DSM OmniCabSim

General documentation, gut shot, schematic links, ongoing circuit tracing, deep thoughts ... all about boutique stompboxes.
bobbass4k
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Post by bobbass4k »

I thought I'd put this up a while ago but apparently I didn't. True story.

A friend asked me to pad the XLR output on his and it would have been rude not to trace it out. This is the original one, and an early one at that (lots of mods/hacks on the PCB to make it functional). The only bit I'm not 100% on is the output buffer, it used a 4066 for switching in a way I haven't seen before and I only had it for an evening so I had to give it back. The caps were SMT, measured in circuit and removed any that didn't make sense to verify so they all be accurate but some tweaking may be required.
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J0K3RX
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Post by J0K3RX »

It works in Multisim.. :wink:

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temol
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Post by temol »

What's the purpose of the U1.2?

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Post by bugg »

temol wrote:What's the purpose of the U1.2?

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My guess is that it was left over from the buffered bypass switching. Doesn't appear to have a functional purpose to me.

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temol
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Post by temol »

I thought it might be left over from the line input (summing opamp)
thx.

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Post by bugg »

Ahh yeah, very well could be. I'm not terribly familiar with the original, just going by the schematic posted.

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Post by bobbass4k »

Yeah U1.2 can probably omitted - the original switching method was a bit weird, there seemed to be more buffers than necessary just because he had leftover op-amps in one of the TL074s. I included 1.2 because that is how the signal chain is when it's on, but it probably makes sense to omit it because then you can use 1 quad and 1 dual and save space.

I've breadboarded this now btw and sounds pretty close (though I don't have the original anymore to A/B).

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Post by J0K3RX »

Removed U1.2, multisim tested, works fine as expected.

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Post by marshmellow »

The last one is superfluous as well.

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Post by Lithium_Grim »

I think all pins of the pots are reversed. That is if you marked pin 1 with a dot in the schematic. The dot should indicate pin 3 not pin 1.
Got this one on the breadboard and testing it. It does work, now I am only A/B it to the videos.

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Post by bobbass4k »

The pots are marked correctly as they should be as the circle indicates pin 3 (clockwise). This is how EasyEDA (and most CAD programs I've used) does it and as far as I'm aware the convention is to mark the clockwise direction (pin 3), I may be wrong though...

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Post by Lithium_Grim »

bobbass4k wrote:The pots are marked correctly as they should be as the circle indicates pin 3 (clockwise). This is how EasyEDA (and most CAD programs I've used) does it and as far as I'm aware the convention is to mark the clockwise direction (pin 3), I may be wrong though...
Ok thanks, did not know that (as I use a different app). Still pin 4 and 11 of the TL074CN are reversed on the schematic. 4 is to Vcc (+9V) and 11 to ground.

I got it bread boarded and on vero and on both it really suck the distortion/overdrive out of the signal. Do you have any pics from the pedal guts left so we can double check the values?

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Post by bajaman »

I simulated this with multisim and found that a few of the component values are wrong, if you want to achieve the responses that DSM gives for this pedal in the downloadable pdf owners manual ;-)
In particular - the punch frequency boost is not centred at 800Hz as the owners manual states, but closer to 400Hz with the given component values.
I have made a few changes to component values that allow the circuit to give similar responses to the owners manual and will publish these shortly. :wink:
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Deluxe OmniCabSim owners manual reduced size.pdf
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bajaman
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Post by bajaman »

here you go :lol:
omnicabsim.png
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Post by Lithium_Grim »

Thank you!! Those are some significant differences. I see you connected the 18k resistor after punch to Vb and not ground as in the original schematic. Why is that?
Also some added resistors (1k5 for mid response and 2M2 parallel to cabsize) and changed values.

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Post by Lithium_Grim »

Also, looks like unity gain is not on 12 o'clock, but more around 9/10 o'clock. I'll try your values later today and see what the changes will be :) Thank you Bajaman!

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bajaman
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Post by bajaman »

the 18k resistor could go directly to ground - no significant difference in performance either way :wink:
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Post by Lithium_Grim »

Tested the new values and they really are an improvement! It is more open now. Before it sounded like a blanket over the sound/amp. Still the cabsize pot does not really do much and also the gain does not do unity at 12 o'clock, still around 10 o'clock.

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Post by bobbass4k »

Re: Bajamans schematic - Thanks man, I wasn't too sure about the punch cap values and I checked the other filters against the manual but did it before I'd settled on the punch section, your values make a lot more sense. Just a disclaimer on the other changes - I think the manual probably uses neat rounded values, or there's been minor tweaks from the early model I traced. I don't want to come off all "precious" about my schematic, because I'm not at all, just noting that baja's uses values/arrangements that are 100% not from the original.

Re: Gutshots, I didn't take any detailed ones as it was SMT caps (bar a couple) and black solder mask, so they wouldn't have been much use, sorry. I'm going to borrow it again and take it in to work to use the good cap meter so I should have an update next week.

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Post by Lithium_Grim »

Thanks man, I cannot wait! Maybe you can also shed some light on how the XLR, through and headphones are implemented so we know for sure what the extra opamps are used for :)

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