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Old 1st December 2004, 03:19 PM   #31
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Quote:
Originally posted by classd4sure
Is it overrated? Good question, maybe not for a 741 Maybe for a high quality audio op amp.
Have you tested a LM324/358? There you can speak about possible improvements! => extremly high crossover distortion!
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Old 1st December 2004, 03:47 PM   #32
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Quote:
Originally posted by peranders

Have you tested a LM324/358? There you can speak about possible improvements! => extremly high crossover distortion!

Hi,

I sure haven't but that's great to know.

How about for a NE5532 such as the OP is using?
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Old 1st December 2004, 03:53 PM   #33
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Maybe this is something:
http://fnt-www.ss.titech.ac.jp/~haji...rtion/opa.html
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Old 2nd December 2004, 11:20 AM   #34
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Default My two cents

I have used both the 5532, 5534, OP27 , OP37, OPA627, OPA637, and a number of AD***/LT**** opamps.
Both in standard topology as forced in class A.

The 5532 may be an Oldy, but for the money it is still a good opamp, same goes for 5534.
Personally i like the Burr Brown sound, BB in all types sound way different from other manufacturers.
A PMI OP27 differs in sound from a BB OP27, that i noticed some 15 years ago when i started with PMI types on a pre, blew them when fiddling with the power regulation and replaced them by BB types, as my DIY friend did from first go.

Powersupply definitely has an effect, as it always has.
I have used OPA627's forced in class A on a pre with a battery powersupply, as with any class A amplifier circuit differences in the powersupply make a difference in sound.
But on any amplifier, AB or A, Grey Rollins wrote some lines on it on another thread stating the same.

I rebuilt two 4 channel class A car amplifiers i use in my Van.
The original KIA4558s i first replaced by OPA2134's, the TIP transistors by Sanken 1216/2922(16 of them).
Later i made a good deal on a hundred OPA627/OPA637's, placed a conversion PCB above the Dual opamp pads in able to use single opamps on the amplifiers.
As car amplifiers make use of switched powersupplies(which i am not a big fan of) the original car battery i replaced by two batteries with higher Amp*hour ratings, fitted under the truck.
On the battery lines are two 1Farad caps, internally a great number of 1000uF/ 4700uF EYF Roedersteins were added, the toroids swapped for larger ones, the switching transistors beefed up.

Both the 2134 and the OPA637 i later used were forced in class A, the sound benefitted much by any powersupply improvement.
Class A car amplifiers are sold as such, in reality the class A percentage is not that impressive.
The amplifiers in my van have been thermally coupled to the roof of the car, the wind over the railing on the raised roof actually cools the amplifiers.
Because of it i was able to raise the class A level, to an extent where sound quality suffered because the powersupply could not handle the load, so the bias had to come down.

My conclusion:
The Burr Brown OPA627 and OPA637 rank highest in integrated audiocircuits, even on switched powersupply car amplifiers.
Forcing opamps in class A the way Carlos described is an improvement, but it is not a wonderdrug.
Put a high level audio opamp in a crap amplifier, force it in class A, you will still have a crap amplifier.
Using an opamp such as the OPA627 also means paying extra attention to every other part of the amplifier.
Force the opamp in class A and even more attention for the entire layout is needed.

I started with a simple Opamp switch, and added a few resitors.
In the end i had to rebuild the entire amplifier, the entire powersupply, had to use several powerlines the size of my D..., even every powerline needed a separate Shaffner filter.
No work, no Play !
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Old 19th September 2016, 04:26 AM   #35
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Is this schematic correct to bias opamp in class A by draw 0.2mA current
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Old 19th September 2016, 06:43 AM   #36
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Quote:
Originally Posted by jacco vermeulen View Post

The amplifiers in my van have been thermally coupled to the roof of the car, the wind over the railing on the raised roof actually cools the amplifiers.
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Old 19th September 2016, 07:33 AM   #37
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Quote:
Originally Posted by supernut View Post
Is this schematic correct to bias opamp in class A by draw 0.2mA current
Is it a dolphin ? You really need to put a bit more effort into drawing what you mean.

(and a 15 volt rail with an opamp biased to zero volts at its output needs a 75k resistor to source/sink 0.2ma)
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Old 19th September 2016, 09:04 AM   #38
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I thought it's a cramp-fish
I have to also say that 0.2mA is really not enough for forcing the OpAmp's output stage into class A.
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Old 19th September 2016, 09:14 AM   #39
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How is about 0.4ma? 15v---10k --- 10nF --- 27k --- output
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Old 19th September 2016, 09:29 AM   #40
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Did anybody notice the 10nF capacitor in series with the resistors?

I bet it will complicate a little sinking current from that Op Amp output.

Agree on the dolphin drawing though
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