Viva Analog tube emulation

[PBE6]

Has anyone tried building the simple circuit at the end of this page?

http://www.lynx.bc.ca/~jc/transferCurvature-TubeSimulation.html

I tried it last night, and after some adjustments I got it working. I'm not sure I hear a big difference between this and any other boost/overdrive, but then again I only listened to it through headphones coming off my Roland Cube 30 amp. The frequency spectrum shows harmonics being generated in a descending linear pattern, dropping by about 4dB for the second harmonic (first overtone) and another 4dB for the third. Does that sound like a 3/2 power law? Even with JC's clear description, this tube emulation stuff goes over my head. =\

[samhay]

You may have seen it, but there's discussion of this circuit going on over at the other forum at the moment.
I have played with it on the breadboard and you can generate some lovely 2nd and 3rd order harmonic content. It sounds nice until overdriven, but I'm not sure if it sounds very different to a Fetzer, etc.

[PBE6]

I have not! I will check for it, thanks.

[PBE6]

Sorry to be a pain, but I tried searching for that discussion on the other forum using "JC", "triode", "tube", "emulation" and "viva analog" in various combinations but I'm coming up blank. Can someone please post a link?

[akc1973]

Here's the link http://www. .org/viewtopic.php?f=28&t=24998

Replace the smileys with you-know-what!

akc1973

[PBE6]

Thanks!

[Hatredman]

Sorry to ask, but I'm puzzled: why the URL obfuscation?

[slacker]

Because for reasons lost in the midsts of time the name of that forum is censored so you can't post links to it. Same way certain rude words are censored.

[teemuk]

The particular forum has strong emphasis on reverse engineering commercial effect units to find out what makes them "tick". So you don't have to buy them when you can build them yourself. Kinda like what we do in here as well.

You know, DIY....

Problem is, they did not exlude effects built by some of the forumites in here.  


I guess the right term for it is "double standard" or something... But it's this forum's policy so why care too much. We all know what forum the "you-know-what-forum" is and at least we aren't censored from visiting it.

[Hatredman]

Hummm, didn't know that. Thanks!

I'm sorta new here, have lots of things to learn.

[aron]

Teemuk,

> The particular forum has strong emphasis on reverse engineering commercial effect units to find out what makes them "tick". So you don't have to buy them when you can build them yourself. Kinda like what we do in here as well.

That isn't the reason. It's all in the archives so you can go look for yourself.

[samhay]

As a gentle nudge to get this thread back on track, I thought I might post the circuit the OP mentioned (left). It can generate quite pleasant low order harmonics, but doesn't clip any better than other FET designs (which some people seem to think sound ok).

The FSB thread has a design of a 2-FET Marshall-inspired overdrive running at 30V and the discussion has mostly descended into a discussion of whether the op-amp biasing causes the FET to behave differently to typical setup with a source resistor that is bypassed by a large cap. In either case, the op-amp biasing does allow you to set an operating point very nicely, which is why I thought I would take that one step further with the circuit on the right, which is a work in progress.

[PBE6]

Very cool samhay! Can't wait to hear what you think of the results.

Since you've presented such a nice design, I thought I'd take the opportunity to ask some dumb questions about it:

1. In the FSB thread, one person made the point that the power supply needs to use two batteries (one for positive swings and one for negative swings). Someone else made the point that the LM358 handily eliminates this need because of its internal self biasing. If I'm using a standard TL072 opamp to offset the source voltage, can I simply:
- create Vref1 = 5 V using a voltage regulator
- create Vref2 = Vcc/2 using a voltage divider
-  bias the opamp by connecting the top of the trim pot to Vref1
- bias the input by connecting the 1 M resistor to Vref2?

(I think in this case Vref1 would have to be slightly bigger than Vcc/2 in order to allow for proper adjustment of the opamp bias up to Vcc/2 + Vp/2..does this sound right?)

2. Is a voltage regulator a better idea than a straight voltage divider? I'm thinking that we don't want the bias point (set at Vp/2) to change even if Vcc does (but I think the input bias Vref2 should in order to maintain headroom, right?). Does this matter?

3. How does the second opamp behave in this circuit? The original used a variable drain resistor to set the output level. The second opamp appears to be set up as a standard non-inverting gain stage that compares the DC voltage on the + input (is this set at Vcc/2 or Vp/2 here?) to the voltage created by the voltage divider consisting of the Level pot and the JFET circuit, and amplifying appropriately. Since this could be accomplished more simply with a single pot as in the original design, I assume the second opamp must be set up to do tube emulation as well. Is that correct? How does this interact with the JFET circuit below it? Could the same effect be created by cascading two of the original circuits as well?

4. Does the filtering cap on the trim pot do anything if the pedal is battery powered? I've always read that battery power is as clean as can be, but is that only in ideal situations?

5. Is it necessary that the output load (basically equal to the output impedance here?) be 100k? Is that to mimic a normal guitar output? I usually try to set the output impedance quite low using a 10k pot. Is this not always a good idea due to impact on battery life - or something else?

[samhay]

PBE6 - There are a lot less dumb questions that most people think.

Edit - what do I think of the result? It sounds ok - more character than an op-amp, but it only really colours the sound if it starts clipping, which is not great. I will keep tweaking for a while, but I am not sure if I need another preamp/gentle booster.

1. The op-amp that sets the source voltage (bottom one) needs to get pretty close to ground. Mine is set at about 1V, which is roughly half Vp. You can do this with a rail-to-rail or single supply (like the 358) op-amp, but not a TL072, 4558, etc. You could use a bipolar supply to get around this problem, which is where the 2 batteries comes in I guess. I think you will run out of headroom if you try to split the 9V supply and use a TL072 (and make it a lot more complicated).

2. I have used one or two 1N4148 diodes in series as a Zener-style voltage reference to keep the bias point constant, but this uses more current than a 100k trimmer and the set point is not super critical anyway. A voltage regulator might work, but most of the common ones can't get that close to ground.

3. Disclaimer - I am still not 100% certain I understand how the second op-amp is working.It is essentially acting as a fancy drain resistor - as you increase the level resistance, you increase the gain - and is very much a work in progress. I tried this as I had a spare op-amp, and it lets you keep the output impedance nice and low. As you put more signal across it, the output swings high and eventually you hit the positive rail, so for maximum headroom, the trimmer is set to about half a volt higher than the bottom op-amp's bias point. Dimitri Danyuk's 'Triode Emulator' uses this approach with a fancy servo to set the +input voltage. This might be worth a try at the expense of about double the parts count.

4. Probably not, but it depends a little on the battery and the capacitor used - a 100n ceramic cap in place of the 10u electrolytics wouldn't hurt.

5. The output impedance is about 100R (set by R4). The 100k R5 is just a pull down resistor and you can make it smaller, but if you do and/or if want to drive really low input devices, then I would suggest you increase the value of C6 by a fair bit.

[teemuk]

3. How does the second opamp behave in this circuit?

It's about the most generic current to voltage converter. Vout is Iin times the feedback resistance.

Practically the opamp directly amplifies drain current in oppose to "usual" configuration where drain current first generates voltage drop across the drain resistor, which is then fed to an opamp voltage gain stage that generates voltage drop across its input resistor and then amplifies this voltage.

Takes voltage and all kinds of source impedances out of the equation basically...

[samhay]

Exactly. Thanks Teemuk.
Where I am struggling is that I don't quite understand what is happening when the output gets near the positive rail (I not proportional to v anymore), as it seems to cause the source voltage to drop.

[PBE6]

Great! Thanks for the quick reply. So it sounds like the TL072 is out, but I think I have a few NE5532's hanging around which I believe are rail-to-rail type opamps.

Just so I understand, using the NE5532 should allow the source offset voltage to get quite close to the negative rail, which would allow me to use the original design as-is without any additional biasing, correct?

I'm trying to build a bass preamp for a friend that he can always leave on to add a bit of character to the sound. I'm going to try and use this circuit to add some harmonics to the clean signal without pushing it into an overdriven state (MPF102s seem to have a decently high Vp to keep the signal clean if I'm extrapolating correctly from Runoffgroove's Fetzer document - a preceding voltage divider may be required as well to keep the signal under control). This signal will get sent to a Tubescreamer-style overdrive section with a small pot inline with the clipping diodes to act as a saturation control so he can dial in just a touch of grit. I'm also going to try and implement some pre-grind bass cut and post-grind bass boost controls to limit the fizz in the distortion without losing body in the sound. We'll see what happens!

[samhay]

Afraid not - 5532s aren't rail-to-rail either, so they won't work with the schematic as drawn.
Sounds like a good basis for an effect otherwise.

[PBE6]

Crap! Ok. Maybe I'll try a 9V into a 5V regulator (and maybe buffer that with a TL072) and then step down the output using a voltage divider with a trim pot. (I know I should just order some rail-to-rail opamps, but shipping takes weeks and I've already got these parts and I have the building bug RIGHT NOW!! lol).

Offhand anyone know how to calculate the maximum source current? I figure this voltage divider should have properly sized resistors so the current through it meets R.G.'s 90%/10% rule of thumb.

[slacker]

You might be able to use opamps that can't get their outputs close enough to ground by adding a small bias voltage to the Fet, by connecting the bottom of R1 to say a 1 Volt reference Voltage, from the FET's point of view that is now ground. From Sam's example your opamp would then only need to go down to 2 Volts which a "normal" opamp can probably manage.