opamps used for unity gain - 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 26th November 2007, 12:14 AM   #1
diyAudio Member
 
Join Date: May 2004
Location: Sydney
Default opamps used for unity gain

Hi,

I was reading another forum and found an interesting subject discussing opamps used for unity gain.

The general comments were opamps when used for unity gain sound bad. The possible cause, I guess, is the largest amount of feedback used.

Note that I am only talking about using unity-gain-stable opamps. I use opamps in active crossover network. Shallen key is common place.

So, is this right? Does it mean we should always add some gains, despite higher noise? High frequency gain or low frequency gain? How much gain? Would it make the opamp clip easily? Any opamps that sound the best for unity gain buffers? opa627, opa2134? ad825? What is your solution to this?

Regards,
Bill
  Reply With Quote
Old 26th November 2007, 02:57 AM   #2
diyAudio Moderator Emeritus
 
lumanauw's Avatar
 
Join Date: Oct 2002
Location: Bandung
Send a message via Yahoo to lumanauw
Quote:
The general comments were opamps when used for unity gain sound bad. The possible cause, I guess, is the largest amount of feedback used.
A post by Nelson Pass here can give a different POV of the problem : http://www.diyaudio.com/forums/showt...437#post606437
  Reply With Quote
Old 26th November 2007, 10:16 AM   #3
Bonsai is offline Bonsai  Taiwan
diyAudio Member
 
Bonsai's Avatar
 
Join Date: Jul 2003
Location: Taipei, Taiwan
I'd say th e comments about stability ar e valid in many cases . . . . but then read the app note and take th e necessary precautions to prevent oscillation/misbehaviour:-

Dont operate non-unity gain stable op-amps at gains lower than specified by the manufacturer

Make sure you isolate capacivie loads adequately - these can cause a pole in the op-amps response and this leads to instability

Decouple thoroughly and close to th e op-amp

Make sure your grounds are clean and are low inductance

For summing circuits (or non-inverting feedback nodes), place the feedback resistors AS CLOSE TO THE INVERTING INPUT PIN AS POSSIBLE. I've seen this rule broken many times and always with the same results - noise, EMC pickup, instability

Finally, to all those people that complain about th e 'op-amp' sound, remember next time you listen to one of your favourite CD's (or maybe vinyl recording) that the signal chain from mike through to cutting head has almost certainly gone through about quite a few op-amps along the way. Welcome to the 'op-amp' sound!
__________________
bonsai
Amplifier Design and Construction for MUSIC! http://hifisonix.com/
  Reply With Quote
Old 26th November 2007, 11:09 AM   #4
sreten is offline sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Hi,

Unity gain op-amps used in high gain circuits do not sound so good.
Non-unity gain op-amps used at unity gain oscillate, not too good.

The idea that high gain op-amps "slugged" for unity gain sound poor
is somewhat of a fallacy but an advanced, (compared to the normal
6dB/octave scheme), 12dB/octave compensation scheme can improve
high frequencies (y filters) by maintaining internal loop gain and
consequently feedback factor at high frequencies.

The same is true for audio gain stages ......

Quote:
The general comments were opamps when used for unity gain sound bad.
The possible cause, I guess, is the largest amount of feedback used.
I'd say this is the "golden ears" imagining things. It is only possibly
true is stabililty is marginal and improved by higher gain. As an idea
it is overly simplistic as the "amount of feeddback" i.e. the feedback
factor is largely constant for a given stability. By increasing gain and
reducing feedback factor (increasing stability) distortion is increased.

Unless of course the op-amp sounds fabulous with the lowest possible
feedback, i.e. max gain and attenuating the output for your required gain.

/sreten.
  Reply With Quote
Old 26th November 2007, 11:28 AM   #5
diyAudio Moderator Emeritus
 
lumanauw's Avatar
 
Join Date: Oct 2002
Location: Bandung
Send a message via Yahoo to lumanauw
Quote:
sound bad
I've got some confusing experience with this.
When one amplifier is nearly perfect (technically), the sound is said "dull, dead, thin, bad".
When there's another amp that has a not so good clipping behavior plus quite high harmonic distortion (2nd, 3rd), the sound is said "good".
Chipamp with PS cap only nF (while the manual suggest about 1000uf/more) and omits the zobel stabilisator is said "very realistic", but I'm afraid that there's a marginally oscilation threshold and the PS junk is actually entering the sound. Confusing for me
  Reply With Quote
Old 26th November 2007, 11:58 AM   #6
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Quote:
Confusing for me
I suspect not.

I know, I know, it's a dull way to look at things, but wouldn't it be useful if, in subjective evaluation, people stopped talking about a box of gain "sounding good" or "sounding bad," but instead talked about whether or not the output sounded like the input?
__________________
And while they may not be as strong as apes, don't lock eyes with 'em, don't do it. Puts 'em on edge. They might go into berzerker mode; come at you like a whirling dervish, all fists and elbows.
  Reply With Quote
Old 26th November 2007, 03:04 PM   #7
diyAudio Member
 
Join Date: Mar 2007
Location: Canandaigua, NY USA
I didn't want to offer the "boring" view, but I just can't help myself any longer ;-) It baffles me why people think they can just pop one of a dozen opamps into a circuit, then listen and declare they know something about the way opamps sound. Where is it written that all those opamps operate at their best in the same circuit? Why do people think all those different opamps exist in the first place? Sometimes the tests are done without a scope, often without a schematic or knowledge of the actual circuit configuration (typically in CD players). "Rolling" opamps is a joke. IMO, almost any of the popular high performance opamps can be stabilized and configured to be indistinguishable in audio applications, if they have sufficient gain bandwidth product, low distortion, and aren't asked to deliver excessive output current or drive an unsuitable load. IMO, opamps are viewed as really simple to use, and the truth is that a certain amount of knowledge and skill is required to get the most out of 'em.
  Reply With Quote
Old 26th November 2007, 04:39 PM   #8
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
Conrad, your thoughts are always in my mind but I have long time stopped arguing about it.

Picture this: Not 100% good recording => MP3 coded => average sound card or even bad => not optimal pcb in general and not in particular for the opamp => bad or average headphones

This is a common setup to judge the characteristics of an opamp.
__________________
/Per-Anders (my first name) or P-A as my friends call me
  Reply With Quote
Old 26th November 2007, 05:50 PM   #9
diyAudio Moderator Emeritus
 
lumanauw's Avatar
 
Join Date: Oct 2002
Location: Bandung
Send a message via Yahoo to lumanauw
Hi, SY,

Quote:
I know, I know, it's a dull way to look at things, but wouldn't it be useful if, in subjective evaluation, people stopped talking about a box of gain "sounding good" or "sounding bad," but instead talked about whether or not the output sounded like the input?
That would be very nice, and will save alot of reviews

What makes me confuse is I like the sound of those "faulty" amps. When I tweaked both the "not so good clipping behavior plus quite high harmonic distortion (2nd, 3rd)" and "Chipamp with PS cap only nF (while the manual suggest about 1000uf/more) and omits the zobel stabilisator" to meet the technical POV, ie: stable towards R//C load, have the best square wave response and best residual (in my distortion meter+scope), the sound becomes "worse". (oops..sorry, I use this kind of term again )
  Reply With Quote
Old 26th November 2007, 06:42 PM   #10
diyAudio Member
 
myhrrhleine's Avatar
 
Join Date: Jan 2006
Location: Avalon Island
NFB is not the cause of 'sound' differences in unity gain op-amps.
whether 120dB of NFB or 130 dB NFB wont matter to the sound.
Stability may be an issue.
__________________
Just because you can't hear it doesn't mean no one can.
  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
Unity Gain op amp circuit? jetbat Solid State 3 5th May 2009 01:29 AM
Heretical Unity Gain line stage PLUS a little gain ? yagas Tubes / Valves 7 24th January 2006 02:30 PM
Unity Gain (AV=1) Zen Amp LS Pass Labs 2 16th September 2004 05:40 AM
Unity Gain Operation (or low gain) Tomo Chip Amps 12 22nd January 2004 02:47 AM


New To Site? Need Help?

All times are GMT. The time now is 08:02 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