Distortion Effects Philosophy and Design

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Post by candidate »

Static is built up on the cable after rubbing it on the carpet on a dry day? I just need to discharge it on the door knob before plugging it into the pedal?
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Post by Nocentelli »

The outer casing of the input jack is surely a better ground point?
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Post by deltafred »

candidate wrote:Static is built up on the cable after rubbing it on the carpet on a dry day? I just need to discharge it on the door knob before plugging it into the pedal?
[smilie=poke.gif]
A pedal should be robust enough to operate in the environment it was meant for (home, studio or stage) without fear of it getting zapped by static. You should be able to use it without having to remember to go through a static discharge procedure first. (IMO, IME YMMV of course)

Protection of inputs from static is nothing new and well understood by EEs, but there again not all pedal "designers" are EEs.
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Post by mictester »

MoonWatcher wrote:
mictester wrote:...has FET inputs. These inputs are susceptible to damage (and destruction) from high voltage spikes. These inputs are exposed to the outside world - this is bad design.
Fair enough. And, nothing new.

But I still find it odd you use the OCD as an example, and not any of the original Marshall pedals that used TL072's, or the numerous DOD pedals that used TL062's with no BJT buffer, or. . . on and on. The number of mass produced pedals using these chips without a bipolar transistor buffer are numerous. By this point, they are staggering. Joyo makes an OCD clone - I've not heard of rampant reports of chip failure. Danelectro made an OCD clone - same thing. At any given price point, users would say something to the effect that "it was fine until it died..." and yet that is not what we hear. If you have proof to the contrary, I'd like to see it.

As long as you are willing to admit that in spite of it being bad design, the percentages of actual chip failure from transient voltage spikes with a FET-input op amp are small enough to not even be something that is more than casually discussed by anyone other than an engineer or manufacturer, or chaps like us, that is fine. You are harping this as bad design - the average user is not.

I want to be clear - I'm not discounting what you say regarding good design principles. It's the mixing of philosophy with personal opinions you think the OCD is a crap design. That's fine. Just say as much. Include the fact that its FET-input chip lacks the buffers. But you should also include the numerous other pedals that also fit this criteria. I understand that you probably want to outright say that Mike Fuller is a buffoon, and I probably wouldn't disagree with you. But I would hope that you could keep your philosophy to . . . philosophy, and not put some jabs in where it pleases you. It waters down the content of what you are saying, at least to me.

I don't think you honestly realize that when you single out specific manufacturers that you cause some of an otherwise captive audience to turn away. Maybe you don't care, but maybe you might.

Again, I'm not saying that Fuller or some of these other clowns don't deserve some kind of response for their misleading marketing, or the half-cocked things that they say. But I come to FSB hoping that there can be educational and positive content in spite of the portions that will never be above reproach.
I'll say this once again:

The OCD was simply chosen as an example not because of its provenance, not because it's insanely expensive for what it is, and not because of Mike Fuller - I chose it as just one example of bad design because I had a dead one on the floor next to me when I was writing the comments. No other reason.

That's it!

Incidentally, if you're going to persist in trying to deeply analyse my choice of devices to criticise or discuss, you'll be wasting your time. Particularly as I've got another Marshall Shredmaster here with a static-blown TL072 in it......
Last edited by mictester on 07 Oct 2013, 22:26, edited 1 time in total.
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Post by mictester »

FiveseveN wrote:I would also like to note that buffers are not the only way to protect an input from static discharge.
I completely agree. However, transistor buffers confer other advantages too, and are really cheap to add to a basic circuit. It must be remembered that all they do is impedance conversion (if they're correctly designed), which could also be considered to be "current gain". They tend to have a voltage gain of slightly less than unity. They do not colour the sound in any way, and even the phase remains the same!

An interesting experiment is to build a conventional silicon Fuzz face (or germanium if you really must) and add buffers. The guitar input "sees" a high impedance (which is what the guitar works best with) and the input to the Fuzz Face is at low impedance. We've eliminated that horrible mismatch..... Now, with the addition of a single pot, you can reintroduce that mismatch in a controllable manner. You'll be surprised. There are some really interesting and unusual sounds to be had there. It's also worth adding an emitter follower buffer after the Volume control, so you can drive long cables without losses! It's also worth looking up "Phatt's" passive pre-tone circuit elsewhere on this forum, and putting the filter part of it between the buffer and the input of the conventional Fuzz Face. Things can get very interesting then!

Other means of static protection include zeners, VDRs, GDTs and even spark gaps! Sadly, none of them are reliably fast enough to protect the front end of a FET op-amp! The best protection is to not expose the vulnerable device to the Cruel Outside World™!
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Post by commathe »

Question:

Is it not possible to keep something buffered on the "hot" side to protect an op-amp or whatever, but keep it true bypass? As in the buffer is only being used when the effect is engaged?

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Post by Nocentelli »

Yes, I don't think mictester is advocating buffered bypass per se, merely putting a buffer between the "send" lug of the footswitch and the effect input.
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Post by mictester »

I also tend to prefer buffered bypass - it helps eliminate the losses incurred when you try to connect a high impedance device down a long cable. The biggest single improvement to most guitar rigs is to give the guitar a high impedance load, and drive the cables at low impedance. A simple "active lead" design is now favoured by many professionals - I use an FET / bipolar hybrid cascode circuit for this. The little amplifier circuit has very low noise, very high input impedance and very low output impedance, and is remotely powered up the cable from a diecast-encased power supply at the other end. there's no tonal change from the guitar output whatsoever, except that the high end isn't lost in the cable any more, so it sounds "brighter".
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Post by commathe »

mictester wrote:I also tend to prefer buffered bypass - it helps eliminate the losses incurred when you try to connect a high impedance device down a long cable. The biggest single improvement to most guitar rigs is to give the guitar a high impedance load, and drive the cables at low impedance. A simple "active lead" design is now favoured by many professionals - I use an FET / bipolar hybrid cascode circuit for this. The little amplifier circuit has very low noise, very high input impedance and very low output impedance, and is remotely powered up the cable from a diecast-encased power supply at the other end. there's no tonal change from the guitar output whatsoever, except that the high end isn't lost in the cable any more, so it sounds "brighter".
Do you have any sources where I can read more about such a design? Especially if it has some technical analysis of exactly what's going on in the circuit? This is the first I've heard about using an audio-cable to deliver power. Are you saying that the power supply is actually at the end of your chain and sends power through all of the pedals? This doesn't make sense to me since input and output caps would prevent this so I must have misunderstood you. I'm aware of the advantages of buffers and the impedance conversion they do, but I find it difficult to understand what makes one buffer better than another: why some are more transparent, why some increase treble response etc.

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Post by mictester »

The enhanced treble response is because the guitar lead shows capacitance. This capacitance appears in parallel with the high impedance of the guitar, so will form a lowpass filter. If you reduce the impedance of the guitar, the turnover point of the lowpass filter rises up into the ultrasonic region, so has no effect on sounds you can hear.

My "active lead" is (effectively) a tiny FET / bipolar buffer pre-amplifier built inside a jack plug, then an ordinary guitar lead (I actually use "microphone" cable because it's quieter). At the other end, the lead has an XLR plug that goes into a "power supply" box. The pre-amplifier is remotely powered up the lead. This has the advantage that the system can be used with any kind of guitar. I make them with right-angled or straight plugs to suit various instruments. The outputs of the "power supply" box are either jack or XLR according to the subsequent rig. The rest of the system "sees" a guitar signal at ordinary voltages, but at low impedance.

I also equip my pedalboards with an output buffer system to drive the long cables that typically go from the front of the stage to the backline. All my pedals have high input impedance and low output impedance, so they can be used in any situation and any combination. I do sometimes make special low noise pedalboards where everything is at low, matched impedances, but these are less flexible (and more expensive because they're unusual).
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Post by Pruttelherrie »

commathe wrote:Do you have any sources where I can read more about such a design? Especially if it has some technical analysis of exactly what's going on in the circuit? This is the first I've heard about using an audio-cable to deliver power.
Check out Tillman's FET cable: http://www.till.com/articles/PreampCable/

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Post by KB »

Hi MIctester

Sorry a bit of a hijack on the thread
mictester wrote:My "active lead" is (effectively) a tiny FET / bipolar buffer pre-amplifier built inside a jack plug, then an ordinary guitar lead (I actually use "microphone" cable because it's quieter). At the other end, the lead has an XLR plug that goes into a "power supply" box. The pre-amplifier is remotely powered up the lead. This has the advantage that the system can be used with any kind of guitar. I make them with right-angled or straight plugs to suit various instruments. The outputs of the "power supply" box are either jack or XLR according to the subsequent rig. The rest of the system "sees" a guitar signal at ordinary voltages, but at low impedance.
I would also like to know more about the active lead and the output buffer you use - if your willing to share. This sounds like something that could be very useful built into a box with a loop as a master system. [buffer - loop - output buffer]. Hmmm maybe even a tuner out? sorry just thinking...

Sorry for the hijack I think we need to start another thread. Back to distortion effect Philosophy and Design... didn't Marshall use the TL072 opamp in the front end of some of their valvstate amps.... I seem to remember something about some of those amps having problems - chips where damaged by to a high input voltage or static or something..... sorry to long ago and too many pivo's....

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Post by commathe »

mictester wrote:The enhanced treble response is because the guitar lead shows capacitance. This capacitance appears in parallel with the high impedance of the guitar, so will form a lowpass filter. If you reduce the impedance of the guitar, the turnover point of the lowpass filter rises up into the ultrasonic region, so has no effect on sounds you can hear.
Finally I understand why I have turning my volume knob down to about 1 and then using a booster!

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Post by FiveseveN »

That's... not how it works. In fact it's the worst thing you can do, SNR-wise.
See http://buildyourguitar.com/resources/lemme/ for more information about how pickups work and loading effects.
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Post by commathe »

FiveseveN wrote:That's... not how it works. In fact it's the worst thing you can do, SNR-wise.
See http://buildyourguitar.com/resources/lemme/ for more information about how pickups work and loading effects.
Awesome article, thanks! I haven't been having many problems with my signal-to-noise ratio but when I'm doing the volume down thing there are no effects other than a booster.

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Post by Chappy »

mictester wrote:To add to the above:

The Tubescreamer was simply designed as a way of trying to emulate the behaviour of a valve amplifier at the pre-amplifier stage. The selection of components wasn't anything to do with achieving the best "sound" - it was purely done to cut costs! The 4558 was chosen because it was relatively hiss-free in the design and recovered from overload more gracefully than other op-amps. This recovery phenomenon is all that accounts for the different "sounds" of various op-amp devices in the circuit.

One myth that can be dispelled right now: any 4558, of whatever vintage or from whichever manufacturer, will sound exactly the same in any given circuit. There is no point whatsoever in spending insane amounts of your hard-earned cash on "NOS" op-amps - it is simply a waste of money.
You are 100% correct on this one. A few years back I had two identical TS-9s. One had the 4558 chip and the other had the 2043 chip. The pedals sounded identical to one another with one exception. When the gain knobs were maxed, the pedal with the 4558 chip still sounded nice. The pedal with the 2043 chip sounded a bit harsh. I replaced the 2043 with a socket and a 4558 and both pedals then sounded exactly the same. I then proceeded to do the 808 mod by changing out two resistors in the output section. That accomplished absolutely nothing according to my ears.

Mictester: What is your take on all of these pedal mods now available on the auction site? $20 for a new chip and a handful of "higher quality" capacitors. Are we really going to hear a difference between a ceramic cap and a polyester one with low voltage signals and frequencies less than 15Khz? I'm having my doubts on this one.

Chappy

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Post by FiveseveN »

Chappy wrote:Are we really going to hear a difference between a ceramic cap and a polyester one with low voltage signals and frequencies less than 15Khz? I'm having my doubts on this one.
You must have missed the 34-page discussion on that :D.
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Post by juanro »

FiveseveN wrote:You must have missed the 34-page discussion on that :D.
"Discussion" like... two monologues in parallel :blackeye

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Post by Chappy »

FiveseveN wrote:
Chappy wrote:Are we really going to hear a difference between a ceramic cap and a polyester one with low voltage signals and frequencies less than 15Khz? I'm having my doubts on this one.
You must have missed the 34-page discussion on that :D.
Fortunately I did miss that one!

I have an old Fisher 500C tube receiver. Back in the early to mid sixties these were some of the best receivers available. Mine still sounds great, and it is full of ceramic caps. So if ceramic caps were good enough for high end Hi-Fi systems in the 60s why is there such a debate over cap choice in low fidelity guitar pedals? I'm not buying it.

Chappy

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