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PostPosted: Tue Mar 11, 2014 10:12 am 
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I had off work last Monday due to snow, and came up with the idea of tossing together a quick overdrive to pass the time. And because I'm not good at doing simple things like that, I instead ended up breadboarding and working on a new design all week. ::)

The Snow Day OD is a FET- and MOSFET-based amp simulation overdrive running on 18V that includes a switchable miniature compressor circuit and soft clipping in a "power tube" section. It goes from glassy "mostly" clean to either open or slightly compressed edge of breakup that feels very similar to one of my favorite amps, up to a medium gain compressed sound. It has lots of harmonics without sounding overly clipped, and a very wide dynamic range.

It uses some principles from several Runoff Groove circuits, Aquataur's mini compression circuit from his mods to the Umble, and Mark Hammer's Stupidly Wonderful Tone Control (which rolls off treble without changing the volume).

Although it's not directly based on any amp in particular and I wasn't going for a particular sound, I did have a couple amps in mind (both Deluxes -- a friend's 5E3 clone and a 1949 Deluxe at Invisible Sound Studios in Baltimore) while I was designing, aiming for a similar feel and gain level to them if not necessarily their tonality.

The pedal runs on 18V internally for a lot of drive but also a lot of headroom, making for plenty of volume dynamics even with the clipping.

Schematic

Image

Video Demo (using my Sheraton)


Audio-only demo (using the DQ caster; no EQ, no effects, direct into the recording interface, also includes stacking with a My Little Klony and a Fatpants).

How It Works

Power section
The power section is a standard voltage doubler. The additional voltage helps coax more gain out of the FETs. FETs always clip when the input signal exceeds a certain voltage, but there is a limit to the voltage gain they can provide at any given supply voltage. They also need to be biased in a particular way to ensure the correct harmonics to emulate tube breakup. (See Runoff Groove's FETzer valve article for more information on the harmonics generated.) The higher supply voltage means that we can use 2N5457s, which have a higher input voltage threshold before clipping, while getting closer to the voltage gain of a J201 (which clips much, much sooner). This helps increase the total dynamic range of the pedal.

D2 is out polarity protection. Z1 is optional as overvoltage protection, but sometimes zeners can go bad when used with charge pumps. Otherwise the standard power filtering is there.

"Preamp"
The "preamp" section is primarily three FETzer valves in a row. Each 2N5457 is biased to 2/3 supply voltage (ended up being about 11V in mine). The bias point helps generate the correct harmonics to get the FETs sounding closer to tubes.

The gain control is right after Q1. R4 provides some minimum resistance (I didn't have room for this on my perfboard layout, but I added it to the schematic. The lowest setting will be clean even with humbuckers. Noon has a really nice edge of breakup sound -- extremely dynamic and picking attack responsive. Since the gain is simply a volume control after the first stage, rolling down your guitar's volume is pretty much identical, so the effect also cleans up nicely and smoothly.

Different FETs could be used with different results. For instance, a J201 in Q2 (with appropriate re-biasing) would be much gainier (and will also lose some clean settings). I got some good results with a 2SK170 in Q3 when using a telecaster.

Just a note: Runoff Groove's newer circuits use clipping diodes after each stage in a way that ensures smooth breakup and to avoid hard clipping the FETs, but I decided to skip that little innovation because I found they were generating clipping earlier than I liked in this particular instance.

Compression!
A miniature compressor stage, based on the one created by Aquataur for his Umble build, is created by R10-14, D1, and C6. D1 generates a negative voltage, which travels through R7 and pulls the gate of the FET negative (by about 2V at max from my measurements), dropping the input level to Q3.

The result is a bit of sag reminiscent of a tube amp at higher gain settings. The compression kicks in and becomes measurable and observable around noon on the gain dial on the "high" compression setting (at least with my strat; other guitars might trigger it earlier). There's an accompanying reduction in distortion and some added perceived sustain. The compression also swallows a bit of treble when it turns on, which creates a smoother sounding distortion sound. It also means that the effect will bright up a bit when you roll off your guitar volume, which is something I've been dying to find a way to do in overdrives that don't rely on guitar loading for a while now.

The attack time in mine is 10mS, and the decay is 100mS. The switch disconnects the compression in the center; one setting has a threshold-limiting resistor (220K) to ease things up on higher gain settings, and the other setting bypasses that resistor and lets through all of the signal.

It's worth noting that there's no capacitor to set the cutoff frequency of the compression, but I did consider adding it. Unfortunately there's only so much signal to go around, and in any case I like that the compression works more on chords (which have more bass and also need more clarity) than on single notes.

Tone (Treble) Control
The tone control, Mark Hammer's Stupidly Wonderful Tone Control, is after Q3. The pot is a continuous 100K series resistance between Q3 and Q4, and as the resistance increases between lugs 1 and 2 (pot turned clockwise -- I'll actually be switching this to work the other way around like in my prototype, though), more treble is dumped through C8. The values were calculated to roll off treble starting at 15KHz down to 685Hz. It can get a little brighter and a little darker than the bypassed signal. It's not extreme (no "playing under water" sounds), but it's effective at controlling the harmonics going into the power tube section. R15 sets the highest cutoff frequency of the treble cut (combined with C8).

"Power Tube" -- Mosfet soft clipping stage
After the tone control comes the "power tube" section. This is a MOSFET amplifier with MOSFETs arranged as diodes in its feedback loop. The MOSFET amplifier part is a pretty standard way to hook it up without needing a Vb reference voltage. I used a MOSFET here because it didn't distort as much as a transistor, and it also sounded a little less harsh when it did overdrive.

I loved the sound of MOSFETs as clipper diodes in the Mossy Sloth, but the arrangement is a little different here. MOSFETs conduct in two directions when you connect the drain and gate together: One way is the body diode (a simple silicon diode -- and no, it has no special sound) and the other way is ... something special that soft clips over a very wide range of a couple volts before it finally hard clips. By connecting them in series at their source pins, the body diode in each one prevents the diode from conducting in reverse, and the result is a very high, soft clipping threshold (about 2V and up).

One last trick to soften the clipping was the use of a limiting resistor (R19) in series with the clipping arrangement, similar to how it's done in certain op amp based designs like the Bluesbreaker and AMZ's big muff mod with a "warp" control. Big thanks to tca on DIYSB for lots of help (and diagrams) in understanding this (plus he suggested the MOSFET as Q4!). Although I don't fully understand everything R19 is doing, but the gist of it is that once the diodes conduct, they appear in parallel with R17, and they will lower the negative feedback. R19 therefore helps set the clipping threshold and ALSO sets how "rubbery" the clipping will sound, AND just for good measure, also acts a bit like a clean blend. Values between 10K and 100K seemed to work with roughly similar overall results, but it's not quite as simple as a lower value = more distortion. I ended up not really being able to decide between 22K and 33K and went with 33K in the schematic simply because there were a bunch of them in the schematic already. So why isn't this a trimpot? I dunno, I guess it could be, but I'm not totally sure it's worth the trouble.

C9 sets the cutoff frequency of the clipping. It's bit enough to definitely pass all the frequencies on the guitar (and might be overkill, but I'm not sure how to calculate it). Smaller values (e.g. 1uF or 100nF) will let through more bass.

Output section
R20 and R21 form a voltage divider, like a volume control permanently set to half. Adjusting their ratio to each other can boost or cut the output. I found it was sufficient to get a small boost to output even at very low gain settings.

An output buffer comes after that, followed by the volume control. The output buffer could be unnecessary (after all, the MOSFET probably has lower output impedance), but I thought it sounded very slightly better with it there. I'm not sure if it added some small amount of distortion, changed the bass or treble content subtly (perhaps cutting the lowest or highest frequencies a bit extra), but I figured it was only four extra parts and I had room on the layout.

Here's my build:

ImageImage

And here's my perfboard layout:
Image

If there's enough interest, I could do a small PCB run. Also, I do plan on making an etch layout, but I want to see first if I need to make a PCB layout (because I might be able to do them both in eagle instead of just making the perfboard layout into an etch image).

As always, I welcome any suggestions or questions about the design.

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I'm Jon. My band. Just me. Vids. PCBs: Bearhug & Cardinal: 1776 Effects. Hamlet & Blue Warbler: JMK PCBs. Flabulanche: Madbean


Last edited by midwayfair on Tue Mar 11, 2014 11:51 am, edited 2 times in total.

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PostPosted: Tue Mar 11, 2014 11:35 am 
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Wow - what a great looking project. So thorough - thanks! 8)

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PostPosted: Tue Mar 11, 2014 11:38 am 
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midwayfair wrote:
One last trick to soften the clipping was the use of a limiting resistor (R18) in series with the clipping arrangement, similar to how it's done in certain op amp based designs like the Bluesbreaker and AMZ's big muff mod with a "warp" control. Big thanks to tca on DIYSB for lots of help (and diagrams) in understanding this (plus he suggested the MOSFET as Q4!). Although I don't fully understand everything R18 is doing, but the gist of it is that once the diodes conduct, they appear in parallel with R16, and they will lower the negative feedback. R18 therefore helps set the clipping threshold and ALSO sets how "rubbery" the clipping will sound, AND just for good measure, also acts a bit like a clean blend...

This is supposed to be R19 instead of R18, right? On the posted schematic, R19 appears in series with the clippers and R18 appears to be the Q4 source resistor.

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PostPosted: Tue Mar 11, 2014 11:49 am 
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Morgan wrote:
midwayfair wrote:
One last trick to soften the clipping was the use of a limiting resistor (R18) in series with the clipping arrangement, similar to how it's done in certain op amp based designs like the Bluesbreaker and AMZ's big muff mod with a "warp" control. Big thanks to tca on DIYSB for lots of help (and diagrams) in understanding this (plus he suggested the MOSFET as Q4!). Although I don't fully understand everything R18 is doing, but the gist of it is that once the diodes conduct, they appear in parallel with R16, and they will lower the negative feedback. R18 therefore helps set the clipping threshold and ALSO sets how "rubbery" the clipping will sound, AND just for good measure, also acts a bit like a clean blend...

This is supposed to be R19 instead of R18, right? On the posted schematic, R19 appears in series with the clippers and R18 appears to be the Q4 source resistor.


Good catch! Thanks.

Tho just to muddy the waters a bit ... that source resistor will also affect the clipping loop. Negative feedback confuses me to no end. :P

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I'm Jon. My band. Just me. Vids. PCBs: Bearhug & Cardinal: 1776 Effects. Hamlet & Blue Warbler: JMK PCBs. Flabulanche: Madbean


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PostPosted: Wed Sep 24, 2014 5:53 pm 
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This is awesome! You can't imagine how much I'd love to hear a bass version... Thanks for this I may attempt it eventually. I'd jump on a PCB of it.


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PostPosted: Wed Sep 24, 2014 7:20 pm 
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static wrote:
This is awesome! You can't imagine how much I'd love to hear a bass version... Thanks for this I may attempt it eventually. I'd jump on a PCB of it.


You could increase C2, but otherwise the bass is pretty much untouched.

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I'm Jon. My band. Just me. Vids. PCBs: Bearhug & Cardinal: 1776 Effects. Hamlet & Blue Warbler: JMK PCBs. Flabulanche: Madbean


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PostPosted: Mon Feb 09, 2015 4:27 pm 
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For anyone who didn't know about it ... Madbean made a PCB project for this design! Go check it out:

http://madbeanpedals.com/projects/Flabu ... e_2015.pdf

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I'm Jon. My band. Just me. Vids. PCBs: Bearhug & Cardinal: 1776 Effects. Hamlet & Blue Warbler: JMK PCBs. Flabulanche: Madbean


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