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Help with a Passive Attenuator?
MUFF WIGGLER Forum Index -> Music Tech DIY Goto page 1, 2  Next [all]
Author Help with a Passive Attenuator?
Exuviae
Hello All-

Here's a silly question, but I am having trouble, so I thought I'd ask.

I need to make a simple Passive Attenuator to go between my Frac Rack and my USB soundcard as the modular is always pegging the soundcard even with the pad reduced and the gain knob all the way down.

I wired a simple jack-pot-jack config together using a 1/4" jack, an A500K pot and an 1/8" jack. I figured this should work but it doesn't. In fact, it doesn't do anything. I've checked and rechecked the wiring and don't see why it doesn't work. I must be missing something.

Help anyone?

Thanks in advance,
Brooks
futuresoundsystems
Have you wired the jack sleeve terminals together?
Exuviae
Yep.
Roycie Roller
I think 500k might be too large a value. In all of the attenuator schematics i've seen, they use 100k, or 50k. Linear (B) for CV, or Logarithmic (A) for audio. Here's a diagram from Doepfer's site-



J1 in the diagram is your jack from the modular- the tip is wired to lug 3 of the pot (P2 in the diagram) (Clockwise end). The wiper lug (lug 2) of the pot is wired to the tip of your jack (J2 in the diagram) going to the soundcard. Then you need a wire for ground which is connected to the Ground lugs of both of your jacks AND Lug 1 (Counter ClockWise) of your pot.
Hopefully that should work!
DGTom
Yeah, I've had good results with 5K & 100K, maybe even try 1K.

This here page may be of some use, maybe this problem could be best solved with a static voltage divider, a couple of resistors;

Dividers made super easy ->
Exuviae
Thanks guys...I got it to work. I changed the pot to a 50K log, which seems a little large still, but it does work now.

Many thanks!
LDT
I am going to make some passive attenuators for CV (euro modular). Which value should the pots have?
wetterberg
anywhere between 10k and 100k is usually suggested. lin for cv, log for audio, etc.
fonik
wetterberg wrote:
anywhere between 10k and 100k is usually suggested. lin for cv, log for audio, etc.

this is kind of "common sense" wink nevertheless i personally like audio for CV too. it gives you fine control for the lower range. but that is the nice thing of DIY: you could roll your own, just as you like.

concerning the value: one should consider the input impedance of the modules you'll be feeding as well. if the value of the pot is less than the input summing resistor, a linear pot acts linear. what you want to avoid is using a pot that's larger, the response would be log then!
i am refering to an article from bernie hutchins. it can be found in the musical engineer's handbook from 1975.
according to his measurements equal values of potentiometer and iput resistor still lead to a slightly (very slightly) log response. nevertheless he suggests using 100k for the potentiometer (assuming there is a 100k input resistor to the summing node), since this would cause less load on the module upfront.
bottom line: in a modular synth environment i suppose the input impedance to be 100k, this is quite common , i think. so one should use a pot <= 100k.
panda30y
Great info Fonik, and I've been able to follow everything on this thread clearly. I just have one completely noob question, how do we determine the input impedence of our circuits? Thanks in advance.
fonik
the input impedance is nothing but the value of the resistor on the input.

back in the 70ies bernie hutchins published the monthly Electro Notes. a lot of the cicuits we see today are based on his designs (i.e. VCOs). he did not invent all of these famous EN cicuits, but he improved them. however, he suggested kind of standard for a synth system. he suggested a standard input impedance of 100k for each module. i <believe> this standard is still followed by many manufacturers. but i don't know.
to determine the impedance you would have to take a look at the schematic (if provided) or at the PCB itself...
panda30y
Thanks for the response, I understand it much better now.
LDT
Just for the record: I made 4 passive attenuators (mostly for CV) today, and 47K linear worked perfectly. Thanks!
wetterberg
pics, LDT!!!!
LDT
wetterberg wrote:
pics, LDT!!!!

I will! It actually looks pretty ok. I have my Asys 6U case in a Gator flightcase, and the top and bottom alu flanges of the Gator case is filled with Cliffs. Bottom has 4x4 multiples and top has 4 attenuators and 4x3 multiple/trunk lines that are connected to my patchbay.
doctorvague
Since the OP was talking specifically about dropping level to a USB soundcard -

You could also wire some cables with resistors hardwired into the connectors. Use alligator clips and try values or use a pot and measure it. When you have the desired amount of attenuation for your card inputs just wire it in.

Knobs are good though.
Suburban Bather
http://www.mouser.com/Search/ProductDetail.aspx?qs=XMFZpxCDm89PpCDHIsG Orw%3d%3d

So, would these be the correct pots to order? Attenuation for the audio and cv inputs on my QMMG would be great. Also how do you orient the pot to know which lug is which(left/right or 1 and 3)?
doctorvague
Roycie explained pretty well

Roycie Roller wrote:
I think 500k might be too large a value. In all of the attenuator schematics i've seen, they use 100k, or 50k. Linear (B) for CV, or Logarithmic (A) for audio. Here's a diagram from Doepfer's site-



J1 in the diagram is your jack from the modular- the tip is wired to lug 3 of the pot (P2 in the diagram) (Clockwise end). The wiper lug (lug 2) of the pot is wired to the tip of your jack (J2 in the diagram) going to the soundcard. Then you need a wire for ground which is connected to the Ground lugs of both of your jacks AND Lug 1 (Counter ClockWise) of your pot.
Hopefully that should work!


As far as exactly what pot, it depends on your format, etc. The one you referenced is a PC mount one - probably not what you want, but it will work. Make sure you get the right shaft type for the knobs you want to use.
Suburban Bather
This is what I'm afraid of. The only DIY experience I have is making cables. I don't have the slightest clue what the hell I'm looking at with schematics and there are way too many variations of certain parts help
Suburban Bather
Ok, how about these pots? Is 10 for $104 a good price or are there cheaper pots available elsewhere?
http://parts.digikey.com/1/parts/737951-pot-100k-ohm-2w-rev-log-taper- kb1041s28.html

Oh, and I'm using both Euro and 5U systems.
wetterberg
pots are usually a buck or less each.
Those are reverse logarithmic pots, which are rare and therefore expensive.
Roycie Roller
Suburban Bather wrote:
Also how do you orient the pot to know which lug is which(left/right or 1 and 3)?


This is a really useful article, and has a picture that shows the orientation of pots.

http://sound.westhost.com/pots.htm
LDT
wetterberg wrote:
pics, LDT!!!!


My attenuators!:



And the trunk lines are connected to the back:
DGTom
SlayerBadger!
Awesome case!
Did you use 16mm pots?
Great job fitting em all in nice & great idea having your I/O & multed up CV/Gate.

Roycie Roller wrote:
Suburban Bather wrote:
Also how do you orient the pot to know which lug is which(left/right or 1 and 3)?


This is a really useful article, and has a picture that shows the orientation of pots.

http://sound.westhost.com/pots.htm


Cheers! I was in Jaycar yesterday & went to the counter w/ a bunch of pots... guy behind the counter had to ask "...and what are these called sir?" WTF!?!! All new low!
Roycie Roller
DGTom wrote:

Cheers! I was in Jaycar yesterday & went to the counter w/ a bunch of pots... guy behind the counter had to ask "...and what are these called sir?" WTF!?!! All new low!


Ewww, that is bad. Sure it wsn't Dick Smith's? My local Jaycar is quite well stocked, albiet expensive, but the workers do know some stuff.
Dick Smith's, on the other hand, has gone the way of the mobile phone dealer & their parts section is shamefully neglected- in fact, one of the workers there told me that they are not going to restock any more parts, & that they are going to close down that part of the business. I'm not sure if that's for all shops throughout the country, and whether they'll continue selling parts online.
To give you an idea- they haven't had any 100k pots for about a year.
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