AMZ Mosfet Boost - Can't Believe I Did This...

[vanhansen]

Hey everyone.  Long time no post.  Been busy with life and band stuff.  Been tinkering with my guitars more than anything else too.  Have I missed anything? 

Anyway, last March I built the AMZ Mosfet Boost on perfboard using Torchy's vero layout as seen here: http://aronnelson.com/gallery/album28/AMZ_Mosfet_Booster_Tonepad. 

Cool, right?  No problem.  Sounds great, I love it.  But I hadn't been using it much lately and always wanted a bit more low end out of it.  It seemed to pull the lows out a little bit, and I noticed it after really sitting with it to do an A/B test with it on and off, clean and overdriven.  Ok, easy enough, just swap the input cap out for a larger value and that should do it.

So last night I get home, open it up to double-check the cap values.  Then I noticed it and had to pull out the layout and schematic to make sure what I saw was true.  Yep, sure enough (and this is where the thread title comes in), I swapped the input and output caps when I built it to perf.  Everything else is perfectly fine.  Just somehow I managed to swap those two caps around. 

Since I was going to change the input cap for a .1uf anyway, I left it alone and changed the .001uf cap at the output for a .1uf cap as well (since that is what it should be).  That right there made a HUGE difference in the low end response.  The basic tone with it off is still there, just that everything is more clear and there.  At unity, this baby is a GREAT buffer.  Bring up the gain level and it really adds some juice to the sound.  Here I was about to play with nothing but a tuner between my guitar and amp but it looks like the AMZ Mosfet Boost may be in the chain.

I know I said it last year when I first built it, but thanks, Jack, for this one.

Now, maybe next time I'll pay more attention to parts placement. 

[zachomega]

I did the same thing while working with a fet the other day.  Instead of a 1.5k resistor on the source, I put a 150 ohm resistor.  Needless to say, the thing didn't bias properly.  Strangely, I had it biased up correctly before I moved the thing on my breadboard.  Sure enough the 1.5k resistor was laying in a pile of resistors I had pulled off of the breadboard to make space. 

Lesson learned?  Clear the entire breadboard whether I plan to use it all or not if I am going to work on a project. 

-Zach Omega

[vanhansen]

At least you caught your error early, and I've done that too.  But this circuit had those caps reversed for 9 months!!    I somewhat felt like an idiot after noticing it. 

[zachomega]

haha...I wouldn't have caught it if it didn't sound so awful. 

-Zach Omega

At least you caught your error early, and I've done that too.  But this circuit had those caps reversed for 9 months!!    I somewhat felt like an idiot after noticing it. 

[vanhansen]

Thank God for hearing.    When I built my Rangemaster I almost put a 39k resistor instead of a 3.9k resistor.  I caught that one during the verification process I do prior to soldering in place.

haha...I wouldn't have caught it if it didn't sound so awful. 

-Zach Omega

At least you caught your error early, and I've done that too.  But this circuit had those caps reversed for 9 months!!    I somewhat felt like an idiot after noticing it. 

[zachomega]

I should definitely take a lesson from this.  Fortunately it was just on a breadboard and took all of 5 seconds to correct once I noticed what I did.  I'm just glad I had enough sense to double check the voltages that were just fine before I played with the layout. 

-Zach Omega

Thank God for hearing.    When I built my Rangemaster I almost put a 39k resistor instead of a 3.9k resistor.  I caught that one during the verification process I do prior to soldering in place.

haha...I wouldn't have caught it if it didn't sound so awful. 

-Zach Omega

At least you caught your error early, and I've done that too.  But this circuit had those caps reversed for 9 months!!    I somewhat felt like an idiot after noticing it. 

[funkbass187]

yeah, even on a bass tha amz mosfet boost is great. it really beefs up my sound. even though i only have it in a raco box, its coming with me to every gig from now on.

[petemoore]

  I often start big on those, then cut lows as necessary later, paralleling it easy, don't ask why I have a seriesed cap under the board...lol. when the wrong thing gets pinned in there by the other right things that went in 'smoothly' just after it?

[vanhansen]

Used this one with the band this past weekend.  At low volumes it's really nice with my rig.  At high volumes, not so nice.  Too much "presence" even at unity making it kinda harsh sounding.  Turning down the treble on the amp didn't help (my amp doesn't have a presence control) and a few of my guitars don't have tone controls.  So, I'll just use it home with another little setup or make it part of the backup rig.  Still, a great circuit and one I highly recommend for anyone new to DIY or not.  It's one worth doing.

[billings]

If you want to tame that "presence" then it would be real easy to either swap out the 10M bias resistor or add a switch to adjust the input impedance to whatever you'd prefer.  Reducing down to 1M or so should help, but it's something you could play around with and maybe go even lower than that.

[vanhansen]

If you want to tame that "presence" then it would be real easy to either swap out the 10M bias resistor or add a switch to adjust the input impedance to whatever you'd prefer.  Reducing down to 1M or so should help, but it's something you could play around with and maybe go even lower than that.

Wouldn't changing the 10M resistor affect the bias of the circuit in such a way that other components would need to be adjusted as well to re-bias the MOSFET?  What about adding a mild low-pass filter or a tone control?

[billings]

If you're using the BS170, then according to the datasheet the gate leaks at most 10nA of current - which makes sense, since it's a MOSFET which usually means an extremely high input impedance.  10nA * 10Mohms = 0.1V, which will not affect the biasing much at all.  And that's worst-case.  Switch it over to a 1M or 500K resister and there should be a mild attenuation of the higher frequencies.

As far as the tone control goes, well I guess you could do that.  I always thought the point of the MBooster was how clean it was, though!  Goes to show you never can tell...  have you tried just rolling off the tone control on the guitar a little bit, or are you using it later in the signal chain?

[vanhansen]

Yes, I am using the BS170.  I'll try a 1M resistor there first and see what it does.  Thing is, I'd have to turn up the amp a good amount and I don't get to do that at home very often.  Might be a weekend thing when we're not practicing and time allows.

If the guitar I was playing had a tone control I would have tried that.    Several of my guitars don't have tone controls and I don't use any pedals really other than a pedal tuner so my chain is as simple as it gets.    Just me, the guitar, amp channel switcher and a tuner.  Though I just got a EVH Phase 90 that I'm enjoying so that'll be getting a trial run in the chain here soon.