February 2007 05:15 PM
current bias j-fet input 2sk170 ,2sj74 at input stage YUTK Solid State 11 2nd June 2005 03:34 AM
Regulator for input stage/pre Mambo Solid State 1 3rd October 2003 03:37 PM


New To Site? Need Help?

All times are GMT. The time now is 05:04 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