[sdiy] DIY Mixer - please help!

[cheater cheater]

>>
>> 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
>>
> 75V?!? Are we really sure about these signal levels? This and the 98V
> level quoted earlier seem insanely high to me. I could be wrong on
> this, never having designed a mixer in my life, but where are you
> going to get solid state devices to handle these levels? I'm pretty
> sure the last time I looked inside a mixer, it was filled with things
> like 1/4W resistors and op-amps in DIP packages. Nothing in there that
> would handle a 75V signal without turning the mixing desk into an
> expensive BBQ.
>

Yes Tim.. it's bedazzling! I just checked the manual for the Manley 16x2:
http://www.manleylabs.com/PDF/PRO_Manuals/asst%20pro/Mixer%20Manual.pdf

Here's what they say about what I understand is their tube summer:

"""
The left tube board takes the mix busses through the 1K MIX GAIN pots,
then a 7:1 step-up transformer. The left signal is amplified by a 25db
tube voltage amp/line driver and the right signal is sent to the right
tube board. The voltage amp is a paralleled (for low noise) 12AT7wa
direct coupled into a White Follower line driver which uses a 6044. It
is AC coupled with a 30 uF audiophile grade Multi-cap into the 1/4"
Unbalanced Mix Output and a Manley 9811 transformer for the Balanced
XLR Mix Output. A tertiary winding on the 9811 is used for negative
feedback through a trim pot for gain adjustment (located near the top
of the board between the tubes). This circuit is virtually identical
to the tube stages used in the Massive Passive and produces +37 dBu
levels ! This stage is quite similar to a Mic Pre with moderate gains.
"""

So it can output up to 54.837244642 Volt RMS

But then the Vari-Mu compressor manual states:
"""Maximum Input +36 dB (52 V RMS) (estimated for 1% THD). Input
attenuator is before input transformer"""

So the 37 dBu will drive the max 36 dBu by one decibel only.. it's
absolutely amazing, right?

And the peak-to-peak on +37 dBu is: 155.103150205V

So there you go

ICs might be out

:)

>>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.
>
> Okay, now THIS I would have to see some proof of. This is starting to
> sound like audiophile hocus-pocus if you ask me. Can anyone else
> confirm or deny this? I know next to nothing about sound
> reinforcement, so I could easily be wrong on the above questions, but
> something just don't seem right...
>

Tim, I'm just repeating what I heard from some mastering engineers
I've spoken to. The point is that speakers don't work the same both
ways - and I can appreciate this idea - after all if you look at a
tweeter dome it's often shaped to 'scoop up' air when it pushes out,
but might not couple air just that well when it pulls back in. I think
it's fairly reasonable to say this might be what's happening. Consider
what happens when you are on a canoe and you're paddling. The paddle
will be curved to 'scoop up' water when you use it one way, and to
have less resistance when you push it backwards.

D.

On Thu, Sep 3, 2009 at 8:28 PM, John Mahoney<jmahoney at gate.net> wrote:
> At 03:21 PM 9/3/2009, cheater cheater wrote:
>>
>> That's another thing. We don't want impedance mismatches. They can
>> easily kill an otherwise good sound.
>
> Don't we *always* want impedance mismatches? A device's output impedance
> should be higher than the input impedance that it feeds into.
>
> ;-P
>
> John
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