different opamps audio distortion diffs in a guitar pedal, are they audibile? - Page 1
Forum section: electronics / projects

author: mcinque
author_rank: Unknown
timestamp: November 05, 2017, 03:51:10 pm
content: I found this video with a guy discussing about "distortion and audio quality" between different opamps he used for his guitar pedal.He get my attention at 02:40 in part 1, when he say that "distortion cannot be measured simply with numbers".The TL072, LF353, LM1458 and TLE2022 he is testing has different levels of distortion but the values on the datasheet are all under the 0,0xxx so I firmly believe something that cannot be heard or quantified without a proper instrument.In part 2 he compares the "different" sound using different opamps.I'm not so technically prepared so I ask you if this can be real or if it can be only a "musician/audiophile" impression.I hear no differences in the audio tests (even with youtube audio limits).https://youtu.be/PhC1ywrBk7Ihttps://youtu.be/DlKAXTMKpjI

author: Zero999
author_rank: Super Contributor
timestamp: November 05, 2017, 06:23:55 pm
content: He is right in that the numbers given in the op-amp data sheets are not useful for building a guitar effects box. This is because the op-amp is deliberately being driven outside of its specifications.To be honest, I couldn't tell the difference between any of the op-amps he tested in the second video, but that might be my inexperience or cognitive bias. I wonder if he could truly tell the difference himself? He should have covered the part numbers and done some blind tests on himself, a few times, swapping the numbers a couple of times, to ensure he's going on sound alone.

author: exe
author_rank: Unknown
timestamp: November 05, 2017, 08:13:21 pm
content: Ha-ha, I think I hear a small difference. Op-amp #1 sounds distinctively different (and "better") to me here:https://youtu.be/DlKAXTMKpjI?t=3m2s. I'm curious if it's my imagination or now.But the thing is we don't know how they are used. I'm no specialist but I've seen in pedals inputs deliberately overloaded to provide "soft clipping" or something. So, I'm pretty sure in pedals difference is possible. Also they mix fet-input and bjt-input opamps, so the signal loading is tiny bit different (but we need to see the schematics).Also these opamp have very limited bandwidth. lt072, for example, may provide audible distortions above 10kHz (see page 402+ for numbers and plots:http://www.nanovolt.ch/resources/ic_opamps/pdf/opamp_distortion.pdf, very good article, imho). Especially if signal slew rate approaching opamps limit.My two cents, I'd like to hear comments from audio engineers (but not from audiophiles).

author: Zero999
author_rank: Super Contributor
timestamp: November 05, 2017, 09:19:31 pm
content: Quote from: exe on November 05, 2017, 08:13:21 pmJust because it's measurable, it doesn't mean it's audible.How much distortion above 10kHz will be noticeable, even to someone under 20, with good hearing? If the amplitude hasn't changed, then it shouldn't be noticeable at all, because the harmonics will be above the audio band. A 10kHz square wave will sound identical to a 10kHz sine wave, of the same peak amplitude.By the way, the data is on page 404, if one includes all the non-numbered pages at the start of the PDF, so enter that into the PDF viewer to skip to the TL072 data.As far as clipping is concerned: generally most op-amps hard clip in a similar manner, although it's not something I've made much of an effort to observe, because it's something I normally try to avoid.

author: Someone
author_rank: Super Contributor
timestamp: November 06, 2017, 12:05:03 am
content: Quote from: Hero999 on November 05, 2017, 09:19:31 pmThe initial clip transition might be very similar on lightly loaded opamps, but saturation recovery and/or drive into heavy (or capacitive) loads shows up enormous differences between opamps.Quote from: Hero999 on November 05, 2017, 09:19:31 pmDistortion is the goal of that product but the frequency (and phase) response still contribute to the sound.Quote from: Hero999 on November 05, 2017, 06:23:55 pmIn this example a well tuned listener would be able to pass blind tests easily, the resultant tones are substantially different. For comparison you might not be able to hear the differences between pickup designs, but the musicians certainly can.Quote from: exe on November 05, 2017, 08:13:21 pm#1 has the most distinct sound from the others, you can also observe the waveform to see how its furthest away. This could be measured with a 3 dimensional "waterfall" type plot showing the full spectrum against varying input levels for a specific frequency (1kHz sine for instance) but that still doesn't directly relate back to how the end users want it to sound so its easier to rely on listening tests.

author: Zero999
author_rank: Super Contributor
timestamp: November 06, 2017, 08:52:46 am
content: Quote from: Someone on November 06, 2017, 12:05:03 amIt doesn't sound like the any of the op-amps are being driven into clipping. If they were, then I'm pretty sure I'd be able to hear it, although it's possible one is on the verge of clipping and it's just below the threshold I can detect but others do. The frequency response being different is a possibility.QuoteAnother factor is the guitarist was aware that he was using different op-amps, so that could affect his playing style, on a subconscious level, so even if a listener can pass a double blind test, it doesn't prove it's anything to do with the op-amp.QuoteJust out of curiosity, have you done any analysis on the waveform? If so, it would be interesting to see it.

author: BrianHG
author_rank: Super Contributor
timestamp: November 06, 2017, 09:35:54 am
content: Quote from: Hero999 on November 06, 2017, 08:52:46 amThey are, either that or there are clamping diodes at the output. The high frequency you are expecting to hear as a result of clipping has been filtered/softened up. If there wasn't clipping, the guitar would sound somewhat more like a normal up-amplified string guitar. That distorted electric guitar sound effect is the point of the effects box.As for slight audible differences, youtube is not the place to be able to hear any fine differences, so, anything audible would be chalked up to just they way each type of op-amp clips as it approaches the rails.Any other differences, because you are dealing with a audio level clipping effect, can just be summed to how hard the player struck the chord...BTW, Youtube usually only passes up to 12Khz reliably. Any higher frequencies are a fluke or an encoder who knows how to force a fixed high bitrate audio down youtube's stream VS the default crap.

author: exe
author_rank: Unknown
timestamp: November 06, 2017, 06:04:37 pm
content: Quote from: Hero999 on November 05, 2017, 09:19:31 pmI don't think so, just checked on:http://www.szynalski.com/tone-generator/.

author: Buriedcode
author_rank: Super Contributor
timestamp: November 06, 2017, 06:49:20 pm
content: Ugh although its not audiophool territory, DIY guitar pedals and opamps has generated many topics in forums. The only time I've heard an opamp in a (guitar pedal) distortion circuit make a difference, to my ears, was in the ProCo Rat which used an LM308 compared with half a 4558. The former sound less harsh (I'm guessing the limited slew rate, combined with much lower GBW with high gain roll off much of the highs, making the buzzy distortion a little less crap).But very few distortion boxes use opamp distortion, its mostly diode clipping, sometimes MOSFET clipping (which should take the fact different opamps clip differently out of the equation). I would say in the vast majority of designs, so long as the opamp is compensated so it doesn't oscillate, and its IO doesn't go near its limits, I don't see it making an audible difference. There is a reason pedals use the TL072, and the 4558, and some even go for the TL062/4 for lower power consumption.I have yet to hear any distortion/overdrive pedal that I would describe as 'magic'. And certainly none that would benefit from a high bandwidth or expensive opamp. The past 20 years have seen a huge resurgence in 'boutique' pedals, some are laughably expensive for what they are, but many are now quite reasonably priced with reasonable design choices (metal film resistors better for high gain distortions, film caps, half decent jack sockets) at about $40-60. But they dont' use rare or expensive components, nor is there anything particularly unique about the designs, its just pedals have nicer graphics now and its as much about kudos than actual tone.

author: Zero999
author_rank: Super Contributor
timestamp: November 06, 2017, 07:00:01 pm
content: Quote from: exe on November 06, 2017, 06:04:37 pmThat's interesting. Me to. Using a square wave, I get get another lower frequency tone simultaneously, which shouldn't happen. Any harmonics should be higher than 10kHz, not lower, thus inaudiable. Look up Fourier transforms. There seems to be some aliasing going on.Don't trust that web based app. Use a proper signal generator!

author: floobydust
author_rank: Super Contributor
timestamp: November 06, 2017, 08:18:42 pm
content: You guys are forgetting some op-amps go into phase-reversal when overloaded, so you get frequency doubling sound-wise. You can overload the input, middle or output stage of an op-amp, each independently. You'd have to see the circuit.Also, some dual/quad op-amps share a common internal bias across sections that does not guarantee the sections are independent. So op-amp #1 getting overloaded upsets op-amp #2 and so on.There are many subtleties in guitar pedals.

author: Zero999
author_rank: Super Contributor
timestamp: November 06, 2017, 09:33:29 pm
content: Quote from: Buriedcode on November 06, 2017, 06:49:20 pmI agree. I'd also consider it to be bad design to rely on any op-amp for distortion, because the nature of the distortion will be variable and won't be specified by the manufacturer.Quote from: floobydust on November 06, 2017, 08:18:42 pmDo you have any evidence to support that?As far as I'm aware, phase reversal occurs, in some op-amps, when the common mode range of the inputs is exceeded, not when the output is overloaded.

author: Someone
author_rank: Super Contributor
timestamp: November 06, 2017, 10:26:58 pm
content: Quote from: Hero999 on November 06, 2017, 08:52:46 amIts all there for you, but seems you've already made up your mind and don't want to put any effort into looking at it further. The differences are audible without any special tools and you can observe several different effects in the waveforms, it might all be faked and the recorded with different preamps or gains, but the end result is still differences between the examples presented.

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 06, 2017, 10:53:31 pm
content: Quote from: Hero999 on November 05, 2017, 09:19:31 pmAnd the Golden Ear Audiophool obverse: Just because we can hear it doesn't mean it's measurable.That seems to be their #1 retort.

author: Someone
author_rank: Super Contributor
timestamp: November 06, 2017, 11:34:22 pm
content: Quote from: Richard Crowley on November 06, 2017, 10:53:31 pmExcept this example is both audible and measurable, its the correlation/agreement between the two which is the difficult part.

author: floobydust
author_rank: Super Contributor
timestamp: November 07, 2017, 02:35:41 am
content: Quote from: Hero999 on November 06, 2017, 09:33:29 pmPhase-Reversal:Op Amp Output Phase-Reversal Analog Devices MT-0364558 phase-reversalOp Amp Applications Handbook by Walt Jung [7-72]Nonlatching Inversion of Transfer Function, Triggered by Exceeding Common-Mode Limit• Sometimes Occurs in FET and Bipolar (Single-Supply) Op Amps• Doesn’t Harm Amplifier... but Disastrous for Servo Systems• Not Usually Specified on Data Sheet, so Amplifier Must be Checked• Easily Prevented:? All op amps: Limit applied CM voltage by clamping or other means? BiFETs: Add series input resistance, R_LIMIT? Bipolars: R_LIMIT and Schottky clamp diode to railI've seen an op-amp overdriven and other on-die sections get upset, it seems to be the common voltage regulator but I'm not entirely sure of the mechanism - I think it's JFET input op-amps, when input current flows, or substrate current, HF oscillation, or latch-up. The dual/quad sections are not always independent, in some op-amps I find.LM358, LM324 Bias Circuitry Common to Four AmplifiersFig. 1 Same for TL071, 72, 74, 84, MC33078, LF412, LM3900 etc. I'm sure Walt Jung mentioned this but can't find the paragraph, it was in an Analog Devices handbook.For the op-amp mid-section (VAS) overload, it's the latch-up recovery time and this would occur when the op-amp CM is within limits but differential overload occurs.For the output stage, it is the push-pull section with NPN Darlington and PNP lateral Sizlaki, they oscillate "bottom side fuzzies" and I hate those '70's designs using this.

author: Buriedcode
author_rank: Super Contributor
timestamp: November 07, 2017, 02:42:34 am
content: Lets not turn this into the other thread (which was about 'high end HIFI systems').From what I saw of the video(s) the guy didn't seem to be selling anything, or 'preaching' (whereby folks hide assertions in questions under the guise of 'lets test it'). He's just trying to experiment to see if he hears a difference. Fair play to him.As many point out though, with A/B testing, you can't really just 'play guitar' through it, since no-one can re-play things exactly, and what you hear determines how you play. And replaying them from a recording of a dry guitar might sound different because most likely the playback output has low impedance, where-as guitar pickups tend to be somewhat high.. which means there could be some dependence on the effects input impedance... then there's cable capacitance etc.. I've seen dodgy hackaday projects where someone made a bot pluck a guitar string, but that's going over the top.Ultimately a sign of a 'good' distortion/overdrive or even effect pedal is one that makes you play more, independent of price 'legend' or popularity. Guitar effects/amps/tone is somewhat different from HIFI tone because you aren't trying to faithfully reproduce a recording - you're creating sound and interacting with it in real time, so there is feedback - how you play determines how it sounds, but alsohow it sounds determines how you play. It is why one really just has to experiment with different setups, EQ, combinations of settings (including guitar volume and tone).Swapping out opamps in circuits I think is way way down the list of 'stuff to try', but as I said, it seems the guy is just curious, and possibly wanting to start a discussion on the matter.

author: Audioguru
author_rank: Super Contributor
timestamp: November 07, 2017, 02:56:10 am
content: Has anyone found the schematic and parts list for this Red Crown pedal? It produces severe distortion that opamps are not designed to produce so I want to know if diodes cause the distortion or if the opamps are clipping like crazy. I heard that the distortion began and stopped at different levels with the different opamps.The LM1458 is a dual 741 opamp (49 years old) with old and bad almost everything. I am old but I am not bad.The TL071 and LF353 are basically the same, fair for hifi.The TLE2022 is not hifi since it has the low supply current (and probably the crossover distortion) of an LM324 and LM358, a poor slew rate, noise and its output current is fairly low.

author: hamster_nz
author_rank: Super Contributor
timestamp: November 07, 2017, 07:42:27 am
content: FWIW, two sorts of guitar distortion is common.Soft clipping is usually done with back to back parallel diodes in the feedback path of an op-amp, maybe with some bypassing. A small input can then make +/- 0.7V swings in the output.Hard clipping is two diodes to clamp the output to approx +/- 0.7 V.Usually the op amp is not actually overdriven or 'clipping' - it is more 'clamping' and 'non linear feedbsck' that boost the harmonics.I would expect the 'brighter' sounding op amps (3&4) slewed faster....Maybe somebody here has a siggen, a four channel scope and a breadboard, and would like to knock up something like one of these with a couple of different op-amps, and see if they act different?

author: Zero999
author_rank: Super Contributor
timestamp: November 07, 2017, 11:46:29 am
content: Quote from: Richard Crowley on November 06, 2017, 10:53:31 pmYes that's BS. Anything audible is measurable. Anyone who believes otherwise is delusional and has already lost the argument when they say that. Test equipment is much more sensitive than the human ear and can pick out imperceptible amounts of distortion. If this wasn't the case, there would be no point in having it in the first place!Quote from: Someone on November 06, 2017, 10:26:58 pmI haven't made up my mind. I sit on the fence. No real evidence has been given that different op-amps in this distortion box really do sound different, but there isn't any evidence to support they all sound the same either! Hence, I'm open the both possibilities, irrespective of whether I'm able to tell the difference.In retrospect , it was a silly question: even if you did analyse the waveforms, ripped off Youtube, it would be a wasted effort, because, as mentioned previously: Youtube doesn't have that good sound quality and the guitarist was aware of the different op-amps, which might influence his playing style.It would be good to see the schematic of the distortion box used. That would make it easier to tell how much of a difference, changing the op-amp would make.

author: Henrik_V
author_rank: Regular Contributor
timestamp: November 07, 2017, 11:54:31 am
content: Quote from: hamster_nz on November 07, 2017, 07:42:27 amOne important 'sound' detail is to choose different or one way double diodes to get unsymmetric -> even harmonic distortion.Changing these diodes (in case of LED , chosse the red, green or blue(s) colour sound) one can expect more changes than in changing the OP.

author: tszaboo
author_rank: Super Contributor
timestamp: November 07, 2017, 12:01:32 pm
content: Quote from: Audioguru on November 07, 2017, 02:56:10 amYeah, but these opamps float around in audioass circles as they were designed in the golden age.I seriously think, if you make a pedal with an ADC-DSP-DAC chain, and place an unconnected socket with a LM358 in it, people will replace it and "hear" the difference.BTW why isn't there any digital pedals. Wouldn't it be infinitely simpler to just design 1 board, and set a different FIR or IIR filter for these folks? It wouldnt even need to become analog between the boxes, just connect DSPs togheter with SPDIF cable or something.

author: Buriedcode
author_rank: Super Contributor
timestamp: November 07, 2017, 03:52:46 pm
content: Quote from: NANDBlog on November 07, 2017, 12:01:32 pmThats exactly why there are a hell of a lot of multieffect pedals, and most modern reverb pedals are DSP-based these days. Not only does DSP/digital allow for endless combinations and storage of settings, its the only sane way of using more than 4-5 effects at once, and is far more economical.One of the main limitations that digital had was latency. If you have 6 effects in a chain, and each one has 10ms latency, you'll notice the delay between plucking a string and hearing the sound. One is much more sensitive to latency when one produces the sound. So faster processors = lower latency > allows for more effects to be layered.DSP/Digital based distortions got a bad reputation in the early 90's, mostly because of a misunderstanding of how stuff works (people describing mid-scooped overdrive as "sounding digital", when it was all analogue) and resistance to modern technology. But in the past decade amp-emulation has come on leaps and bounds. So whilst the obvious timing-based effects (chorus, delay, reverb) often went digital, it took a lot longer for people to 'accept' digital distortion units, despite being much simpler devices. Analogue pedals whilst still used, seem to be more of a 'retro' thing with much of the professional recording either just using an amp, or using modellers (be they hardware or software based).Also note, whilst some OD/distortion circuits have diodes in the feedback of the opamp, the most common config is just back-to-back on the output of the opamp. Hard clipping, with a bit of post EQ is the basis for hundreds of pedals. Although guitar bandwidth is pretty low, I suspect very old opamps (like the mentioned 741) will sound 'different' when used in high-gain circuits given the bandwidth limitations. This of course, isn't necessarily a bad thing - diode clipping without a low pass filter, or plugged straight into a HIFI amp sounds dreadful, so the low pass effect of the limited bandwidth and slew rate removes some of the harshness.There are other topics of discussion that tend to polarise people... Silicon vs germanium transistor fuzzes, tubes, vs solid state... $60 guitar cables vs $5 cables etc..

author: floobydust
author_rank: Super Contributor
timestamp: November 07, 2017, 06:50:53 pm
content: A guitarist's sound can define a band, so they are always looking for unique cool sounds. So a million analog pedals exist with much empirical work to "get that sound". There's many patents on effects pedals.It's not simply one pedal - several are used on a "pedal board". Compression, distortion, EQ reverb, chorus etc. all in a chain.DSP can't touch the complexity - a pedal board has thousands of sonic possibilities - musicians just want to "stomp" on a pedal to change settings, rather than DSP and fiddle with menus and keypads. If the UI was better, then more digital might be used.Tom Scholzdesigned the electronics forBostonand for 1976 it was a totally unique sound, just amazing for those of use who remember the LP.

author: Buriedcode
author_rank: Super Contributor
timestamp: November 07, 2017, 07:41:26 pm
content: I'm not sure where you got the impression that a DSP can't cut it.. Fractal systems Axe FX II can do 8 effects at once, in any order, and emulate different amps, cabs, rooms, with a vast number of parameters for each effect. Individual pedals are all well and good, but they get unwieldy with more than 8 on a board, and rarely do guitarists have more than 4 in a chain.I'm not suggesting analogue is dead or anything, not at all, just that digital provides a whole other level of control, and one doesn't have to write down settings. It has been this way for well over 20 years. I'll admit its much more intuitive to adjust knobs than go through menu systems, but recently things have got better (the Zoom G3 for example).And "The Tom Sholtz" tone was mostly overlaying guitar parts with lots of EQ and chorus. His 'Rockman' was great at the time, but its a one-trick pony.

author: Someone
author_rank: Super Contributor
timestamp: November 07, 2017, 11:47:03 pm
content: Quote from: NANDBlog on November 07, 2017, 12:01:32 pmIts the same reason analog RF microphones took a long time to reach the market and still aren't dominant, latency in live sound (particularly musical performance) can be extremely critical and adding a few ms a few times in the chain adds up to unacceptable performance. As for simply FIR or IIR filters there are already pedals which can do this:http://www.logidy.comBut they can't reproduce the processing of non LTI responses, which is the majority of the pedal market.

author: tszaboo
author_rank: Super Contributor
timestamp: November 08, 2017, 12:50:55 pm
content: Quote from: Someone on November 07, 2017, 11:47:03 pmYes, I understand that latency is critical. I've tried singing while listening to my voice with a delay, it just wasn't working. If I hear my own voice with some 1-2 sec delay (skype eg.) I forget how to speak.But 10ms delay is 48 samples at 48KHz sampling. For sure the only reason for having that much delay is because the filter is chosen incorrectly. Or too many filters are in series.I didn't want to turn this into a Digital vs Analog discussion. For sure today, digital can do anything that analog can, otherwise we would use analog computers.Quote from: Buriedcode on November 07, 2017, 07:41:26 pmAdding a knob to a DSP to tune the filter values... I'm sure someone did this already.Quote from: floobydust on November 07, 2017, 06:50:53 pmLuckily you cannot patent software or connecting 2 diodes on this continent. Or rounded rectangles.

author: Zero999
author_rank: Super Contributor
timestamp: November 08, 2017, 01:30:45 pm
content: Quote from: NANDBlog on November 08, 2017, 12:50:55 pmYes you can, in some countries.

author: tszaboo
author_rank: Super Contributor
timestamp: November 08, 2017, 01:36:24 pm
content: Quote from: Hero999 on November 08, 2017, 01:30:45 pm"on this continent".

author: Buriedcode
author_rank: Super Contributor
timestamp: November 08, 2017, 02:38:45 pm
content: Quote from: NANDBlog on November 08, 2017, 12:50:55 pmYeah, I'll try not to keep banging on about digital. Ultimately, digital multieffects have to do that - multiple effects. Its not uncommon for an amp modeller to include compression, several EQ both pre and post-overdrive, cab emulation, and even room reverb.. and that is considered to be ONE effect. When the user starts adding more compression, chorus, reverb and delay, you can see just how many stages the DSP has work on. A simple filter is fine, but 10-20 in series?It required quite a fast DSP to cope with this... and also users want to be able to turn effects on instantly (in an analogue both there is a DPDT kick switch which switches the effect in and out of the loop) and so don't want to have to wait for the DSP to 'catch up' my loading in different patches.Latency isn't really a problem these days, we were discussing why digital - specifically digital amp modellers or overdrive units - didn't take off in the 90's as quickly as other effects (chorus, flanger, phaser etc..) and it was mostly the above issues, combined with the fact there were plenty of versatile analogue solutions (Marshall JMP-1, ADA MP1, with digital pots for storing settings).Quote from: NANDBlog on November 08, 2017, 12:50:55 pmhttp://www.musicradar.com/news/guitars/16-of-the-best-multi-effects-pedals-for-guitarists-640907You can see that starting about 10 years ago, they switched from having navigation buttons (up,down, left, right, enter etc..) to using rotary encoders for data entry. As they got cheaper, they started adding more, so each knob could be specific to one parameter, so the they could be labelled and the user didn't have to keep track of what did what. In fact, they look much more 'basic' these days, despite the internal DSP's being waaay more advanced.I'm not suggesting everyone should go digital here, just that, there is still a lot of stigma attached to using digital gear for some reason. The problems were mostly ironed out 20 years ago, and I for one find it awesome that for $150 I can have access to almost every guitar effect ever made.