'New' Jfet low-gain overdrive idea...

[liquids]

I mentioned the idea when thinking out loud in this thread: http://www.diystompboxes.com/smfforum/index.php?topic=74554.msg606454#msg606454

See, I like a lot about the LTD Silver--it was the first low gain overdrive that I liked and was sold on.  I need a pedal that is 'hardly' even an overdrive.  A neck pickup lead tone that is clear and responsive to dynamics-- dynamic in the way that a clean tube amp set on the edge and sound clean when picking single notes normally, but just starts breaking up with accented single notes if you dig in, or play multiple notes/intervals, etc.  Absolutely no Mid hump, because besides for having humbuckers, I need it to mesh well with my Fender's clean sound, and it needs to 'cut' just right etc.

I wanted a pedal devoted to that sound but never found anything that sounded good till I breadboarded the LTD silver.  But recently I thought, rather than op amps, what if I could take what I know about Jfets, mu-amps, the BSIAB, the BMP tone control...and make an overdrive for myself based on those, that does this sound how the LTD silver did?

I messed a bit yesterday and today, and am already pleased with it!

I can't make a schematic easily, so it will take me a few days or a week to post if for your guys to try it if you need a schematic... but for starters, I'll explain it for everyone interested.  I'm interested to get feedback on how this works for people with different setups than mine, and likewise for opinions on others mods/tweaks etc.  It's based on the BSIAB / Blues Booster for starters so use that as reference, but it it has a few twists...three nearly identical low gain mu-amp stages, and a modified the BMP tone stack between stage two and three...here's what I have so far:

Standard input, .1uF cap in, 1M input impedance,  etc.

Mu-amp stage 1: MPF102s, standard arrangement other than no cap on source to ground-- just a 4.7K resistor.  A  .1uF cap between the upper Jfets gate and source; .1uF cap out of stage one into...

a 47K and 470pf cap in parallel, feeding a 1K resistor into a 500K audio variable 'gain' pot to ground (like with a BSIAB), with a 470pf cap across it to preserve high end, all feeding the next stage....

Mu-amp stage two: same as the above stage but with 2n5457s; .1uF cap output feeding the tone stack...

BMP tone stack, with a 1uF cap on the 'bass' side so there isn't any noticable bass roll of, with a very small 470R resistor connected to a 10K pot to ground for a mid / AMZ presence control; 47K resistor on the other side of the BMP tone control and a .0047uF cap to ground off the resistor for subtle high end control...so you have a 'treble' tone pot and a 'mids' control, but overall this stage cuts a lot of gain/mids, in favor of having an EQ that sounds about flat with some control, and a bit of high end control...

This feeds into a third Mu-amp stage to recover gain, which is the same as the first and second stage, but with J201's and a 1uF cap between gate and source on the upper Jfet. .1uF cap out into a 100K audio volume....

I realize a schematic is worth the thousand above words. Overall, I find it excellent for my neck humbucker with the mids low.  For a slight overdriven amp sound with bridge humbucker, you'll want more mids.  The Volume knob may need to be max if mids and gain are kept low.  It does shave a noticeable amount of flabby bass to my ears, but that can be tweaked by increasing all the cap values to taste...

I'll get to a schematic ASAP but I'm slow and not so good with them.  I do look forward to hearing what people think either way!   

[alanlan]

Sounds good, but I don't associate mu-amps with "low gain".

[R.G.]

Anything's low gain if you put enough input series degeneration in series with it.

[liquids]

Okay, I was able to hack a schematic together more quickly than expected...but I'm sure someone could do a better one if there is interest: http://sites.google.com/site/teaandfiction/Home/overdrive 

As a side note, you can see I of course gave credit to Ed and Jack...but if this is 'too close' to their designs and not acceptable to post for some reason, I'd want to know. 

I'm having a blast with this already.  There's lots of mods and tweaks I can see already depending on the player and equipment...but this is how mine is starting for now given my equipment and needs...

Next I need a a vero layout, so that after I've tweaked values for weeks and feel confident about it being how I want it I'll actually be all set to solder.   

[alanlan]

Okay, I was able to hack a schematic together more quickly than expected...but I'm sure someone could do a better one if there is interest:

You might want to play around with the value of the caps connected between the source and gate of the upper JFETs - with 0.1uF you will not get so much gain at lower frequencies.  It's a matter of taste of course.

Mu amps provide extremely high gain as long as they are not loaded very much - they have a very high output impedance so the potential gain can easily be lost by the next stage in the circuit and will lose top end if driving a capacitive load (like a cable for example).  A source follower or emitter follower on the output will give you a better drive capability - again, all down to preference, but it's something to think about.

Anything's low gain if you put enough input series degeneration in series with it.

Still, a mu-amp wouldn't be the first thing to spring to mind if you want just a few dBs of gain would it?

[liquids]

You might want to play around with the value of the caps connected between the source and gate of the upper JFETs - with 0.1uF you will not get so much gain at lower frequencies.  It's a matter of taste of course.

I definitely understand that.  This is being designed (primarily) using neck humbuckers, bear in mind. so I like the focus in the bass offered buy it, and the way high strings gain up more than the lower string.  With humbuckers, oftentimes the output of the low strings overwhelms the higher strings so the opposite is true.  This somewhat counters that tendency.

I just got started on this to be honest, but I liked it right away.  As per my initial write up, I  agree, to those who want more bass or more even breakup, that's the first place to tweak.   Currently the .1uF caps work for me so far for the sound I'm looking for, but I'm going to continue tweaking and toy with increasing their value here and there until I know where too much is too much. 

I'm very interested in other players preferences given variations in equipment, given that my rig is not necessarily a "standard" setup, or the mid-point for a stock schematic for DIYers.   But you have to try it, hear it and play it...I found that things that looks good on paper don't always work out tonally--the opposite is also true. 

The way Mu amps provide extremely high gain as long as they are not loaded very much - they have a very high output impedance so the potential gain can easily be lost by the next stage in the circuit and will lose top end if driving a capacitive load (like a cable for example).  A source follower or emitter follower on the output will give you a better drive capability - again, all down to preference, but it's something to think about.

Please share more about this!  I did not know that they have a high output impedance.  I was under the impression that the 100K volume output by itself would give the circuit a reasonably low output impedance, and avoid loading the input of anything downstream.    "Source follower" or "emitter follower" for "better drive capability" are clear as mud to me.      Please tell me more or link me to death so I can learn more...

Still, a mu-amp wouldn't be the first thing to spring to mind if you want just a few dBs of gain would it?

Well, I don't just want a few DB's of gain, persay.  I want something that gently overdrives naturally and gracefully.  I play with my fingers primarily.  My limited experience and equipment have shown me that clipping the input of a single gain stage doesn't really create the sound I'm looking for, no matter how little clipping there is. The typical hot-as-possible input stage with a gain pot, slamming a second stage is a bit closer, but still not it.. Mu-amps are the starting point as I kind of understand them, and likewise they don't need trimpots like a single JFET stage, so it offers a bit of simplicity.  I happen to like them, and their basic sound, so that's what I started with.   

Actually, I was expecting to have more success with a few really lowish gain 4049 stages clipping each other a bit, which may work as well, but I was surprised at how much I liked this immediately. And again, there may be many other ways to do it, but trying to get the sound of a cleanish amp on the edge that sounds basically clean with normal single notes, but starts to overdrive with intervals or hard picked single notes etc -- clean to slight overdrive given input dynamics -- has not been too easy.  So it's more complicated than just needing a few dBs of gain per say.   Almost every 'low gain' overdrive I've ever tried, especially tubescreamer types, while great in other ways, for this type of sound are:

A) Way too dirty no matter how little gain I dial in
B) Sounds too full and 'midrangey,' and hence sound vastly different in EQ than my clean tone to not sound like an 'effect'
C) Not natural sound and responsive enough to input dynamics to simulate going from basically clean-to slight breakup just with picking strength / amount of notes played
D) Melds naturally with my Fender Super, which is typically set Bass-(2-4), Mids 8-9, Treble 6-8, bright switches off.

I feel the Barber LTD Silver did it all of that rather well particularly with the EQ tweaking, but I'm thinking I'm liking this better already for my own personal needs.  So, I'm interested to see if and how anyone else finds this circuit useful and specifically what tweaks other players prefer, or if it only work for my gear and to my ears.   Try it, please? 

[JDoyle]

If nothing else, I like the way you used your parts!

Specifically, the use of the higher Idss 2n5457 to drive the tone stack, and the use of lower Idss J201 on the output, having a lighter load of 100k, to get the 'primary' distortion. (Though without a buffer in-between the pot and the output jack, whatever follows the circuit will be in parallel with the output pot, and the total load on the output of a muamp is a large factor in its response.)

The only quibble I would have would be the use of the MPF102 as the input transistor - the spread in Vgs(off) and Idss are enormous for that part, and unless you can specify a closer range, and whoever builds it feels like testing and sorting*, it will be really hard to ensure future replication of what you are listening to from your circuit. Not that any part of the last sentence negates any success you have had with your circuit in any way!

Congrats!

Regards,

Jay Doyle

* However, I would suggest strongly that anyone who is going to use JFETs should at least spend some time sorting a few - if only for the educational experience of building a test jig out of what you have on hand, and seeing how a JFET reacts in real time to your specified conditions. And knowing you have matched transistors of any kind is always something that comes in handy...

[liquids]

Uh...thanks.  That was a happy accident I suppose, because I didn't know any of that.    But now I have a good reason to leave the JFET type in stage two and three as is. Thanks for the feedback!

I use MPF102s at the input because it seems to my ears that they don't clip as easily from a hot pickup, at the input stage.  They had better volume knob clean up on my BSIAB.  I guess I may deal with any compromisesthere , but would agree to guys using weaker pickups to also demo it with other JFETs in the first stage if they want more gain (that's one way of doing it), or just to hear the differences.   

[liquids]

Some tweaks...

Tone control tweaks: I made the treble side cap .001uF since I want it as subtle as possible and only want a way to roll off high end fizz with higher mid and gain settings.   Also, testing it against a record, the bass was a little full for what I'm looking for, and since ideally for most sounds I'm running the tone pot full treble side anyway, I made the 'bass side' cap .1uF because it offers a little bit more of a subtle roll off with higher tone pot settings, and dialing it back shaves only the highest treble and adds a bit of bass back...


realized that I am conciously subconsciously trying to get something along the lines of John Mayer's Lead tones on Continuum as a reerence...   Likewise, moving all the caps on the upper JFETs to 1uF filled out the bass to closer to 'bypass' levels. I'm not using that, but it's a useful easy mod so long as if you use electrolytics, positive side to gate, negative side to source...Tested it with my Strat now too.  It's definitely single coil approved!  IT has plenty of gain -- especially with reasonable 'mid' settings.  Too low gain to break up with single coils was a fear, but not anymore.  And I still prefered the .1uF caps for stage one and two with single coils despite the bass loss...

I made the mid pot Audio taper, as  the 1K range is most noticable, and from 4.7k to 10K is really just one notch.

I should mention that the range I prefer is the 1k-2k range on the mid pot, so setting it low is what I'd say the "stock" sound, and where humbuckers shine.  It has a subtle mid dip to the ears..  With single coils this is useful.  For warmer mids and a slight mid hump, full on 10K with single coils makes sense.  With Humbuckers it gets too throaty, but at 5k-10k on the Mid pot, it becomes more of a 'power amp breakup" crunch sound for your bridge pickup and those full on chords, mids and treble to taste. 

Also, realize the mid pot is like a master 'gain range' control too. If you keep it set low you'll never get much dirt, which is what I prefer, especially with pickups that are hot and provide plenty of 'gain' on their own.

[petemoore]

Please share more about this!  I did not know that they have a high output impedance.
  Neither did I really.
  The BSIAB has BTB Mu amp stages...
  If you have a pedal following, or long cable, the ability of the output to drive the load may come into play.
  AMZ has loads of buffers and loading / impedance info.
  A couple good textbook buffers are the last stage of the BSIAB II or Tubescreamer {GEO.
  One way to figure that out if impedance losses are happening is use any pedal with buffers [boss, ibanez, anything with 'tripswitch bypass' like that]. Put between two circuits with input/output mismatch, the buffers low output impedance [buffer is 'on' when the effect is in bypass mode] will drive cables and inputs in question, compare that to no buffer, if there's a difference in sound, then it's impedance related.
  But probably could use a buffer if driving a cable.

[liquids]

Yeah, as standard practice I'm feeding this right from the breadboard setup into a Boss pedal for the 'generic' buffering, much like in my live rig so I can't speak to it doing that or not, I'll have to try it since it could be an issue for others not running buffers.... 

If I do follow the current circuit with something like the AMZ Simple JFET buffer, or, say, a Tilman Fet preamp tweaked for unity gain...won't these kind of 'buffer' stages also get clipped by the preceding stages, and hence distortion will increase?  That's an issue.

Likewise, if I use the buffer concept, is it better to put the volume pot between the 3rd mu-amp stage and the buffer, or have the volume pot after the buffer stage?  I would think, if striving for unity gain, that following the volume with such buffer stage would limit the gain the said buffer stage sees and hence reduce any clipping of that stage. Does putting the volume pot between the third mu-amp stage and a buffer stage defeat the purpose altogether?

[liquids]

Re: the buffer-- Pete said, "as far as headroom, buffers offer 1:1 voltage gain, and have greater headroom than a transistor trying to do voltage gain"...so I'm going to try it.

My thought would be to start by using a J201 since this is most peoples preferred JFET around here...any good reasons to choose differently?  I'll try it and update the schematic if the results seems more positive than negative.

[liquids]

No one seems to be that interested, but I'll write this up anyway..

Using a JFET buffer of Jack's on this page http://www.muzique.com/lab/buffers.htm--the one used at the input of the Dr. Quack, to follow the 3 mu-stage circuit as mentioned--wasn't anything impressive to my ears. 

With the circuit's hot signal due to the third mu-amp gain recovery stage slamming the buffer, I dare say it was creating more clipping.  Putting the volume control between the last mu-amp stage and then feeding the buffer seemed to work alright, as the volume would in theory limit the signal level out of the 3rd mu stage to near unity gain and then go to the buffer.  If you are really concerned about buffering, it may make sense to do it this way, but I noticed no improvements.  I say this because I usually test my pedals with a BOSS buffer following, for standard reference and my current rig, and I run my signal into a rack unit delay/reverb etc that really sucks tone if not feed a buffered signal, so I need something there.

So for my tests I left the rack unit but removed the boss buffer from the lineup, so that there was no buffering going on at all.  With everything off, there was of course the tone suck from the rack unit and cables etc.  With the circuit on and nothing else, it certainly sounded buffered again to my ears.  Then I went back and forth with the sound of the circuit with and without the added buffer following the volume pot, and to my ears, there was no real difference--if anything, it was a tad cleaner without  the buffer, but I don't doubt that was just my imagination--I think they were the same.  As mentioned, the circuit it sounded surprisingly nice and clear and buffered feeding the low input impedance rack unit to my ears, as it were.  So this buffer stage is obviously an optional add on, but I'm leaving it out, especially as I always run my gain pedals into a buffer of some kind.

Last but not least I added some tweaks to the circuit, and I think I'm complete. The "treble peaking circuit" was the last thing I got to - it basically was unnecessary as the 47k resistor I had there originally doesnt do much, but that is just what I started with.  In a high gain circuit like the BSIAB II with the gain high, changing this resistor between 470K and 47K is even pretty subtle.  With gain lower it makes more of a difference in tightness. With a real low gain near-clean sounding circuit such as this, it makes an interesting difference.  Going to 470K like the standard Marshall preamp really tightens and clears up the bass and midrange comparatively, of course with the loss of overall apparent gain as the resistor increases, yeah.  I considered making it a trimpot, but I think I will make it a good 'bass' kind of control on the front end and round out a flexible but entirely usable treble/mid/bass EQ section nicely--500K should do, I found 1M overkill.  I'll update the schematic to reflect these final tweaks soon as I can. and even if no one else tries this, I couldn't be happier with this pedal!  I hope someone else stumbles across it and likes it, or better yet - improves it to their own liking. 

[jimma]

No one seems to be that interested, but I'll write this up anyway...

I'm definitely interested and have been checking in on it. Thanks for the updates. I have a few things to finish and after that I'm gonna start experimenting with what you've laid out.

[gght]

Still watching also....... I like the idea!

[liquids]

Cool...this should inspire me to update it like I said I would. 

[liquids]

Okay, I updated the schematic it for the few of you who expressed interest.  And I think I'll call it something silly like the Liquidrive, but if you can come up with a good name, I'm all ears.   

It would be a lot better if someone with a good schematic program did it to look a lot nicer.   I like it so much that I'll get to vero this...maybe sometime this year--I think!      I actually may need it at a gig next month so maybe necessity will make it happen sooner...

[aziltz]

me likey.  i will have to build this!

you designed it for your neck pickup, but described the things i like to do with my tele bridge pickup and fingers.

[liquids]

you designed it for your neck pickup, but described the things i like to do with my tele bridge pickup and fingers.

I'm thinking you will really dig this!  If/when you build it (or anyone), contact me or post here if you want help on tweaks for sounds you can't seem to get.  I really think that if you're looking for that picking-strength regulated clean to gritty type overdrive, it will be here.

[Toney]

Looks good.

With regards to JDs comments, did you find much audible variance when taste testing other mpf102s at the front end?
I may draw this up on the weekend if I have time, perhaps with an optional output buffer.

Don't be disheartened by a lack of responses, I think a lot of people look but don't post these days.

I really like to see people messing with designs and putting it out there !