Piezo/Mag combination questions...jazz box project Dec 16, 2011 22:45:44 GMT -5
Post by reTrEaD on Dec 16, 2011 22:45:44 GMT -5
blend is such that there is signal in to the centre lug, signal out one side, ground at the other...in the current scheme.
It's not common to have a blender after an active buffer. So we're in new territory. The common way of doing a blender has the pots wired "backward" as you just described, with the input on the wipers. Best to avoid that.
I think either John's scheme with a single-gang 100k linear blend or the more recent scheme with the 500k stewmac blend pot wired CORRECTLY will be better than the stewmac wiring.
I promise you will like this one.....!
So, mag pup goes to buffer. Then dual-gang master volume, 100k log. (its 100k so it can be followed with a 500k blend pot). Then to the Stewmac blender. On the diagram, Iï¿½ve put a heavy line on the half of each half pot that has low/zero resistance. So the signals after the blend pot, with volume at max and blend half way, are each the full pickup signals. Then there are resistors, Iï¿½m suggesting 47k or 100k but that could be reviewed, then buffers to two outputs. So you have your two stereo outputs, each under master volume and blend-pot control. Actually in stereo mode, I would call the action of the ï¿½blendï¿½ pot a ï¿½balanceï¿½ pot ie, it is adjusting the levels of two signals but not mixing them.
Then, the final feature, the mono stereo switch ï¿½ which joins the two channels to combine them. It happens after the resistors, so no distortion due to shorting active circuits together. In mono mode, you have two outputs of the blended mix. The gradualness of the blending depends on the resistor value ï¿½ actually 100k instead of 47k will give a more gradual blend.
The mono/stereo switch could be part of one of the output jacks (needs an isolated switch as shown, but it could be adapted to use a simple switched jack). But, I think it would be much fun to have a separate mono/stereo switch. You could use it at a poignant moment to make a leap from a centrally placed mono blend to the two channel stereo image.
ReTread, Ash, newey and others ï¿½ what do you guys think?
I'm wondering if a blendpot that has full output at the midpoint will cause a different set of problems in this application. In "stereo" mode, when the control is rotated from center, one side is reduced in volume, but the other doesn't increase. It just says the same. This is a little different from the way a "balance control" operates. Not necessarily bad, but not what one might expect.
The other problem will be in "mono" mode. As we move from midpoint toward the mag end of the rotation, the mag signal at the wiper isn't increasing. But the resistance at the piezo wiper will first rise to a maximum of 125k, then decrease to zero. If we look at it from the standpoint of the wiper resistance being in series with the bottom of the voltage divider, the volume of the mag signal will have a bit of a hump somewhere between midpoint and full rotation toward mag.
A blend pot with a gradient from end to end would probably be better than one with full output at the midpoint. At least in this application.
I guess we should wait til 4real can tell us what's in those stewmac blenders.
fwiw, I made a spreadsheet based on this diagram.
It's meant to evaluate the interaction of tying two wipers together. The Y axis of the graph is the output voltage at the load when the left hand pot is rotated. I can select the voltage on either source, both source resistors are inputted from a single field, as is the value for both pots.
The value of the right hand pot is at a constant value (percentage of rotation) I have a range of 20 values (5% increments) for the rotation of the left hand pot.
The result is a graph. Since the increments are linear, the results should be linear. Any deviation from a straight slope suggests "abruptness" or "flatness" in that portion of the rotation.
Here's an example:
This might be useful along the way.