Scott Stites/Buchla 291 adaptation

[ericcoleridge]

Ok, I finished one half of this VCF, and everything appears to be working very well. This filter sounds awesome, I'm very happy with it!


But, I was trying to feed it an oscillator signal from my Dual Oddy VCO, and the output signal from the 291 is very very low. I believe the 291 expects a 10v p to p signal, is this correct? Is this why my output signal is so low (I think the Oddy VCO outputs a 5v waveform)?

[guitarfool]

[funkyfarm]

(maybe too soon, but I feel that 3904 have a far more "agressive" response...3566 seem to smooth all the things down...tests are running...)

[Photon]

Luka
I don't believe it! I've spent the last couple of days trying to incorporate Scott's updates into the Verbos layout. But you finished first.
Your posted layout doesn't seem to be full resolution when I downloaded it. Can you re-post the full-sized?

Thanks!

Peter

[ericcoleridge]

[Scott Stites]

My 291 is loud as hell - I'm not sure if the difference between 3.3V and 3.9V would be all that great. If it doesn't make a difference, I have a feeling something somewhere is not quite right. If the new zeners make no difference, maybe we can put our heads together on this and get it going.

I don't think they were meant for 10Vp-p at all - seems to me, Buchla audio is fairly tame in level (line level?), but the CVs are pretty large. Maybe a Buchla person can correct me on that if I'm wrong. The filter handles 10Vp-p just fine - Vactrols are quite forgiving.

Cheerios,
Scott

[ericcoleridge]

I'm waiting on some more Vactrols to complete the second half of this dual filter. Maybe that will help me understand what might be going wrong on the first VCF. Thanks for your response.

[funkyfarm]

I've edited my last post as I didn't know 2N3566 was obsolete.

2N3904 is a perfect sub in this design.
291 sound doesn't really rely on the obsolete part.

I'll post sample so you can compare.

This filter did not work here for months just because I've heard somewhere online that 3904 and 3566 were pin to pin compatible.

I now understand this is wrong : a transistor is pin compatible with an other within a specific case (TO-92 TO-220...)

TO-92 2N3566 or PN3566 is pin-compatible with TO-92 2N3904.

MY LAYOUT IS NOT FOR TO-105 CASE TRANSISTORS.

you'll have to bend two pins and put it backwards...

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[numbernone]

I have not gotten back into mine to fix the BW control, but can confirm it is LOUD. Although it does not self oscillate it seems to go right to the edge and really pumps.

[Scott Stites]

Yeah - in fact, the bass frequency amplitude can really catch you off guard. I remember having some conversations with Peter Grenader about that. There's sort of an amplitude hump down there.

I used PN3566.

Cheerios,
Scott

[ericcoleridge]

I finally got both sides of my Buchla 291 Dual VCF up and running. Unlike some here, mine self-oscillates very strongly. I used 3904s instead of 3566s, but I transistor socketed them, so I'll probably see what difference, if any, the 3566s make.

This is definitely an unusual sounding filter. Unlike some Band Pass filters I've heard/used, this one (or mine at least) will not totally filter out the input (like a Low Pass, High Pass, or some Band Pass typically would). In this sense, it's more like Phase Shifter type of filtering. Not that it sounds like a Phase Shifter, just that it has that kind effect.

But, it's also extremely unstable sounding, especially with the resonance more than 2/3s up. In this way, it reminds me of the kind of raw, rough response of the dual MS20 filter... the way it blisters and breaks up at certain settings.

As others have reported here, my 291 is extremely loud--even with the input trim all the way down (which is where I'll leave it set). I'm really glad I built this filter, it's definitely adding new tonal variety to my synth, especially in the searing, cutting, biting area of sound.

[numbertalk]

Hi Luka,

Do you still have a PDF for this for etching that you could post? Much appreciated.

[numbernone]

Hmmm I would like to also see this modded layout.

Also, what is the error on the input mixer? Mine are flummoxed and I have just bypassed them. Im sure it simple but sadly this filter is so fun that I have not taken the time out to pull it and troubleshoot it.

[Luka]

hey yeah i have a layout
it has added audio mixer, +/_ cv processor and the hp&lp output buffers

ill post it up soon
im just going to get it built first before i release it to the community
ive given it to a couple of people but kinda feel a bit bad about it as it hasnt been tested, it should be fine but i would like to give it a thorough test before i allow it to get into your hands just incase.

[numbertalk]

[funkyfarm]

http://mypeoplepc.com/members/scottnoanh/sitebuildercontent/sitebuilderfiles/291%20.jpg

Hi !

What to do to add a simple non-reversible cv input on CF ?

a 47K or 68KR in "parallel" with R45 ?
The same thing as for FM input but without any cap to block DC ?

[Junk Rhythm]

[Luka]

i cant seem to upload from my browser at present. ill keep trying.
might have to wait till after the 16th when our plan kicks over
my housemates destroy our limit so quick

[numbernone]

After fidgeting with my second build of the 291 a bit, I think I have it a bit more in line. I primarily use the HP tap, on which the BW control introduces a massive wave of what I normally think of as resonance (self osc levels). Can anyone else confirm this? I mean, it is like bottom of the ocean speaker destroying WHOOOMP! Modulating the cut and BW with various sources makes for just truly mind melting tones.

I Love this filter more everyday.

[Luka]

ok here you go
funny the net seems to work in the mornings
i really have no idea how internet speeds work, they seem to work outside of my comprehension

it is a single filter instead of a dual as i found transferring pcbs this size were harder to find small errors. the previously mentioned mods to the filter are just taken from other modules. the cv processor is from the cgs vco. the mixer is a standard summing mixer and the hp/lp taps have a standard opamp buffer. ill re-draw my mod schems to make it all a bit smoother. the buffer and mixer came from scott stites' version but his schems had an error in the mixer.

The fillter core is untouched, i made maximum effort to leave it alone so i couldn't introduce errors. So the mod section is isolated at the top. This means you need to use fly wires to connect it up. The good thing about this is you can use the mods how ever you wish or not at all.

im still waiting on some final parts to complete mine and totally verify the layout but from initial tests all the IC are attached to correct power lines. The only layout issue i had was that the ferrite beads are really close to the power connector. I seemed to have flattened my layers and cant edit in illustrator so if anyone has photoshop i would recommend them shunt them down a bit if you can. No probs if you cant you can still get them in.

Let me know if there are any issues

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[jwilsey]

Thanks Luka for posting up the PDF. I am going to try your layout in the next 2 weeks.

[Luka]

well the filter sounds awesome!
just powered it up tonight and it worked straight away
my xoxbox sounds sweet through it

ill populate the mod section this weekend and make sure it all works

[Photon]

[Luka]

nice photon!
ive got mt panel and pcbs done too

its in the to-be wired pile now

[Photon]

Thanks Luke. I hope to have some time this week to wire it. I'm really looking forward to playing with this.
You'll have to post some pics of yours when its finished.