[sdiy] DIY Mixer - please help!

[cheater cheater]

That's another thing. We don't want impedance mismatches. They can
easily kill an otherwise good sound.

D.

On Thu, Sep 3, 2009 at 8:05 PM, John Mahoney<jmahoney at gate.net> wrote:
> 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
>
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