[sdiy] DIY Mixer - please help!

[John Mahoney]

At 09:28 AM 9/3/2009, cheater cheater wrote:
>[snip]
>Also I'm not very happy with using divide-down/resistors/whatevers in
>the summing bus input. The first problem is that it changes the way
>the mixer works depending on how many channels are in use (and the
>additional complexity of added switches/relays/etc is not worth it).
>The other problem is, as I understand it, that such approaches either
>require an active element (which adds distortion) or a passive network
>(which can easily create problems in impedance matching)
>
>For example Manley seem to be using solid-state strips, but the
>summing bus is a tube based circuit, which can easily run on high rail
>voltage, since tubes like high voltages, and also distorts nicely,
>since tubes, well, distort nicely. But I think that a similar approach
>can be had with a discrete design based on solid state circuits, I
>might be wrong of course (probably am).
>
>So the question is - how do you make a summing stage that can work on
>high voltages that reach, say up to 75V without distortion, and then
>progressively starts being less and less linear, behaving like a nice
>soft clipper? I think it would be nice if the behavior when you're
>near the rails were similar to what I described.
>
>I think it would also be nice to be able to dial in the symmetry
>yourself by inputting a DC bias - instead of the circuit changing
>symmetry in itself. For example I sometimes see this used for clipping
>off 'spikes' on just one side of a signal. I understand that one trick
>is that if you leave the spikes only in the 'negative' part of the
>signal, they would happen when the tweeter 'pulls' and not when it
>'pushes', giving you a mellower high-end.
>[snip]

You are hinting at the possibility of "something for nothing", but I 
don't think that a resistor-free mixer exists. At some point, you 
need to mix several signals together, and -- as far as I know, which 
could easily be wrong -- that requires a summing bus that uses resistors.

Seems to me that to achieve the massive headroom you want, you need a 
passive summing bus. Then you need a master attenuator to set the 
bus's output level as it is fed to the next active stage, adjusting 
it to drive the active stage to the desired level (overdrive or not). 
Perhaps the bus's output attenuator will play games with the input 
impedances, though.

John