Mufu 12V

The MuFu 12V 2-4 Watt Amplifier is a simple guitar amplifier that sounds a bit like an older tube amp (I think so anyway). It gets it's name from the Mu Follower stage preamp.

I started off with an expremely simple source follower design using a 12V atx power supply, an IRF510 mosfet, and two 10 ohm power resistors in parallel for 5 ohm. This amplifier design was for testing effects. I basically just wanted a buffer to drive a speaker, so that all the voltage gain would come from the guitar effect. When testing my MuHaHa effect, I wanted to know how much of the sound was produced by the MuHaHa circuit, and how much was produced by the Ruby amp I was using for testing. I figured the best way to do this was to build an amp for testing that had little clipping of it's own.  Testing effects this way gives an incomplete picture of how an effect can color the sound of a guitar, but this amp should give me a good idea.

 

Fixed_Load_12v_IRF510_Source_Follower

 

I decided to modify this design by replacing the power resistors with a constant current source. I figured an easy way to do this was by using a current mirror using a couple more IRF510's.  I used the 5 ohm(paralleled 10ohm power resistors) load from the previous circuit to set the reference current. Doing this would make my class A design a little more efficient and would squeeze out a little more output power.

 

CCS Source Follower

 

 When I plugged in my MuHaHa effect to test it out, it sounded like crap. I thought maybe the problem might be with the MuHaHa effect or maybe it's combination with the test amp. I decided to add a preamp stage to see how the amp would sound with a non-clipping or mildly clipping preamp. I used a single Mu-follower stage (basically, my MuBu Booster Circuit). It still didn't sound right. I switched from using the 4 ohm speaker that I scavanged from a PC soundsystem to a 6 ohm car stereo speaker. This was a big improvement. Although not suitable for modern Metal, it had classic rock like sound. 

 

RTC-AMP-001%20-%2012v%202-4%20Watt%20MuFu%20-%20Prototype%201%20schematic

 

From this, I've learned about damping factor, somthing I should have learned about a long time ago.  Damping factor is the load impedance(in this case speaker impedance) divided by the output impedance of the amplifier. Because the speaker impedance is never constant, the damping factor is obviously not always constant, but using the rated speaker impedance to calculate the damping factor is probably pretty good. I thought it was always best to have the amplifier output impedance match the speaker impedance for maximum transfer of power. However, most solid-state amplifiers have damping factors much greater than one (maybe 100-1000). The damping factor of this amp is less than one with a 4 ohm speaker, and maybe a little over one with a 6 ohm speaker. 

Here's why damping factor is important. When the speaker cone moves, it has momentum.  Moving a cone with momentum to a specific position requires a different amount of force than moving it to that position from rest. If an amplifier doesn't compensate for the momentum of the speaker cone, then the speaker will not accurately  represent the input signal that the amplifier "tells" it to represent. Because the speaker is an inductive device, movement of the speaker cone generates a voltage. The amount of current generated by this voltage depends on the output impedance of the amplifier. The current generated by this voltage will oppose the movement that created it. I think this is called electromagnetic drag. This compensates for the effects of the momentum of the speaker cone. An amplifier with a low output impedance will allow for more of this opposition current than one with a high output impedance. This means that trying to match output impedance to speaker impedance might allow for more transfer of power, it also allows for less damping of unwanted speaker movement due to speaker cone momentum.

That doesn't mean that  a low damping factor is always a bad thing. Some speakers have lighter speaker cones than others. The speaker enclosure and other mechanical factors can provide dampening. Damping factor is less critical for higher frequencies. And sometimes the effects of poor damping are even desired.

Older tube amp designs and simple modern designs have low damping factors. Even tube amps that use feedback to increase the damping factor have damping factors lower than most solid-state amplifiers. I'm beginning to think that the low damping factor of these amplifiers are a big part of what creates the warmth of a tube amplifier. The MuFu amplifier has a similiarly crappy damping factor.

Using MOSFETs for voltage gain in the preamp circuit and current gain in the power amp  circuit also make the amp a bit "tubish". While MOSFETs do NOT clip quite the same as vacuum tubes, they do clip much more gracefully than bipolar junction transistors. Also, using a Mu follower configuration further softens the clipping (at least that's what I've read, and it sounds good to me). I think the same is also true by using a constant current source as a reactive load for the source follower stage, though I haven't really properly researched or tested this (I don't have the right equipment). 

 There are a few things I might do to improve this design. One thing this amplifier lacks is tone control. It would be good to add a tone stack. It might make sense to use  an lm317 as a current source instead of the MOSFET current mirror. This would make the amplifier much more efficient and reduce requirements for the power supply. I might also experiment with using jfet Mu Follower stages for the preamp.

Keep in mind that I don't have a lot of experiance with a wide range of amps. I might be full of crap, but to me this is a good(notice I didn't say great) sounding amplifier for classic rock or blues.  If you build one, let me know what you think.

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