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Passive Matrix Mixer: 47k or 100k Pots?
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transponderfish



Joined: Feb 09, 2012
Posts: 16
Location: 3rd from the sun

PostPosted: Sun Aug 04, 2013 2:32 pm    Post subject: Passive Matrix Mixer: 47k or 100k Pots?
Subject description: Not sure which value to choose
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Hello all,

after having taken apart an old mixing console I have plenty of knobs and pots. Values are 47k log, 100k lin and log.
First thing I want to build is a 5 in / 3 out mixer matrix.

I tend to use 47k log for this purpose, as I think 100k is better in Lunettas and noisemakers. (and the plans on the Doepfer DIY page for a matrix mixer have 47k, so I might build these buffer-amps someday)

I plan to use 10k summing resistors and use the matrix with stompboxes (of which I own plenty) and Lunetta/WSG/Cacophonator etc as inputs. Output will be mainly fed into the soundcard. I am mainly interested in doing feedback-loops.

Does this sound reasonable or am I missing something?
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JovianPyx



Joined: Nov 20, 2007
Posts: 1225
Location: West Red Spot, Jupiter
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PostPosted: Tue Aug 06, 2013 10:21 am    Post subject: Reply with quote  Mark this post and the followings unread

I would not do a passive mixer for audio. Especially if you plan to drive multiple outputs. All you need is a single opamp for a mixer. There are many schematics on the interenet.

Passive mixing invites problems with impedance. Yes, with an opamp, a power supply is necessary, however, the opamp will also cause the circuit to be stable with constant input impedance and it's output will be able to supply the current for multiple outputs.

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transponderfish



Joined: Feb 09, 2012
Posts: 16
Location: 3rd from the sun

PostPosted: Wed Aug 07, 2013 7:46 am    Post subject: Reply with quote  Mark this post and the followings unread

JovianPyx wrote:
I would not do a passive mixer for audio. Especially if you plan to drive multiple outputs. All you need is a single opamp for a mixer. There are many schematics on the interenet.

Passive mixing invites problems with impedance. Yes, with an opamp, a power supply is necessary, however, the opamp will also cause the circuit to be stable with constant input impedance and it's output will be able to supply the current for multiple outputs.


Hmmm, yes, I did expect problems doing this, but on the one hand I trust in Nicolas Collins' book, on the other hand I intend to create experimental noise.

I googled this topic, then deleted my question, if I should use shielded cables for the connections between jacks and pots: nobody seems to do that, so I won't care either - even if I build it into a wooden cigar-box (which gives me no shielding at all). I will have enough space for some opamps if I want to add them later.

Is there any danger for my (external multitrack) soundcard involved? Or do you mean there is danger of frustration for me? Cool
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JovianPyx



Joined: Nov 20, 2007
Posts: 1225
Location: West Red Spot, Jupiter
Audio files: 157

PostPosted: Wed Aug 07, 2013 8:02 am    Post subject: Reply with quote  Mark this post and the followings unread

transponderfish wrote:
Is there any danger for my (external multitrack) soundcard involved? Or do you mean there is danger of frustration for me? Cool


Just frustration with things not working as you expect or desire such as level not being high enough or unexpected filtering of your signal.

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