Crackle NOT Okay!

8 01 2009

Recently, the Four Banger became a bit of a small hit at The Gear Page. I’ve gotten a good number of orders and I shall be quite busy for the next few weeks. Thanks!
But…. some people seemed to take issue with my inclusion of the circuit inspired by the ZVex Super Hard-On (the section labeled “S” in the Four Banger). Despite the work I put into designing the Four Banger from scratch, including that simple cheap little thing makes me an unoriginal thief in the eyes of certain people.

Since I’m not interested in having to defend myself against being called a “cloner”, I decided to throw out the SHO entirely and design something new to put in there. It took me about three days of experimenting, but finally I came up with what I desired: a circuit that captures the same sound and response as the SHO without that awful crackling noisy gain control!

Behold!
Here it is, the thing so many DIYers have tried to do, but none have ever accomplished, the Crackle-Free SHO!

Now, I admit, it’s not exactly like the SHO, but your ears will have a very hard time telling the difference! When you look at the gain control, you see that the input impedance of the circuit goes down as gain goes up. This is a key ingredient to the SHO-style high gain sound. The input impedance of the original SHO changes like this too, because of the relationship of the feedback resistance vs. gain. Freestompboxes member “estragon” was kind enough to give an excellent explanation about the change in impedance which I shall quote here:

…the input impedance of the SHO is not 5 Mohm as the popular belief dictates. No, it varies big time with the bias pot. At min gain, the end of the upper 10Meg resistor is tied to an inverted copy of the input voltage, not ground, so overall input impedance actually becomes 3.3Mohm when you do the proper math to find out the equivalent thevenin resistor (this has been verified in the sim as well). As the bias pot is reduced and gain increases, something very convenient happens: the input impedance starts going down, down to 140 kohms at max gain. This certainly helps taming the high frequency resonance of any guitar pickup (around 2-5kHz), thus taming highs and helping to the musicality of the clipping. Again, this is because the upper 10Meg resistor is effectively tied to a voltage source that is an amplified version of the input voltage, and not a simple ground point.

So, I have managed to be able to control both the gain and the input impedance in a way which is analogous to the action of the SHO, but with a new method that doesn’t make noise!

What causes the noise in an original Super Hard-On’s gain control, you may ask? It’s not actually the control itself that’s making the noise – attempts at using top quality low-noise controls have been fruitless – it’s the transistor that makes the noise. What happens is that as the gain control is adjusted, the transistor’s bias changes; and because of this change, it switches on and off momentarily while it stabilizes. The two resistors which connect to the Gate (10M in the SHO, 2M2 in my version) allow the circuit to automatically bias to the center of the voltage swing “window” which it has available. But it doesn’t do this automatically, it takes a moment for it to happen; and in the meantime, the transistor is cutting out and crackling. It’s very interesting to hear if you turn the gain control of an SHO very quickly from minimum to maximum gain; the sound will cut out completely for a second and slowly fade back in… very odd behavior, and hardly something I would consider desirable!

So, now we don’t have to put up with that noise any more, because since my version has the Source at a constant level, there is no crackling, popping, fading, etc… because the bias is constant. Instead of using the ratio of Drain to Source resistance to control gain, my circuit uses the ratio of Gate series resistance to Drain/Gate feedback resistance to control gain – just like how you see in an Inverting Op Amp circuit. It’s just as simple and elegant as Vex’s design… except it’s how he should have done it in the first place so he wouldn’t have had to waste so much time and effort convincing people that a noisy junky-sounding gain control is “Okay”!

Where can you go with this? There are many possibilities for this kind of circuit. Just look at Vex’s product line; a substantial number of his things are just various numbers of SHOs all together. With my circuit, I urge that you try something creative; even if you’re keeping it simple, you can do fun things like add an ultra-high impedance input buffer which would allow you to scale down the values of the gain stage’s Gate resistors and Gain control, so you don’t need to use the high values to get a high input impedance. You’ll see how useful that is if you ever try to find 5M controls to use for a super hi-Z version…. High values of resistance contribute noise as well, so there are many advantages to using a buffer with it.

When I find the time, I will put up a DIY project with a printed circuit board layout you can use for your own experiments. And I reiterate, this is now the boost which is used in my Four Banger in the spot previously inhabited by the SHO.

-MC

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