Op Amp Evaluation -IMHO

[WGTP]

Notice â€" Highly Subjective Op Amp Evaluation by lo-tech Guitarist

This saga started out with a modified version of Gus’ Muff Fuzz #1 or Gary’s Mockman.  Both add a 470K/1meg resistor across the diodes, as a mod to stop gating.  Since the Muff Fuzz runs one op amp into the other which has diodes in the feedback loop, it seemed like a good test bed.  If you are going to use dual op amps, you might as well use both of them!  I was going to leave it alone, but I couldn’t.  I was getting too much noise, but really liked the sound.  This is the first distortion with diodes in the loop I have tried, and I like the smooth rich distortion.  The diodes in the circuit such as GE's, SI's and LED's make a much more noticeable difference than the op amps.  I think I can hear a difference when using a Jfets or Mosfets as clipping diodes.

I initially tweaked the diodes and found that using LED’s raised the output, but the noise did not increase, plus the LED’s had a nice chunk to them.  The down side is that the distortion level is lower, so I used an MPF102 Jfet for the other diode which has a slightly higher clipping threshold than a single SI (I tried a Jfet/Mosfet combo, but it was too fuzzy for my taste, the Jfet/SI combo is nice, I’ll probably need some switches). This got more distortion, plus more even ordered harmonics due to the asymmetrical distortion produced by dissimilar clipping thresholds.  The LED is 1.8v and the Jfet .7v.  I hypothesize that having diodes with higher clipping thresholds provides the op amp with more opportunity to clip, whereas using lower threshold diodes results in the diodes clipping more and the op amp less.  I may just be dreaming.

As I sorted thru the op amps I had, thanks to one of the “elders” of the forum, I was also watching to see which op amps others were using.  I started requesting samples from various manufactures and found that Texas Instruments had most of what I wanted and was vary gracious and quick in providing their samples for the test.  I also searched thru their list of op amps and found others to try.  (Thanks TI.com)  

I needed some consistent way of measuring the 15 or so op amps I had.  Each time I thought I heard something different; I would later be unable to duplicate the findings.  Most of my observations have question marks by them. It's difficult to hear differences between 2 seemingly different designs of op amps like a 4558 and TLC2272 cmos.  It's more difficult to hear a difference between 2 op amps of the same basic design.

I came up with 2 tests besides the basic and highly subjective listening test.  I have found that one evening something sounds great and the next day it doesn’t.  

First was the plug it in and see how much “Noise” it made test.  I had members of the family assist and was surprised to find that some noise, even though the same perceived volume was judged as harsher than others.  Smooth and flowing vs. harsh and irritating seemed to be the difference.

The second test was the plug it in and see how quickly and how much “Feedback” it got while the guitar rested on the bed opposite the amp test.  I think this relates to gain.  The 4558 was adjusted to slowly develop feedback and with no changes to the set up different op amps were substituted.  Another unexpected finding was that the CMOS op amps would feedback on a lower harmonic than the others.  Does this mean more gain at certain frequencies?  Were the harmonics odd vs. even???

In all tests the TI RC4558 served as the constant with which the others were grouped.  I had to group them in clusters to make testing reasonable.  I use a standard group that included the 4558, a Jfet input group which includes the TL072, a CMOS group which includes the rail to rail op amps and the high performance group that includes the Burr Browns.  Each time the 4558 was used to compare the others in the group.  The results of the tests are below.

If you don’t stand in exactly the same spot when you test, that has more impact on the sound than different op amps do.

I did find that most of the op amps with high gain were also noisy.  This would seem to be typical, the OP275 being tops in both categories.  I think the Muff Fuzz circuit with 1 meg resistors in the loops seems subject to noisiness.  Lying on top of my SS amp didn’t help either.  I’m sure it will be quieter once I get it into a junction box.  Still obvious differences existed between op amps.

The TI RC4558 is not a bad choice and I can see why it is commonly used.  When maxed it breaks up nicely and it isn’t nosier than some others with the same “gain”.  The noise was judged as being harsher than some others though.  At times it seems to have sort of a swirl to the distortion that is a little different, but at other times it doesn’t.  2 commonly recommended op amps, the LF353 and NE5532 do sound sweet, but have less gain than the 4558 and less bass.  That can be remedied.

Distortion:  For me that is what it is all about.  The 4558 has plenty and maybe the 4559 has more.  The higher gain op amps don’t necessarily break up more or have higher levels of distortion and sustain.  Some seem to stay clearer, perhaps due to there design for hi-fi circuits/slew rate.  Some sound more “robust” or have more balls than others.  Some may emphasis different harmonics, but again the diodes have much more noticeable effects on the nature/character of the distortion.  You don’t have to have your family/friends come in and help you hear the difference.  If you don’t stand in exactly the same spot when you test, that has more impact on the sound than different op amps do.

I have concluded that most of the difference between the op amps was due to more or less gain, distortion, highs and/or lows.  These are all things that can be addressed with different caps or different gain settings.  This seems to account for most of the differences I could hear repeatedly.  The ones I thought had sweeter smoother highs, turned out to have less gain and/or bass, which smoothed out the highs and could be accomplished by using smaller input caps.  I found that I could use a larger input cap with some op amps that would sound too woolly with others or some diode combinations worked better for the same reason.  The cmos op amp’s that go rail to rail seem to be a bit woollier than some others which works better with smaller caps or SI diodes.  The Jfet/Mosfet’s as diodes worked better with less wooly op amp cap combo’s.  This is based on my particular taste and could differ from person to person.

Output:  I haven’t turned the output pot very far at all, so I’m assuming that they all have plenty of drive for a tube front end, although this version doesn’t have a gain control and doesn’t get really clean.

It's fun searching, even if it is subjective and tone is pretty much that way.   My double picking and jazz scales are much better than before I started.  

OP AMP’s

Bi-Polar Op Amps
RC4558   Old standby good gain and distortion, some noise, Mojo???
RC4559   Turbo version of 4558, cool   
NE5532   Very Quiet, sweet highs, less gain???
LM833   Nice w/big bottom??

Jfet Inputs
TL052      Originally sounded superior, but now like the rest
TL072      Another std.  sounds good less mid’s than  4558
LF353      frequently recommended, sounds good, quiet/less gain
LF412      Little stronger than LM353

High-Performance
OP275   Lots of gain and noise.  Probably fine enclosed.  The hi-fi
 -  -  -      Op amps seem to be slightly smoother/creamer sounding???
OPA3140   Quieter than 2604 probably less gain, nice
OPA2604   Almost as much gain as 275 but quieter may not break up as much

Cmos â€" Rail to Rail
CA3260   Mosfet in Cmos out spec.s sound cool
TLC2262   Cmos may sound fatter w/sweeter highs??? I will probably use
TLC2272   one just in case.  Usually lower noise.
TLV2372   Sounds nice

:roll:

[Paul Marossy]

Gee, your fingers must be tired  :lol:

Thanks for the opamp taste test, it's interesting info. That's interesting about the OP275, I used it to good effect in some of my guitar effects. It's touted as being the first opamp to use the Butler front end, etc., but it's a little overrated. The NE5532/5534 is a pretty good all-around performer, too. I have even had good results using a lowly 741 and 1458. But that may be due to how I set my controls - I usually have the gain at max. or close to and the volume not much louder than the bypassed signal. I'm sure the noise would be very apparent with the volume at high levels, though.

[Watcher]

That's some nice work, WG.

[Fret Wire]

Quite a test you've done there! I'm always happy to hear I'm not the only one who likes the NE5532. I dont think my ears are good enough to hear the difference between different brands of the same chip. Like you said, it stands out more in certain pedals than others.  Kind of a reminder that we should all pitch in on testing stuff instead of waiting for someone else to expend all their time, effort, and money for our benefit. On the other hand, it's fun! Thanks!

[Peter Snowberg]

Great taste test WG!

Nice work, from another 5532 head out here in effects land.

Take care,
-Peter

[brett]

Great testing.  Informative.
(Hey, count me "in" as another 5332 user.  They're cheap and easy to get, as well as giving high sound quality)

[aron]

Thanks for the reviews!

This is good information. I'm sure well have our favorites with regards to op amps.

[onboard]

:shock:  I'll need to read all of that when my attention span is a little longer!

Seriously,  shows a true regard for the community.

[Gary]

Good detective work, WGTP.  Useful info in the style of the old forum.  Much appreciated.  Now I can stop lusting after those exotic op amps.

[Gus]

Interesting
 I tend to like the lf353 and ne5532.  To my ears they seem to have more gain.  I think sometime grit is heard as gain.

 My ear,brain hear the ne5532 having more gain.  

 
Gus

[WGTP]

I'm sure my testing method are subject to some valid criticism, but that is the best way I could think of with limited resources and knowledge to test the little beasts.  I too was suprised that some of the more commonly recommended devices feedback at a lower level than the 4558 and even more suprised that the cmos feed back on lower harmonics than the others.  I hadn't even considered that as a test criteria.  They may have more hi end roll off???  Compared to the 2604, the cmos sound less brilliant.  I know the 5534 made my rat sound better compared to a TL070 and the 353 sounds good as well.

More feedback could be a problem at high volume levels and may not be desireable.

[WGTP]

I tried the "feedback" test again tonight with interesting results.  First pass I used an SI/mosfet diodes and the results from low to high:  353, 5532, 4558, 2272, 2372, 2134, 2604.

Then I tried it with 2 LED's for diodes and the results were different.  The difference in feedback levels were much closer and the 2 CMOS op amps, the 2272 and 2372 both feedback on lower harmonics, AS DID THE 353??? The 2372 sort of tried both harmonics before settling on the lower one.  The order from low to high:  4558, 2372, 353, 2272, 5532, 2134, 2604.

[Peter Snowberg]

I find that most amazing that the 2272 and 2372 both feedback on lower harmonics. I wonder if they're just better at lower frequencies and those just dominate?

Great info!

Thanks again!

-Peter

[Ed G.]

I had seen some stuff about the OPA2604 a while back and when I made a mouser order, I tried ordering one. It cross-ref'ed to a TSH22, they were about a buck and some change each. I don't know if they are as good as the Burr-Brown chips, but for some reason today I had time to kill so I started swapping out chips in my pedal, and they each sounded very similar, and I was separating the 'good' ones from the mediocre ones so I could narrow it down. The TSH22 was the last one I tried and once I plugged it in, I knew I didn't want to listen to the other ones again.
For some reason, this one 'felt' good. It responded to my playing in a way the others didn't, including JRC4558, JRC4559, RC4558 and 4559 (TI), TL072.
This may not be the case in all pedals because my circuit has an unbuffered front end. Something like a tube screamer with a buffer at the input may interfere with this 'response' effect, or it may not, I don't know.
Anyway, you're right about the 'creamy' part, it has a very smooth high end to it. It has a nice character to the tone I can't quite describe.
I guess now I have to try out the BB OPA2604 and see if it gets even better, but I'm pretty happy for now.

[Paul Marossy]

If I remember right, the OPA2604 type opamp has a very wide bandwidth of ~10mHz. In a solid state circuit, this is necessary for more high order harmonics, which sounds smoother. And it should also sound a little more like a tube circuit.

[Jay Doyle]

If I remember right, the OPA2604 type opamp has a very wide bandwidth of ~10mHz. In a solid state circuit, this is necessary for more high order harmonics, which sounds smoother. And it should also sound a little more like a tube circuit.

Well, 10MHz is a wide bandwidth and it means that it will certainly have full gain throughout the aural spectrum, but we only need to go out to 20kiloHz as that is the limit of human hearing.

I also don't know how it would make it sound more like a tube circuit, could you explain Paul.

Thanks,

Jay

[Paul Marossy]

Well, maybe more properly said, "it could" make a circuit sound more tube-ish.

Read this article: http://www.diyguitarist.com/PDF_Files/TubeEmulation.pdf

Most opamps have a gain-bandwidth of around 3mHz. A 12AX7 has a gain-bandwidth on the order of about 8mHz. The OPA2604 is around 10mHz.

I interpret that, in terms of harmonics, as being a lot closer to a preamp tube than the typical opamp. Therefore, what is interpreted as "sounding smoother" is probably because one can hear the high order harmonics that we wouldn't normally hear from a generic opamp. What I'm not sure of is how strong the second order harmonics would be compared to the third order. That's what would make it sound very tube-ish - a very strong 2nd order harmonic with the 3rd, along with a little compression, which may or may not be present in a distortion circuit. It seems like they all compress the signal at least a little bit.

Tubes also produce far more of the high order harmonics than SS devices do.

Just my opinion, of course.  

[Jay Doyle]

Well, maybe more properly said, "it could" make a circuit sound more tube-ish.

Read this article: http://www.diyguitarist.com/PDF_Files/TubeEmulation.pdf

Ok, I read it and although seemingly well researched, I find it a bit questionable. Most specifically this sentence:

After adjusting the tube bias, the input and output gains and the emulator grid-to-plate capacitance, the waveforms for a variety of input levels and frequencies show a good match at various overdrives, frequencies and loads

I read this to mean: "After I changed everything around, I was able to come up with a particular configuration with the tube that matched the circuit I was using, thus proving my point."

Most opamps have a gain-bandwidth of around 3mHz. A 12AX7 has a gain-bandwidth on the order of about 8mHz. The OPA2604 is around 10mHz.

And a 2N3904 has a GBP of 300MHz    and there are reasons for the difference between discrete single stage transistor/tube circuits and op amps. The phase shift in an opamp is a complex thing, not so in a transistor/tube stage. An opamp has cascaded gain stages each producing their own rolloff due to the Miller capacitances in each stage. A discrete amp only has one to worry about.

I interpret that, in terms of harmonics, as being a lot closer to a preamp tube than the typical opamp. Therefore, what is interpreted as "sounding smoother" is probably because one can hear the high order harmonics that we wouldn't normally hear from a generic opamp.

A higher GBP doesn't mean that you can hear the harmonics better only that those harmonics can be amplifed to be louder. For example, assuming a total open loop (no feedback) gain of 100db, a 1MHz GBP can amplify a 20kHz signal (the limit of human hearing, give or take, so the highest harmonic we have to worry about) by 20db, where as a 10MHz GBP can amplify it by 40db; it is still there in the 1MHz GBP, just not as loud. But that isn't all that important as we are worried about the production of harmonics by the distortion of the stage, in which case both the 1MHz and the 10MHz can produce them with ease.

In any case, higher order harmonics are what contributes to the "harsher" sound of a transistor amplifier clipping. The difference lies in the tubes natural response to squash the signal instead of hard clipping like a transistor. To quote Boscorelli:

In a squarewave, harmonic energy exceeds that of the fundamental, with much of the energy clustered above the 5th harmonic. In a squashed wave, harmonics roll of rapidly, less than 2% of total energy falling above the 5th harmonic.

What I'm not sure of is how strong the second order harmonics would be compared to the third order. That's what would make it sound very tube-ish - a very strong 2nd order harmonic with the 3rd, along with a little compression, which may or may not be present in a distortion circuit. It seems like they all compress the signal at least a little bit.

Right, it is all of these factors, along with the shifting bias point and squashing before hard clipping, that make the tube sound. IMO, a tube is a tube is a tube   . You can't emulate it with SS, so why try? I just try to make SS sound good.

Tubes also produce far more of the high order harmonics than SS devices do.

Actually I think it is the opposite, thus the "harsh" sound of transistor clipping.

Just my opinion, of course.  

Me too.  

[Paul Marossy]

Hmmm... points well taken.

But, from everything I have read thus far, and the curves I have seen comparing tubes to transistors, tubes produce a lot more high order harmonics, odd and even order - but more even than odd. Transistors produce a strong third order harmonic, and everything is else is small in comparison. Odd order harmonics sound harsh to the human ear, and even more pleasant, right? If it were not for the strong 2nd order harmonic, tubes would probably sound a lot more harsh. If you really, really clip a signal with a transistor you basically end up with a square wave with slightly rounded corners. Would a tube react the same way?

Perhaps I am mistaken comparing transistor devices to opamps? Although opamps contain a bazillion transistors and diodes....

Anyhow, that's just they way I see it.  

[shredgd]

Just my 2 (euro)cents...

To those who like NE5532 opamps and therefore suggest to use them: be aware! They almost double the current draw in my Son Of Screamer! (6.3ma when idle, instead of 3.4ma using a TI4558)

Giulio