Inverting attenuators

Coriolis · Joined: Apr 11, 2005 Posts: 616 Location: Stilling, Denmark

No idea... Sad

C

fonik · Posted: Tue Oct 10, 2006 1:49 pm Post subject:

okay, i thought about it: you don't want any attenuation on CV used for 1V/oct. the VCO would get out of tune. so you'll have to pay attention to the linearity of the circuit=no amplification/attenuation.

Coriolis · Joined: Apr 11, 2005 Posts: 616 Location: Stilling, Denmark

IE: trimming the center-tapped pot to be 0V right in the middle, and not touching offset either?

C

fonik · Posted: Tue Oct 10, 2006 10:37 pm Post subject:

he, he, no! no attenuation (center taped pot in the middle) would mean 0 Volts on the output no matter what you put in!
what i meant was that you want a "perfect matched" OPA: center taped pot to the right (fully open) and you get out what you put in. therefore the resistors (pots/trimmer) on the input of the first OPA and the resistor in the feedback path have to be matched to guarantee a 1:1 amplification (i.e. 4V in, 4V out). one could put a 2nd trimmer (T2) in line with the resistor in the feedback path so that P1=R3+T1=R4+T2

cheers,
matthias

Coriolis · Joined: Apr 11, 2005 Posts: 616 Location: Stilling, Denmark

Fonik, and everybody else: Sorry for digging this thread up again, but I've been working on a pcb layout for that last schem fonik posted, and thought I'd run it by the forum. It's not verified - perhaps someone here can take a look at it and point out some obvious faults (if any)? I haven't even breadboarded this thing yet, but I just bought this neat layout program Sprint-Layout, from Abacom (same people who make Uncle K's beloved Lochmaster), and I just had to get my hands dirty... Wink

Well, here you go. Anybody spot anything yet?

C

Edit: Layout removed - was incredibly faulty... Embarassed

fonik · Posted: Sat Oct 21, 2006 2:56 pm Post subject:

looks very good, i didn't looked for errors though...

you gave me a credit on the layout - to be honest i don't want to adorn myself with borrowed plumes: it's a peter grenader circuit based on a chris macdonald design slightly improved by me:)

BTW and FYI i already made a pcb layout for the voltage processor, including the LEDs, optimized for eurorack format with cliff jack sockets - breadboarded version works like a charm:
http://www.modular.fonik.de/Page15.html

cheers,
matthias

Coriolis · Joined: Apr 11, 2005 Posts: 616 Location: Stilling, Denmark

Guess you're right about the credits - I'll remove them! Wink

I can already see a few things that need reworking. Power rails and gnd should be grouped together, so a standard connector can be used (2.54mm=0.100 inch between holes). Other than that?
I think I'll just build one and see. Since you've already done a layout, there's no need for another one - I just hadn't seen yours. Smile

Ch

fonik · Posted: Sun Oct 22, 2006 5:05 am Post subject:

Fernando · Posted: Thu Feb 08, 2007 1:44 pm Post subject:

fonik · Posted: Thu Feb 08, 2007 2:16 pm Post subject:

Fernando · Posted: Thu Feb 08, 2007 2:24 pm Post subject:

thanks mathias,

I meant to add a trimmer to the offset pot, not to the processing pot...
The idea is using a center detent pot for the offset control but use a trimmer to compensate and have 0V at the detent

fonik · Posted: Thu Feb 08, 2007 3:14 pm Post subject:

oh, now i see!
funny, i thought about this before (but after building the modules!). i'd tried to put the offset between +V and -V instead of +V and GND and then put trimmpots between the +V/-V and the pot to calibrate the offset to 0V at the center detent. I'm just guessing - try it on the breadboard!

+V
I
T1
I
<
>POT- to circuit
<
I
T2
I
-V

hope you can see what i meant?
BTW did you see my BUCHLAesque module on my site?
buchlaesque

Fernando · Posted: Thu Feb 08, 2007 3:46 pm Post subject:

>hope you can see what i meant?
thank you (and excuse such a basic q)

>BTW did you see my BUCHLAesque module on my site?
Yes. In fact I had that circuit in my files... both the original and Grenader's (I'm refering to the processor)

I would have inverting attenuators w/offset almost anywere!

It would be great to fabricate small pcbs with the processing pot as a holder so you can add it where you need it (and add the extra offset pot if you can/like, as well as an output to use it as an independent processor...)

Peter Grenader sold boards in that fashion but for a short period of time.
He called them the Gizmotron.
By comparing the schem to a picture of the actual Gizmotron from his old site I assumed this was the circuit in the Gizmotron: (see atachment)

Another interesting circuit that can work as a single or multiple (mixer) inv. att. is at the top of Ken Stone's VCO schematic:
http://www.cgs.synth.net/modules/cgs48_vco.html
A 1k or whatever output resistor should be added instead of the 100k (wich is part of next block)
IIRC, it has some gain at both extremes of each proc. pot and that more gain can be achieved by lowering the value of a given input resistor.
Just a dual op amp and you can make an at/inv input or mixer!

P.S. should "Attinvoff"/"Attinvofflag" be a good name for such a tool? =:))
btw, how do you spell that stoned emoticon?

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Funky40

sorry i'm maybe i lttle late,

mosc

Scott Stites · Posted: Sat Feb 10, 2007 5:43 pm Post subject:

You can use a pot with a center detent and have the detent calibrated for zero volts.

IMO, the main advantage of the inversion switch is that it will give you a finer range of adjustment (full travel of the pot). However, in those cases, I'll just patch an inverter in and avoid using up space with a switch. My personal panel standard uses less room for a connector than a switch. Mixers generally provide an inverted output, and I often put in a tap there, regardless of the function of the mixer - I find I don't use 100% of my modules 100% of the time, so why not put some freebie functions in them for the times the main function of the module is not being used?

Also, snagging the non-inverted output of a mixer is a real plus, too. Mixers are always the key ingredient in my more complex patches.

Cheers,
Scott

Funky40 · Posted: Sat Feb 10, 2007 7:39 pm Post subject:

Scott Stites · Posted: Sat Feb 10, 2007 9:16 pm Post subject:

I get them at Mouser.

P/N is 313-2000F-XXXX where 'XXXX' is the value.

313-2000F-100K is 100K, 313-2000F-10K is 10K, etc.

Cheerios,
Scott

fonik · Posted: Sun Feb 11, 2007 4:24 am Post subject:

Funky40 · Posted: Sun Feb 11, 2007 4:55 am Post subject:

@Scott
Yeah, thanks,
i have to get anyway some jack sockets from mouser when i have money next months.

@fonik

sorry, where is your file ?

fonik · Posted: Sun Feb 11, 2007 4:59 am Post subject:

Fernando · Posted: Sun Feb 11, 2007 7:19 am Post subject:

I think it highly denpends on what you want to do... how do you make your magic (music) Your aproach.

I like to explore small interesting control zones I find of musical interest (no matter what I'm controlling) so I need to be able to scale and offset a control signal to get there...
I do not use cv to generate specific pitches and then other voltages to model the timbral characteristics.

If you want to have perfect control of the pitch within the western traditional temepered scale (or whatever), the procedure and tools are different. So having too many "control doors" open may become a problem. (I'm talking only about pitch here)

I mean I think there are two main aproaches to build your music (very generally speaking of course). One based on instrumental (western) tradition, where everything is built over pitch and then the timbral shape of that voice. And another aproach (let's call it post-Schaeffer) where music is built from the sound up, made possible thanks to the existence of the synthesizer (= all the sound generating and processing tools available)

Fidgit · Joined: Jul 13, 2006 Posts: 42 Location: CH

i'd refrain from using pots with center detent. while it may seem a nice feature at first sight, later you realise that it doesn't allow you to adjust values only a little bit off the zero/middle value.

the pot always "falls back" into the detent or the gap in the middle. uncool...

Scott Stites · Posted: Wed Feb 14, 2007 7:46 am Post subject:

Hmmm, never had that problem with the pots I use. However, I don't like'em because I hate that little bump in the middle when you're sweeping the pot.

Cheers,
Scott

CJ Miller · Joined: Jan 07, 2007 Posts: 368 Location: 127.0.0.1

Center detents can be removed from most pots! By "most" I mean those which are in a metal frame, typically held together by tabs. Sealed pots though are of course made to stay enclosed, so likely can't be rebuilt.

I have done this on Alpha pots before, and just last week on some nice 50k Alps sliders which were in a crappy equalizer. The detent is simply a small metal bearing which stays in a recessed area. Then when the pot is moved the resistance element gets caught on the bearing at its midpoint. I just gently pry the tabs open, drop the bearing out, and re-assemble. No more detent, only smooth travel.

This may well not work with all pots, but there were no problems with those I've tried. Make sure to use spares if one is unsure!