Good opamp for that1646

[sahib]

IIRC NE5534 is stable for minimum gain of five ? Don't have time to look through the datasheet right now. But you can easily compensate it to be unity gain stable.

Easier to use a voltage divider on the input to attenuate the signal first and then run it at gain of 5 (or 6 to be safer).

 

[sahib]

Thank you guys. Actually I'm reading the manual as we speak. I think it looks good but also a bit more sensitive when it comes to PCB design . Another question I have is if I can run it at +/-18V. Absolute maximum is +/-20V but +/-15V is suggested.

Indeed the absolute maximum rating is +-20V but you might be fine with +-18V.

 

[warpie]

But what would be the benefit of using a 5534 plus the resistors for the voltage divider and for setting the gain? Just cost?

Also the 5534 is inferior as far as I'm concerned.

I have some 5532s, 5534s and op97s laying around so I can try them all but I think the OP97s might be the most wise choice (?)

 

[Newmarket]

Easier to use a voltage divider on the input to attenuate the signal first and then run it at gain of 5 (or 6 to be safer).

Don't need to do that - compensation needed is just one 22pF capacitor.

btw it seems my memory was wrong and it is stable at gains of three and above.

 

[Newmarket]

But what would be the benefit of using a 5534 plus the resistors for the voltage divider and for setting the gain? Just cost?

Also the 5534 is inferior as far as I'm concerned.

I have some 5532s, 5534s and op97s laying around so I can try them all but I think the OP97s might be the most wise choice (?)

You don't need to attenuate the signal for the 5534. Just need a 22pF capacitor to compensate it for unity gain. This is standard practice.
OP97 looks unsuitable for audio. Compare the specs' (and lack of ) to any well known 'Audio' OpAmp eg TL071 ; OPA134...

 

[sahib]

But what would be the benefit of using a 5534 plus the resistors for the voltage divider and for setting the gain? Just cost?

It is a work around if the opamp you want to use is not unity gain stable.

Also to remind you that 1646 has a gain of 6dB.

Also the 5534 is inferior as far as I'm concerned.

Of course you are entitled to your own opinion. But, think about all those classic consoles and outboards.

I have some 5532s, 5534s and op97s laying around so I can try them all but I think the OP97s might be the most wise choice (?)

Try all of them. Will do no harm. 5532 is internally compensated. I would imagine you'll be using one half.

 

[warpie]

In fact I also have some OP297 and LF353N but I'd probably stick to the 5534 with an 22pF (or so) cap. I'm reading the "small signal audio design" book now and Douglas also doesn't recommend the OP297 for audio unless DC precision is very important. I guess both the OP97 and OP297 would be good for DC servo applications.

In fact, I'm just looking for something better than the 5534 in terms of performance but not overkill.

 

[Newmarket]

In fact, I'm just looking for something better than the 5534 in terms of performance but not overkill.

What aspect of 5534 performance are you looking to improve ? Note that the 5534A devices have a more tightly spec'd voltage noise IIRC. LM4562 is an obvious contender. Also take a look at OPA1612 (SMT only). I haven't used it myself though.

 

[warpie]

What aspect of 5534 performance are you looking to improve ? Note that the 5534A devices have a more tightly spec'd voltage noise IIRC. LM4562 is an obvious contender. Also take a look at OPA1612 (SMT only). I haven't used it myself though.

Well, any possible aspect I guess I was reading that the LM4562 is very sensible to RFI so it's best to be shielded and I really don't wanna go down that road. Would it be a huge improvement to the 5534?

I don't know. I might go for the 5534 and call it a day...

 

[Newmarket]

Where did you hear about LM4562 RFI problem ? News to me. But presumably the whole electronics will be inside some form of shielded enclosure or assembly ?
Not a huge likely improvement I'd say. But that's due to how good a 5534 is for this application.

 

[warpie]

Where did you hear about LM4562 RFI problem ? News to me. But presumably the whole electronics will be inside some form of shielded enclosure or assembly ?
Not a huge likely improvement I'd say. But that's due to how good a 5534 is for this application.

The LM4562 represents a real advance on the 5532. It is, however, still a good deal more expensive and is not perfect –it appears to be more easily damaged by excess common-mode voltages, and there is some evidence it is more susceptible to RF demodulation (small signal audio design)

https://www.audiosciencereview.com/...pecially-of-the-lm4562-lme497x0-family.10687/
and it some other places I can't remember right now.

I think I'm just gonna go with the 5534 and call it a day

 

[warpie]

So, before trying my luck on the breadboard I think I could do with some advice...

Does this look OK? A few questions I have:

1) Do I need R1, R2 and C1?
2) Do I need a coupling capacitor between the 5534 and the 1646?

 

[JohnRoberts]

It is a work around if the opamp you want to use is not unity gain stable.

A work around? A de-compensated op amp can deliver more open loop gain than unity gain stable compensation. If you are not operating it at unity gain you are leaving performance on the table. The gain for stability consideration is "noise gain" so a unity gain differential will already have a noise gain of +6 dB, more than half way to +10dB. Note: technically a feedback capacitor can look like 100% NF at high frequency, but optimizing stability can be a little more complex. Using unity gain stable op amps means not having to tweak.

JR

Also to remind you that 1646 has a gain of 6dB.


Of course you are entitled to your own opinion. But, think about all those classic consoles and outboards.



Try all of them. Will do no harm. 5532 is internally compensated. I would imagine you'll be using one half.

 

[abbey road d enfer]

1) Do I need R1, R2 and C1?

Not strictly, however adding a resistor in the non-inverting input is a good opportunity to also add a small cap (100pF) to ground so that no rubbish can bother the 5534.
And I wouldn't really bother with R2/C1.
BTW, you need to ensure that the input signal has proper galvanic connection to ground.

2) Do I need a coupling capacitor between the 5534 and the 1646?

No. A cap there does only ruin low frequency CMRR.

 

[warpie]

Thanks, The reason I included the R2 is because I've seen in some other schematics and also it's being suggested in the NE5534 datasheet for unity gain...

So, does this look better?

No. A cap there does only ruin low frequency CMRR.

I'm mostly concerned about the offset from the output of the 5534.

BTW, you need to ensure that the input signal has proper galvanic connection to ground.

Not sure what you mean I'm afraid.

 

[abbey road d enfer]

Thanks, The reason I included the R2 is because I've seen in some other schematics and also it's being suggested in the NE5534 datasheet for unity gain...

It has to do with settling time and transient behaviour and recovery from overload; you shouldn't worry about that as long as you use it reasonably.

So, does this look better?

10k adds noise. 1k is good.

I'm mostly concerned about the offset from the output of the 5534.

Offset is generally a non-issue. Even if you drive a transformer, a few mV in whatever direction is not a problem.

Not sure what you mean I'm afraid.

The circuit that drives the input must have conduction to ground, in order to set the correct operating point.
E.G. if you have a capacitor at the output of the circuit that drives the input, you need to add a resistor to ground.

 

[warpie]

It has to do with settling time and transient behaviour and recovery from overload; you shouldn't worry about that as long as you use it reasonably.

OK so from what I understand, what you're saying is that it doesn't harm to have R2/C1 but I can as well skip it, correct?

Offset is generally a non-issue. Even if you drive a transformer, a few mV in whatever direction is not a problem.

So, it's more of a choice between higher offset Vs better CMRR and the latter is preferable?

The circuit that drives the input must have conduction to ground, in order to set the correct operating point.
E.G. if you have a capacitor at the output of the circuit that drives the input, you need to add a resistor to ground.

OK I think I get it. Does the value of the resistor depends on the capacitor value? I guess it does because it's a hpf...

 

[abbey road d enfer]

OK so from what I understand, what you're saying is that it doesn't harm to have R2/C1 but I can as well skip it, correct?

Yes.

OK I think I get it. Does the value of the resistor depends on the capacitor value? I guess it does because it's a hpf...

It does somewhat because the higher it is, the higher the offset; typically a value between 10k and 47k is advisable.

 

[Newmarket]

It has to do with settling time and transient behaviour and recovery from overload; you shouldn't worry about that as long as you use it reasonably.

afaik the primary function of a resistor there is to protect the diodes between the + and - opamp inputs from excessive current due to an abnormally high differential voltage there eg if the feedback loop cannot be closed. (See datasheet for the detail on these internal diodes - also means it's not a good idea to use opamp with this structure as a comparator)
I don't see the need for it here.
btw - I saw the stuff about LM4562 and interference/noise. Interesting. Not had time to comment.

 

[warpie]

afaik the primary function of a resistor there is to protect the diodes between the + and - opamp inputs from excessive current due to an abnormally high differential voltage there eg if the feedback loop cannot be closed. (See datasheet for the detail on these internal diodes - also means it's not a good idea to use opamp with this structure as a comparator)
I don't see the need for it here.
btw - I saw the stuff about LM4562 and interference/noise. Interesting. Not had time to comment.

OK, so you also suggest leaving R2, C1 out. Interesting. As I said I've seen it it many circuits (including some Neve) and that's why I used them.

Anyway, I'll give a try on the breadboard and probably come back with more questions

I'm also trying to find more information about correct bypassing caps. Some people suggest only a cap between the rails, some other suggest one cap to ground for each rail and some other suggest both options. Go figure...

BTW, thank you all for your help! Much appreciated.