[sdiy] AC coupling caps on MS20 clone

[Mattias Rickardsson]

On 7 August 2010 07:57, David G. Dixon <dixon at interchange.ubc.ca> wrote:

"Yes, but what if the filter has a three-input mixer?  (You see,
that's my dilemma...)"

Good! Single-input audio modules - requiring standalone mixer modules
which means both extra hardware, scattered controls, and reduced
ergonomics - is also *not* good design imho. :-)

 "Oh gawd!  Just use the switch, Mattias!"

:-D
Sometimes the best solutions come out of overcomplicatifyizing
followed by reduction. ;-)

On 7 August 2010 12:02, megaohm <megaohm1 at gmail.com> wrote:
>>
>>>  3) level pot with pull-switch, to select AC or DC coupling.
>>
>> Too complicated/expensive for my tastes (which are simple and cheap).
>
> It's no more complicated than a pot and a toggle switch. If you use
> quality toggles (NKK for example) then a P/P pot is less expensive
> than a pot and toggle.

True, however the pot switch will probably be of lower quality.

>>> The pull-switch solution is my personal favourite, but it might
>>> interfere a bit with another clever thing discussed here some weeks
>>> ago when it came to "bipolar" level pots... where the true null point
>>> can be hard to find, and a pull-switch could come in handy when the
>>> signal is supposed to be totally off.
>
> Another benefit is you get the full rotation of the pot for level
> control instead of just half the travel.

True, a longer travel is better almost all the time.

The resistor-shunted "bipolar fake log" pot has advantages over the
resistor-shunted "fake log" pot however: it has a flatter curve around
the center null point, whereas the non-bipolar is steeper at its end
null point and then flattens to a log-like behaviour. Easier to set at
low levels. I'd prefer "bipolar fake log" any day, regardless of its
shorter travel - but it needs an extra buffer amp.

http://www.elby-designs.com/documents/tailoringpotentionometers.pdf

>>> - With mono plug half-in, BOTH jack tip and jack ring goes to plug tip.
>
> The reason I don't like this idea/solution is because all patchcord
> plugs are different. When half inserting into a stereo jack the tip...

Great, you just helped me to avoid those deceptive design thoughts and
focus on something better and more reliable. :-)

> Have you thought of this solution:
> Use just one cap and one toggle switch.
> Connect all inputs (after the level pots) to one side of a cap. Other
> side of cap connects to summing point of op amp. Use the toggle switch
> to short the cap when you don't want AC coupling. This solution saves
> you two toggles and two caps.

It doesn't get much simpler that this, and it's hard to justify the
more advanced ways of doing it. :-)

/mr