MuHaHa

MuHaHa_Web_Logo
 

MuHaHa is my first guitar effects pedal design. It creates some wicked distortion suitable for Heavy Metal. It's name is supposed to sound like an evil villian laugh on a cheesy cartoon or movie - "Mu ha ha ha ha".  That's because it has Mu Follower amplifier stages and creates some "evil" distortion.

Originally, I was working on a design for a headphone amplifier. While doing some research, I ran across an article(which I can't see to find any more) that describes using jfets in a Mu Follower configuration. I thought this would be great for a headphone amplifier, because each amplifier stage would be midpoint biased regardless of the batteries voltage, and it would be a good way to get plenty of voltage gain from jfets.

Here's the schematic for my first prototype:

 MuHaHa_Prototype_2_schematic

Up to the last stage, this circuit looks a lot like the Booster 2.5 circuit at  http://www.diystompboxes.com/wiki/index.php?title=Featuredpedals. I DID NOT get the idea for using Mu Follower stages from the Booster 2.5 circuit, but I did copy the tone control and RF input filter from the circuit. I used MPF102's for the second stage simply because when I was experimenting with different components, the circuit seemed to sound a little better with MPF102's in the second stage. There is a lot of variation among jfets, even of the same type from the same factory, so a different pair of MPF102's might not have sounded better than another pair of J201's.

Originally, I used a LM386 for the output stage. I basically liked the way it sounded, though as a headphone amplifier, it would sound much better with some speaker cabinet simulation. It seemed to work much better as just a distortion pedal. I decided to build it to work as both.

When building the circuit on my breadboard, I was using a 12V ATX power supply from an old dead computer. When I tried the circuit with a 9V battery, it didn't sound quite as good. I couldn't use 9V batteries for 12V and I didn't want to use a bunch of 1.5V batteries. I decided I needed to use two 9V batteries for 18V. The LM386 version that I was using isn't rated for 18V, so I needed a different output stage. 

 I decided to try a Mu Follower stage with 2n7000 MOSFETs. In retrospect, a simple source follower stage would have been fine. While low batteries would throw off the bias of such a stage, it would probably not be that big a deal. In fact, a simple source follower without any voltage gain stages preceeding it makes a great  sounding clean tone headphone amplifier. 

 I don't know if my MOSFET stage is original or not, but I can say with certainty that I came up with it independantly. So far I can't find any examples of MOSFETs in a Mu Follower topology. I'm sure someone's done it before though. It's hard to come up with anything electronic that hasn't been tried before. Maybe people with more knowledge and experiance than I know better than to use MOSFETs in a Mu Follower stage. Whatever the case, I like the way it sounds. 

In fact, it's the last stage that really creates the sound of the MuHaHa circuit. The distortion from this stage seems to me to be more edgy than with the JFET stages. The next version of this circuit, which I'm working on right now, uses two of these stages with no preceding JFET stages.

While I like the sound of this circuit, and even built it into a pedal, there are some problems with it that will be addressed in the next version. It really has too much gain. To be usable, either the drive pot or the volume pot needs to be turned down almost all the way.  Also, there really needs to be a volume pot after the last stage.The volume pot is really more of a second drive pot. Also, this really doesn't work well at all as a headphone amplifier circuit. This turned into something completly different than what I intended to create. I'm glad it did.

Dispite it's problems, it does have one advantage over the second version. You can get a variety of sounds from this circuit by experimenting with different volume and drive settings. With a low volume setting, you get the sound of distortion from the second stage filtered through the tone control. With a low drive setting and a high volume setting, you get a hard, edgy distortion from the MOSFET stage. 

Comments