EHX -Electric Mistress 18V volume drop fix (proved on a V5)

dudeduck · Post by **dudeduck** » 05 Jul 2012, 08:25

Hi

You might remember me opening a thread about a recently purchased a v5 green logo Electric Mistress and asking for help regarding a polarity swap
Well it worked and the pedal sounds amazing, really good lush tone that gives my A/DA a run for it's money
But the volume drop made it absolutely useless for band practice through a small fender vibrochamp
So i browsed through the web and found very little info to fix this other that adding a booster circuit

Lazy as I am I reviewed Blondegraemy's fix for his v3-v4 and traced it to my v5
So anyway heres a quick and proven fix for V2-V5 electric mistresses
http://www.metzgerralf.de/elekt/stomp/m ... ematic.gif

The parts we are going to be refering here are universal for all electric mistresses but the pcb location might change you'll just have to trace from the output
So on the schematic there are 3 resistors a 470 10k and 13k
The 13k resistor goes to a 47n capacitor
What you'll do is put a 10k resistor in parallel with the 13k
and a 22n in parallel with the 47n capacitor
you could also replace the 13k for a 23k and the capacitor for a 69 i think
but i dunno if those values are even made

So anyway this fix isnt really a major discovery it's been there ever since blondegraemy uploaded his video explaining it
so all credit goes to him
I just tought about explaining it a little and posting pictures to help the v5 pcb users

Anyway Enjoy

PS Its 3 am here in Mexico and my camera sucks at night
Better quality photos tomorrow Cheers

dudeduck · Post by **dudeduck** » 05 Jul 2012, 10:22

Sorry I made a mistake you could replace the 10k in parallel with the 13k for a 5k6 resistor
but it's really just easier to put it in parallel

Fender3D · Post by **Fender3D** » 05 Jul 2012, 13:45

Sorry mate, but I'm a little dumb...
Shouldn't this mod change the dry/wet ratio?
How you and blondegraemy claim it's the same sound with just a "slight increase" in volume?
Don't you need a resistor in parallel with the 10K one?

dudeduck · Post by **dudeduck** » 05 Jul 2012, 18:39

Hi Fender 3D
Yes as mentioned you need a 10k resistor in parallel with the 13k already there
about the wet/dry ratio you could be right i dunno
I couldnt find any differences ear-wise about the dry wet ratio, however I could be biased because i never used it with my other larger amps
only through the vibro champ in wich the volume drop was so severe i couldnt what the pedal was doing in detail
I really dont know if this is the ultimate fix but it worked for me and maybe some more people like me that cant afford international shipping to howard davis workshop
and or want a "less scary" fix.

Also I just played around with it for a few minutes now that its daytime and i can play at my usual volume levels
and the volume drop it's still there but now its very slight and its definetly more useful on amps with less power like my champ
I will be playing through it more though, and post any differences in sound i notice regardinng the dry/wet ratio

Cheers Man

Fender3D · Post by **Fender3D** » 05 Jul 2012, 19:52

The 13k on the dry signal + the 10k on the wet are the output mixer.
In a flanger you have the best effect when both the dry and wet signals have the same level, so they can cancel each other by the phase difference.
Thus, if you decrease one path (dry in your mod) you must decrease with the same amount the other path (wet).
I don't know how much you can lower those resistors, before the output impedance influences too much the signal entering the BBD (that's why a buffer is a better option), but I'm sure you must decrease both path.
Try decreasing the 10k connected to VR2.

dudeduck · Post by **dudeduck** » 05 Jul 2012, 23:37

Then what is the purpose of the 22n capacitor?
does it filter more frecuencies to compensate?
Also isnt the dry the part that goes straight from the input before the input capacitor?
or do you mean inside the effected part of the circuit?
I'm not sure im making much sense, seems we have a languague barrier here

dudeduck · Post by **dudeduck** » 07 Jul 2012, 08:25

So after an extended playthorugh with the pedal i notice that indeed the dry tone of my guitar
is louder than the effected signal I could try reducing the resistance on that 10K after Vr2
but i wonder about what you waid about the impedance influencing the bbd to much
what's the worst case scenario?

Also if adding a buffer at the output wich in particular would be recomendable for this case?
Schematics would be apreciated
Thanks

Fender3D · Post by **Fender3D** » 07 Jul 2012, 15:48

mate, english is not my mother tongue, but I don't think our misunderstandings come from language, rather than flanger's working technology knowledge.
I mean, if you'd know why flanger "flanges", you would have understood why I talked about mixer and dry+wet signal.
No offence, but if you don't realize how important the wet/dry ratio is, for a proper flanger or phaser effect, you (and blondegraemy) can say the effect is working just because it somewhat "moves" or modulates applied signal.
I bet EH technicians spent several days before nailing those 13k AND 10k resistors...

You can't say

dudeduck wrote:So after an extended playthorugh with the pedal i notice that indeed the dry tone of my guitar is louder than the effected signal...

unless you get a scope or signal meter and MEASURE how much wet and dry signal you have.
Wet signal will be checked at VR2-7k5-10k junction, dry at 47n-13k junction.
Dry signal will be slightly higher than wet in those test points.
When I'm talking about signal, here and before, I mean the signal at IC1b's pin1.

To understand what I said about impedance influence try this:

: mix; mix.bmp (9.11 KiB) Viewed 3912 times

Then connect 2 guitar on left sockets and amp to right.
Play one guitar and turn down volume on the other.

You may use any mixer, the simpler would be that used in P90 (it uses 150K, just swap resistors for your needings), otherwise any inverting op-amp configuration.

Fender3D · Post by **Fender3D** » 07 Jul 2012, 17:25

Fender3D wrote:When I'm talking about signal, here and before, I mean the signal at IC1b's pin1...

Read this as "When I'm talking about DRY signal, here and before, I mean the signal at IC1b's pin1...

Where the **** is the edit button ?????????

dudeduck · Post by **dudeduck** » 07 Jul 2012, 23:00

Fender3D wrote:
Fender3D wrote:When I'm talking about signal, here and before, I mean the signal at IC1b's pin1...
Read this as "When I'm talking about DRY signal, here and before, I mean the signal at IC1b's pin1...

Where the **** is the edit button ?????????

I hear ya
I too looked for it after i realized just how stupid my statement before the whole language barrier was
And yes i understood, pretty late unfortunately
Well so what you mean is that the impedance resulting from decreasing the 10k after vr2 could affect the bbd
In what way?
would it just mess with the regen and make the color now more pronounced or could it turn the sad1024 into self oscillation?
(those reticon's are expensive/rare)
Also what was the porpouse of increasing capacitance of the 47nf capacitor in blondegraemy's mod?

I just said that i noticed the dry sound more in terms of what i heard and not thorugh the use of oscilloscope
I'm just a guy with a flanger trying to make it more useful and learn something in the way not an electronics prodigy

So how about that buffer then?

Thanks

Fender3D · Post by **Fender3D** » 08 Jul 2012, 00:41

lol
impedance is the analog circuits' boogey man...
it will always lurks in darkness ready to mess with your filters/levels you name it...

Since you'll likely have a high impedance pedal/amp after that mixer, using 2 lower resistors will let signal almost freely pass (try my posted schematic).
It won't harm your BBD, but it will alter the effect (very probably it will be like having feedback control somewhat "up", it might affect the signal entering BBD, and maybe other side effects).
I don't know why blondegraemy raised the cap value...
Look for P90 schematic, it has a PNP just before output.
PNP has 2 150k input resistors, these are the equivalent of EM 10k and 13k resistors, and a 150k from base to collector.
You may raise EM's resistors to 100k and 130k then adjust the 150k resistor to obtain desired level.

Cheers

dudeduck · Post by **dudeduck** » 08 Jul 2012, 03:21

Ok so something like this?

or this?

Also a couple more questions since blondegraemy's reasons for increasing capacitance are unknown should i just revert it to the original value?
And why cant the original 10k and 13k values be respected and decrease the other values in the same measure (15k betwen the collector and base)
Lastly would it be better to use a pot as a variable resistor between collector and base to reach unity gain and then measure and replace for a fixed resistor?
This lat question is based on what you said about EH placing thos values for a reason

dudeduck · Post by **dudeduck** » 08 Jul 2012, 03:37

One more thing they dont sell 2n4125's in my town
i could scavenge one from something but i do have a PNP 2sa495y
dunno if it would be suitable though

Thanks

Fender3D · Post by **Fender3D** » 08 Jul 2012, 15:19

It was too late last night for a proper reply...
Someone here around said "Do not post at 3AM"...

So..... after a simulation, try this:

: EM buffer.bmp (17.28 KiB) Viewed 3888 times

Any PNP will work. You may trim the 50k and set desired level.

You owe me a beer

dudeduck · Post by **dudeduck** » 09 Jul 2012, 10:39

it's been done but I still have a volume drop even with a total resistance of 150k betwen base and collector

Fender3D · Post by **Fender3D** » 09 Jul 2012, 11:29

with 100k input resistor and 150k feedback resistor you should gain a couple dB.
If your volume drop is higher than this, maybe you have other issues....
Raise the 82k, then report.

osa · Post by **osa** » 10 Jul 2012, 15:01

Make an LPB 1. Attach it to the output at the board. Use a trimpot to adjust volume to taste. Simple.

dudeduck · Post by **dudeduck** » 11 Jul 2012, 10:01

osa wrote:Make an LPB 1. Attach it to the output at the board. Use a trimpot to adjust volume to taste. Simple.

Done and done
I really didnt want to make a circuit or anything i just looked fof an easy fix
why? couse I'm really lazy
But this whole ordeal has taught me that I shouldnt be
Otherwise i would have put a linear gain device in the out on the first place
but really your comment was really the last push i needed

Thanks Man

Skreddy · Post by **Skreddy** » 17 Jul 2012, 06:14

It's a simple matter of changing the resistors on the opamp to change the volume; an extra gain stage is unnecessary. Here's how I modded my v4 for example to take care of the volume drop and reduce the noise quite a bit (still has a way to go, but I've left off at this point)...

Fender3D · Post by **Fender3D** » 17 Jul 2012, 14:22

Skreddy wrote:It's a simple matter of changing the resistors on the opamp to change the volume; an extra gain stage is unnecessary....

This is true until you don't overdrive BBD