Homebrew distortion unit

[NorthernLightX]

Hi all,

I've just finished testing my homebrew distortion unit. It has a lot of tweaking options, probably more than most midiboxers need, but feel free to adapt the schematics to your liking.

The circuit has been tested with a synth and I find it to be quite useful. It could be a nice addition as a preamp or insert FX for the MBHP_SID and MBHP_FM modules, therefore I post it under User Projects. If the mods deem it to be misplaced here feel free to move it around.

The documentation is available in both OpenOffice .ODT and in plain .TXT

The schematic looks like this:

The eagle schematic file can be found here:

If you would want to make a (guitar) pedal from this schematic you could use this artwork:

[update]

Everything is relocated to the WIKI: http://www.midibox.org/dokuwiki/northernlightx

[/update]

Artwork for a 1U 19" module will be created somewhere in the near future.

Many thanks go out to Mendelt for endless help designing and documenting the project.

I am still waiting for a few parts to arrive to finish the pedal, and will post pictures of the building process in this thread.

Any comments are appreciated :)

Cheers,  Alex.

[TK.]

[ x ] sound example with MIDIbox SID

[ x ] please :)

Best Regards, Thorsten.

[NorthernLightX]

I would love to post a sound sample but I haven't figured out how to send out midi AND record an audio stream at the same time with my soundcard (phonic firefly) yet. Will try to record something tomorrow.

[TheAncientOne]

Two thoughts:

1) remove the grounding switch from the output jack, or it will short the op-amp to ground when the jack plug is removed.

2) Put an end stop resistor (2K2 or so) in the ground side of R5, to avoid overloading the opamp output stage whe the wiper is at the grounded end.

Just a couple of ideas

[NorthernLightX]

Two thoughts:

1) remove the grounding switch from the output jack, or it will short the op-amp to ground when the jack plug is removed.

Why is that a bad thing?

2) Put an end stop resistor (2K2 or so) in the ground side of R5, to avoid overloading the opamp output stage whe the wiper is at the grounded end.

I have studied a lot of distortion schematics and the hard clipping diodes are always shunt directly to ground. R5 lets you tune between hard clipping and soft clipping (diodes in the feedback loop) or overdrive (no clipping diodes at all) depending on the position of S2. So please explain the benefit of not overloading the opamp.

Just a couple of ideas

And they are appreciated ;)

Cheers, Alex.

[TheAncientOne]

Why is that a bad thing?

Because it can make no difference to operation if the output lead is removed, but if the input is still driven the last op amp is driving a short, which may harm it in the long run, and will certainly not help your battery life,

I have studied a lot of distortion schematics and the hard clipping diodes are always shunt directly to ground. R5 lets you tune between hard clipping and soft clipping (diodes in the feedback loop) or overdrive (no clipping diodes at all) depending on the position of S2. So please explain the benefit of not overloading the opamp.

I agree that clipping diodes are usually to ground, but the way you have arranged them, there is no limit on current flow from the op amp if the pot is at the end. The op amp is effectively driving a very low resistance about 0.7V above ground. A small resistor will not have much audible effect (unless the op amp overload is part of the sound you want), but again is kinder to the op amp and batteries.

I need to get some time and try it out, and I'll post back then. I've done a matched pair of tube screamer clones, (for stereo etc), and have a number of others I need to fit to a rack panel, including a tonebender clone made with 3 of my last real Mullard 0C75's, (I used to make a lot of fuzz/FX boxes for guitarists years ago). I can really recommend PAIA's "Stack in a Box", a real valve with a low voltage supply - dirty but warm. http://www.paia.com/prodimages/siabsch.pdf. I modded it to use a DC regulated heater and had a transformer suitable to get the higher voltage without using the voltage multiplier, so got less background noise, (and it's pertty quiet as is). You can do a lot of tweaking of the tone stages to get a particular 'cabinet' sound if you like.

Best wishes

Mike

[DrBunsen]

Have you added this to the Wiki?

[NorthernLightX]

Have you added this to the Wiki?

Not yet, I will do that when everything is finished. Right now there are some options which are not very useful for synth use (the tone stack and level pot, a mixer can do that much better) and a low-pass filter to cut out noise could be a nice addition - the current schematic is for guitar. As soon as I think the thing is finished I will add everything to the wiki.

[NorthernLightX]

Because it can make no difference to operation if the output lead is removed, but if the input is still driven the last op amp is driving a short, which may harm it in the long run, and will certainly not help your battery life

I believe you, so I will change that.  ;)

Put an end stop resistor (2K2 or so) in the ground side of R5, to avoid overloading the opamp output stage whe the wiper is at the grounded end.

I agree that clipping diodes are usually to ground, but the way you have arranged them, there is no limit on current flow from the op amp if the pot is at the end. The op amp is effectively driving a very low resistance about 0.7V above ground. A small resistor will not have much audible effect (unless the op amp overload is part of the sound you want), but again is kinder to the op amp and batteries.

When creating the audio samples today I noticed a steep drop in audio signal when that pot was at minimum or maxiumum value so your story turns out to be quite relevant. I will add this resistor to the test setup soon, and listen if there are any audible effects. When tweaking the pots the thing sometimes produces resonant filtered sounds, I like to keep those :)

Thanks for the feedback!

Cheers,  Alex.

[NorthernLightX]

[ x ] sound example with MIDIbox SID

[ x ] please :)

Best Regards, Thorsten.

I made a very odd setup, but managed to create some sound samples with the SID. here's what I did:

Laptop with fruityloops -> MIDI interface -> MidiBox SID V2 -> Distortion -> PC with windows soundrecorder

It actually worked quite good :)

Unfortunately soundrecorder decided that I should record in 22KHz 8bit, so a lot of the samples I made were bad quality, and I canned them. I then re-recorded the last loop I recorded at 44KHz 16bit, while tweaking the pots of the distortion unit.

[edit]

soundsamples removed, not good examples of current build

[/edit]

The first 2 bars are with a neutral tone setting, drive at 0. Some things you will notice:

- Bass gets cut out when the drive is turned up

- Noise also increases when the drive is over 60%

I hope you like the sound, the only effect used is the distortion. I would have liked to put some delay in, but this way it is easier to hear the distortion do its work.

Cheers,  Alex.

[stryd_one]

I made a very odd setup, but managed to create some sound samples with the SID. here's what I did:

Laptop with fruityloops -> MIDI interface -> MidiBox SID V2 -> Distortion -> PC with windows soundrecorder

It actually worked quite good :)

LOL Why didn't you record into fruity? Hey whatever, it works! That aside, doesn't seem odd to me, sounds like a standard kinda setup?

Unfortunately soundrecorder decided that I should record in 22KHz 8bit

Grrrr.

I then re-recorded the last loop I recorded at 44KHz 16bit, while tweaking the pots of the distortion unit.

Thankyou sir ;)

The first 2 bars are with a neutral tone setting, drive at 0. Some things you will notice:

- Bass gets cut out when the drive is turned up

Is it bass getting cut out or highs getting brought in? It seems like there's some filtersweeping going on there so I'm really not sure... Or

- Noise also increases when the drive is over 60%

I hope that a distortion unit would do that from about 1% ;) I assume you mean the background noise.. I wonder if you are able to properly monitor your input levels with windoze sound recorder, maybe it's clipping a bit? Sure doesn't sound that way, but it is a disto demo ;)

the only effect used is the distortion.

No filtersweeps? It sure sounds like it! around 25 and 40 seconds sounds like no distortion I've ever heard... Heck pretty much the whole thing is effected isn't it?

It might just be the distortion being nice, but it even sounds like the ringmod or sync or something is modulating at the end (low end growls around a bit)... I was listening quickly (I'm in the middle of Funcrusher!) on a so-so system, trying not to be loud and annoy the neighbours, so I dunno, this might all be BS.

[Sasha]

I hope that a distortion unit would do that from about 1%

;D ;D ;D ;D ;D

[NorthernLightX]

LOL Why didn't you record into fruity? Hey whatever, it works! That aside, doesn't seem odd to me, sounds like a standard kinda setup?

I couldn't figure out how to do that. Since I needed the sample quickly this was the shortest solution.

I hope that a distortion unit would do that from about 1% ;) I assume you mean the background noise.. I wonder if you are able to properly monitor your input levels with windoze sound recorder, maybe it's clipping a bit? Sure doesn't sound that way, but it is a disto demo ;)

I mean unwanted noise. It could be background noise from the SID, from the dodgy jack connector I used in the box, from the less than optimal cables I soldered year ago, or from recording a mono signal through multiple connector-adapters into a stereo port. Either way, I think proper cabling could bring down the noise, and a variable low pass filter (a pot and a capacitor is all it takes) instead of the tonestack and level pot would be an improvement for synth use. The sounds were recordes of a setup intended for guitar.

No filtersweeps? It sure sounds like it! around 25 and 40 seconds sounds like no distortion I've ever heard... Heck pretty much the whole thing is effected isn't it?

It might just be the distortion being nice, but it even sounds like the ringmod or sync or something is modulating at the end (low end growls around a bit)... I was listening quickly (I'm in the middle of Funcrusher!) on a so-so system, trying not to be loud and annoy the neighbours, so I dunno, this might all be BS.

The SID patch might have had a LFO's assigned to CutOff and/or Resonance as default, but I did not use any outboard effects (filters, reverb, delay, whatever) other than the distortion, nor did I tweak any settings on the SID.

Cheers,  Alex.

[NorthernLightX]

This weekend we tested the thing extensively with guitar and basguitar, and a few quirks came to light:

- The M5218AL opamp I used is not so suitable as an overdrive opamp

- The potmeters I used for my test setup could be a bit dodgy

Also some things in the clipping section were changed, as can be seen in the new version of the schematic. R5 was reduced to 10K, and the soft/hard clipping routing was altered. In the previous version the switch could select overdrive or soft clipping, and the pot could turn to hard clipping. Now the switch selects between soft and hard clipping, and to pot turns to standard overdrive, much better now ;D.

I also adjusted the wiring of the output jack as per TheProf's advice. I did not put in the suggested resistor, because clipping to ground has a nice sound, and if you want to be gentle to your opamp just don't turn the pot all the way.

I already uploaded the latest version of the schematic in borth Eagle and .PNG, docs will follow shortly.

Cheers, Alex.

P.S.

The current version is most suitble for guitar. In the near future, when the stompbox is finished, we will make some small adaptions to make it more suitable for synth use. Things to consider are:

- reduce imput impedance to 10K instead of 1M, to reduce unwanted noise

- leave out the Tone section and Volume section, because a mixer can do this much better with it's EQ and Gain setting

- make the thing stereo

Oh by the way, the opamp picked instead of the M5218AL is the TL082.

[edit]

Reminder: you have to press F5 to see the revised image in the tread.

I also uploaded the revised docs just now.

[NorthernLightX]

Just for the record: there are going to be some minor changes to the design (adding 2 caps and 1 resistor), mostly to protect the opamps and the equipment after the stompbox. Testing those will occur this weekend, and I hope to be able to post some pictures of the construction of the stompbox too :)

[Jaicen]

Just out of interest, have you tried an LM308 op-amp??

It's not particularly well suited for audio, but when it's driven rail to rail (as in this design), it has a nicely overdriven sound because its slew rate is so low. In practice, this means that it has frequency selective distortion which is very nice in my opinion. FWIW, the 308 is used in the old ProCo Rat guitar pedal, so chek that out for a taste of what it sounds like. My guess is a 303 through one of these would sound immense!

[NorthernLightX]

Just out of interest, have you tried an LM308 op-amp??

It's not particularly well suited for audio, but when it's driven rail to rail (as in this design), it has a nicely overdriven sound because its slew rate is so low. In practice, this means that it has frequency selective distortion which is very nice in my opinion. FWIW, the 308 is used in the old ProCo Rat guitar pedal, so chek that out for a taste of what it sounds like. My guess is a 303 through one of these would sound immense!

Hi Jaicen, no, the only opamps we tested were the M5218AL and the TL082. In theory any dual opamp with the same pinout (and the pinout is very standard) could be used. Builders of this schematic are encouraged to pick the parts they find sounding best, that's the reason we did not specify the clipping diodes in the schematic too.

I believe anything through this sounds immense, the sound samples I recorded were not so hot compared to what I hear the thing produce last weekend ;)

For synths a few changes need to be done though, and we will speciy those in the near future. Mendelt also wants to make a special version for bassguitar, all findings will be posted here and I will make a wiki article eventually.

Cheers,  Alex.

[NorthernLightX]

Progress to report:

The schematic is final now. Two 100uF caps where added, one to protect the first opamp when doing hard clipping, and another other to protect the equipment connected to the output jack from receiving DC current. A 47K resistor was also added to keep the output jack at 0v, also to protect the equipment connected to the output jack.

The 100nF cap between the Tone and Volume pots was removed from the schematic because we couldn't remember why we put it there originally (:D), and removing it proved to have no influence on the sound.

On a personal note: Tonight I made an attempt to apply Lazertran to the pedal case I'm making. I might have been too hasty turning up the heat, as there are quite a few bubbles on the durface now. I thought I had a bottle of turpentine, but it turns out to be ordinary white spirit, which can't be used to remove the plastic lazertran residue. Sticker remover removes both the residu and the lazertran, I found out  :-\ (luckily not on the front panel)

I'll keep you guys posted if there are any updates.

[NorthernLightX]

Somewhat unclear photo's, but it is finished at last  ;D

Sound is very fat and tweakable. I will try to record some more demo's shortly. Lazertran was a disaster. Only a very faint layer stayed on the metal, most likely because I brushed the aluminium. Next time I'll polish it instead of brushing, flat surfaces work better with lazertran I read. Anyway, I hand-painted over the faint lazertran decal and I like how it came out.

Final schematic and documentation will be updated soon. The capacitor we removed does have a function after all: if you use assymetric clipping this cap protects the second opamp from DC. This will of course be documented, and finally be put into a WIKI page.

Cheers for now!

[MTE]

Nice One  :D

ROAAARRRR  ;D  ;D

Best Regards