About op-amps use. - 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 18th May 2015, 12:03 PM   #1
diyAudio Member
 
Join Date: Sep 2009
Location: Stavanger (NORWAY)
Default About op-amps use.

Hi !
reading around i found this intriguing comment about using high value of uF very close to op-amps.

Quote:
... Here's some of what have learned about op-amps.... Put 6800uF at every power pin of the op-amps, all the op-amps. This will make your op-amps disappear, no more noise, no more distortion! Bass, as good as the circuit can deliver...
Ok .. maybe 6800 uF is a little on the high side.
But i see very few uF used normally. I have listen to a preamp based completely on opa604 with 2x1000 uF close to the opa.
It sounded both clean and not particularly solid-statish ... i would say very good indeed. Clean as solid state can be but also not hard or harsh.
What do you use normally ?
Thanks a lot, gino
  Reply With Quote
Old 18th May 2015, 12:11 PM   #2
diyAudio Member
 
abraxalito's Avatar
 
Join Date: Sep 2007
Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though...
Blog Entries: 121
Send a message via MSN to abraxalito Send a message via Yahoo to abraxalito Send a message via Skype™ to abraxalito
How much to use depends on the particular opamps, their loading and the circuit toplogy. Run balanced with very light loading so the output stages don't come anywhere near leaving classA and you probably won't need that 6800uF. But it might come in useful for filtering a noisy reg, in conjunction with a series L.
__________________
When irony first makes itself known in a young man's life, it can be like his first experience of getting drunk - Robertson Davies
  Reply With Quote
Old 18th May 2015, 12:38 PM   #3
diyAudio Member
 
Join Date: Sep 2009
Location: Stavanger (NORWAY)
Quote:
Originally Posted by abraxalito View Post
How much to use depends on the particular opamps, their loading and the circuit toplogy.
Run balanced with very light loading so the output stages don't come anywhere near leaving classA and you probably won't need that 6800uF.
But it might come in useful for filtering a noisy reg, in conjunction with a series L.
Hi and thanks a lot for the helpful advice. I think i understand now.
If the opamp works within class A it draws always the same amount of current.
I guess that bias current is what in the datasheet is called idle/quiescent current ?
How can i be sure that this current is not overcome ?
For instance i see an opa209 here with 2.5 mA of quiescent current. This is nothing ... it is clear that it will work always above that level.
Instead the AD811 that is considered an "hot" opamp is around 15mA.
No opamp around 40-50 mA ??? just to be sure ...
and in that case for a preamp no reason to go too high with uF if i understand well.
However .... is it possible to increase bias current for an op-amp ?
or is it fixed ?
Now i start to understand the reason of class A.
Thanks again, gino

Last edited by ginetto61; 18th May 2015 at 12:49 PM.
  Reply With Quote
Old 18th May 2015, 12:49 PM   #4
diyAudio Member
 
abraxalito's Avatar
 
Join Date: Sep 2007
Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though...
Blog Entries: 121
Send a message via MSN to abraxalito Send a message via Yahoo to abraxalito Send a message via Skype™ to abraxalito
Its not necessarily the case that if an opamp stays in classA that it'll always draw a constant current from the supplies - that's because the load current has to go somewhere. Hence my stipulation of running balanced which cancels the load current effect.

How you can be sure - short of measuring the current taken from the supplies in the real-world circuit, I don't know how to be sure. I think its fairly safe to assume that a 50uA output current isn't going to cause the output stage to leave classA. The quiescent current spec'd in the DS isn't all going to flow in the OPS, only a smallish proportion does.

When I want more current from an opamp OPS than 50uA I'll put in a discrete classA buffer to handle it or alternatively hang a CCS from the opamp output in the case of an opamp with decent drive capability (not a TL072!).
__________________
When irony first makes itself known in a young man's life, it can be like his first experience of getting drunk - Robertson Davies
  Reply With Quote
Old 18th May 2015, 01:42 PM   #5
DF96 is online now DF96  England
diyAudio Member
 
Join Date: May 2007
Quote:
Originally Posted by ginetto61
If the opamp works within class A it draws always the same amount of current.
No. Class A means the current averaged over a complete AC cycle remains unchanged, whatever the signal amplitude. The signal current may appear (often will appear) on the supply rails, but as it is AC it will average to zero.

Local supply rail decoupling works if it provides a lower impedance for AC than the supply rail itself. At higher frequencies supply rail inductance means that small caps can help. At lower frequencies the resistance dominates, but this will be small so you would need a rather large cap very close to the chip to make any difference - but a large cap will bring its own problems, as it has inductance too. No point whatsoever in putting more than a few uF at the supply pins of a normal opamp - to do its job the cap must be physically small.
  Reply With Quote
Old 19th May 2015, 06:59 AM   #6
diyAudio Member
 
Join Date: Sep 2009
Location: Stavanger (NORWAY)
Quote:
Originally Posted by abraxalito View Post
Its not necessarily the case that if an opamp stays in classA that it'll always draw a constant current from the supplies - that's because the load current has to go somewhere. Hence my stipulation of running balanced which cancels the load current effect.
How you can be sure - short of measuring the current taken from the supplies in the real-world circuit, I don't know how to be sure. I think its fairly safe to assume that a 50uA output current isn't going to cause the output stage to leave classA. The quiescent current spec'd in the DS isn't all going to flow in the OPS, only a smallish proportion does.
When I want more current from an opamp OPS than 50uA I'll put in a discrete classA buffer to handle it or alternatively hang a CCS from the opamp output in the case of an opamp with decent drive capability (not a TL072!).
Hi and thanks a lot for the very interesting and a little difficult for me to understand explanation.
But i have two very trivial questions:
1) what are the drawbacks of putting more uF close to the op-amps ? value like 10-20 milliF i mean. Will there be problems ?
2) i see in the dataheet a value of max current often from 20 mA up. Is it not enough for a line stage ? and for an headphone stage ?
Thanks a lot again. Gino
  Reply With Quote
Old 19th May 2015, 07:03 AM   #7
diyAudio Member
 
Join Date: Sep 2009
Location: Stavanger (NORWAY)
Quote:
Originally Posted by DF96 View Post
No. Class A means the current averaged over a complete AC cycle remains unchanged, whatever the signal amplitude. The signal current may appear (often will appear) on the supply rails, but as it is AC it will average to zero.
Local supply rail decoupling works if it provides a lower impedance for AC than the supply rail itself. At higher frequencies supply rail inductance means that small caps can help. At lower frequencies the resistance dominates, but this will be small so you would need a rather large cap very close to the chip to make any difference - but a large cap will bring its own problems, as it has inductance too. No point whatsoever in putting more than a few uF at the supply pins of a normal opamp - to do its job the cap must be physically small.
Hi and thanks for the valuable advice. The guy seemed quite convinced actually. The only way would be to see the V changes at the caps pins.
If they do not sag during peakes no problems at all.
I understand that for line stage the delivery requirements are quite light.
Low current delivery is needed.
Thanks again, gino
  Reply With Quote
Old 19th May 2015, 07:12 AM   #8
diyAudio Member
 
abraxalito's Avatar
 
Join Date: Sep 2007
Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though...
Blog Entries: 121
Send a message via MSN to abraxalito Send a message via Yahoo to abraxalito Send a message via Skype™ to abraxalito
Quote:
Originally Posted by ginetto61 View Post
1) what are the drawbacks of putting more uF close to the op-amps ? value like 10-20 milliF i mean. Will there be problems ?
10mF is going to be a fairly bulky beast so practically impossible to put it up close to an opamp unless you move Muhammed to the mountain (so to speak). Meaning perch the opamp between the pins of the cap, using the cap itself as the mounting of the opamp.

Quote:
2) i see in the dataheet a value of max current often from 20 mA up. Is it not enough for a line stage ? and for an headphone stage ?
Certainly enough for a line stage, enough for a few higher impedance headphones. But that's a quantitative spec, tells you nothing about the sound quality when delivering that amount of current. Some DSs spec a current limit and a lower figure as 'maximum linear output current' or something similar.
__________________
When irony first makes itself known in a young man's life, it can be like his first experience of getting drunk - Robertson Davies
  Reply With Quote
Old 19th May 2015, 10:36 AM   #9
diyAudio Member
 
Join Date: Sep 2009
Location: Stavanger (NORWAY)
Quote:
Originally Posted by abraxalito View Post
10mF is going to be a fairly bulky beast so practically impossible to put it up close to an opamp unless you move Muhammed to the mountain (so to speak). Meaning perch the opamp between the pins of the cap, using the cap itself as the mounting of the opamp.
Sorry silly me. I wanted to say 1-2,2mF.
That can be done quite easily. Sorry i said a stupidity.
I will attach a picture with 1000 uF caps surroundigs some OPAs this evening as an example of what i have in mind.

Quote:
Certainly enough for a line stage, enough for a few higher impedance headphones. But that's a quantitative spec, tells you nothing about the sound quality when delivering that amount of current. Some DSs spec a current limit and a lower figure as 'maximum linear output current' or something similar.
I understand and thanks a lot for the important advice.
Especially now that op-amps come almost always in smd it is quite clear to me that they are not power parts. They are not even heatsunk.
I have seen op-amp with up to 250-500 mA of current delivery without a metal plate to heatsink them properly.
I have had bad experience with heatsinks ... nowadays everything seems to be designed to stay in a cell phone or in a tablet ...
Thanks a lot again and sorry for the mistake.
I had 1000-2200 uF in mind as max values. No more.
Kind regards, gino
  Reply With Quote
Old 19th May 2015, 10:51 AM   #10
AndrewT is online now AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Quote:
Originally Posted by ginetto61 View Post
...................I have seen op-amp with up to 250-500 mA of current delivery without a metal plate to heatsink them properly. ......................
high current IC (opamps and Buffers) will require a heatsink.
This could be using the multiple legs/leadouts to a very big copper plane on the PCB, or could be a contact surface on the bottom side of the chip that requires a big copper plane to dissipate the heat.

Some are required to be soldered to the copper plane.
__________________
regards Andrew T.
Sent from my desktop computer using a keyboard
  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
Making more powerful amps. Voltage ratings of my op-amps are being exceeded Plecto Solid State 21 3rd October 2013 03:16 PM
Why do solid state guitar amps need so many op amps? razorrick1293 Solid State 2 8th June 2011 10:06 AM
Burr Brown op amps,in amps,power op amps for sale magneticnorth Vendor's Bazaar 1 23rd September 2010 10:50 PM
FS PA02 Power op-amps +/- 18v, 5 amps modifry Swap Meet 0 11th August 2004 07:45 PM
Op Amps in Solid State Guitar Amps MrGuitardeath Instruments and Amps 1 11th February 2004 07:43 PM


New To Site? Need Help?

All times are GMT. The time now is 04:09 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2015 DragonByte Technologies Ltd.
Copyright 1999-2015 diyAudio

Content Relevant URLs by vBSEO 3.3.2