freestompboxes.org

We made a few of these and sold them on ebay in late 2007. I remembered about it when reading another thread and OTA's came up.
I thought I'd let anyone who wants to take a look at it, build it, or do whatever with it.

download/file.php?id=25725&mode=view

I limited my self to a dual opamp, and the dual OTA because I wanted to fit it in a 1590B enclosure. Otherwise i probably would have used two more opamp stages rather than the Jfet input and especially the BJT output buffer... Or I could have just replaced the output buffer with an opamp, and used the other stage to make a reverse sweep function... lots of possibilitites, but only so much space to work with.

What the knobs do:
B100K in the upper middle left- resonance
B100K in the upper right- mix between LP and BP output
A100K range
B10K Center frequency

Known issues-
When the resonance pot is turned all the way up, it may oscillate. Perhaps adding a small resistor in series with the positive feedback would be a good idea here...
The mix pot is a bit scratchy, which should be pretty obvious. Not really loud scratchy, but if your amp is all the way up, you will notice it.
The decay is a but "gurgly" I actually kind of liked it that way with this perticular filter. Changing the 4.7uF cap to a 10uF would be more to some people's liking.

So there it is... a fairly simple envelope filter that doesnt rely on optoisolators, and sounds pretty good too.

The OTA circuit was basically lifted from the LM13700 data sheet, but I added the positive feedback for resonance.

Some known issues:

looks cool, brian.
i have a lot of respect for you guys with your low parts count circuits.
do you have soundclips?

hi Brian
thanks for sharing this with the World - hopefully one or more of the members here may build this and may even improve upon it
Once again - thanks for sharing it
bajaman

bajaman wrote:hi Brian
thanks for sharing this with the World - hopefully one or more of the members here may build this and may even improve upon it
Once again - thanks for sharing it
bajaman

Imrovements on filter circuits can be a lot of fun, and complex or as simple as you want to make them.

Taking the filter section as a building block, and coming up with different ways to create a control voltage could easilly create a number of cool projects.

This one is also probably could use some work... as noted above it was a bit compromised by the limitations I set my self up with.

A couple years ago I put a filter with 12 pots and a few switches on breadboard. It really was beyond overboard, because it would not have been all that user friendly. I tried to build a prototype, but after building the PCB with about 200 parts on it, it didnt work. I looked back at the artwork a few weeks later and realized i had merged two nets in to one, and couldnt make a fix very easilly. I still have the thing.... it's probably the most parts ive ever squeezed in to a stompbox... it looks impressive i guess would have been nice if it had worked.

prehistoric wrote:looks cool, brian.
i have a lot of respect for you guys with your low parts count circuits.
do you have soundclips?

Sorry, had to search the site for them... i couldnt find it last night.

http://subdecay.com/gurgle.mp3

It's kinda long, and after listening, you'll understand why it has gurgle in the name.

I'm not 100% sure that this clip was from the final schematic... I think it was still on breadboard when i recorded it.

great clip! the gurgling is a nice effect, and one not really commercially offered as it's own feature before as far is i know.


envelope filters are really nice. i like to bury them in the middle of a chain of fuzz/octavedown/loop etc so their effect is masked but still causing a harmonic blip in the chain, can make some demonic vowelly sounds

prehistoric wrote:great clip! the gurgling is a nice effect, and one not really commercially offered as it's own feature before as far is i know.


envelope filters are really nice. i like to bury them in the middle of a chain of fuzz/octavedown/loop etc so their effect is masked but still causing a harmonic blip in the chain, can make some demonic vowelly sounds

Actually, after listening to the clip again, I'm not sure how great it is. Seems more subtle than i remember.

Regardless, there is a pretty useful building block in between all the compromises noted above.

When i had this on breadboard, i also tried an LFO controlled dual filter with opposite sweeps. sort of a take on the anderton antiwah. Sounded pretty good, although it was a bit of a large circuit for what it actually did.

Im more of a filter after fuzz kind of guy though... That reminds me i need to get back to an old idea i had a couple years ago....

Hmmm i was just looking at this schematic, and noticed that the envelope follower section was screwed up.

One of the resistors is not marked with a value. I'll post a corrected drawing soon.

The sound sample sounds pretty unique. I like it! Looking forward to seeing the updated scheme.

fixed version posted.

have fun with it

the links are broken.. could someone be so kind to relink it im really curious !!

THNKS!!!!!

mhond wrote:the links are broken.. could someone be so kind to relink it im really curious !!

THNKS!!!!!

It got blasted when we restructured our website last year.

has anyone got a copy they could send me pm me if you do or repost it possibly cheers. thanks in advance

Sorry about digging up an old post but could some one repost the schematic?

rocklander wrote:

dauserarms wrote:Sorry about digging up an old post but could some one repost the schematic?

+1 ... schematic anywhere anyone?

Please note that Brian pulled this schematic:

Brian Subdecay wrote: A few notes-

This could be easily improved, as i made some minor compromises to fit it in to a 1590B case. In a perfect world the input and output buffers would be opamps.

the filter is essentially taken straight from the lm13700 datasheet, with positive feedback added for resonance, and values tweaked a bit for stompbox use.
We sold a few of these as prototypes as the gurgle and moan. it sounds good, but some tweaking could be done in the envelope follower to smooth things out a bit.

Sorry i don't have everything marked on here. I'll get to that some other time.