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

 

[Matt Syson]

Thanks for posting the schematic Ian; It saves having to look at the boards I have and work it out (again). I also have the updated mini boards that used a NE5534A op amp used on the later produced desks such as Broadcasting House (the studio that formed Radio 4 material IIRC.
YES you CAN run a 5534 at plus and minus 24 Volts which I discovered when revisiting the studio a year after it was commissioned and discovered the 5v6 Zener 'droppers' had been forgotten on the motherboard. OOPS!

 

[ruffrecords]

Thanks for posting the schematic Ian; It saves having to look at the boards I have and work it out (again). I also have the updated mini boards that used a NE5534A op amp used on the later produced desks such as Broadcasting House (the studio that formed Radio 4 material IIRC.

Back in 1974 I was a 23 year old recent graduate who got his dream job designing mixers at Neve. For many years I had been an avid reader of Tape Recoder magazine which morphed into the pro audio focused Studio Sound so I knew the names of most of the mixer manufacturers in the UK but I had never come across Calrec. It was not until one of our salesman took me to BBC Television Centre that I saw my first Calrec mixer. It looked very much like a Neve, even the paint was almost identical. The knobs were different but the EQ was much the same as were the routing modules. But, of course, they would be because that is when I realised the BBC wrote the spec (in detail) and sent it out to tender. But I still thought it strange that Calrec did not seem to advertise itself.

YES you CAN run a 5534 at plus and minus 24 Volts which I discovered when revisiting the studio a year after it was commissioned and discovered the 5v6 Zener 'droppers' had been forgotten on the motherboard. OOPS!

Cheers

Ian

 

[JohnRoberts]

Thanks for posting the schematic Ian; It saves having to look at the boards I have and work it out (again). I also have the updated mini boards that used a NE5534A op amp used on the later produced desks such as Broadcasting House (the studio that formed Radio 4 material IIRC.
YES you CAN run a 5534 at plus and minus 24 Volts which I discovered when revisiting the studio a year after it was commissioned and discovered the 5v6 Zener 'droppers' had been forgotten on the motherboard. OOPS!

+/- 24V exceeds the published max voltage spec (+/- 22V). I hope you added the missing zeners.

JR

 

[ricardo]

It was not until one of our salesman took me to BBC Television Centre that I saw my first Calrec mixer. It looked very much like a Neve, even the paint was almost identical. The knobs were different but the EQ was much the same as were the routing modules.

Puh..lease Ian!! I don't go damning N**e sh*t with faint praise .. or at least most of the time

Calrec never advertised cos their order books were always full from the Broadcast organisations ... and after Abba bought the first M series for their Stockholm studios, from selected 'music' customers too.

That B 210 module was designed for an EVIL purpose dear to John Robert's heart. It could destroy any input stage in its time ... including the obligatory release of Holy Smoke from the victim.

 

[JohnRoberts]

That B 210 module was designed for an EVIL purpose dear to John Robert's heart. It could destroy any input stage in its time ... including the obligatory release of Holy Smoke from the victim.

I'm not sure creaming input stages was dear to my heart, BUT it reminds me of a story... The lads in our service department would often call upon me for help with unusual problems. Peavey sold a couple decent vacuum tube studio SKUs under the AMR brand. One a nice vacuum tube mic preamp (the VMP2) was pretty popular with Mackie 8bus console customers. The problem for them was that the VMP's hot output creamed the 8bus' inputs. I provided the lads in Peavey service with a simple passive (resistive) pad to knock off about 10 dB of level, so the Mackie 8bus customers could use their Peavey (AMR) mic preamps, without overloading their console inputs.

JR

 

[ruffrecords]

Puh..lease Ian!! I don't go damning N**e sh*t with faint praise .. or at least most of the time

LOL

Calrec never advertised cos their order books were always full from the Broadcast organisations ... and after Abba bought the first M series for their Stockholm studios, from selected 'music' customers too.

Calrec was founded in 1964 to make microphones - and very good ones they were too. At that time, there were only two broadcasting organisations in the UK - the BBC and ITV. And that remained the case until just after I joined Neve in 1974 when commercial radio began in the UK.

Cheers

Ian