New Project -- 8/8 BA mixer, help wanted w/buss design

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Moderator: Joe Malone

poserp
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Location: Los Angeles, California

Post by poserp »

I ought to learn how to do schematics; although I'm a programmer by profession I'm still a bit befuddled by SPICE. Perhaps I can put together something in Visio... I don't know why I didn't think of that before, anyways I'll post some drawings in a day or so.

poserp
Posts: 40
Joined: Thu Mar 08, 2007 10:11 am
Location: Los Angeles, California

Post by poserp »

...O.k, here are a couple. The first is a detail view showing the circuit for one of the four mixing transformers on input:

(Note: old links removed, updated schematics are in the next post)

Excuse the cross-posting, I need to consolidate the various threads from the various boards that are running concerning this project. Anyways, that should give you an idea of what I'm working with.
Last edited by poserp on Wed Nov 21, 2007 6:22 am, edited 1 time in total.

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Joe Malone
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Post by Joe Malone »

Image
OK just from a quick look.
1. Bottom 3 inputs on above circuit will be out of phase with top 3 inputs if pin 4 is a centre tap.
2. 300ohm is series with ground is not a good idea as it will not help increase the input impedance if the input is driven by a solid state output and if it is driven by a floating transformer output the 300R will let the top and bottom inputs cross modulate each other badly.

Image

1. Input 1 & 4 will be in phase and Input 2 and 3 will be out of phase if pin 4 is a centre tap.
2. Same 300R to ground problem as above.
3. Third transformer is not needed at all as the outputs of input transformers can just be wired in parellel as long a T Atten is used as this alwaya leaves a resistyor in series to the transformers.
4. Really only one transformer is needed as all the T atten outputs can just be wired together and then feed into one transformer. This would make all inputs in phase and give the same impedances.
Joe :-)
JLM Audio
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poserp
Posts: 40
Joined: Thu Mar 08, 2007 10:11 am
Location: Los Angeles, California

Post by poserp »

I've made some changes based on the schematic in this paper:

http://music-club.rutgers.edu/headfonz/ ... onsole.pdf

See pdf page 4, Figure 6 specifically. I noticed that they have the resistor on the middle pin set to half the attenuator resistance. Also, they put the resistor between ground and the middle pin, instead of the other way around:

Image

If I understand what you're saying about the second schematic, the T attenuator acts in the same way a mix resistor does in a passive resistor mixing network, thus two channels can be tied to the same input post of one ttransformer. Is that correct? I could use the same effect to double the number of channels, then, in the first schematic as well. Here's that schematic for reference since image links have changed. I've added dots to both schematics to match Edcor's phase indications:

Image

poserp
Posts: 40
Joined: Thu Mar 08, 2007 10:11 am
Location: Los Angeles, California

Post by poserp »

Another update. For christmas I got a couple of tools (including a metal punch for doing faceplates) and a rack case, among other goodies. I've tested the circuit posted above with slightly different routing and no resistor on the ground pin on input into the mixing transformers. I tested half of the full mixer circuit and everything worked as planned. Now it's on to finishing building the stepped attenuators (I've built 7 so far, 1 more to go), component mounting and final wiring and testing. I'll post pics once I have everything mounted in the box. I'm looking forward to pairing this with my Baby Animals and a couple of the eq kits once they're released.

poserp
Posts: 40
Joined: Thu Mar 08, 2007 10:11 am
Location: Los Angeles, California

Post by poserp »

Finito... for now anyways. There are a couple of minor (to me) issues, but the thing generally works. I may mess around with the design a bit more to see if I can lower the signal loss (currently about 18 db) and make some other improvements (there are certain settings which cause volume changes and channel interactions, but these are easily avoided in use):

Image

Image

Image

Image

Features as built: There are three switchable inputs per channel. 1 and 2 are mono, split between two bridged-t stepped attenuators to control l and r volume (and therefore panning) seperately. The third input is meant for DAW stems, hardwired l and r, with volume control available for each side seperately. The stepped attenuators allow manual recall of volume/panning, or they can be set full-on and volume/panning can be controlled in software.

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