Tone stack after a boost pedal?

[amonte]

Here's the skinny...   

I've got a Vox Pathfinder amp that I started using as a "preamp/booster" in front of a TS-9 pedal.  It works fairly well, but I was always nervous about using the headphone out on the Pathfinder and putting it in front of one of my larger amps.  Plus, it's a pain to lug the Pathfinder around just to get that sound when I could probably get the same thing out of a pedal.

I had thought about going with the English Channel or Vox EXP, but I'm not really looking for an amp sim or even a distortion pedal - just something that will take my guitars input and give it that sparkly, mid-cut sound that the Pathfinder has. 

I was able to secure a schematic to the Pathfinder, but it's hard to read in parts.  I was thinking of going through the Pathfinder circuit board and seeing if I can get a better handle on some of the component values.  In the meantime, I went forward with my idea by using a MOSFET boost in front of the Vox AC30'ss tone stack.  Not the same thing, but a decent starting point.  The only problem is that the tone stack cuts a HUGE amount of volume out of the signal.  My plan for tonight is to try and put a second MOSFET boost stage after tone stack.  Will this yield decent results?

My other problem is that I'm not the most patient person, and I'd like to build this thing with parts I have lying around this weekend - I've got a bunch of different NPN transistors, a handful of JFETS and some MOSFETS as well...but no op amps...  The rest of the components (caps/resistors/etc.) I should have handy. 

Are the MOSFETs the best choice for this application?  Well, the best choice among the parts I have? 

Any assistance would be greatly appreciated.

[GibsonGM]

Someone else has probably done similar, and will also reply.  My 2c, sure, try a 2nd recovery stage after the tone stack.  The only way I've ever gotten good results is to just experiment...no quick and easy way, everyone will have their own opinion and way of doing things that sounds good with their rig.   

My solution to a similar issue was just a simple LPB-2 with a buffer before it.  If I'd wanted to add a tone stack, I could have done so, and would have used 1 of my options that are available....buffer with some gain, tone stack, LPB.  Or, buffer, LPB-2, tone stack, recovery stage.  Tweaking gains etc. at each stage for optimum results.  Just to make sure nothing's screwy, I measure my outputs at each step to avoid things like your concern about the headphone out killing your amp.    How about playing with a booster ckt's in/out caps to shape your tone rather than an entire tone stack?  Or a boost with simply more gain to 'withstand' the tone stack insertion loss?  Another possibility, and really depends on what your desired output is (clean, overdriven, etc).  Personally, I'm not a stickler for whether I'm using opamps, NPN's, Mosfet or whatever, I just try what I have around and usually get good results.

The endless search for tone, LOL.  Good luck!   

[amonte]

Thanks for the tips, GibsonGM!

I will try adding a second MOSFET stage, and if that doesn't work, try adding a buffer in front as well.  The reason I want the tone stack is so that I can have the flexibility to alter the tone and also to get the "mid scoop" that the stack provides.

[amonte]

Question - if I'm using a MOSFET input stage with a high impedance, would the buffer in front of it do anything?  Would it help at all?

[tcobretti]

I had typed out a response here, but upon rereading the thread decided it was unhelpful.

A gain recovery stage seems like a good idea in your case.  Good luck!

[petemoore]

  Also you might want to take a look at Superfuzz's notch filter, mods to that may provide some mileage.  Something like replace the switch with a pot, then diddle with the freq response/values.

[amonte]

Well, you guys were right.  The recovery stage did the trick.

So the basic design of the pedal right now is MOSFET Boost Stage => Vox AC30 Tonestack => MOSFET Boost Stage.  I was going to do a Gain and Volume control, but I'm trying to keep the knob count down and the Gain didn't seem to make that much of a difference.  I was also considering adding a cut control, but again, to keep things simple, I'll probably just add some cap values to ground. 

I actually decided against going with the traditional AC30 brilliant input value (500pF) because I wanted the pedal to be a bit more versatile (though I will probably at least try it in there before I take this off the breadboard) and also because I was concerned that with such a small value, even the recovery stage wouldn't be able to bring back that much volume.

The recovery stage works well - there's enough gain so that the pedal can act as a booster as well, but it doesn't give a HUGE boost...but more than enough to push an amp a little bit. 

Only two concerns at this point, one of them minor.

For the input/output caps, I've got .047uF/.047uF on gain stage one and .1/.1uF on gain stage two.  As an added side effect, the pedal actually seems to be boosting the bass a hair, so I'll probably either leave it (it will have a bass and treble knob), or bring these down a little.

The other concern is the noticeable HUM when the pedal is on.  Would/could this simply be because I am working on a breadboard and all of the components are "exposed", or is this a result of having the tone stack followed by a second gain stage?  Should I be concerned or just wait what it sounds like when it's in a case?  Any ideas on how to battle the hum?

[amonte]

anyone?  I'd really like to get rid of the hum if at all possible.

[GibsonGM]

Anytime I've got boost going on, on a breadboard, I pick up all kinds of junk, Amonte - AM radio, computer monitor interference and so on...that's probably the culprit for you too!  You can only know after you get things tweaked, boarded, and in an enclosure.  RF loves breadboards.   Of course, this can't tell you if you have a missing ground or something (but try to find one anyway)...it may hum after being enclosed, which will tell you where to look for the hum ;o) 

I recommend star grounding (or as close as you can get).  And being aware of how much gain your last stage is really adding...is it way over the top? That can undesirably amplify noise...finding the right balance of gain for these things is important (to recover from insertion loss, but eliminate amplifying noise).  Tracing the signal with an audio probe might show you where (from what stage) the noise originates...   

Isn't it cool how boosting ckt's changes their bandwidth, etc?  I've done away with my tone control ideas several times due to the effects of active devices in the chain...solved my problems that way a few times, along with cap changes.  I sometimes include a boost/buffer type setup in pedals just for that purpose (fatter sound, more 'sparkly' highs).   If you've got high input Z from the mosfet, a buffer isn't really necessary, no...

[amonte]

Thanks again for the reply, GibsonGM!

I hope you're right about the breadboard getting interference from other sources.  In terms of gain, both stages are adding a decent amount (not sure how much), but the tone stack is really taking a lot of signal out. 

I will let you know how it turns out when I get it off the breadboard.

[GibsonGM]

Glad to help, Amonte...lots of steps, as you can see!  Hopefully it's quiet once enclosed, but if not, this is the place to figure out why 

[amonte]

You know, I have to admit, of all the times that I tried to build something "off the cuff", this is the first time it ever really did what I expected or wanted.  It's very gratifying.  Granted, it's not a complicated pedal by any means - MOSFET Booster => AC30 Tonestack => MOSFET Booster.  But it gives lots of that mid-cut Vox sparkle.  I have two P-90 guitars (a Les Paul Jr. clone and a Reverend Club King 290) and both are a bit too dark in the bridge.  This does the trick nicely...well, so far it does...

[GibsonGM]

I had similar 'issues' with my LP (490/498 pups)...built an LPB-2, but it was too mid-rangey...so, added an opamp buffer to the front end and played with caps.  I'm really happy with the results (sort of AC-30ish in itself, I run into a Fender Hot Rod Deluxe). I thought I'd have to do a tone stack, but it's fine just using amp controls.  You never know what will work with your combination of guitar, pedals, and amp!  I tend to overcomplicate, and have to cut myself back and keep it simple, LOL.  Glad yours worked out!