IC chip tutorial?

[shawnee]

Hello again. I was wondering if any of the experts would give us some info on the meaning of the letters in the IC chip designations. Example: at Mouser there are 9 versions of the TL072 in stock.
TL072BCP, TL072BCPE4, TL072CP, TL072CN, TL072IP, TL072ACN, TL072CPE4, TL072IN, TL072ACP.

I know that a higher Common Mode Rejection Ratio (CMRR) is desirable in audio circuits but does it matter here? What about the Input Offset Voltage? Would that be better to have higher or lower? I see most guys using the 'CP" chips but there are chips with better specs. Is it because of cost or is there another reason?

I remember when I was building an amp that the parts list aclled for a ceramic cap in one place. When I asked about it the response was "it's not a full range audio amp. you need a little grit in places to keep the amp from sounding sterile and too hi-fi. All amps have some amount of distortion even if you think it's totaly clean. That little bit of distortion can be the character and warmth in the clean tone."

Any thoughts or comments on this in regards to pedals?

[stecykmi]

lower input offset voltage is better, it's basically some small voltage that's needed to make the output "perfect" 0 volts. usually the inputs of an opamp aren't quite balanced so the result is some small voltage is needed if you want to make the output 0. there are ways to compensate for it, but i can't recall exactly how to do it.

and all the different TL072 are basically different packages, different standard operating temperatures, and different degrees of accuracy. For the audio realm, none of these are too important other than the package (usually you want DIP-8).

What is important is Slew Rate, which is basically the rate at which the output of an opamp can change. The TL072 datasheet lists a slew rate of 13 V/us meaning in 1 microsecond (1 x 10^-6 seconds), the output voltage can change by 13 volts. The OPA2134 datasheet lists 20 V/us, quite a bit faster, but the chip is also 10x as expensive. The LM741 (the 741 is one of the earliest cheap opamps, and has been in production for something like 40 years) list a slew rate of a measly 0.7 V/us, but the chip can also be bought for a few cents.

A slow slew rate isn't always bad for distortion, but there's a reason people swap out IC's in mixers and pro-audio.

[shawnee]

GREAT information. Thank you very much. So if you had to choose between a lower input offset voltage and a higher CMRR, which would you choose?
offset-3mV, CMMR-75dB    or    offset 6mV CMRR-80dB.

[jkokura]

Every once in a while Misha busts out some crazy technical knowledge that blows all our answers away!

The simple answer, for those who aren't following (not that I found Misha to be hard to understand but...):

The letters after an IC code don't mean much for operational use. Usually the sufixes do indicate something, but for our purposes there is rarely any difference between a CP or a CN or whatever... at least functionally. As Misha mentioned, the most important part is the package! Always make sure you get a DIP-8 as mentioned, not SMD.

Perhaps more helpful, and it should be coming at some point, is a tutorial on how to read or get datasheets. ALL the differences can be seen between the IC by looking at a datasheet. For example, check the TL072 datasheet here: http://www.datasheetcatalog.org/datasheet/texasinstruments/tl072.pdf

Notice right on the first page we have a list of all the IC's that datasheet pertains to? Also notice the chart of available options? Read through the rest and you'll quickly see how the datasheet will tell you the differences between the IC's.

As for your later questions, the more technical answer Misha gave you should help answer that. However, I will add that often times it really is about the ears. Get a TL072CN and a CP, and then tell us if you can hear the difference! Usually I just go for whatever is cheaper as long as the datasheet says it's a compatible IC...

Jacob

[stecykmi]

GREAT information. Thank you very much. So if you had to choose between a lower input offset voltage and a higher CMRR, which would you choose?
offset-3mV, CMMR-75dB    or    offset 6mV CMRR-80dB.

I would say neither would make much of an audible difference because they're both pretty small... 75dB is somewhat on the high side in comparison to most opamps, so I suppose the second opamp is better. But we don't really use opamps as differential amplifiers very often, so it's almost irrelevant.

I suppose if you were using an opamp in a differential amplification choose the second (i don't think there are any in Madbean builds), otherwise choose the first.

If were forced to only buy one opamp, choose the cheaper one.

[shawnee]

So a higher CMRR is only uselful for noise reduction in a differential circuit?

[stecykmi]

As far as I know. There's probably something else out there that I don't know about, but that's the primary ramification of CMRR.

[shawnee]

OK, thank you very much for your insight.