Opamps and Gains

[mudihan]

I am upgrading an old cd player (Ariston CD Player, $800 in 1990), whose analog output stage has two dual opamps. I like the sound of OPA637 a lot, but it is only stable when the gain is >5. There is no way I can find a schematic of this player and therefore I can't figure out the gain. But I did do a test-run with OPA2228s, which are also said to work when the gain is >5. For eight hours straight, the chips seemed to perform fine as the sound coming out of them was great, without any audible distortion. The chip was not hot also, unlike the two pairs of AD843s I put in previously, which became too hot to touch in about 10 min. (I suspect that there is the class-A bias).

So here are my 2 questions:

Based on the information that the player uses one TDA1541A (S1) chip and has an output about 3V (stronger than normal CD/DVD players), can someone estimate the gain for the opamps in the output stage?

If a opamp is outputing good sound, does that mean that it is stable? Or stability can not really be heard?

Many thanks!

[darkfenriz]

It is hard to tell. How much circuitry is around?
Maybe an opamp works simply as I-V converter. If so I would use unity gain stable one or add a small capacitor between output an -input. How about opa627 ?

[mudihan]

Thanks for your reply. They not I/V converters (which I have replaced with a couple LT1364s). I am certain that those are final stage of amplification -- they are next to the RCA outs and the traces on the PCB show that too.

I am currently using OPA627s, but I prefer the sound of OPA637 more -- to exaggerate a little bit, it's like comparing a river (637) to a creek (627).

The capacitor between output and input is used to avoid oscillation, am I right? Is instability mainly about oscillation? Would such a capacitor make OPA637 stable even when gain is less than 5?

Thanks a lot!

[darkfenriz]

Originally posted by mudihan

The capacitor between output and input is used to avoid oscillation, am I right? Is instability mainly about oscillation? Would such a capacitor make OPA637 stable even when gain is less than 5?

Yes, I think so.
cheers

[mudihan]

Originally posted by darkfenriz


Yes, I think so.
cheers

That really solves the problem!

Cheers!

[macboy]

someone estimate the gain for the opamps in the output stage?

Why don't you figure it out? To do this, you need to figure out the value of two resistors. Here's how:
1. locate pin 6 and pin 2 of the opamp (assuming a single opamp). There will be a resistor connected between the two. Locate it and determine it's value; we'll call it Rf. (Do not use an ohmmeter; calculate the value based on the color bands or digits printed on the resistor. If you need help determining the value of the resistors, google it.)
2. from pin 2 to either ground or the signal input, is another resistor. Determine its value, we'll call it Ri.

If the resistor from pin 2 went to ground, the opamp is a non-inverting amp, so gain = 1 + (Rf/Ri)
If the resistor went to the signal source, then the opamp is an inverting amp, and gain = -Rf/Ri

If you find that the gain (absolute value of gain) is at least 5, then feel free to use OPA637 or OPA228. Otherwise, you should use OPA627 or OPA227, which are the unity-gain-stable equivalents to the others.

Do you realise that the OPA637 is a single opamp and the OPA2228 is a dual? I guess not. They are NOT pin compatible. You can only replace a single with a single or a dual with a dual. The AD843 is also a single. You didn't mention what was in there originally. I don't suppose you would recall?

The capacitor between output and input is used to avoid oscillation, am I right? Is instability mainly about oscillation? Would such a capacitor make OPA637 stable even when gain is less than 5?

Well, the cap is there to reduce gain at high frequency, creating a low pass filter. But you have to be careful because this cap can actually cause a opamp (which is not stable at unity gain) to oscillate. This is because it can reduce the high frequency gain (aka noise gain?) below the stable gain level (and indeed, even below unity).

Instabiliy is all about oscillation, and this oscillation can be caused in many ways.

That cap will not (necessarily) make the OPA637 stable, though you may get lucky. Generally, a cap is required across the inverting and non-inverting input pins. You can probably find some great tech notes at TI concerning opamp stability.

[AndrewT]

Hi,
I have not seen a schematic for the Ariston.
But being made in the UK, like my Arcam, it may do things similarly.

In the Arcam the 5534 is used as a 2pole filter and the gain is near 1.5 times without any Ccomp.

[sreten]

Originally posted by darkfenriz

Yes, I think so.
cheers

Hi,

I don't. The op-amp needs an internal (to the open loop)
compensation capacitor. if the op-amp doesn't have pins
for this it cannot be made unity gain stable with anything
connected output to input outside the feedback loop.

/sreten.

[mudihan]

Originally posted by macboy


Why don't you figure it out? To do this, you need to figure out the value of two resistors. Here's how:
1. locate pin 6 and pin 2 of the opamp (assuming a single opamp). There will be a resistor connected between the two. Locate it and determine it's value; we'll call it Rf. (Do not use an ohmmeter; calculate the value based on the color bands or digits printed on the resistor. If you need help determining the value of the resistors, google it.)
2. from pin 2 to either ground or the signal input, is another resistor. Determine its value, we'll call it Ri.

If the resistor from pin 2 went to ground, the opamp is a non-inverting amp, so gain = 1 + (Rf/Ri)
If the resistor went to the signal source, then the opamp is an inverting amp, and gain = -Rf/Ri

If you find that the gain (absolute value of gain) is at least 5, then feel free to use OPA637 or OPA228. Otherwise, you should use OPA627 or OPA227, which are the unity-gain-stable equivalents to the others.

Do you realise that the OPA637 is a single opamp and the OPA2228 is a dual? I guess not. They are NOT pin compatible. You can only replace a single with a single or a dual with a dual. The AD843 is also a single. You didn't mention what was in there originally. I don't suppose you would recall?


Well, the cap is there to reduce gain at high frequency, creating a low pass filter. But you have to be careful because this cap can actually cause a opamp (which is not stable at unity gain) to oscillate. This is because it can reduce the high frequency gain (aka noise gain?) below the stable gain level (and indeed, even below unity).

Instabiliy is all about oscillation, and this oscillation can be caused in many ways.

That cap will not (necessarily) make the OPA637 stable, though you may get lucky. Generally, a cap is required across the inverting and non-inverting input pins. You can probably find some great tech notes at TI concerning opamp stability.

Yeah, actually I know. Two positions use dual opamps. I used Browndog adapters for the AD843s, and will be using the same adapters for OPA637s as well.

As far as looking at the resistors, I have also tried that but couldn't figure all the traces. I will try it again.

[macboy]

I used Browndog adapters for the AD843s, and will be using the same adapters for OPA637s as well.

OK, good. I was suspicious when you mentioned both single and dual types, but didn't mention adapters. You might be surprised how many people just solder in an opamp that they had read somewhere was the bestest-sounding-ever, without understanding the single/dual issue (or any of the many other issues...)