Opamp Buffer - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 7th June 2003, 10:51 PM   #1
diyAudio Member
 
EchoWars's Avatar
 
Join Date: Dec 2002
Location: Left of the Dial
Default Opamp Buffer

An amp I am working on has a preamp/tone control like the pic below. The output is fed to a differential pair, not shown, obviously...

I am going to add pre-out.main-in jacks, and plan on breaking the signal at the output of this circuit. However, it appears to me that the drive capability of this circuit will be pretty poor, so I planned on adding a OPA2604 as a buffer between the two stages.

Honestly never used a opamp as a simple buffer (unity gain). A friend emailed me and said to install a resistor in the feedback loop equivilent to the output impedence of the circuit feeding the amp, although he didn't say what this circuit might require. (10K parallelled with 27K....somewhere around 7K? Or feedback resistor not needed?) Shooting for a reasonable degree of DC precision...

Any thoughts on simple buffer circuits and perhaps the best way to incorporate one into this?
Attached Images
File Type: png ka-3500_preamp.png (4.8 KB, 1574 views)
  Reply With Quote
Old 8th June 2003, 06:46 AM   #2
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
Download the datasheet of BUF634. At the end you have the simplified schematic of the circuit (Diamond buffer) or you could also check my QRV-03 headphone amp and the output stage. The korean guy below is very satisfied with it.

You could also check Walter Jung's article.
http://www.elecdesign.com/Globals/Pl...tent/2800.html

http://www.headphoneamp.co.kr/bbs/vi...esc=asc&no=604
http://www4.head-fi.org/forums/showt...threadid=35074
__________________
/Per-Anders (my first name) or P-A as my friends call me
  Reply With Quote
Old 8th June 2003, 08:29 AM   #3
diyAudio Member
 
EchoWars's Avatar
 
Join Date: Dec 2002
Location: Left of the Dial
Yep, I've read most of that at one time or another.

The BUF634 is a fine chip, but way overkill for what I want. Plus it means ordering more parts from a supplier where the minimum order is $25 US and the BUF634 is about $7 US each, and I'd need two. I have several OPA2604's just sitting here, and I'd like to put one of them to use. They would work fine for my purposes in this mid-fi amplifier. I was just hoping to gain some insight into using an audio opamp such as the 2604 in a unity gain application.

So far, the only suggestion I've gotten is the resistor in the feedback loop. Is this adviseable? What value?

Any other ideas?
  Reply With Quote
Old 8th June 2003, 09:55 AM   #4
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
Does your circuit work? Have you calculated the bias for each stage? This 5 pF? Do you need this?

I think driving capability for the tone control network seems to be low. Also the wire between base and emitter (= short circuit), what the purpose, error maybe? If you want a very simple buffer, just use an emitter follower.
__________________
/Per-Anders (my first name) or P-A as my friends call me
  Reply With Quote
Old 8th June 2003, 08:15 PM   #5
diyAudio Member
 
EchoWars's Avatar
 
Join Date: Dec 2002
Location: Left of the Dial
I don't know where that wire came from....between the base of the second transistor and the 150K resistor...pls ignore, looks like a drawing artifact of some sort.

This is a fully functional gain stage/tone amp on a commercial product. Not terribly high-end, but the integrated amp here was bought to utilize the amplifier section, which is quite good sounding. So, to use the amp section I need to add the preout/main-in jacks. The buffer is not necessary for the main-in section, and I won't use the preout much, but I can see that unless I add some sort of buffer to the output of this circuit that the preamp section will be nearly useless, and I'd like it to be functional. Hence the questions about using a simple opamp such as the 2604 as a buffer.
  Reply With Quote
Old 8th June 2003, 09:38 PM   #6
diyAudio Member
 
EchoWars's Avatar
 
Join Date: Dec 2002
Location: Left of the Dial
Here are three basic unity gain circuits. The third is an example of what my friend told me...to make the resistor the approximate value of the output impedence of the supplying stage.

What are the pluses and minuses of using any one of these?

You replies are really appreciated. Thank you much...
Attached Images
File Type: png opamps.png (1.8 KB, 1228 views)
  Reply With Quote
Old 10th June 2003, 04:43 PM   #7
diyAudio Member
 
Tube_Dude's Avatar
 
Join Date: Mar 2002
Location: Aveiro-Portugal
Default One or two...

Hi EchoWars...

The first exemple in your figure doesn´t have unity gain ,but the gain of two..

Regards
__________________
Jorge
  Reply With Quote
Old 10th June 2003, 05:32 PM   #8
MBK is offline MBK  Singapore
diyAudio Member
 
Join Date: Apr 2002
Location: Singapore
Standard application is "no resistor". It will work with no massive problems. Data sheets for op amps will say, to be picky, match impedances on op amp inputs IF the series resistance on the (+) input exceeds a certain value. For the OPA 2134 this value is 2k. For the OPA 2604 I don't know.

In your case: if this is an emitter follower buffer, in other words, zero or just a few dozen ohms between preceding stage output and the (+) input, then the feedback path can be a straight wire. If there is a series resistance exceeding say, these 2k, then make the feedback path from out to (-)in a resistor of equal value to the (+)in series resistance.

The main effect will be in lowering DC offset.


MBK
  Reply With Quote
Old 10th June 2003, 05:37 PM   #9
MBK is offline MBK  Singapore
diyAudio Member
 
Join Date: Apr 2002
Location: Singapore
OR:

use an inverting buffer: the circuit as is to (-)in, a 27k//10k from -(in) to output, and (+)in to GND.

Usually overshoot and capacitive drive capability as well as DC offset problems are lesser in inverting configuration.

MBK
  Reply With Quote
Old 10th June 2003, 06:22 PM   #10
diyAudio Member
 
EchoWars's Avatar
 
Join Date: Dec 2002
Location: Left of the Dial
Default Re: One or two...

Quote:
Originally posted by Tube_Dude
Hi EchoWars...

The first exemple in your figure doesn´t have unity gain ,but the gain of two..

Regards
Yes.....saw my error after posting, was too lazy to change.

MBK, the datasheet on the 2604 does not indicate at what point the series resistance should be matched, but in the circuit in my first post, it appears that the output impedence is somewhere around 10K. This seems like a large value for a feedback resistor in a unity gain stage. If I have examined the supplying stage incorrectly (certainly possible, since I'm obviously rusty on this), I'd appreciate being corrected.

I cannot invert. There is no latter stage where I might invert again, so I'm stuck with non-inverting. Perhaps I may stick a trimpot in the feedback loop and see what it does to offset, which is my main consideration...from there I might read a value and decide on a resistor...
  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
UcD400 opamp buffer tomahack Class D 3 1st November 2006 10:56 PM
PMA's opamp/buffer pcb garbage Digital Source 1 2nd August 2004 03:09 PM
Opamp + buffer, how to bypass buffer at HF for unity gain? Christer Solid State 10 29th October 2003 07:50 AM
opamp with/without buffer....??? tbla Solid State 11 13th September 2003 04:18 PM


New To Site? Need Help?

All times are GMT. The time now is 11:30 PM.


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