Good DC-Precision OpAmp

[EchoWars]

I have an Adcom GFA-555II amplifier with a bOrked opamp in the input section. It is simply being used as a DC servo for the diff-input pair. The only ID on the chip says "Adcom 3A". Not too informative.

Examination shows that it is a standard pinout for a single channel 8-pin dip opamp. Neg is connected to ground, and positive is at 20V.

So, seems as if I could just pop in a reasonable replacement as long as it's DC characteristics are good.

Mostly, we all here discuss audio opamps. How about something for this application with good DC specs, pin-configured as a single-channel 8-pin dip? Any suggestions??

Thanks guys...

 

[peranders]

OP07, OP97, OPA277 maybe?

You might check it further if the inputs are near ground and pin 4, then you must check for a type which has common range down to negative rail (pin 4).

Remember though that for audio almost any opamp will do. Offset of 1 mV isn't much in audio amps.

 

[PMA]

OP27

 

[peranders]

Echowars, why do you want to change the opamp since it's use only for DC? Your listening to DC

 

[Dan Banquer]

DC Servo

A good DC precision op amp is the LT1012CN8. Very low bias currents also. Due to the very low bias currents you can use a larger value resistor and a smaller value capacitor for RC networks, this is usually cheaper and saves board space.
d.b.

 

[carlmart]

Years go by and Erno Borbely keeps using the LF411 on his servos. Why not try that one?


Carlos

 

[mlloyd1]

Yep. Similarly, I've been using the old AD711 for years in this (as well as other) role. It works well as a servo device. I got a bunch cheap years ago.
mlloyd1

Years go by and Erno Borbely keeps using the LF411 on his servos. Why not try that one?
Carlos

 

[tbla]

i have even seen the opa627 beein' used as servo opamp.......would it be superior to others in that position...?

 

[jan.didden]

Red alert!!

It is not true that any old DC opamp will do! This opamp (if it is the servo) will have to handle audio freqs. The integrator function means that it will have to supply a charging current to the integrator cap from the signal it is fed.

At the output, especially at low freqs, you will find a replica from the signal, albeit low level. However, this low level signal is then injected directly in the amp. Any distortion in the servo output (AC) means amp distortion.

Please think this through, don't make it too easy on your self.
What happened to those people that claim to be able to hear a servo? For once, I can accept this claim.

Jan Didden

Edit: With almost all currently available opamps having very low offsets, paradoxically, DC precision probably is the least of your worries.

 

[SY]

Jan, I generally use a trick of putting a passive RC pole at the output of the integrator, with its frequency set a decade or so above the integrator time constant (that will put the pole at typically 50-60 Hz). That way, the effect of the high frequency characteristics of the servo opamp are minimized.

 

[jan.didden]

Yes, SY, that helps. You can also put this in front of the integrator to limit hf signal into the opamp. But these two methods can give problems with stability (low freq motorboating) and/or irregular freq response. After all, you're messing around with the feedback loop. I don't say it can't be done, but some care and experience is required. But my main point was that selecting the opamp solely based on DC performance is the wrong priority.

Jan Didden

 

[EchoWars]

Echowars, why do you want to change the opamp since it's use only for DC? Your listening to DC

One opamp's output is stuck at 18.5V (i.e., it's broke😉). The rest of the circuit appears OK.

The output is tied directly to (via a 1.5M resistor) the base of the first diff-pair transistor. Inverting input tied to the amp output, Non-inverting to the base of the second diff-pair transistor.

With a single supply (+20V), seems the output of the opamp should be only able to swing to 10V. Output stuck at 18.5V seems to indicate that it is screwed.

So...what opamps will work correctly with a single supply in this application? Seems that I can get the LF411 pretty easily and cheaply...might it do?

 

[skilpad]

Hi,
You could try Analog Devices AD822
We use it often and have so far been very satisfeid with the performance on AC and DC signals.
Not cheap though.

 

[EchoWars]

As a direct replacement, the AD822 is a no-go (dual amp). However, the AD820 might do, since no one is interested in chiming in about the LF411. Not willing to experiment here, so unless I can get an idea about the LF411, I'm tracking down a AD820.

 

[SY]

FWIW, I use an LF412 (dual 411) in my crossover as a servo; works just fine.

 

[EchoWars]

Small update....found a couple of bad resistors, so I wanted to see if the thing worked. Popped in a couple of 741C's just to get it running, and the damn thing sorta works....

...sorta. The amp is functional, but DC offset is at 2V(!). I have a couple of AD820's on order, and I'm hoping that they will do a better job than the generic 741's. If not, then time to replace the diff-pairs.

 

[EchoWars]

Wanted to thank skilpad for recommending the AD822. I did finally get the single channel version in today, the AD820. Popped it in, and offset dropped to less than 1mV(!)

I am impressed. The original Adcom part was lucky to keep offset under 10mV. The AD820 appears to be the perfect part for the job as I can barely measure the resulting offset. Very nice. Much appreciated.

Thanks again....🙂