Walt Jung biasing opamps in class A

[John McDougal]

I agree but it is debatable if an operational amplifier is what you really want for audio. They were originally designed for use in analogue computers for military applications

Well, that is an entirely different debate which has a different set of criteria.

I think you need a lot less bias current than that. The BA283 output stage runs at about 80mA and that is single ended so a push pull class A should require no more than 40mA quiescent but that still equates to nearly 1W of dissipation with a 24V rail.

the BA-440 doesn't have the benefit of transformer loading to increase maximum voltage output and so needs to drive a transformer primary impedance which is one quarter that of the BA-283.

The real reason there are no class A op amps is clearly the quiescent dissipation is way too high

For anything under about a 2K load this is true.

 

[JohnRoberts]

I agree but it is debatable if an operational amplifier is what you really want for audio. They were originally designed for use in analogue computers for military applications

As I have shared numerous times over the years I personally find high impedance differential inputs extremely useful inside large console design for keeping multiple local grounds sorted. Transferring signal between circuit blocks can back reference or forward reference between different grounds (signal 0V) as needed.

I am surely repeating myself but class A delivers limited benefit for obvious reasons.

I think you need a lot less bias current than that. The BA283 output stage runs at about 80mA and that is single ended so a push pull class A should require no more than 40mA quiescent but that still equates to nearly 1W of dissipation with a 24V rail.

I'm not sure push-pull works that way (but I am not familiar with BA283). Class AB (push pull?) generally involves a class A current that is a tiny fraction of max output, not 50%.

My tube class A output stage runs at 6mA quiescent and will output +26dBu into a 600 ohm load via a 2:1 step down transformer. The Neve needs a 10dB step up so its quiescent needs to be 16dB higher which would be just under 40mA.

I am not a tube guy..

Even so, the real reason there are no class A op amps is clearly the quiescent dissipation is way too high.

yup...

JR

I am not sure. I am at the wrong PC right now. I will check later.

Cheers

ian

 

[ruffrecords]

Isn't the PQ-14/15 the only class A Calrec?

I was thinking of the series B200 series (pdf attached) used in the PQ1061

Cheers

Ian

 

[Brian Roth]

I was thinking of the series B200 series (pdf attached) used in the PQ1061

Cheers

Ian

I didn't see that PDF....

Bri

 

[ruffrecords]

I didn't see that PDF....

Bri

Ooops. Fixed.

Ian

 

[Markku01]

I read a comment referencing Walt Jung and how he had an article about biasing opamps( example 5532) to be class A operation.
That all you had to do was add a resistor to the opamp output going to the negative rail.

Does anyone have any details on this or the article the reference in the comment?

I would be curious as to what it achieves outside of heat increase?

I personally don't think you need to do anything for the op amp output to run class A. Especially if it has a bipolar PSU. The cross over from positive to negative is controlled by the op amp internal components. It is after all the same basic output configuration as a fully complementary transistor power amplifier.

 

[Zeltec Service]

FYI, the B183/283 amplifiers are pure class A, a 2N3055 pulling against a 47Ω resistor. The output stage also has gain. One other technique for lessening the op-amp crossover glitch is to offset the power rails in order to move the disconnected area away from the lowest level program to where it's less apparent. The only approach I can think of is to have the output transistor, connected to the positive rail, act as a constant current source when not passing its portion of the audio signal. In other words, never turning completely off. I'm about to see what I can do with the new CMOS op-amps, OPA991 and 992, rail to rail output up to ±20 volts. The output can be turned off and go high impedance so outputs can all be wired together for use as a multi position, high level, switch. Also internal RF Rejection.

 

[ricardo]

I was thinking of the series B200 series (pdf attached) used in the PQ1061

These were basically Class B (or AB for da pedantic) though I supposed if you used B210 (running 10mA total) with a 10k load, you could claim it was operating Class A

 

[John McDougal]

These were basically Class B (or AB for da pedantic) though I supposed if you used B210 (running 10mA total) with a 10k load, you could claim it was operating Class A

Yes exactly.
Except, as designed, the B210 drives a 600 ohm load so, class AB

 

[ruffrecords]

These were basically Class B (or AB for da pedantic) though I supposed if you used B210 (running 10mA total) with a 10k load, you could claim it was operating Class A

A class A push pull stage operating at 10mA quiescent can sink or source up to 20mA peak or 14mA rms without ever leaving class A operation . Into a 600 ohm load, 14mA rms is +20dBu.

Cheers

Ian