MOSFET Driver after other pedals (impedance question)

[allanshookphoto]

I just built the MOSFET Driver, and it sounds great! Except... when it is after another pedal (and that pedal is on), the treble goes crazy. Sizzling bacon high end. I didn't know my amp could preproduce that much high end! Turn the preceding pedals off (all true bypass), and it behaves just fine. With the MOSTFET Driver first in the chain, its perfect. So, I suppose this is a case of the input impedance of the MOSFET driver not matching the output impedance of the pedals before it? I'm wondering, if I had a buffer before it (a tuner pedal), it would probably do the same thing? Does anyone have any experience with this pedal? Is it just one of those pedals like a Fuzz Face that really needs to be first in line?

 

[Coda]

A buffer would most likely help, and should be on all boards. Buffers = good…

 

[allanshookphoto]

A buffer would most likely help, and should be on all boards. Buffers = good…

Thanks, I am meaning to get a Polytune, and I'll see if the buffer makes a difference to the MD. However, if the output of a buffered bypass pedal is low impedance, and the output of a pedal that is "on" is also low impedance, then isn't my first-in-chain, always-on OD buffering the signal? So, if the fact that a pedal that is switched on before the MD causes it to go treble-crazy, then a buffered bypass tuner should do the same thing. I'm just going over the logic in my head, I've only been at this guitar/amp/pedal building thing for a short time, I'm still learning.

 

[Chuck D. Bones]

Specifically, which pedals cause problems when connected in front of the MOSFET Driver? Are you overdriving the input of the MD with these other pedals? How are the bias trimmer set? I have a pretty good idea what's going on here...

And the $64,000 question: is the MD built per the Build Docs, no part subs or mods?

 

[allanshookphoto]

OK, so I'm resurrecting this old thread. The MOSFET driver still sounds great, however, ONLY if it first in the chain after the guitar. If it is after a buffer, or after an "on" pedal, it is thin and crispy, sounds like crap. Has anyone else found that the MOSFET Driver must be first? I had a Bad Stone Phaser and Sweet Baby OD before it. I like to sound of Bad Stone before dirt. I have always had the Bad Stone before the Sweet Baby OD and Muroidea Distortion, and that works perfectly.

 

[almondcity]

I find there is something goofy with this pedal as well. Where it goes in my chain makes a lot of difference but I can't tell you what it is

 

[lrgaraujo]

Impedance is too big of a word for my small brain, but I do find the value of R3 (10k) kind of small when compared to other buffers. Too small of an input impedance might be responsible for this chain-placement-dependent behavior

 

[jesuscrisp]

Impedance is too big of a word for my small brain, but I do find the value of R3 (10k) kind of small when compared to other buffers. Too small of an input impedance might be responsible for this chain-placement-dependent behavior

You're absolutely right though, 10k input impedance is TINY. try bumping that up to 1M and see if it helps.

 

[Feral Feline]

Since the thread is resurrected, maybe answer that $64,000 question.

 

[jesuscrisp]

Since the thread is resurrected, maybe answer that $64,000 question.

Looking at the schematic it makes me think that building it exactly according to build docs is the issue here.

 

[Feral Feline]

I struggle with input impedance, because it's always about some Z-formula with measuring across poles and blahblahblah-techno-bable crappola.
Never is it explained in layman's terms.

How does one calculate the impedance for the MOSFET Driver? The input resistor of 10k surely doesn't represent the input impedance?

The Electro-Harmonix Deluxe Big Muff Pi, for example, has an input impedance of 130kΩ;
most Muffs have an input resistor of 33k or 39k and input cap of 100n.

According to RG Keen, as posted in this DIYSB thread:

The simplest good way to measure input impedance is to hook up a large value pot, perhaps 1M to 5M, in front of the pedal as a variable resistor, from 0 ohms to full. Then put a certain amount of signal in; 100mV should be fine. Leaving the signal source alone, turn up the pot until you only read half the original input voltage (i.e. 50mv if you put in 100mv). Now measure the pot resistance. It's equal to the input impedance of the effect at that test frequency.

Input impedance really ought to be measured at a number of frequencies, as it varies, but most people measure it at some middle frequency, like 500Hz or 1kHz and let it go at that.

The MD has a 10k input resistor followed by an input cap of 33n, giving a corner freq of 482.5Hz and after that my mathematically-challenged brain goes numb-er than my butt on a 14hr Greyhound bus ride (those seats are NOT anatomically correct for humans).

Bless RG for putting it close to layman terms here:

Input impedance is literally the input voltage divided by the input current. If you put in one volt of input signal, and it requires one milliampere of input signal to get this to happen, the input impedance is 1V/1ma = 1K ohm.

and here:

Everything the input signal touches contributes to the input impedance. The only question is at what frequencies.

The ferrite (bead??) has an impedance that's essentially zero at audio frequencies. It's there to hold radio signal outside.

The 1M to ground pulldown would make the input impedance 1M for audio if it was the only thing there.

 

[bean]

You're absolutely right though, 10k input impedance is TINY. try bumping that up to 1M and see if it helps.

Not only that but remove R2. That forms a voltage divider with R3. It will solve the issue I think.

 

[lrgaraujo]

How does one calculate the impedance for the MOSFET Driver? The input resistor of 10k surely doesn't represent the input impedance?

I find it very useful to evaluate impedance with LTspice. Simulation of the input stage of the mosfet drive results in the following:

 

[allanshookphoto]

Since the thread is resurrected, maybe answer that $64,000 question.

Yes, the build is meticulously built to the specifications with no subs or mods. I triple checked everything.

 

[allanshookphoto]

I've tested all the values, and double checked the schematic. I don't think it's a build issue.

 

[Drayve85]

I would go with Beans answer! I built a vero MOStortion and haven’t played with it too much because it didn’t sound ‘right’ to me anymore. I may have your issue with it. I’ll check it out today and see. Not sure if they’re at all similar.

 

[jesuscrisp]

I would go with Beans answer! I built a vero MOStortion and haven’t played with it too much because it didn’t sound ‘right’ to me anymore. I may have your issue with it. I’ll check it out today and see. Not sure if they’re at all similar.

Completely different circuit

 

[Drayve85]

Completely different circuit

Ok thanks!

 

[allanshookphoto]

Looking at the schematic it makes me think that building it exactly according to build docs is the issue here.

At some point, I have to eliminate the possibility that the issue is improper build or assembly. I can only assume that it is behaving in the way that the circuit is designed to behave. I guess the only way to prove that is to order another board, and all new components, and compare them.

 

[jesuscrisp]

At some point, I have to eliminate the possibility that the issue is improper build or assembly. I can only assume that it is behaving in the way that the circuit is designed to behave. I guess the only way to prove that is to order another board, and all new components, and compare them.

I would seriously just consider the advice from bean and what I said: snip out R2, swap R3 for 1M. At this point I'm rather sure it's just faulty design.