EHX - Electric Mistress 18v Version 2 1976

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EHX - Electric Mistress 18v Version 2 1976

Postby bettsaj » 06 Jun 2018, 13:27

I'm planning my next pedal build, and this is going to be a biggy.

I want to build as accurately as possible a version 2 18v Electric Mistress. I've started researching this pedal as much as possible, apart from correcting the notorious volume drop i want to make it as authentic as I can..... Pretty much as I did with my Colorsound Powerboost and Violet Rams Head clones. if need be i'll print and etch my own board ideally cloning from an original, and build it using as many vintage correct parts as possible.

around Christmas time i was lucky enough to post something on the Farnell forum regarding this, and a kind gentleman in Holland contacted me and said he had 2 or 3 SAD1024 IC's that he didn't want. All are NOS and surplus to requirements. Long story short he sent them to me for free and I just paid the postage. With that in mind I've started to assemble the components required for the build, but i'm still having issues with a PCB. This is where you guys come in...... Has anyone here got a PCB trace of a 18v EM board, or can you point me towards one. I've looked everywhere and have hit brick wall upon brick wall.

Secondly, looking at the schematic, does anyone know what the workaround is for correcting the volume drop on these pedals. If i can alter slightly the PCB layout (when I eventually get one) to accommodate the modification I can do that at design stage.

1976-electric-mistress-v2-schematic.gif
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Re: Electric Mistress 18v Version 2 1976

Postby AuditoryDriving » 07 Jun 2018, 23:40

Adjust the IC1b resistors for more gain or replace them with trimpots! I cannot take credit for this, i think it was Skreddy who first proposed it,there s a post of his about it somewhere. I've done the same on my dem and it worked like a charm :)

Good luck!!
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Re: Electric Mistress 18v Version 2 1976

Postby MetalPose » 11 Jun 2018, 09:16

I guess increasing the gain before SAD1024 will cause clipping.
I increased the two 47nF capacitors (after the SAD1024 and for the dry path) and decreased the 13K and 10K resistors to the same ratio in my V2 18V Electric Mistress. I could scan the pcb of my pedal if that helps.
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Re: Electric Mistress 18v Version 2 1976

Postby bettsaj » 11 Jun 2018, 11:33

MetalPose wrote:I guess increasing the gain before SAD1024 will cause clipping.
I increased the two 47nF capacitors (after the SAD1024 and for the dry path) and decreased the 13K and 10K resistors to the same ratio in my V2 18V Electric Mistress. I could scan the pcb of my pedal if that helps.


A scan would be great! I already have a few that I got off the net but they're not all that clear to be honest.... it's the trace side i'm interested in, and also the PCB dimensions
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Re: EHX - Electric Mistress 18v Version 2 1976

Postby MetalPose » 12 Jun 2018, 10:18

I scanned it but it's blurry, i hope it works for you.
If you create the pcb, could you share it here?
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Re: EHX - Electric Mistress 18v Version 2 1976

Postby MetalPose » 12 Jun 2018, 10:19

I forgot to add dimensions,
11.5 cm x 10.3 cm
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Re: EHX - Electric Mistress 18v Version 2 1976

Postby DrAlx » 13 Jun 2018, 12:53

I spent a long time reworking that circuit and fixing all its problems. In particular the noise and volume drop.
Search for "18V Electric Mistress with reworked LFO & VCO" and you'll find all the details on one of the other sites.
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Re: EHX - Electric Mistress 18v Version 2 1976

Postby MetalPose » 13 Jun 2018, 18:47

DrAlx wrote:I spent a long time reworking that circuit and fixing all its problems. In particular the noise and volume drop.
Search for "18V Electric Mistress with reworked LFO & VCO" and you'll find all the details on one of the other sites.

With shielded cables and TL072, mine is pretty silent and with the mod that i’ve done in addition to the blondegraemey’s mod, it’s unity.
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Re: EHX - Electric Mistress 18v Version 2 1976

Postby bettsaj » 11 Jul 2018, 10:59

I have been lucky and found someone that was selling just and original v2 EHX board, minus the SAD1024 chip. Needless to say I bought it in a heart beat. I already have 2 SAD1024 chips so it was easy to just stuff one in the board, and rewire it up.

20180710_195957.jpg


20180710_200010.jpg


here's the end result....

Lash Up.jpg



All worked as it should so i'm going to use this board as the basis to make a board layout for heat transfer. I need to photograph the trace side at hi-res, load it in Adobe Illustrator and then basically trace the circuit. Once I've done that i'll print it to PDF at 100% scale.
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Re: EHX - Electric Mistress 18v Version 2 1976

Postby bettsaj » 12 Jul 2018, 08:38

Just a thought, after speaking to someone on another forum about this volume drop issue.

In the original circuit, both the direct signal and the delayed signal are reduced (roughtly 25:1 and 20:1 respectively) and then summed together to get the output. surely you don't need to then turn round and amplify the end result - You can just make the reduction a little less. What about changing the 470 resistor at the output to a 910? That will give you about twice the signal out and keep the direct and delayed signals in approximately the same balance. this wasn't my suggestion, but it does make some sense.
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Re: EHX - Electric Mistress 18v Version 2 1976

Postby marshmellow » 12 Jul 2018, 14:11

Increasing the resistor will increase the level, true. But your numbers are off, because you are ignoring the 47n capacitor to ground. Its impedance is roughly 3,4k at 1kHz. So the level drop is less than you state, and doubling the resistor won't give double the volume. The problem is by changing the resistor value you also shift the lowpass frequency of the filter that is formed by the three resistors and the capacitor to a higher frequency. You basically get a treble boost. But the output treble filter must match the pre-filter network around IC1B in front of the SAD, or you won't get an even response.

You can try a small increment, but I doubt you will get it to what you will perceive as unity without making it sound trebly or harsh.
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