Walt Jung biasing opamps in class A

[ruffrecords]

you know that in this thread
https://groupdiy.com/threads/modifying-existing-op-amp-circuits-to-operate-in-class-a.62082/
is the information to take a 5534 and turn it into a true class A opamp with easily selectable idle current to suit the expected load.

I just checked that thread. I could into find anything about turning a 5534 into a true class A amp.

Cheers

Ian

 

[dai h.]

hi,

this post:

https://groupdiy.com/threads/modify...uits-to-operate-in-class-a.62082/#post-787856

talks about taking the output from a 5534 from pin 5 to an external output stage.

 

[ruffrecords]

hi,

this post:

https://groupdiy.com/threads/modify...uits-to-operate-in-class-a.62082/#post-787856

talks about taking the output from a 5534 from pin 5 to an external output stage.

An external class A output stage not quite the same thing as making an op amp itself work in class A

Cheers

Ian

 

[Dylan W]

An external class A output stage not quite the same thing as making an op amp itself work in class A

Cheers

Ian

In this case it is, because pin 5 comes out after the VAS and before the AB output buffer. FWIW they use this technique in a lot of the new RND products, which sound absolutely fantastic.

 

[merlin]

There is information about doing this in Douglas Self's book "The Design Of Active Crossovers".

For reference:
"13.10 Reducing 5532 Distortion by Output Biasing" page 389 in Design of Active Crossovers.

"For the 5532, the current must be injected from the positive rail; currents from the negative rail make the distortion emphatically worse... there is a general reduction in distortion across the whole of the 5532 output range, which seems to indicate that the 5532 output stage is better at sinking current than sourcing it, and therefore injecting a positive current is effective at helping out."

Self wrote about this after he had already written the first edition of Small Signal Audio Design, so I guess he just forgot to add it to later editions.

 

[ruffrecords]

In this case it is, because pin 5 comes out after the VAS and before the AB output buffer. FWIW they use this technique in a lot of the new RND products, which sound absolutely fantastic.

But you are not using the output buffer, you are replacing it so it is not the same.

Cheers

Ian

 

[ruffrecords]

For reference:
"13.10 Reducing 5532 Distortion by Output Biasing" page 389 in Design of Active Crossovers.

"For the 5532, the current must be injected from the positive rail; currents from the negative rail make the distortion emphatically worse... there is a general reduction in distortion across the whole of the 5532 output range, which seems to indicate that the 5532 output stage is better at sinking current than sourcing it, and therefore injecting a positive current is effective at helping out."

Self wrote about this after he had already written the first edition of Small Signal Audio Design, so I guess he just forgot to add it to later editions.

It's definitely not in the edition I have and I could find no reference to it on his web site.

Presumably he measured how much the distortion was reduced?

Cheers

Ian

 

[living sounds]

It's definitely not in the edition I have and I could find no reference to it on his web site.

Presumably he measured how much the distortion was reduced?

Cheers

Ian

Yes, there are AP printouts in the book.

 

[merlin]

It's definitely not in the edition I have

It's in the first edition, are you saying it's been removed from a later edition?

 

[ruffrecords]

It's in the first edition, are you saying it's been removed from a later edition?

I am saying I cannot find it in the first edition of Small Signal Audio Design (2010)

Cheers

Ian

 

[squib]

I reckon adding two transistors, a couple of resistors and diodes to a 5534 is a very neat way of making a high performance true class A opamp with the freedom to easily set the idle current to suit expected loads.

 

[Speedskater]

Audio Amateur magazine touched on the subject in some of their projects.

 

[ruffrecords]

I reckon adding two transistors, a couple of resistors and diodes to a 5534 is a very neat way of making a high performance true class A opamp with the freedom to easily set the idle current to suit expected loads.

Back in the very early days of op amps like the 709 and the ubiquitous 741, the most pressing problem for audio circuits was their poor noise performance and gain bandwidth product. Both of these were significantly improved by adding a discrete transistor (or two) at the input (one of the better designs being the Tweed Audio mic pre designed by David Rees). Later op amp deveelopments improved the noise performance to the point where an external transistor was no longer necessary. I wonder why this was never extended to the provision of class A output stages?

Cheers

Ian

 

[JohnRoberts]

I reckon adding two transistors, a couple of resistors and diodes to a 5534 is a very neat way of making a high performance true class A opamp with the freedom to easily set the idle current to suit expected loads.

What exactly is a class A opamp? FWIW the old school singe supply discrete transistor audio paths were "class A" and not very low distortion.

Back in the very early days of op amps like the 709 and the ubiquitous 741, the most pressing problem for audio circuits was their poor noise performance and gain bandwidth product. Both of these were significantly improved by adding a discrete transistor (or two) at the input (one of the better designs being the Tweed Audio mic pre designed by David Rees). Later op amp deveelopments improved the noise performance to the point where an external transistor was no longer necessary. I wonder why this was never extended to the provision of class A output stages?

Cheers

Ian

+1 not just the input, but adding discrete bipolar devices to the output side could be beneficial (i've added discrete devices before and/or after).

"Class A" is a poorly understood buzz word. AFAIK there is no such thing as a class A op amp (operational amplifier). There is such a thing as a "class A" power amp (that doubles as a room heater because it is grossly inefficient).

JR

 

[ruffrecords]

"Class A" is a poorly understood buzz word. AFAIK there is no such thing as a class A op amp (operational amplifier). There is such a thing as a "class A" power amp (that doubles as a room heater because it is grossly inefficient).

JR

There are plenty of class A discrete op amps - the Calrec range for instance or the Neve BA440.

Cheers

Ian