Question about api 325 as a summing amp

I´ve been thinking about the use of a api 325 as a summing amp. From what I have understood from application notes and schematics, the gain is adjusted by either placing a fixed value resistor or a pot between two of the pins of the card.

What would be the best way to acomplish an insertpoint pre fade with the 325?

I have been thinking about using a fixed gain and adding the fader after the the card post insert but then the multiple outs of the transformer goes to waste so to speak. I guess adding this between the opamp and the transformer isn´t possible right?


Here is the documentation BTW:

http://www.danalexanderaudio.com/api.html

Fredrik,

You say "summing amp", do you mean a "mix-bus amp" for example, so you can mix a lot av input modules together in one point ??

In this case, it is very simple to change the 325 card to a "virtual mix-bus amp"

1. connect card connector 5 (+ not inverted input) to ground.

2. connect a 220 uF capacitor (positive to the card side) to card connector 8. (- inverted input)

3. Use the capacitor negative side as the mix input, there you shall connect a 47 kohm resistor in series for every signal source you want to add to the mix-buss.

Every mixed input (include the 47 kohm serial resistor) have unity gain sensitivity.

Note that the amp are phase reversed from in to out, so if you want it in phase, you must do a phase reversing on the cards output connector.

--Bo

[quote author="Bo Hansén"]Fredrik,

You say "summing amp", do you mean a "mix-bus amp" for example, so you can mix a lot av input modules together in one point ??
[/quote]

Yes that´s what I meant. Thanks Bo :thumb:

I have an additional question about this arangement. If I would like to add separate passive level control to the sources being summed to the 325 would a buffer between the attenuator and the 47k resistors in the summing network be a solution?

Fredrik,

You not need extra buffer.

Connect the potentiometers "wiper" to the 47 kohm serial resistor input.

Use 10 or 22 kohm logaritmic potentiometers.

--Bo

Ok Bo, thats good news. Thanks

Ok, back to the idea of adding an insert point before the master fader. I have made this block diagram to ilustrate how I´m thinking here. The 325 symbol is representing the whole amp including the transformer. Any thoughts on this, would it work or are there any potential problems you can spot etc.

http://twin-x.com/groupdiy/albums/userpics/325_block.pdf

Would it be a problem to keep the signal balanced after the transformer? I´m assuming that the output from the trasformer can easily drive any type of insert on the send. But can the fader and distribution be made while keeping the signal balanced?

As an alternative I´we been thinking of the possibility to use two 325 in series where the first one is used as a mix bus amp without a transformer on the output. The balancing on the insert is done with THAT or SSM type driver/recievers. The second 325 has the fader on the gainport and the distribution is made by using all three outputs on the transformer.

Also, if anyone knows how I can make the image ( pdf ) viewable directly in the forum that would be nice. Thanks