[sdiy] DIY Mixer - please help!

[Jerry Gray-Eskue]

This is an untested concept but should work OK. I cannot give you any sort
of THD distortion number for this setup. It is a simple thing to breadboard
and check out.

I have not had all my coffee yet so I may have the (Diode(s)+) in backwards,
but they look correct at the moment...


signal ground --- Resistor In ----+------ Resistor Out-------------+
                                  |                                |
					    +--Zener+----+Zener--------------+
                                  |                                |
                                  |    ---------------------       |
                                  +--- - input             |       |
                                      |              output--------+-------
      Signal In ---------------------- + input             |
                                      |                    |
                                      |       Op Amp       |
                                      ----------------------

Except for the Zener Diodes, this is a standard non-inverting amplifier with
a Gain of:

Volts Out = (1+(Resistor In/Resistor Out))* Signal In Volts.

I tend to use resistors around 100k, just say away from small values <5k and
very large values >1m or you might get some side effects.

The Zener diodes will limit this to around the Zener voltage.
The power supply is a split voltage +12 / -12 or +15 / - 15
Signal ground is also the power supply ground (0 volts).

Without the Zener Diodes you are limited to the output swing of the Op Amp
related to the power supply rails, this varies but is around a couple of
volts. You should be able to +10 to -10 out with a +- 12 v power supply. You
may want to use a 9 or 10 volt Zener.

To add additional diodes you need to do this:



signal ground --- Resistor In ----+------ Resistor Out-------------+
                                  |                                |
					    +--Diode(s)+--+Zener-------------+
					    +--Zener+-+Diode(s)--------------+
                                  |                                |
                                  |    ---------------------       |
                                  +--- - input             |       |
                                      |              output--------+-------
      Signal In ---------------------- + input             |
                                      |                    |
                                      |       Op Amp       |
                                      ----------------------

The plus (+Zener)	(+Diode(s)) indicates the cathode.



-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of cheater cheater
Sent: Friday, September 04, 2009 7:47 AM
To: synth-diy
Subject: Re: [sdiy] DIY Mixer - please help!


Jerry,
that sounds very interesting. Do you think you'd be able to whip up an
absolutely minimal schematic that contains just all the components for
the op amp and compares the approaches with the diodes, and without
them? It's hard for a newbie like me to picture what's going on in the
design you mention.

Thanks a lot
Damian

On Fri, Sep 4, 2009 at 2:44 AM, Jerry Gray-Eskue<jerryge at cableone.net>
wrote:
> <<How would an op amp/vca need to be designed in order to gradually set
> on distortion as the signal nears rail voltage? And 'controlled'
> distortion before that - while keeping noise levels at a minimum until
> the 'controlled' distortion happens, and making the thing as linear as
> possible until that point?>>
>
> OK, it may be a bit tricky, but basically you want the op amp signal path
> the work normally when you are not near the rails, which is what they do
> anyway, so that part is easy. The trick is to add in a "Soft Clip" as you
> approach the rails.
>
> One way to get a soft clip is to add opposite facing Diodes parallel in
the
> feed back loop of a gain stage to reduce gain as they turn on, the problem
> is they start conduction way too early, around .4V and are fully on at .5V
> to .9V typically about .6 Volts.
>
> However you might use Back to Back Zener Diodes in the feed back loop,
> controlling the "Clip Limit" by the Zener voltage. An additional benefit
is
> that while one Zener is in Breakdown the other is in forward conduction
> giving a longer soft clip region.
>
> The other approach would be to use the opposite facing Diodes approach,
but
> use Strings of diodes to get the turn on voltage you want, unfortunately
> this is about 16 diodes in a string for 10 volts. LEDs have a greater
> forward voltage drop, about 1.2 but still would require 8 in a string.
>
> The Hot ticket may be to use the Back to Back Zeners and if the clip is
not
> soft enough start dropping the Zener voltage and adding stacked diodes to
> put more "Curve" in the clip.
>