EHX - Crash Pad Schematic Redraw (Percussion Effect)

[Scruffie]

Title says it all, left out the power supply as most people aren't going to bother getting the correct transformer, although I may draw it seperatley if requested.

Thanks to Stephen Giles for Providing the Original 



Info on the unit thanks to our own EH Man - http://electroharmonix.vintageusaguitars.com/index.php?option=com_content&view=article&id=22:the-different-drummer-pt-2&catid=13:electro-harmonix-articles&Itemid=45

[Scruffie]

Can't win 'em all, Small Mistake Corrected thanks to Analogguru and additional Note added on a questionable junction (C8 / Q8).

[liquids]

I just found this potential gem yesterday.  I've used LTspice to simulate this, and FYI it only seems to work like a resonant filter when the connection in the above schematic is made.  I also have seen the hand drawing, and it would be kind of odd to go to great lengths to draw it that way if Q8's base simple needed a cap to ground!

Of note, I find that the Q seems most resonant when the trim+R19 is 10k.  YMMV, this is just in an ideal world known as spice, but that trim seems pretty much useless beyond the 8l-12k range anyhow...and even that may be pushing it, unless you want something very subtle...

While yet to try it in reality, I'm hoping to breadboard it soon. And I'm hoping that in combination with a synthy sub-octave/square wave sound, it will kind of be a 'quick fix' for everything I love about the microsynth filter without the obsolete parts.  And a smaller build at that, believe it or not.   Though it may need an input gain control (to avoid clipping?) and a nice volume control /re-buffer at the end...excited, but trying not to get my hopes up.

[StephenGiles]

It would be interesting to adapt the trigger circuit for guitar, and and to have the circuitry to extract the voltage proportional to the strength of the note played in order to replace the 9v connection of the Start Frequency pot - not the first time I have mentioned this of course!!

Reason - the sweep would be largely independent of the input, after it's initial peak , and free of jitter.

[liquids]

Is there an echo in here?  Stephen you are on a relentless quest.  I personally like the predictable and consistent filter of the microsynth in most cases, so this could be ideal for me.

With something like the above, would it work to not use an envelope filter to feed the 'start freq' an envelope voltage...maybe something like the 555 timer  gate/reset thing in the subcommander or an optocoupler...where the initial envelope yields a pulse or something which creates a voltage which can vary what the input of pin 10 as per the schematic or 'start frequency.'   I have no concept of that but I imagine you could do it with so much time spend thinking of it!  And I dare say, if not you, who? 

[Scruffie]

Before you build it, I seem to recall I made some changes to the schematic because of some problems (could have been one of the other EHX Percussion pedals mind)... let me dig it off my old hard drive and re post it, if I don't do it in a day or two, send me a PM to remind me 

[liquids]

I'm only bread boarding it...which I did just now...of course, it needs troubleshooting for my errors so time remains for you. 

[StephenGiles]

Is there an echo in here?  Stephen you are on a relentless quest.  I personally like the predictable and consistent filter of the microsynth in most cases, so this could be ideal for me.

With something like the above, would it work to not use an envelope filter to feed the 'start freq' an envelope voltage...maybe something like the 555 timer  gate/reset thing in the subcommander or an optocoupler...where the initial envelope yields a pulse or something which creates a voltage which can vary what the input of pin 10 as per the schematic or 'start frequency.'   I have no concept of that but I imagine you could do it with so much time spend thinking of it!  And I dare say, if not you, who? 

There could be some mileage in what you say. I need to be tied to a chair with a breadboard in front of me

[Scruffie]

Here we go... all I can see is i've corrected the LM324 Power Connections... maybe there's something else, anyway, use this one  



Edit: This seems to have also replaced the image at the start of the thread (in my browser atleast) how odd.

[blooze_man]

Can't win 'em all, Small Mistake Corrected thanks to Analogguru and additional Note added on a questionable junction (C8 / Q8).

I pretty sure the q8 base is connected to the diode chain. In this schematic a curved line is used to show paths are not connected and a dot when they are.

[Scruffie]

I pretty sure the q8 base is connected to the diode chain. In this schematic a curved line is used to show paths are not connected and a dot when they are.

I realise this and in both versions I have of this schematic, Q8 is not connected, either with a curved hump or just straight out skipping past... whether the schematics are correct, well that's up for debate.

Edit: Wait a minute, this has already been rectifyed in the corrected schematic, I thought you meant straight after Q8 where the hump is, the reason that question is there on that version of the schematic (See the other two for the correct version) it was hard to see on the schematics so I left it in question.

[liquids]

I couldn't get it to 'trigger' right away but that filter is pretty intense, as sweeping the 'start frequency' pot worked.  It distorts pretty easily with a guitar signal unless you really cut the signal...kind of typical for filters...likewise it still distorts as the Q is pretty hot.  And it self oscilates well before the resonance is full out.

Anyhow, I also wanted to note that its probably 18db (3 pole)?  When you sweep it to a low frequency, you just plain get no signal since the cut off is so strong.  That being said, before I've even finished getting this fully working, I'm imspired to go back to working with OTAs for their simplicity, but using a 3 or 4 pole filter version (microsynth is 3 pole I believe)...and instead try to subcommander's arrangement for filter triggering.

[StephenGiles]

I couldn't get it to 'trigger' right away but that filter is pretty intense, as sweeping the 'start frequency' pot worked.  It distorts pretty easily with a guitar signal unless you really cut the signal...kind of typical for filters...likewise it still distorts as the Q is pretty hot.  And it self oscilates well before the resonance is full out.

Anyhow, I also wanted to note that its probably 18db (3 pole)?  When you sweep it to a low frequency, you just plain get no signal since the cut off is so strong.  That being said, before I've even finished getting this fully working, I'm imspired to go back to working with OTAs for their simplicity, but using a 3 or 4 pole filter version (microsynth is 3 pole I believe)...and instead try to subcommander's arrangement for filter triggering.

Erm  .........it was not designed to work with guitar, a mic was glued to the underside of a pad which is whacked by a drummer!! Some serious changes to the trigger circuit are required to make it work successfully and reliably with guitar - for which I have lost my notes from well over 20 years ago unfortunately.

[liquids]

Erm  .........it was not designed to work with guitar, a mic was glued to the underside of a pad which is whacked by a drummer!! Some serious changes to the trigger circuit are required to make it work successfully and reliably with guitar - for which I have lost my notes from well over 20 years ago unfortunately.

I realized that.  And It wouldn't be hard to modify for guitar, said notes unnecessary with some simple tools and understanding.... 

I just wanted to report my findings as it doesn't seem that anyone else has given it a go or tried or reported what it may sound like with guitar, even though I put it aside after one evening and didn't even attempt getting the triggering to work, which was probably due to breadboarding error.  And I'm abandoning it not because it wasn't plug and play, but because it inspired some ideas in working with OTA filters I haven't tried before...

[StephenGiles]

Erm  .........it was not designed to work with guitar, a mic was glued to the underside of a pad which is whacked by a drummer!! Some serious changes to the trigger circuit are required to make it work successfully and reliably with guitar - for which I have lost my notes from well over 20 years ago unfortunately.

I realized that.  And It wouldn't be hard to modify for guitar, said notes unnecessary with some simple tools and understanding.... 

I just wanted to report my findings as it doesn't seem that anyone else has given it a go or tried or reported what it may sound like with guitar, even though I put it aside after one evening and didn't even attempt getting the triggering to work, which was probably due to breadboarding error.  And I'm abandoning it not because it wasn't plug and play, but because it inspired some ideas in working with OTA filters I haven't tried before...

I did actually get the triggering to work with guitar, which fired the sweep generator nicely. I would have made some notes at the time. However I had no luck with the VCF, but from memory this was on the last evening on one of our past holidays with my dear departed mother in law in the west of England, and the components became rather mangled in and pulled out of the breadboard during the journey home. I flew into a rage when I found this back home and ripped out all the components!!

[slacker]

I just wanted to report my findings as it doesn't seem that anyone else has given it a go or tried or reported what it may sound like with guitar,

I haven't tried this exact circuit with guitar but I built a pedal using essentially the same filter, that might give some ideas on tweaking it to work nice with guitar. It can also be modded easily to get hi pass and band pass modes.

http://www.diystompboxes.com/smfforum/index.php?topic=45148.0

[StephenGiles]

Excellent, many thanks.

[JuliaTruchsess]

Can't win 'em all, Small Mistake Corrected thanks to Analogguru and additional Note added on a questionable junction (C8 / Q8).

Sorry I'm late to this party, but in case it might help anyone, C8 and Q8 are connected to the center of the diode string, I just checked my original copy. And anyway, it wouldn't work if they weren't

julia

[Scruffie]

Can't win 'em all, Small Mistake Corrected thanks to Analogguru and additional Note added on a questionable junction (C8 / Q8).

Sorry I'm late to this party, but in case it might help anyone, C8 and Q8 are connected to the center of the diode string, I just checked my original copy. And anyway, it wouldn't work if they weren't

julia

Always nice to hear from you Julia thanks for the clarification!

I've been looking at another of your (I believe i'm right in saying) designs recently, the ol' guitar synth... it's a big one for sure but I heard a demo and well... with so few units out there, it would be rude not to try one in person and give it a new lease on life 

[analogguru]

Can't win 'em all, Small Mistake Corrected thanks to Analogguru and additional Note added on a questionable junction (C8 / Q8).

Sorry I'm late to this party, but in case it might help anyone, C8 and Q8 are connected to the center of the diode string, I just checked my original copy. And anyway, it wouldn't work if they weren't

julia

Exactly - as I already wrote somewhere, one has only to look at the original schematic - thats the Steiner VCF - and read the accompaining explanations from the inventor Nyle A. Steiner and it should become clear. Here is a scan of an article from 1974:
http://user.xmission.com/~dingebre/N-Steiner-VCF-1974.pdf