Mantic Vitriol  [traced]

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Mantic Vitriol

Postby bobbass4k » 15 Feb 2017, 18:33

Apologies for posting without a schematic but I was on the forum anyway so I thought I'd get the ball rolling. I took the plunge on one of these last week as it looked interesting, actually with no intention of tracing it out, but I was curious and tracing it out has kept me out of trouble for the week.

I'm writing the schematic up properly now but it's basically the DOD FX86 Death Metal Distortion with added Gain/Distortion control, the mids control omitted and some value tweaks. The cap choices seem very deliberate, there's a few polarized 220nF, a few mylar 18nF but then one polybox 18nF and a 330nF tant.

Quick gutshot -

Image

I'll try and get the schematic up tonight.

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Re: Mantic Vitriol

Postby bobbass4k » 16 Feb 2017, 14:59

Here's a rough write up, electrically I'm pretty confident everything's fine, it's just a bit cramped and the proper voltage symbols seem to have disappeared from library so I used Vaa for +V and Vcc for the bias Voltage, I'll try and spruce it up tonight.

It's mostly SMT but with a lot of through-hole caps, some of the choices are pretty odd, how integral they actually are to the sound is debatable, but here's the list (all others are SMT MLCC or SMT/Leaded Polarized Electrolytic) -

C4,C12,C18 - Mylar Polyester
C17 - Polybox
C6 - Metallized Polyester
C7 - Polybox
C8 - Through-Hole MLCC
C13 - Non-polarized electrolytic
C16 - Through-Hole Tantalum
C19,C23,C24 - Polarized Electrolytic

Cap values marked with * are assumed from the FX86 Schematic as I wasn't able to measure them, all other cap/resistor values confirmed except C18 - It read 1.8nF but the meter was reading a bit high that day so I've assumed it's 1.5nF. There was no name on the FX86 schematic I used but thanks to whoever did it, big help.

I'm gonna breadboard it over the weekend, the omission of the mids control seems a bit odd, especially when there's a spare op-amp to use for the gyrator, I'm gonna add it on the breadboard and see how it works.

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Re: Mantic Vitriol

Postby bmxguitarsbmx » 17 Feb 2017, 17:22

Good work! :)

looks like a dod "grunge" pedal, which is a boss metal zone without the mid control.
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Re: Mantic Vitriol

Postby bobbass4k » 22 Feb 2017, 11:09

D'oh... I drew the op-amp gyrator for the low EQ wrong, I mirrored the op-amp to make it fit better and forgot to move everything else around... I'll fix it tonight. On the plus side, breadboarded it (with the fixed gyrator) and it works a treat. Added the mids control from the FX86 in too and it sounds really good, I'll add it on to the schematic as an option. I'll work on a stripboard layout this weekend.
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Re: Mantic Vitriol  [traced]

Postby bobbass4k » 23 Feb 2017, 01:24

New version, fixed a few things, added notes and optional mids control. In theory you can add as many bands to the EQ as you want, you could have a full blown 8 band EQ if you wanted, though that would be weird. The component values in the EQ gyrators are easily tweaked if you want a different voicing, here's a gyrator calculator i knocked up a while ago for an EQ build if anyone wants to tweak around with values - https://docs.google.com/spreadsheets/d/1pLr6ogImA3BYj2rfudnTNGCQLLWuUj8w-9EqtEJJLaY/edit?usp=sharing
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Re: Mantic Vitriol

Postby Belanger88 » 06 Mar 2017, 03:02

thank you:)
Imitation is suicide
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Re: Mantic Vitriol

Postby Frank_NH » 06 Mar 2017, 22:08

Thanks so much for the schematic. I was wondering about the use of the NPN BJT "buffers" in the gain and tone stages - what is the purpose of those versus a simpler resistor/capacitor network on the negative input side? I've seen them elsewhere (e.g. Zoom Tri-metal) but haven't found a good explanation online.
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Re: Mantic Vitriol

Postby bobbass4k » 07 Mar 2017, 00:30

They're Gyrators rather than buffers - active cut/boost band-pass filters, there's a quick Jack Orman explanation here - http://www.muzique.com/lab/gyrator.htm. The advantage here is that they're active - they can boost frequencies rather than just attenuate them out. With a simple RC filter you can only ever get as much bass or treble as you put in, but with a gyrator you can boost or cut a particular "band" of frequencies. I'm surprised they aren't used more in distortion pedals to be honest, the active bass boost is what makes this so great for bass, you get all the high end bite and a targeted, focused bass boost. This circuit is a bit odd in that it uses both BJT's and Op-Amp gyrators for some reason (the FX86 uses all BJT's). With the schematics on the Jack Orman link you can use the calculator on the page, or my google sheets calculator I posted earlier to see what each filter is doing, and you can tweak about with values for the EQ section (the low and mids controls) - centre frequency and bandwidth are very interactive and it's important to check how altering one will effect the other.
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Re: Mantic Vitriol

Postby Frank_NH » 07 Mar 2017, 05:41

Ah yes, I understand now. Thanks for that. So it looks like they use the gyrators not only for tone control but to actively boost the mids before the clipping stage. Interesting. May have to do some breadboarding to experiment with this...

One more question - what is the impedance of the gyrator? Just wondering how to calculate the gain for a op amp stage with a gyrator.
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Re: Mantic Vitriol

Postby johnk » 08 Mar 2017, 02:13

i just drew up, etched and tested a PCB for the schematic posted and it sounds great. thanks for posting the schematic!

here's a pic of the completed PCB:

Image

here's the layout:

Image

and here's the etch for it:

http://johnkvintageguitars.homestead.co ... OL_PCB.jpg

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Re: Mantic Vitriol

Postby johnk » 08 Mar 2017, 18:06

PCB and etch updated!
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Re: Mantic Vitriol

Postby EmmG » 07 Apr 2017, 14:47

Frank_NH wrote:Ah yes, I understand now. Thanks for that. So it looks like they use the gyrators not only for tone control but to actively boost the mids before the clipping stage. Interesting. May have to do some breadboarding to experiment with this...

One more question - what is the impedance of the gyrator? Just wondering how to calculate the gain for a op amp stage with a gyrator.


Undoing this pre gain mid boost is usually the first "mod" people do to their Metal Zones, which this basically is.. or a Grunge /Death Metal.
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