EQD Monarch

[chongmagic]

I have hard some good things about the sound of this drive.

 

[music6000]

I've seen a sketch that puts a 1M resistor from gate to ground on 2nd & 3rd stages. I helped Chong get his pedalparts Orange King working by adding the 1M to the 2nd stage and shorting out the 1uF coupling cap between the 2nd & 3rd stages. Now I'm thinking that rather than add a 1M resistor to the input to the 2nd stage, short out the 1nF cap instead. I reviewed the Orange Graphic amp schematics, on which this was allegedly based, and I think the latter approach makes more sense. I'm gonna breadboard this and try out the fixes for myself.

Is this the same one that chokes up & dies when the Treble pot is Cranked.
One chap installed a 150k resistor between the 1nF Cap & Treble 3 and said it fixed that issue.

 

[Chuck D. Bones]

Here is an LTSpice sim of the Monarch. I did three things that deviate from the broken design.
1) Shorted out C12.
2) Replaced the JFETs with 2N4393 because I don't have a model for 2N5457 and these are close enough.
3) C10 is either shorted out (red trace) or grounded (cyan trace).

In the pedalparts schematic, R6 is 330R. Changes the gain of the 1st stage.

This is with all controls at noon, except VOLUME is dimed. Grounding C10 reduces the Presence significantly.

 

[Chuck D. Bones]

With GAIN at zero, the effect of grounding C10 is less pronounced.

 

[Chuck D. Bones]

And with GAIN dimed...

 

[Chuck D. Bones]

Let's assume that the people who traced the EQD board made no mistakes. We're not going to know what the EQD designer had in mind by putting in C10 and C12. I've seen plenty of strange stuff in EQD designs. We're also not going to know how many of these pedals failed test at EQD and got thrown on the scrap heap. For this circuit to bias properly, the JFET gates all need a DC path to ground. C10 and C12 block that path and all we're left with is "leakage biasing." Leakage biasing makes sense with leaky germanium transistors, but is just plain wrong with JFETs.

I don't see any reason to include C10 or C12 on the board, they do more harm than good and just because they were on the BOM isn't a good enough reason to keep them.

One other point about C10: grounding it it has the same effect as dialing down the TREBLE control. If you set TREBLE to zero and the sound is still too bright, then increase the value of C15.

 

[reubenreub]

Absolutely brilliant work yet again, Mr. Bones! I think removing both of those caps and replacing with jumpers is a great solution. Looks like I need to learn how to use ltspice because this is wonderful.

 

[Chuck D. Bones]

Simulation is no substitute for a listening test, but it does help understand why circuits do what they do.

 

[chongmagic]

Circling back to this for a moment, you know Stew Mac sells the Monarch EQD kit, I wonder why it is never in stock?

Correction, they did have it, https://www.stewmac.com/dropped_ite...cs/EarthQuaker_Devices_Monarch_Pedal_Kit.html

Here are the build docs I found for it: https://www.stewmac.com/freeinfo/i-2203/i-2205_monarch.pdf

No schematic listed whatsoever.