Class A biasing of opamps

I read in a diy article years ago that you could class A bias an AD711 by running a 6k resistor between the output and the negative supply rail. (assuming that elsewhere you were taking care of the offset)

2 questions:

1) will this work also for an NE5534
2) If using the NE5534 in a phono stage with a feedback loop, will the offset screw up the feddback system?

I can't see how you can have a high gain opamp and feed back a DC offset from the output without it bolliksing things up.

Advice?

BTW, I'm using the 5534 due to it being both cheaper and lower noise than the 711 in this application but would like to class-a it if possible. It's reported to make a significant positive improvement.

TIA

Drew
 
1 making the output stage going in class A is nothing unusual.

Check here

http://www.linear.com/prod/datasheet.html?datasheet=202

2 Don't know exactly what you mean but you don't makes things worse with this arrangement, only warmer or even hotter chip.

Note that this type of arrangement is only wise if the load is small (not heavy). Typical application is between a buffer like in the example above. The RIAA-network is rather heavy load if you want low noise so the advantage will probely be little. NE5534 has very small distortion figures without this class A thing. I have seen values of down 0.001%.
 

djk

R.I.P
2001-02-04 4:23 am
USA
Feedback will keep the difference between the inputs at 0V. If you put a 5K pull down resistor to a -15V rail all that will happen is the output transistor on the +15V rail will turn on a little harder to keep the output at 0V. How much harder? 15V/5K=3mA. how much heat? (15V ^2)/5K=45mW.
 
djk said:
Feedback will keep the difference between the inputs at 0V. If you put a 5K pull down resistor to a -15V rail all that will happen is the output transistor on the +15V rail will turn on a little harder to keep the output at 0V. How much harder? 15V/5K=3mA. how much heat? (15V ^2)/5K=45mW.

I ment heating the chip! A couple of milliamps isn't much if you include your load but this current can just about enough to reduce distortion if you use a buffer.

The best solution is to use a current generator, a JFET or a BJT. Check my QSXM3 RIAA amp for an example of this.
http://home5.swipnet.se/~w-50674/hifi_files/qsx/qsxm3r0schema_p1.pdf
 
lower noise eh?

Could be, I haven't looked at the '711 datasheet recently. But opamp noise is a fickle thing, and often misunderstood... ei, or the input voltage noise is most often quoted when comparing opamp noise specs. However, one must be aware that opamps also have current noise at their input terminals, and depending upon your circuit impedances, the total noise of the circuit could vary dramatically from the opamp's ei, since this does not necessarily dominate the noise of a circuit.

If I recall correctly, the output voltage noise will be something like:

en = sq.rt.[ ei^2 + (R*in)^2 + 2RkT ]

where:
en = output noise voltage
ei = input voltage noise of opamp
in = input current noise of opamp
R = source impedance seen by input (should be matched at each input for best performance)
k = Boltzmann's constant
T = temperature in degrees Kelvin (approx. Celcius + 300)

That's all just off the top of my head, so I could be off by a little, but that's the basic form of the equation. So, next time you're checking out opamp specs, pay attention to the current noise as well, and consider the impedances which will be in your circuit. If the datasheet doesn't list current noise, you can get a ballpark idea by looking at the input current spec. Most bipolar opamps will have input currents in the pA range, while FET input opamps will often have input currents in the fA range!

Anyway, 5534 is pretty low noise, but I hope some DIYers out there will find this info useful next time they're trying to build a low-noise circuit.

As for output stage biasing, a FET current source is probably your best bet... costs a few cents more than a plain resistor, but should sound better in most cases.
 
5534 vs 711

What function is that NE5534 being used for?

Both chips maybe pin interchangeable, but are quite different beasts. The 5534 is low noise, the 711 is not.

The 5534 is now cheap, but in the past it was not. The 711 was never too expensive, but now it probably costs a bit more. The reason being that now there are several 5534 types, but only Analog makes the 711.

But before wondering whether to force the 5534 into class-A, which will probably warm it a bit, you should wonder if it's up to the task proper to a modern chip in your application.



Carlos
 

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
The Saint said:
Putting an Opamp into class A with a resistor on the o/p to any supply rail was originally shown by Walt Jung of Audio Amateur Magazine fame.
An active current source works great as well.
The concept works and it does make an improvement...

This technique predates Walt Jung. Barry Thornton was using
resistors about 1970, and the Forte preamp (Pass) used
current sources in the early 80's.
 
Re: 5534 vs 711

carlmart said:
But before wondering whether to force the 5534 into class-A, which will probably warm it a bit, you should wonder if it's up to the task proper to a modern chip in your application.

DrewP, what are you going to use this NE5534 for?

I agree with carlmart, look for a modern chip instead.

OPA134 is a good NE5534 replacement and also not so expensive.
 
Re: NE/SE5534 vs AD711

Calmart asks a (the?) very important question. :)

Another beneficial characteristic of the xx5534 type parts (vs. the AD711) is a beefier output stage. I've used the AD711 for a long time and after listening to it years ago in some of my prototype circuits, I wouldn't use it in line driver applications without a buffer. It's better than the TL07x stuff in this regard, but not by a whole lot. Nowadays, I like the AD711 in DC Servo applications, power supply regulators, music sythesizer modules (oops, that one slipped out), etc....

The AD711 is a pretty good GENERAL PURPOSE BiFET op amp, not among the best choices for high perfrmance audio line amps, IMHO ;).

carlmart said:
What function is that NE5534 being used for? ....
Carlos
 

BrianGT

Retired diyAudio Moderator
2002-01-21 2:42 pm
near Atlanta, GA
www.chipamp.com
jean-paul said:
Like OPA637 for phonoamps.

I have a bunch of opa627 and opa637 that a friend gave me (20 of each). Can I use these in place of opa2134 (assuming that I modify the wiring to use two single opamps instead of one double) in my active crossover? Are these opamps much better in terms of sound quality over the opa2134?

I read over the datasheets, but I couldn't find the difference in the two (627 and 637). Anyone know?

Thanks,

--
Brian