A summing amp with more transformers

[bjosephs]

So, I promptly lost track of this project last summer for a number of reasons. I periodically go back to it and get a little stymied over the approach I could take to the individual buffers on each channel. I get stuck between designing the PCB and hoping I like the result or leaving it a bit flexible for future experiments. That got me wondering why I need buffers... If I use a virtual earth summing amp, can't I just make my mix resistors equals to the optimum load for the transformer? Something like the attached 4-channel example. And even with a non-VE summing bus wouldn't it be fine as long as the parallel combination of the "other" bus resistors, plus the series mix resistor, equals that optimum load on each channel?

 

[ruffrecords]

It will be fine in both cases. The VE case is self evident. For the passive bus you need to bear in mind the impedance of the source device reflected to the secondary of the transformer. Normally this is a very small value so you are safe in the assumption that the parallel combination of the other bus resistors in series with the mix resistor should be equal to the optimum transformer load. The thing you need to be aware of is that this no longer holds if nothing is plugged into the other inputs.

Cheers

Ian

 

[ruffrecords]

But they're so small!

To be clear I'm not looking to mangle things so maybe it will be the right balance. Care to share how you plan to implement them? 1:1 or 1:.5? Are you using the terminations from the datasheet?

Apologies for the late reply. I somehow missed that question. I am using it as a 10K:10K with the secondary loaded by a 10K pot.

Cheers

Ian

 

[bjosephs]

Thanks for confirming.

The thing you need to be aware of is that this no longer holds if nothing is plugged into the other inputs.

I plan to tie the tip and ring together with a switching jack. I think that should make the unused input look like a low impedance so as to not affect other channels when unplugged.

 

[bjosephs]

am using it as a 10K:10K with the secondary loaded by a 10K pot.

With the datasheet damping network or are you letting them ring? My scope says they ring at around 54kHz, lower than I would have thought but probably not a big deal. I know some folks ascribe some of that vintage goodness to underdamped transformers.

 

[ruffrecords]

Thanks for confirming.



I plan to tie the tip and ring together with a switching jack. I think that should make the unused input look like a low impedance so as to not affect other channels when unplugged.

THat is an ideal soultion. You would be supriesed how many people don't do that.

Cheers

Ian

 

[ruffrecords]

With the datasheet damping network or are you letting them ring? My scope says they ring at around 54kHz, lower than I would have thought but probably not a big deal. I know some folks ascribe some of that vintage goodness to underdamped transformers.

It is a tradeoff. If you dampen the ringing you can lose HF response with higher impedance sources.

Cheers

Ian