Messages

Wonderful etch, cap'n!

I will be the lone voice of dissent here and say that it does make a difference in the sound (not just the volume).

Take a typical Rat circuit, for example. The sound of LED clippers is not the same as silicon diodes. There is less compression (and thus sustain) with LEDs, and the conduction "knee" is more abrupt both off and on. Silicon diodes give a more "distortion" because they are clamping down a lot tighter on the op amp's output. If you correct for volume differences, the difference is sustain and clipping threshold (aka "dynamics") is apparent. Go ahead and give it the old Pepsi challenge. Make two marks near the volume pot where the two sets of diodes have the same output. Switch diode sets and move the volume pot to keep the two volumes the same.

If they don't sound different to you, then this will be first time ever that I heard something that wasn't ragingly obvious to others.    I have the least golden ears of anyone building pedals (ever).

Are you also correcting for gain differences? Are you saying that LEDs would be different from silicon diodes when the ratio of gain:Fv is the same?

You're right that silicon diodes are faster than LEDs, but LEDs are still pretty fast -- they're fast enough to switch an H11F1 to fully on, which is half a millisecond.

Cool build, and a really nice finish. What are the knobs? They look a little like the boss, but without the skirt.

You can work this out on your own by understanding how the switches work.

A rotary has common poles in the center (up to four), and then the ring of poles around the outside connect to them in each position of the switch.

3P4T
Center A : 1, 2, 3, and 4
Center b: 5, 6, 7, and 8.
center C: 9, 10, 11, 12

So position 1 would be A1, B5, C9. Position 2 would be A2, B6, C10. Etc.

A toggle has the common poles in the center, and the outside lugs connect to them:

1 4 7
2 5 8

and

2 5 8
3 6 9

The best way to do this? Draw out the switching schematic for standard bypass, and then draw it how it would be hooked up to the 3PDT. Then draw out how it would look in series.

While I'm sure there might be someone out there who has already made a diagram, you'll understand it a lot better if you do it yourself. Or maybe there isn't a diagram and you'll be able to contribute one.

Also the P1, not sure if PPP has them. It's shaped like a 125B but with a volume more like a 1590BB.

I'd suggest a 3-pole rotary for what you want. An on-on-on toggle will not work the way you're hoping.

Speaking of those switches, are they able to be board mounted along side pots?

They're a little taller than 16mm pots with covers, but only a millimeter or two. In this case it was a few extra millimeters because I had the leads bent over and sticking up a bit.

I'd want to dry fit them with full-height electrolytics before I try to get something using them in a 1590B.

Thanks for the kind words about the painting. It's always good to know that any given piece of art strikes a chord with someone.

John: 'Contemplatively' is the real word.

Your IC is fine. It's just made by a different manufacturer, and almost any dual op amp will work in this circuit in any case.

1. Which 10K pot? If it's the volume pot, no it's not supposed to act that way, and you need to check that you didn't mix up two of the wires.

2. Some settings on the trimpot are going to be noisy, but squeals are not normal. What you've described sounds like positive feedback, where the output of the signal is fed back to the input. Check all your off-board wiring.

You should post voltages and pictures.

Well heck I guess a flippant answer is in order...In my 20 years of playing professionally I always tried to make life as simple as possible, playing in a power trio and playing keyboards and singing, the last thing I wanted to do was try to match volume in a distortion pedal using a knob on the floor... Actually I didn't use distortion pedals in the 60s and 70s, I had a batterey powered opamp in my guitar that would blow out the front end of any amp, along with all the pedals and such, I was using... Maybe I'm just odd?

So I want to understand this. The type of diode sets the headroom of the IC and the IC does all the clipping? That is not how I think it is...Or the diodes have an inherent clipping profile which includes their rise time and forward voltage which is controlling the gain of the opamp as well as supplying the shape of the clipping?

So what you are saying is that if I lower the voltage AFTER the clipping occurs with the LEDs and lower the overall gain of the opamp it will be the same as using silicon?

Please help me to understand this as I have not gotten any answer that makes sense to me... Basically all I have heard is you have to live with it. I'll go with that if that is how it is...

Thank you guys for your replies I really do appreciate it and am trying to learn...

Daniel Shattuck

Diodes have a property called forward voltage: when you pass voltage through them, there's that much signal loss. When you pass voltage across them (your guitar signal in this case), they will clip the signal to no bigger than the forward voltage. Things do get slightly complicated when they're in the feedback loop of the diode, but this is still *generally* true.

Let's say you have a completely clean sine wave (you never do with guitars, but for example). Your guitar signal is perhaps a volt. Now you amplify it so that it's 4V peak to peak. That's still less than the power rails on the op amp (4.5V in either direction), so it's still clean.

Now you take a pair of silicon diodes. Their forward voltage is 0.6V, normal for a generic silicon diode. You put them in the feedback loop. Now, when you amplify the signal, it will never get larger than 0.6V peak to peak, because that's the limit set by the diodes. What happens to the rest? It's clipped off at the top. It's soft clipping in an op amp, so there's some weird compression stuff going on otherwise, and there's some clean bleed with a HUGE input signal, but for the most part your signal won't exceed that limit set by the diodes.

Essentially, the diodes are acting as an artificial limit on the headroom of the circuit, sort of as if you'd used an imaginary 1V or 3V battery instead of a 9V. In general a circuit simply can't reproduce a signal that's larger than the supply voltage provided by the power supply, and most of the time you're luck to get 90% of the total voltage (that's total voltage -- positive AND negative swing, so think 4V peak to peak as max on a good day). There actually is an exception to this, but it's not something you can just tack onto any given circuit. Anyway, the more your signal exceeds the headroom of the circuit, the more it's clipped, until you get a complete square wave. The clipping is heard as distortion.

Think about the difference in size of a signal that's 4V compared with one that's 0.6V -- it's almost 1/8 the size! That translates directly to a corresponding loss in volume. If you use two silicon diodes in series on either side, you'll get a little over a volt, still MUCH less volume than the diode-less signal size we were discussing.

LEDs have a much higher forward voltage, about 1.7-2V. That's three times the size of most silicon diodes, and still higher than your two diodes in series on each side. Again, that voltage difference corresponds directly with (a) less clipping at the gain levels and (b) more volume at similar levels of clipping.

In other words, if you want distortion, you are going to sacrifice volume. If you want volume, you are going to sacrifice distortion.

But let's go back to your original question. You want to "maintain the same clipping [character]" -- why are you switching the clipping diodes if you already have the clipping character you want? What exactly are you trying to accomplish? The reason for the diodes in the first place is to create the clipping. Are you switching between less clipping and more clipping?

You also say that you're trying to "keep things as simple as possible" -- are you switching the diodes in mid-set? You could mark your pedal for the two settings you use and you could set the volume at the same time you switch the diodes without having to fish around for the new setting.

But this goes back to you saying that you want to keep the same clipping sound ... the different sound of the clipping is directly related to the forward voltage and the size of the signal (gain) before it's clipped. LEDs clipping a signal three times their forward voltage will clip a signal pretty much exactly the same as a silicon diode clipping a signal three times their size, but the LEDs will be louder. The only reason to switch the diodes is (a) you need more volume at lower distortion levels (LEDs) or (b) you need more distortion at lower volume levels (silicon). Sorry if it sounds like I'm repeating some stuff, but I'm just trying to hammer home the fact that the diodes are only there to create a certain amount of distortion at a certain gain level.

Like I said, it's probably possible to change the output volume at the same time you change the diodes, but I'm not sure exactly why you'd do that -- and it's not going to be perfect in any case, because the volume change won't be identical at every gain and volume setting. You would still end up needing to change the volume most likely.

doze diodes.

ffffffffine.

Dude, that's so beautiful all around.

Or you can use this thing I've described here:
http://www.diystompboxes.com/smfforum/index.php?topic=105261.0

9V --_-------_
        |         |C (Q1)
        R1--->]B
        |         |E >--C1
        R2
        |D (Q2)
in--> ]G
        |S
        R3>Ground

Q1: NPN BJT. Q2: N-channel FET


The gain on that stage is not quite as high as the SSRP, but it's a ton of DC gain, it's not overly distorted, and it has very low current draw.

EDIT: The omega can be adjusted to have clean output as well. http://www.runoffgroove.com/omega.html

ANOTHER EDIT: I realized, though, that you were hoping not to redesign. Sorry. There's not a whole lot you can do within the topology, assuming the FET's bias is settling properly.

It's basically a Mu Amp -- Make it an SSRP instead. http://www.geofex.com/Article_Folders/modmuamp/modmuamp.htm

The SSRP is the version I used in the Clipper Ship. I think you know how much gain that had. But among other things, the SSRP is also less noisy and has fewer loading issues (I stuck a tone control AND volume control on it and it still had over 20dB.

Measure the voltage at the input; see if there's a DC offset (i.e., it should read 0 -- even while switching). Do the same on the output. Find out where the switch pop is originating. If it's not at either, try removing the LED.

There could be something else going on that's got something connected to power that shouldn't be. Is the pot insulated from the circuit? Is the transistor securely attached (i.e., is it soldered or socketted)?

You could put a resistor in series with the diodes ... and consequently raise the forward voltage of the diodes to that of LEDs and make it pointless to use different diodes.

Or you can put a limiting resistor in to change the gain by a set amount when you switch the diodes ... and either lose the cleaner settings on the diode side or some volume/gain on the LED side, making it pointless to have the different diodes.

You could try to come up with some sort of calculation to switch the resistor on the output so that the volume of the whole pedal is lower when the LEDs are in the circuit ... but then you lose some maximum output from the LED side, which is sort of the whole point of using LEDs (so you can get more output from the pedal with less distortion).

You might see where I'm going with this. The whole point of the diode differences is to change the headroom of the circuit, so anything you do to adjust the gain is going to negate the reason you have a diode switch. And I might sound flippant, but this question comes up a lot, and I'm not sure I really understand the reason people don't just use the obvious solution: If you want the volume to be the same, adjust the volume knob. That's what it's there for.

cray cray

Annnnd I read this in the proper accent. Thanks for the laugh!