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Laurie
Double Platinum Member
Canada
4854 Posts |
 Posted - 11/01/2009 : 15:46:55
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While chasing down something unrelated I found myself watching this youtube video again:
http://www.youtube.com/watch?v=bpTv2jAree8
And it got me thinking about what is really being said here. "the opamp doesn't have much to do with the overdrive sound".
I think the "overdrive sound" part of that statement is glossed over a bit. If you have a pedal that uses clipper diodes (like nearly all OD pedals) then the statement is probably/possibly true. This is because the clipping diodes come into play with "output swings" of the opamp of less than a volt (or two if using LEDs or series diodes).
Consider though a pedal that isn�t using clippers. If the output of the opamp swings near (or to) the supply rail as part of the normal operation of the pedal, then the way the opamp handles this �opamp clipping� is critical to the tone.
In my experience, the JRC4558 actually does sound pretty good under these conditions...
If the opamp clipping is �spikey� then the sound will be harsher. If the opamp is �gentle� then the sound will be smoother.
This is NOT a plug for my pedal, but I did take this into account when I was designing my HZD pedal� if you drive the crap out of it, it will distort for sure (any amplifier will). So for a pedal that is all about tone, the opamp better sound nice when it does distort. For that reason I chose the JRC4558D. It does actually sound nicer when driven to these extremes (more like the smooth clipping example). I determined that it sounded better by trying a few others that didn�t� If you did an A-B test on a HZD pedal driven to distortion with different opamps you would have noticeable differences�
http://members.shaw.ca/lauriepedals/HZDinstructions.pdf
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Goran
Double Platinum Member
Sweden
2203 Posts |
Posted - 11/01/2009 : 17:38:56
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Thanks Laurie, on one of my modded TS I have a three way 2 pole (on/off/on) to let assymetrical 1N4148, symetricak LEDs and off no diodes at all. And this setting works very nice with a couple of opamps, right now some Burr Brown.
I like the distortion/crunch better witout clipping diodes morre that any od the diodes. |
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 11/01/2009 : 17:57:30
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quote:
Originally posted by Goran
I like the distortion/crunch better witout clipping diodes morre that any od the diodes.
If I recall, you are using tube amps? If that is true, the crunch will be coming from the pedal acting as a pre-amp and driving the amp input harder. Without the diodes, the pedal can swing to the supply rails (maybe 7V peak-to-peak) and then into opamp clipping... So the sound is coming from a driven preamp in the amp plus (maybe) opamp clipping.
If you use the diodes instead, the OD sound is coming only from the clipping diodes.
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Goran
Double Platinum Member
Sweden
2203 Posts |
Posted - 11/02/2009 : 09:27:01
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quote:
Originally posted by Laurie
quote:
Originally posted by Goran
I like the distortion/crunch better witout clipping diodes morre that any od the diodes.
If I recall, you are using tube amps? If that is true, the crunch will be coming from the pedal acting as a pre-amp and driving the amp input harder. Without the diodes, the pedal can swing to the supply rails (maybe 7V peak-to-peak) and then into opamp clipping... So the sound is coming from a driven preamp in the amp plus (maybe) opamp clipping.
If you use the diodes instead, the OD sound is coming only from the clipping diodes.
Sure I use tube amps, but the sound I get when using a tubescreamer without clipping diodes is not purely there because of the pedal pushing the tubes into distortion.
If I set the volume on the pedal the same as bypassed there is a hint of distortion that I like a lot, if it�s not the op-amp I don�t know what it is�..
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 11/02/2009 : 16:25:11
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| Hmmm... might be. I'd have to take a closer look at the gain structure of the TS. |
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Goran
Double Platinum Member
Sweden
2203 Posts |
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Laurie
Double Platinum Member
Canada
4854 Posts |
Posted - 11/03/2009 : 14:58:16
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Just checked out the TS schematic. If the diodes are removed completely, the opamp that the diodes are connected to has more than enough gain to drive into opamp clipping (my quick calculation gives a gain of over 100 with the drive maxed).
So, the distortion you hear is definitely from the opamp (as per my original post in this thread), so changing opamps will give different flavours... supporting my theory 
If you have the opportunity, I'd be very interested if you could try a JRC4558D and let us know how it sounds...
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Goran
Double Platinum Member
Sweden
2203 Posts |
Posted - 11/03/2009 : 15:15:41
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quote:
Originally posted by Laurie
Just checked out the TS schematic. If the diodes are removed completely, the opamp that the diodes are connected to has more than enough gain to drive into opamp clipping (my quick calculation gives a gain of over 100 with the drive maxed).
So, the distortion you hear is definitely from the opamp (as per my original post in this thread), so changing opamps will give different flavours... supporting my theory
If you have the opportunity, I'd be very interested if you could try a JRC4558D and let us know how it sounds...
I have tried the JRC4558D (it�s standard in TS-7) and I didn�t like it as much as others when getting distortion from only the op-amp.
I have 15-20 different op-amps that work in tubescreamers, it�s hard to say that some is better than another. In a way most sound good but different. Combined with different diodes there are a lot of possibilities�. And what you like one day don�t sound as good the next day�
LM833 is one I like for the clarity but when driven hard it sounds a bit harsh.
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Pontifications about opamps
