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Easel/208 Envelope problem and solution
MUFF WIGGLER Forum Index -> Buchla, EMS & Serge  
Author Easel/208 Envelope problem and solution
tarandfeathers
As some folks might be aware, there is a problem with the behaviour of the Music Easel envelope generator in sustained mode - at short settings of the duration slider, the envelope will not sustain indefinitely, but will slowly decay away. This is, as far as I can tell, a universal problem affecting all RF versions of the 208 (and most probably all original 70s modules as well, no one has yet confirmed whether or not the BEMI models do it but I wouldn't be surprised if they do.)

There were a couple of circuit revisions during the original lifespan of the 208 which were efforts to improve the behaviour of the circuit, but a proper fix was never implemented (I assume because the production numbers were so low, it was not considered worthwhile to spend much time on the issue).

This always bothered me (I quite specifically wanted to be able to set a very short envelope for very snappy playing but to also be able to hold notes for extended durations, tens of seconds up to minutes) so I recently spent a bunch of time working on understanding the function of the EG circuit, and coming up with a fix.

In essence, the envelope is based around an integrator, with some logic control switching on the different "stages" of the envelope to charge or discharge it. The duration stage is split into two parts, one that charges, and one that discharges, and, in sustain mode with a key held down, the second duration stage should oscillate on and off, keeping the integrator topped up with charge and holding the envelope high until the key is released, at which point the decay stage switches on. The problem is that whether or not the oscillation starts up depends on the duration slider setting on the front panel being long enough. Don Buchla added a "feedback" cap to the circuit to improve things but it does not fix the problem entirely, and can in fact lead to a level of instability where upon releasing a key the second duration stage and decay cycle back and forth forever and the envelope never closes.

I have prototyped a fix which completely resolves this problem. It involves some additional logic sniffing the status of all the envelope stages, and an electronic switch to add Don Buchlas feedback modification only at the appropriate time. I have done extensive testing with this and having the fix attached only fixes this behaviour and does not modify the timing or behaviour of the envelope in any other way.


Here's a scope trace of the envelope logic without the fix, and then one with it





I wanted an elegant way of implementing this, so I have designed a small board (32x29mm) which replaces IC3 and IC5 on the envelope card.



In order to make it fit, this board is all SMD with a double sided load, and since the pin headers that mate with the card must be precisely lined up to match the spacing of the footprints on the card, the best option seems to be to make it available as an assembled unit, ready to fit, so that's what I'm doing. Price will be £30/€35/$40US plus actual shipping cost from the UK to wherever you are. Boards will be shipped in an antistatic box/foam, inside a jiffy bag, and will include sockets to fit into the envelope card.

I have a final prototype arriving sometime in the next week so that I can check the spacing is perfect and then I will order a run and make them available. I will be setting up a website to deal with orders, feel free to let me know if you are interested but I will not be processing or keeping track of anything on here or through PMs. Unless I have more than 50 expressions of interest before I place the order, this will be a run of 50 and will be a one time run that will not be repeated.

Points of note:

*This will physically fit card 3 of 208r2 and 208r2.1 modules. It should fit original 70s 208s and another forthcoming easel clone being developed by jhulk and DonT but I have no way to check this, buy at your own risk.

*If you have used wiper type IC sockets on you card 3, the board will plug straight in, but this is a one time deal as the pin headers deform the wipers so you will have to replace the IC sockets if you want to go back to the unmodified behaviour. It will not fit in turned pin/machined IC sockets, but as noted will be supplied with suitable sockets to make it plug in/replaceable.

*It will not fit directly in the 208r1 or the BEMI easel and I will not be providing any support for doing so, but it should be possible to do if you know what you're doing. Again, buy at your own risk.

*Feel free to argue about whether it is "authentic" or not, but critically it produces the envelope behaviour I personally want, and it is my belief that this gets the envelope working as Don Buchla originally intended. It's my hope that having the boards fit straight into the IC3 and 5 positions makes the modification reversible enough for any cork sniffers out there, but you are most welcome not to buy if you feel that Don Buchla is an untouchable genius and nothing he did could possibly be improved :twisted:

*Apologies in advance for cross posting this on a couple of threads, I want to make sure it reaches the relevant parties.
synthi
applause It's peanut butter jelly time! Rockin' Banana!
tarandfeathers
Envelope sustain boards are now up for sale, along with various other 208/Buchla odds and ends, at http://www.dunningtonaudio.co.uk



Note that these are currently being manufactured and will be shipped in early September.

I'm writing an article that goes into some depth about the design of the envelope generator, the nature of the problem and how it is fixed. That should be up in a couple of weeks, for anyone who is interested.
papz
I just installed it in my 208 and it works perfect, great work, thank you tarandfeathers ! thumbs up
captnapalm
papz wrote:
I just installed it in my 208 and it works perfect, great work, thank you tarandfeathers ! thumbs up


Me too. Thanks! screaming goo yo

If anyone in US is considering this but is hesitating because you already soldered in components and don't have a desoldering station, I'd be happy to desolder your old sockets/chips for no charge. You could just mail me that board. It's so dang easy with the Hakko 808...
auxren
Took me 15 minutes to install it. Removed the old sockets with some chipquik.
Easy peasy and it works as expected.
davebr
I bought one and installed it. It works great. I chose to solder it directly to the PCB rather than use sockets and had to fold the disc capacitor under it flat against the PCB.

Dave

@green
The BEMI version definitely does this. Always thought it was a "buchla thing".
captnapalm
@green wrote:
The BEMI version definitely does this. Always thought it was a "buchla thing".


It is; it just followed the Easel in all its variations through to the new BEMI one. MY ASS IS BLEEDING
Clemdu
davebr wrote:
I bought one and installed it. It works great. I chose to solder it directly to the PCB rather than use sockets and had to fold the disc capacitor under it flat against the PCB.

Dave



As always great picture Dave !
I take this opportunity to thank you for the tremendous help your website was when I was building my 208. we're not worthy
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