Modding EH Micro Synthesizer

[orni]

I'd like to try modding my Electro-Harmonics Micro Synthesizer. I find the sound of this box quite 'ducky' and it is due to the filters.
I've read somewhere that the input filters are responsible of this sound.

Here is the schema:

http://filters.muziq.be/files/schematics/eh_microsynth.pdf

What should I do on C1, C2, C3  to suppress the filtering?

And would replacing the RC4558 op amps a good idea?

[Mark Hammer]

There is a mods article in DEVICE at my site (http://hammer.ampage.org  check on page 9 or 10). Most of the mods are directed at making it more reliably usable, though, rather than changing the tone.

[Bill Bergman]

swt had some mod/fixes, but I think they were for the DIY version. Not sure it would apply to a original unit.

[swt]

Bill is right...the mods are for the diy. I like the original pedal anyways...

[orni]

Could you have a look on the schematic and tell me how to remove the low pass filter at the input? I love this pedal but it sounds like a duck  :?
Would simply remove the  C1, C2 and C3 caps work?

[niftydog]

could you expand on the term "ducky"?

This interests me but I'm not sure we're talking about the same kind of sound that I have in my head!

[orni]

I mean by 'ducky' a sound as produced by a duck. Nasillard.
For instance, if I set all sounds to zero except the 'guitar' trim, it is a very low-fi sound.
In the same site mentionned above, there was a review of the Micro Synth. And basically what the reviewer said was about this sound, very certainly due to the low pass filter. I would like to remove this filter, making the 'guitar' pot sound more like the original sound of the guitar.

[nelson]

Bill is right...the mods are for the diy. I like the original pedal anyways...

There is a diy version of this pedal!!??   :shock:

WHERE!? Who cloned/built it?

[Bill Bergman]

I and SWT have built one. I pretty sure some other people have also. It's on three pcbs. Francisco made layout for one of the boards and I made the layout for the other two. SWT came up with some fixes and improvements.

[Mark Hammer]

Could you have a look on the schematic and tell me how to remove the low pass filter at the input? I love this pedal but it sounds like a duck  :?
Would simply remove the  C1, C2 and C3 caps work?

C1, C2, and C3 need to stay where they are.  They form a 3-pole lowpass filter that provides a number of other portions of the circuit with a more "intelligible" signal.  Remember that the pedal has to be able to correctly identify the note so as to be able to generate an octave down that is actually an octave down from the note you are playing, not a harmonic or relative of that note.  Indeed, I wish more octave down pedals HAD such filtering on the front end.  They might behave better.

It is possible the filtering you want to tinker with is the actual VCF itself, which is formed around A10, A11, and A12.  This forms a 3-pole lowpass filter (not quite Moog but better than a 2-pole) whose range is set by C25/C26/C27.  If these caps are made smaller in value the uppermost sweep of the filter is set higher.  Making them .0022 (2200pf) might get you what you want.

[Bill Bergman]

That's interesting Mark,
it really does seem to track better than some octave downs I've tried.

[nelson]

I and SWT have built one. I pretty sure some other people have also. It's on three pcbs. Francisco made layout for one of the boards and I made the layout for the other two. SWT came up with some fixes and improvements.

Any chance I could get the PCB layouts/transfers?


I was dreaming that maybe I could draw a PCB for it.....at some point, but if there is a layout available I could use that time to make the EH Bass synth. Having the guitar synth layout would go a long way to helping me do that too.

[Bill Bergman]

I think there is just a cap or two difference between the bass and guitar version though I could be wrong. I have the layout but it's at home. I could send it to you tonight.

[nelson]

I think there is just a cap or two difference between the bass and guitar version though I could be wrong. I have the layout but it's at home. I could send it to you tonight.

Sounds GREAT Bill.

I will PM you my address.

If it is just a few caps I could just use one synth for both instruments, switching the caps. I will check my schems.

Thanks!

[swt]

i have them at home also...let me know if you didn't get them...

[Gila_Crisis]

here's how to do the stop freq expression pedal mods, quite simple. by shotgunn!
note the expression pedal needs to have a 100k lin pot to work right

I used a TRS 1/4" phone jack, with DPDT switching capabilities.
The mod requires 5 wires from the jack to the PCB, and three traces that must be cut.

The cut traces are - 10V, and two Ground traces. The first ground trace to be cut is the one between the Start and Stop freq faders. Now on the other side, (the right side, not the bottom) there is a small ground trace goinf off to a resistor. This too, must be cut and replaced with a jumper from the resistor back to the ground of the Start Freq fader.

The -10V supply goes to the common terminal of one pole of the switch. (Wire #1)

Ground goes to the common terminal of the pole on the switch. (Wire #2)

On the normally closed side of the -10V pole, we want the -10V to be connected to the top of the Original EH fader. (Wire #3)

On the normally closed side of the Ground pole, we want the Ground to be connected to the bottom of the Original EH fader. (Wire #4)

On the normally open side of the -10V pole, we want the -10V to be connected to the outside lug of our Volume Pedal pot. Which side only matters to control which direction of pedal travel will be the High end of the sweep.

On the normally open side of the Ground pole, we want the Ground to be connected to the other side of the Volume Pedal pot.

With the VP cable removed the Normally Closed contacts complete the connections from -10V and Ground to the top and bottom of the Original Fader, respectively.

With the VP cable inserted the Normally Open contacts complete the connections from the -10V and Ground to the outer lug on our Volume Pedal pot.

I chose the leave the wipers of the two pots conected. Because the 'unused pot has no Voltage, or Current because both ends are open. This way the unused pot is completely removed from the Voltage and Ground. Without Voltage and ground it is virtually removed from the circuit. I say virtually because it is still connected by the pots wiper (the middle lug). (Wire #5)

[Canucker]

I and SWT have built one. I pretty sure some other people have also. It's on three pcbs. Francisco made layout for one of the boards and I made the layout for the other two. SWT came up with some fixes and improvements.

Any chance I could get the PCB layouts/transfers?


I was dreaming that maybe I could draw a PCB for it.....at some point, but if there is a layout available I could use that time to make the EH Bass synth. Having the guitar synth layout would go a long way to helping me do that too.

THIS!!!!! I was dreaming that I could build one of these things. The first summer I "actually" learned how to play guitar I found a seventies version for $2 at a yard sale. I actually said to the guy "if it doesn't work can I come and return it?". It worked and then I foolishly traded it away for a couple guitars, a metronome and twenty bucks. The friend who I was attempting to start a band with was in electronics and building a distortion for a project. We both hoped that we could build one of these some day.