Big Muff Opamp with James - crazy vol!

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tonymcbony
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Post by tonymcbony »

Hi folks,

Been tinkering with a circuit of mine for a while and trying to get around a design knowledge problem I have.
Basically I have an Opamp Big Muff running into a James Tonestack and I wanted to use a TL074/Quad Opamp so I used the last Opamp stage to act a boost.
I *almost* got there – the design "works" but I tried to be clever and have a pot in the FB loop of the opamp to act as a volume, with unity as 0 and boost from there.

The flaw is that at really high EQ settings (Both Treble and Bass Maxed) the James tonestack is barely doing anything so at the lowest Vol it can go, the pedal is loud as hell, but for low EQ settings you really need that extra grunt.

Here's my current schem:
Screen Shot 2022-09-19 at 9.32.36 AM.png
I tried just replacing R20 as the a 100k pot to ground with lug to going to output but it never went silent – in fact I think I accidentally created a LPF filter!

Would love some advice here – I feel this should be really simple but it's showing big gaps in my DIY slopped together knowledge

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blackboarcult
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Post by blackboarcult »

I'd look at the Fender Frontman 212r schematic (clean ch)... Has a clever way of inserting a vol pot in the feedback loop of an opamp :thumbsup

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tonymcbony
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Post by tonymcbony »

blackboarcult wrote: 19 Sep 2022, 03:20 I'd look at the Fender Frontman 212r schematic (clean ch)... Has a clever way of inserting a vol pot in the feedback loop of an opamp :thumbsup
D676D9F1-2197-4CCE-B154-FF3C5DE8B5FD.png
That’s really interesting!

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Post by blackboarcult »

You can find further examples in old SS Peaveys (Renown, Special, Audition etc.). The approach is the same, although they use it at the beginning of the circuit, IIRC.
Ref. the Fender one, if you were to clone that stage, make sure to tinker with the values of the resistors and capacitors on the inverting input, I remember them affecting the amount of treble across the pot range. Easiest thing is to throw it in Spice and check the freq response, output impedance etc...
Hope you'll find a suitable solution :wink:

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phatt
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Post by phatt »

Hello, I just simulated your circuit and I'll add my observations;
Input Z is very low for guitar, You are likely better served by making U1 a buffer input as you can make up gain in later stages.

Regards the tone problem;
You have little treble response. the issue seems to be partly due to C2 & C3. the way it is now the treble starts to roll off at 100hz which is useless.
Try C2 at 1n8 and C3 at 3n3.

Then the tone stack needs to be reworked as the Z of that is likely out of wack, hence little control so I'd guess values need to be changed.
Also you need to set the gain for the output stage then use a simple divider pot for level out.
I would just make that another buffer stage driving a pot for output level as it's already too loud.
frankly I would not use this type of Tone circuit as there is no mid control which IME is critical for OD circuits :|

If you want I can redraw the whole thing with a better Tone Circuit.
Sharper minds here might see things I've missed.
Keep going as you will learn as you go :thumbsup
Hope it helps, Regards, Phil

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tonymcbony
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Post by tonymcbony »

phatt wrote: 20 Sep 2022, 07:22 Hello, I just simulated your circuit and I'll add my observations;
Input Z is very low for guitar, You are likely better served by making U1 a buffer input as you can make up gain in later stages.

Regards the tone problem;
You have little treble response. the issue seems to be partly due to C2 & C3. the way it is now the treble starts to roll off at 100hz which is useless.
Try C2 at 1n8 and C3 at 3n3.

Then the tone stack needs to be reworked as the Z of that is likely out of wack, hence little control so I'd guess values need to be changed.
Also you need to set the gain for the output stage then use a simple divider pot for level out.
I would just make that another buffer stage driving a pot for output level as it's already too loud.
frankly I would not use this type of Tone circuit as there is no mid control which IME is critical for OD circuits :|

If you want I can redraw the whole thing with a better Tone Circuit.
Sharper minds here might see things I've missed.
Keep going as you will learn as you go :thumbsup
Hope it helps, Regards, Phil
Wow, thank you so much Phil!!!
Simulating the circuit is above and beyond, I appreciate that so much!

I’d love to try any suggestions you have, particularly around the tone control.

If you care to draw that up I would definite love to learn from what you’re thinking.

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phatt
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Post by phatt »

Yep sims although not always perfect can certainly help you find how/where tone is won and lost.

You can try this out but understand I've not built this circuit but having built many similar circuits over many years It a fair bet this should work & sound better.

If you want more bass make R9 56k. Lower C5 value for more top end.
If you have this on a bread board then don't be afraid to experiment. :thumbsup

Some here will recall my PhAbbtone circuit of which I've now made so many for local guys I've lost count but it's over 30 and I've not had any complaints.

Yes I use this tone circuit in my rig as I have done for many years.
The McBony Phatt mods
The McBony Phatt mods
Have fun, Phil.
Attachments
Old /Mod tone compare
Old /Mod tone compare

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tonymcbony
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Post by tonymcbony »

phatt wrote: 20 Sep 2022, 12:46 Yep sims although not always perfect can certainly help you find how/where tone is won and lost.

You can try this out but understand I've not built this circuit but having built many similar circuits over many years It a fair bet this should work & sound better.

If you want more bass make R9 56k. Lower C5 value for more top end.
If you have this on a bread board then don't be afraid to experiment. :thumbsup

Some here will recall my PhAbbtone circuit of which I've now made so many for local guys I've lost count but it's over 30 and I've not had any complaints.

Yes I use this tone circuit in my rig as I have done for many years. McBonyMuff-PhattModsSchem.png
Have fun, Phil.
:worship:

Oh amazing! Can’t wait to crack out the breadboard tonight!

Off the bat I like the fact there’s more sensible pot values :mrgreen:

Will give it a go and report back

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Post by phatt »

Glad to help,, if you want more distortion then reduce the value of R5, Below 10k will likely cause problems.
Phil.

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