#### qwerty870

##### 100+ Head-Fier

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Burr Brown opamps

- Thread starter qwerty870
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Check out Tangent's Website, he has done a lot of research into various op-amps.

Formerly with Tangentsoft Parts Store

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Also do a search here and at Headwize. There's no shortage of opinions on op-amps in the archives.

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Those burr-browns are used in the cmoy pocket amp!

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Originally posted by andrzejpw Those burr-browns are used in the cmoy pocket amp! |

Just about any opamp can be used in the CMOY circuit.

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Originally posted by qwerty870 |

Thjough I havent done an A/B test yet (no oun intended) the B series are supposedly tested for higher tolerances. In otherwords a pair of Bs are guaranteed to match within a very high degree of precission whereas the A seris may not mate as well. Are these differences audible? I would tend to doubt it but I was a disblelieve on differences on ICs.

The only difference between the two is about $15 a pair otherwise and I have decided I'd rather spend the money for the peace of mind.

If I were building several amps I would order the As to save a few bucks and I would be careful as possibly to sonically match (though this may not be necessarry).

If you only need 2 it's only $15 more for the Bs and you shouldn't have any regrets later (should I have gotten the Bs instead, I wonder if they would sound even better?).

For the voltage rails you describe the 627s or 637s would be a better choice. If the gain is over 5 and there is some bandwidth limiting in place (or can easily be put in place) you might consider the 637s over the 627s. It's just more money.

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my preamp provides only 6db of gain. Is this enough for the OPA637s?

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Originally posted by qwerty870 my preamp provides only 6db of gain. Is this enough for the OPA637s? |

If my numbers are correct, and I have every reason to doubt them, you would need a gain of at least 7dB for the 637 to be stable.

Here are my numbers for a G of 5 (the minimum stable gain for the 637):

dB=10 Log(5) = 6.989dB

So, be safe and stick with the 627.

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Originally posted by Nezer If my numbers are correct, and I have every reason to doubt them, you would need a gain of at least 7dB for the 637 to be stable. Here are my numbers for a G of 5 (the minimum stable gain for the 637): dB=10 Log(5) = 6.989dB So, be safe and stick with the 627. |

D'OH! Like I said, I figured I was wrong.

That formula is only valid when the 5 from above is a power ratio. The more generic formula to decribe the gain relation for voltage or current is this:

dB = 20 Log(Vout/Vin)

In this case a G of 5 is a 5::1 ratio of gain so we can say:

dB = 20 Log(5)

which yields a dB of 14 and *well* above your 6dB. For the 637 to work you would need to change the Gain (unless limiting the bandwidth effects the gain stability of the opamp, I dunno what I'm talking about half the time). So the 627, which is unity gain stable (or rather, stable when G=1 or 0dB), would be a better choice for your application where G=2 (or 6dB).

This is making more sense to me all of a sudden.

Now, how does one reversethe equation to calculate G based on dB of gain? I never went very far in my math classes as they bored me.

I can get:

dB/20=Log(Vout/Vin)

But what happens to the Log when moved over?

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Formerly with Tangentsoft Parts Store

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my preamp provides only 6db of gain. Is this enough for the OPA637s? |

Um, you're confused. The gain of the upstream source hasn't got anything to do with it. The OPA637 chip itself has to be configured for a gain of 5 or more to be stable. The higher the gain you use, the more stable it will become. Since a gain of 10 is perfectly reasonable for a headphone amp, I would recommend you did that rather than go with the minimum gain you can get away with.

If a gain this high is a problem get the OPA627 instead. It is stable down to unity gain.

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how does one reversethe equation to calculate G based on dB of gain? |

The inverse of a logarithm is exponentiation. If:

X = log_subY_(Z)

then:

Z = Y^X

The subY part is the "base" of the logarithm. Since decibels are figured using base-10 logs, the inverse function is:

G = 10 ^ (dB / 20)

[size=xx-small]EDIT: Fixed the equation.[/size]

That is, divide decibels by 20, and then raise 10 to that power to get the gain factor. Any calculator with a base-10 log function will have a 10^x function as well .... and now you know why!

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