OP275 Opamp - Is It Really That Special?

[Paul Marossy]

There's one guy out there building pedals which use an OP275 opamp, which he claims makes his pedals so touch sensitive. His pedals ARE quite touch sensitive, but I question if it's just because of the opamp.

The OP275 uses a "Butler front end", which is a hybrid of FET and BJT. Would this really respond differently than say an FET opamp like a TL072? I do like the OP275, it is very quiet and has a "very high slew rate", but I have also seen where people make claims that an NE5532 actually outperforms the OP275, etc.

I'm just wondering if it's more to do with the circuit design, or if there really is something to this. Anyone have any opinions to offer?

[Ice-9]

Although I have to agree different opamps do make a difference to things, I really think the surroundig circuitry design is what make THE big difference. For instance I have auditioned circuits using an OP275 that aren't any more touch sensitive as a circuit with a TL072. Do you have any links to the guy your talking about ?

[Paul Marossy]

Yeah, I've done an A-B test and didn't find that the OP275 made things any more "touch sensitive" than any other opamp I tried in the same circuit.

[familyortiz]

From opamp to opamp, if you have suitable bandwidth, slew rate, etc., then other parameters like offset voltage and noise specs will lead to subtle differences which may or may not be noticable. What I do like about precision, low noise opamps is that it can simplify your design by eliminating coupling caps and other passive components. Also, what is there not to like about low noise?

[Paul Marossy]

Also, what is there not to like about low noise?

+1

[WGTP]

IIRC ZVex uses one in his little tube amp for the ?pre-amp?  Seems like the Black Cat OD used one.  An Op-amp Distortion. 

[Johan]

perhaps he is hooping that people will confuse it with NKT275 and found a way of hyping and cashing in on a, for him easy to get, but not stocked by radioshed, product?...stranger this have hapened in the internet...
J

[12Bass]

Hard to say... from my own listening, the OP275 has an interesting, colored, sort of sound... thought it might be low-level distortion that I was hearing, and maybe a tad mid-scooped.  But then, most of my testing has been in audio reproduction, not distortion pedals, where it was not among my favorites.  I've found that something like an LM4562 or OPA2211A has less apparent coloration.  Actually, I kind of like the somewhat low-fi fuzziness of the TL072, if character is desired.  Probably best to try it and see.... 

[Gurner]

Got to say....I'm firmly in the "you're all talking tosh" camp, when I hear of people saying that "such an such an opamp has a tighter low end or more pronounced mids" etc.

Get a scope on there......put an audio signal through it & the output of most correctly configured opamps will look *identical* to the input.....& if it looks the same on a scope, it really should sound the same (I say should cos the brain is the wildcard here) ...there may be more discernible noise on some opamps vs others, but really, IMHO you're all being sold snake oil.

[Hides-His-Eyes]

low-fi fuzziness of the TL072

lolwut

[artifus]

Got to say....I'm firmly in the "you're all talking tosh" camp, when I hear of people saying that "such an such an opamp has a tighter low end or more pronounced mids" etc.

Get a scope on there......put an audio signal through it & the output of most correctly configured opamps will look *identical* to the input.....& if it looks the same on a scope, it really should sound the same (I say should cos the brain is the wildcard here) ...there may be more discernible noise on some opamps vs others, but really, IMHO you're all being sold snake oil.

a/b: record signal via op amp circuit to daw track. swap opamp and record same signal to new track. match levels if necessary. invert phase/polarity of one track with both tracks playing panned central. hear/see difference, if any.

[12Bass]

low-fi fuzziness of the TL072

lolwut

Compared to an OPA827, AD797, or OPA211?  Yup.... Those old 70s chips are not very clean.  Doesn't matter of course if it is used in a distortion pedal, and may in fact be desirable.

See here for empirical measurements: http://www.sg-acoustics.ch/analogue_audio/ic_opamps/index.html

[Hides-His-Eyes]

low-fi fuzziness of the TL072

lolwut

Compared to an OPA827, AD797, or OPA211?  Yup.... Those old 70s chips are not very clean.  Doesn't matter of course if it is used in a distortion pedal, and may in fact be desirable.

See here for empirical measurements: http://www.sg-acoustics.ch/analogue_audio/ic_opamps/index.html

calling an op-amp like that "lo-fi and fuzzy" just seems pompous, if I'm honest. You think a guitar going through a TL072 buffer and into an amp is going to end up sounding 'lo-fi and fuzzy'?

There's a massive difference to me between "not quite the best chip-op-amps money can buy" and "lo fi and fuzzy"

[artifus]

lo-fi and fuzzy? we're amplifying an electrical signal created by a metal wire vibrating in close proximity to a magnetic coil! 

[familyortiz]

12Bass is making a good point... if you want some noise or distortion, then a non precision opamp might work well for you.
In short though, "touch sensitive?" I would say that this is not the right phrase to apply here.

[Paul Marossy]

Hard to say... from my own listening, the OP275 has an interesting, colored, sort of sound... thought it might be low-level distortion that I was hearing, and maybe a tad mid-scooped.  But then, most of my testing has been in audio reproduction, not distortion pedals, where it was not among my favorites.  I've found that something like an LM4562 or OPA2211A has less apparent coloration.  Actually, I kind of like the somewhat low-fi fuzziness of the TL072, if character is desired.  Probably best to try it and see....  

According to the OP275 data sheet, the distortion is supposed to be very, very low from 30Hz to 1kHz. According to one individual, it's a "high-quality op-amp that gives a distinctive linear output waveform". If that is true, then it seems to me like what comes out of the opamp ought to be a pretty accurate reproduction of what goes in, and maybe is a little more "touch sensitive" than other dual opamps. I've looked at the waveforms on a scope, they do look pretty linear, even when massively distorted. There is a touch of pulse width change when really distorted, which I understand in an indicator of some even order harmonics being present.

Also, "the output is capable of driving 600 ohm loads to 10V RMS while maintaining low distortion. THD + Noise at 3V RMS is a low 0.0006%".

Maybe that is part of what gives the Black Cat OD-1 it's touch sensitivity? I define touch sensitive as something that very closely follows your playing dynamics. For example, a Boss Metal Zone is not something I would put in that category. It sounds way over the top no matter how hard or soft you pick. In comparison, the Black Cat OD-1 with an OP275 does a great job of following what you do.

[familyortiz]

Paul,
I think you hit the nail on the head... it is more the circuit design, not the opamp.

[WGTP]

Minimally educated speculation.  As long as things stay linear, I'm doubt I can hear the difference with my damaged old ears, but when things become non-linear, i.e. distortion, there may be differences in the harmonic content generated, i.e. harmonics/overtones, that I can hear.  Since different op-amps have different spec.s and respond to resistance, capacitance, transconductance, bias, etc., i.e. Circuit Design differently, (isn't' that why there are so many different ones) there may be something going on that is audible.  The different circuit topography of the op-amps may distort differently, respond to overload differently, take longer to recover, etc.

I have noticed that a higher level of even ordered harmonics seems to add midrange for example.  I think I have a heard a swirl to the 4558 distortion that isn't present in some of the higher quality op-amps.  I'm probably not a good test subject though. 

[amptramp]

You can improve the linearity of a JFET op amp pretty easily.  Most of the non-linearity comes from non-linearity of the capacitance from the input gates to the substrate (which is connected to the negative supply).  This capacitance has a non-linear behaviour as a function of common-mode input voltage.  Thus, it pays to operate at the highest voltage possible.  But if better performance is needed, the negative supply should be fed through a PNP transistor emitter follower whose base is driven at the common mode voltage.  Using the same feedback resistors (or resistor ratio) from the output to the base and the base to the negative supply as the output to the inverting input and the inverting input to ground will ensure the substrate voltage tracks the common mode input, keeping the capacitance constant.

This information is from the Analog Devices Applications Reference Manual, App Note AN-232.  They show an AD744 driving an AD811A connected as a unity gain-buffer.  The negative supply for the AD744 is obtained from a 2N3906 PNP transistor.  They show 7.5K feedback resistors to the AD744 inverting input and the transistor base from the AD811A output and a 1.82K resistor from the inverting input of the AD744 to ground and 1.82K from the transistor base to the negative supply.  The collector is at the negative supply and the emitter goes to the negative supply of the op amp.  They found the harmonic distortion went from 0.1% at 10 KHz and 0.2% at 20 KHz to under the noise level when the bootstrap negative supply was used, less than a tenth of what it was with the negative supply of the op amp connected to the negative supply.

No reason you couldn't do this with a TL071 and get "audiophool" performance.

[12Bass]

According to the OP275 data sheet, the distortion is supposed to be very, very low from 30Hz to 1kHz.

Actually, Groner's measurements (posted above) suggest that it is not that clean, at least when compared with newer high-performance op amps.  

BTW, my comments about the "low-fi" nature of the TL072 were in context of audio reproduction, not in a pedal.  I mentioned this in the post.  In a pedal, I'm not sure that distortion artifacts matter that much, and might even make it sound "better".  However, I'm fairly confident that I could reliably pick up the different chips if comparing, say, a pair of OPA827s against a TL072 in a high fidelity playback system.  The OPA827 sounds subjectively cleaner and clearer, and this is supported by Groner's tests.