Electrodyne ACN Summing Mixer

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RSRecords

Well-known member
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Jun 8, 2009
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Hi all.  I'm currently designing a 16 ch summing mixer based around 2 Electrodyne ACN-1P's.  CH. 1 -8 will have LCR panning and level adjust. 9-16 will just be hard panned stereo inputs.  For outputs I'm thinking of two spa62s that I've been saving into Jensen output transformers.

I'm hoping to get some feedback on my drawing. Anything I'm just forgetting? Obvious pitfalls? Something gonna blow up??

Do you think it is a waste to only use 12 of the potential 21 inputs per ACN? I could easily enough add another 8 stereo inputs for 24 total.



Thanks


edit: corrected schematic in later post.



 
Just to be clear, your inputs will all be unbalanced?

Not sure how you worked out the LCR pot divider but you should be aiming for about 3dB drop at the centre.

is the ACN a virtual earth mixer>

Cheers

ian
 
ruffrecords said:
Just to be clear, your inputs will all be unbalanced?

Not sure how you worked out the LCR pot divider but you should be aiming for about 3dB drop at the centre.

is the ACN a virtual earth mixer>

Cheers



ian
Click to expand...
All inputs will be unbalanced.  I couldn't work out a cost effective solution for balanced inputs since the ACN-1Ps are unbalanced in and out. The ACNs actually put out a decent level but I thought the output transformer would benefit from another amp.

I used an online t-pad calculator for the 3db Center attentuation. What would be the preferred way to calculate those values?

I'm am pretty sure its a virtual earth mixer.  I was able to track down that the input impedance of 10k.
 
RSRecords said:
All inputs will be unbalanced.  I couldn't work out a cost effective solution for balanced inputs since the ACN-1Ps are unbalanced in and out. The ACNs actually put out a decent level but I thought the output transformer would benefit from another amp.
Click to expand...
OK, understood. Just wanted to be sure i had read your schematic correctly.
I used an online t-pad calculator for the 3db Center attentuation. What would be the preferred way to calculate those values?

I'm am pretty sure its a virtual earth mixer.  I was able to track down that the input impedance of 10k.
Click to expand...
As the Electrodyne ACN-1P is a virtual earth mixer with 10K input impedance it seems reasonable to conclude that the 10K input resistors are built in to the Electrodyne ACN-1P.

For a T attenuator, you need to have a known input impedance and a known load impedance. The load impedance is 10K but the input impedance varies as the 10K input pot is adjusted. As you have a virtual earth amplifier,  the gain is given by the internal feedback resistor  divided by the 10k  input resistor. So, to lower the gain by 3dB, all you have to do is increase the input resistor by 3dB. SO you can simplify your switching arrangement to simply adding a 4K3 series resistor in each channel when in the centre position.

Cheers

Ian

P.S. I could not find a schematic for the Electrodyne ACN-1P so I do not know if the assumption that the 10K resistors are already built in is correct.
 
ruffrecords said:
OK, understood. Just wanted to be sure i had read your schematic correctly.As the Electrodyne ACN-1P is a virtual earth mixer with 10K input impedance it seems reasonable to conclude that the 10K input resistors are built in to the Electrodyne ACN-1P.

For a T attenuator, you need to have a known input impedance and a known load impedance. The load impedance is 10K but the input impedance varies as the 10K input pot is adjusted. As you have a virtual earth amplifier,  the gain is given by the internal feedback resistor  divided by the 10k  input resistor. So, to lower the gain by 3dB, all you have to do is increase the input resistor by 3dB. SO you can simplify your switching arrangement to simply adding a 4K3 series resistor in each channel when in the centre position.

Cheers

Ian

P.S. I could not find a schematic for the Electrodyne ACN-1P so I do not know if the assumption that the 10K resistors are already built in is correct.
Click to expand...

The 10k input resistor network is built into the ACN-1P.  I don't have a schematic either just a pinout that I attached below.

Thanks for the input::) regarding the the center position attenuation.

Just to clarify, you are saying to replace R2,3&4 with a 4k3 resistor. And also replace R5,6,&7 with a 4k3 resistor?

What do you think about the 10k Pots feeding the output amps (R10 and R13)? Would it be more prudent to use a passive fader approach for the main outputs and just leave the amp at a fixed gain?

Thanks for the help, it's much appreciated.





 
RSRecords said:
The 10k input resistor network is built into the ACN-1P.  I don't have a schematic either just a pinout that I attached below.

Thanks for the input::) regarding the the center position attenuation.

Just to clarify, you are saying to replace R2,3&4 with a 4k3 resistor. And also replace R5,6,&7 with a 4k3 resistor?
Click to expand...
Delete R4 and R7. Replace  R2 and R3 with a single 4K3 resistor. replace R5 and R6 with a single 4K3 resistor.
What do you think about the 10k Pots feeding the output amps (R10 and R13)? Would it be more prudent to use a passive fader approach for the main outputs and just leave the amp at a fixed gain?

Thanks for the help, it's much appreciated.
Click to expand...

R10 and R13 look fine to me. However, there is an error in the op amp drawing. You appear to have positive feedback.

Cheers

Ian
 
oops goofed up. I'll attach a corrected drawing with negative feedback.


corrected schematic with some de-cluttering as well.
 
RSRecords said:
oops goofed up. I'll attach a corrected drawing with negative feedback.


corrected schematic with some de-cluttering as well.
Click to expand...

Better but you now have the -ve input of the op amp grounded.

Cheers

Ian
 
hitchhiker said:
On my Sphere console the output of the SPA62 goes through a 100 uF then a 47 ohm resistor then the JT123.  Also has a 47k to ground after the 47r  (primary transformer side) .
Click to expand...

Oh cool. What Sphere do you have? I was using some  partial Eclipse A docs .

Does anyone know the benefit of adding the 47k to ground?  Just not sure what that would be doing. I'm not saying it's not needed I'm Just curious.
 
It's a small 4 bus console that was stripped of the input EQ's when I got it.  The output cards are 920 EQ's with the EQ section bypassed. These cards are  two SPA-62  with fader between.  WRT the 47k I would guess that the designer (John Hall) was controlling the output impedance that the SPA-62 would see?  Many others here that could critic if it was a good idea or not?
 
RSRecords said:
Oh cool. What Sphere do you have? I was using some  partial Eclipse A docs .

Does anyone know the benefit of adding the 47k to ground?  Just not sure what that would be doing. I'm not saying it's not needed I'm Just curious.
Click to expand...

If the transformer is not on the PCB then it ensures there is a dc path to ground for the output cap to charge up through. Maybe they tested the PCBs in production without the transformer attached.

Cheers

Ian
 
ruffrecords said:
If the transformer is not on the PCB then it ensures there is a dc path to ground for the output cap to charge up through. Maybe they tested the PCBs in production without the transformer attached.

Cheers

Ian
Click to expand...

That does make since. And it's certainly easy enough for me to add.

I'm working on an updated schematic. I really appreciate everyone's input and guidance thus far. I do a bit of repair work but not a lot of designing and drawing.

Thanks!


P.s. Pretty sure sites like this are why the internet was invented...
 
Updated schematic attached.

I think I have the neg. feedback figured out for unity gain. I'd appreciate any feedback on the subject.


I'm not starting to figure out a meter buffer schematic.  I'd rather diy it than buy a JLM board (as nice as those look.)I'm going to use a nice pair of vintage  Simpson VU meters. Does anyone  have a simple buffer circuit for real vu meters? something I could breadboard?

Thanks!
 
ruffrecords said:
Not sure why you have the 10K resistors in series with the pot wipers.

For unity gain you just connect the output to the -ve input. -ve input goes nowhere else.

You need to read up on basic op amps. e.g.

http://www.electronics-tutorials.ws/opamp/opamp_3.html

Cheers

Ian
Click to expand...

Fair enough, that's a good website. Thanks for the link.

  I think I am over complicating things a bit. The 10k resistor in series with the 10k pot is straight off the sohere  documentation.  I am going to do the neg feedback the same as it is in the docs and see how that goes.

hitchhiker said:
The gain set on my SPA-62 output amps are 56k with a 22pf cap.  Not unity but is unity gain what you need?
Click to expand...
I'm not sure how much gain I'll need, probably not much because the ACN does output unity or even 10db of gain.
I've attached some pics of the documents I'm using. I'm pretty sure they are from an eclipse a (maybe c). This attachment has the part values. The next will have the output amp.
 

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