Federal AM864/ Vari-mu stereo linking? Re: the pim /mod the Poor Man 660 thread

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Rompstomp89

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Have been toying around with the idea of stereo linking a pair of Federal AM864 builds.  What would be the best way to approach this?  I want to stay away from needing external power supplies and relays and such.  emrr had an interesting idea in using the collins 26U-2 stereo linkage circuit in a fairchild clone. I attached his thoughts on this post.


emrr said:
There's been no mention of stereo linking method to date, and it's not on the PM schematic. The Fairchild simply ties the two control voltages together.  Here's some food for thought.

Many limiters use diode isolation to ensure dual mono and stereo operation match in timing. The Fairchild fails to correct for this, for better or worse. As is, in the original design, if you toggle between linked and unlinked the time constants will change because of the nature of paralleling networks. We do not have the complexity of time constant switching found in the Fairchild, so it's not as if I'm suggesting we alter something that already mimics exact original operation. As far as I can tell, the only thing possibly changed by addition of diodes is the relatively minor need to overcome the diode voltage drop through additional drive on the threshold control. 

NONE OF THIS HAS BEEN ATTEMPTED IN THE FIELD WITH A PM660!

Here's a comparison of the mono Collins 26U-1 with the stereo 26U-2 for example.  This is not to be taken as a cookie-cutter drop-in, given the differences in design.  The Collins being the more common type of side chain driven from a high impedance source with fairly high bias voltages present, and the PM660 being a side chain driven from a low impedance source with fairly low bias voltages present.  The Collins manual describes the 1N459's as preventing feedback into the 1 mfd caps, to avoid increase in attack time from effective lowering of 1 mfd value. 


2771971717_e76b746cff_o.jpg


The unattached connection from the 0.01 cap below runs to the link switch, then to the other identical side chain and audio channel. 
2772818784_97a4faf058_o.jpg
 

emrr

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26U-2 is in the Tech Docs now.    Diode iso works well, have added it to several other vari-mu pairs. 
 

Rompstomp89

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emrr said:
26U-2 is in the Tech Docs now.    Diode iso works well, have added it to several other vari-mu pairs.

Thanks emrr!

Looked at the 26U schematic.  from what I see its as simple as linking the two center taps at the L and R inputs? Im such a noob when it comes to all of this stuff.  I can follow a schematic for the most part but I have no idea of the reasoning behind what I'm doing ha.  My goal is to have one of my units as a slave. so all the volume, attack, release and threshold knobs would work the same for both.  Is that even possible? Like I said, I'm a noob.  Have you done this to an AM864?

Also, Can you shoot me somewhere to understand this diode iso as well?

Thanks so much in advance!

Rob
 

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