The Aefea Drive - a simple one-chip diversion

[Mark Hammer]

I was prompted by some suggestions I made on another thread concerning the drive structure of the Marshall Bluesbreaker, and lured by the prospect of not having to think about how much frustration I was having with the building and wiring of some MFOS and EFM modules (more when they're done and working).  The result was a thing I built last night and drew today that I'm calling the Aefea Drive (pronounced "ay-FEE-ya").  It stands for "Almost everything for everybody...almost". (With my luck it'll probably turn out to be a swear-word in Greek or Farsi!)

The basic concept is that of a single clipping/gain stage that can be used in a manner that generates more typical asymmetrical clipping, or very soft clipping, and shades in between.  The Bluesbreaker, and its cousin - the Analogman King of Tone - use a resistor in series with the clipping diodes to produce a softer clip.  I decided to make that clipping quality continuously variable.

The 4k7/220nf an 3k3/47nf networks exploit the Proco Rat trick of providing differing amounts of gain for the lows and low mids, and the upper mids and highs.  The 3k3/47nf network provides a bit more gain for content above 1khz.  At absolute maximum gain, the gain is 228x for stuff above 1026hz, and 160x for stuff below that.  So, not a high-gain monster, but clips harder than a TS-9.  Not nasally, but relatively bright.  The 100pf feedback cap provides a treble cut beginning around 2.1khz at maximum gain.

The stuff after the 10uf output cap is borrowed from the FY-2 thing I contributed as a prize for the anniversary contest.  The 1k/20k/6k8 path is where the low end passes to the output.  The 20k pot is configured as a SWTC, and forms an always-in-circuit variable lowpass whose rolloff point ranges between around 76hz and 1.6khz.  When combined with the 4n7 cap, it forms a midscoop network whose scoop-point can be moved around.  Without the 4n7 cap, it's just a variable lowpass.

The switch shown is a 3-position SPDT that either connects the 4n7 cap, the 36k resistor, or nothing, for 3 different tonal modes.  Between the three modes and variable lowpass there is a fair amount of tonal flexibility, particularly when combined with the different clip qualities.

Now, back to the op-amp.  The 250k pot is wired up as a variable resistor, and is in series with one side lug of the 500k pot.  The other side lug of the 500k pot goes to the clipping diodes, and has a 22k fixed resistor in parallel with it.

When ALL of that pot's resistance is between the wiper and the diodes, that side of the pot, plus the 22k resistor, puts roughly 21k in series with the diodes, and yields mild coloration.  Since the max resistance beteen the inverting pin of the chip and the output is now entirely dependent on the 250k pot, max gain is limited to 76x for stuff above 1026hz, and 54x for stuff below that.  So, boost but not gobs.  The 21k series resistance will assure the diodes don't yield too much harshness.  Note that when gain depends ONLY on the 250k pot, the treble cut begins around 6.4khz at max gain, which is bright but helps to keep some hiss out.

Rotating the 500k pot in the other direction reduces the resistance in series with the diodes (making for a harder clip), and ADDS series resistance to the 250k pot, increasing gain.  The upshot is that the two controls can be used to produce restrained soft clipping or much harsher and harder clipping, at higher gains.  Four knobs - hardness, gain, lowpass shift. volume - and one toggle.  I've got it built into a piece of perf 9 x 18 holes, so it'll do a 1590A if you want to use those little plastic-shaft 9mm pots.

Like I say, almost everything for everybody...almost.    I'll see if I can record some sound samples, once it's boxed up.  It's nothing earth shattering, or particularly distinctive, but gets a pleasing array of tones.  The output level is compromised by the passive tone network, but there is enough audible boost to keep people happy.  Your basic stock Tube Screamer is not exactly an output monster either, remember, and that hasn't stopped it from being popular and useful.

[bluebunny]

Thanks for sharing, Mark - looks very interesting (doubtless joining "the list").  And comes with a stealable logo too!

[deadastronaut]

nice one mark, look forward to the clips man..

[bool]

This is very extremelly similar to a circuit I used to design for myself in 90's (slight diffs in implementation and pot connections) and from what I gather at a first glance, this one would benefit greatly with addition of a single resistor (4k7-10k) and a generic NPN bjt as an output buffer (i.e. after the filter).

Imo it's a no-brainer.

[Seljer]

I made layout that could be easily made into perf because most connections are pretty short. Its 15x15 holes (41mm x 41mm), smaller if you skip the traces to the row of potentiometer connections at the top and wire the controls directly to the components. Unverified for now so try to spot anything iffy you can!

Ignore the numbering in the potentiometer connection and use your head the determine the proper order of lugs.



Transfer if anyone wants to etch: http://www.aronnelson.com/gallery/main.php/v/Seljer_001/transfer.png.html

This is very extremelly similar to a circuit I used to design for myself in 90's (slight diffs in implementation and pot connections) and from what I gather at a first glance, this one would benefit greatly with addition of a single resistor (4k7-10k) and a generic NPN bjt as an output buffer (i.e. after the filter).

Imo it's a no-brainer.

Or use a dual opamp and use the second half as a buffer

[bool]

Dual opamps vs bjt buffer... imo a bjt (or a darlington, or any other discrete that would "fit", etc) adds some indescribable "goodness" to the signal that is missing when you use an opamp as a buffer.

If you are a "opamp swapper", it's less handy though. One factor more to deal with.

[Mark Hammer]

I made layout that could be easily made into perf because most connections are pretty short. Its 15x15 holes (41mm x 41mm), smaller if you skip the traces to the row of potentiometer connections at the top and wire the controls directly to the components. Unverified for now so try to spot anything iffy you can!

Ignore the numbering in the potentiometer connection and use your head the determine the proper order of lugs.



Transfer if anyone wants to etch: http://www.aronnelson.com/gallery/main.php/v/Seljer_001/transfer.png.html

This is very extremelly similar to a circuit I used to design for myself in 90's (slight diffs in implementation and pot connections) and from what I gather at a first glance, this one would benefit greatly with addition of a single resistor (4k7-10k) and a generic NPN bjt as an output buffer (i.e. after the filter).

Imo it's a no-brainer.

Or use a dual opamp and use the second half as a buffer

Actually, it occurred to me that using a dual op-amp may be appropriate, given the passive signal loss in the midscoop/tone network.  I suppose the buffering aspects are helpful too.

And yes, I didn't think this was anything particularly original or innovative, merely a complete functioning circuit for those who wanted to dicker around with varying degrees of hard-vs-soft clipping.  I suppose another way of doing it might be to have two different diode or even diode/cap networks in the feedback loop, with a 10k pot to pan between them (the wiper of the pot goes to the inverting pin, and the outside lugs go to each of the diode networks).  That would yield degrees of soft  and hard clipping for each type of diode network.  Basically, an extension of what you see here - http://www.muzique.com/lab/sat2.htm - but in the feedback loop.

[Seljer]

Did some math, with the components values chosen, it's barley losing 2dB in the tone section so its not that hard of a hit. Sure lower output impedance may be nice, but 100k on the end is still less than a bare guitar.

You wouldn't really need any extra components to add a buffer by using a dual opamp, just leave the tonestack at Vref and move the 10uF cap and volume pot to after the unity gain stage. Its probably more interesting to incorporate the opamp section into the tone stack itself.

[smallbearelec]

Hi Mark--

I just posted a revised "platform" for the 60 x 60 mm Bare Box perfboard in the Gallery. Would it be OK if I (or someone else) does a layout for this geared to the 6060?

[Mark Hammer]

OK?  I'd be honoured.

Four pots and a 3-lug toggle works perfectly for the 5-pot board. 

[smallbearelec]

Four pots and a 3-lug toggle works perfectly for the 5-pot board.

As I noted in my other post today, I ran into an issue with combining pots and switches on the five-pots board: The bushing of the standard sub-mini toggle switch turns out to be a hair too short. I will probably have Taiway make what I need at some point. Meanwhile, Just use the five-pots board as an easy template for drilling the hole centers and use 9mm or 12mm pots.

SD

[lungdart]

Simulated this in SPICE for a while, then built it on a breadboard. Was making a lot of different distortion sounds by dialing up that 500k log pot. I wasn't able to build the tone stack, because I didn't have a 20k pot on the shelf, so I had to use a different tone stack.

Non-the-less, this thing has some killer sound! Good work.

[Mark Hammer]

Thanks.  And I will reiterate: there are a lot of different circuits where the interactive gain and soft-clipping/hardness pots can be applied.  You can do it to any TS-type circuit, not just a Bluesbreaker.

[Kipper4]

Intresting
Thanks Mark.
I think it might be worth trying some transistors in the place of the diodes too.
High time I took Gus's drive off the breadboard and have a play with this.

[Quackzed]

  ok that diode / feedback loop configuration is hurting my brain.
theres always 500k in parallel with the diodes, 500k to 750k with that 250k pot/variable resistor... right?
then theres also a minimum of 22k in series with the diodes a la warp style soft clipping... wait no, it can be zero series resistance in series with the 500k at one extreme , no warpish resistance in series...

would you mind walking me through  what the feedback loop parallel resisance and series resistances are doing?
not that i think anything is wrong , i just cant get my head around it...

[Mark Hammer]

No problem, Gil.

Let's remove the 22k to make it easier.  We have up to 500k that an be added in series with up to 250k for the feedback resistance.  Whatever portion of the 500k pot is not placed in series with the 250 pot goes in series with the diodes.  Since it doesn't take very much of that "unused" 500k to effectively shut down the action of the diodes, there is no point in having all the 500k on that side of the wiper.  So, I stuck a resistor in parallel with that leg to limit how much the resistance in series with the diodes could be.

There.  Make more sense now?

[Quackzed]

ahhh. ok. thank you. i see it now.wiper to one side of the 500k is a variable resistance in the feedback loop (with the 250k) , and the other side to wiper of the 500k is at the same time, inversely, in series with the diodes. and to keep the diodes series resistance side from getting needlessly high, the 22k is there as a reasonable limit of series resistance for the diode side. whew. that was crossing my eyes there for a while...

[Kipper4]

Thats my little relaxing diversion for the evening.
I built this and am dying to try it but its now 2am.
Im out of those switches but i can bodge something up in the morning for test purposes.
I cant wait to hear it. Its ages since i did a drive circuit.
Thanks Mark

[Mark Hammer]

I properly boxed up a 2nd build, and I am VERY pleased  This thing has a LOT of sounds.  Takes a bit of getting used to, to know where to set the controls, but once you get the hang of it, just tons of flexibility.

A few updates to the schematic posted earlier.  The tone-mode section with the 3-way switch  shows a 4700pf cap.  Realistically, that maybe should be 3900pf, for a more balanced scoop, but i'll let others determine what they like.  The 36k resistor should be 30k to produce the biggest difference between the middle and side switch positions.

I'm still debating about whether I need to change from a linear taper for the 500k pot, but for now linear seems fine.

The tone mode, in tandem with the shift control, just yields so many tonal options.  On my Nashville Tele, I can dial in Brad Paisley-aproved middle+bridge cluck with just a hint of grit, using the variable midscoop switch position (4700pf bypass cap switched in).  I won't profess that it makes it possible to sell your Crook/Dr.Z setup, but I was finding myself thinking "Hey, I've heard that sound before!"  Switch to the bridge pickup, roll back a bit of treble on the guitar, dial in the soft clipping, add a bit of gain, and you have your Tom Petty rhythm sound.  Turn the gain up a bit with your neck pickup and the clipping softness dialed down just a hair; set the switch to the 30k bypass resistor, and adjust the Shft control until you find the right amount of mids, and you've got your TS-9 tone.  Crank the gain, turn the hift all the way up, and set the mode switch to the mid position and you have beautiful warm and round "woman tone" with just enough bass.

I just love it when I make educated guesses about where certain rolloffs ought to be, build it, and the sound corresponds to the numbers I had in mind.  And this is one of those times.

I'll try and make a video demo soon.  Unfortunately, I have a budget webcam whose software seems to have been uninstalled and I have no idea what the brand of the unit is to go to the corporate site and download the drivers again.    I may try to pick up another webcam tomorrow.  You won't like my playing, but you will like what this can do, and the number of different personalities it can take on.  If you have a chance to try it out  and provide some feedback, I'd be much obliged.

[bluebunny]

Just one question, Mark: there are two places where wires cross at right angles - one leading to the inverting input of the opamp, and one after the output of the opamp.  The latter is clearly a four-way join.  Is the first one all joined up too?  I think "yes", but wanted to be sure.