Fishman "Powerchip" - Need A Schematic

PRR · February 10, 2011, 01:34:10 AM

> "A piezo works by producing a very low level positive signal. It produces no negative signal."

Uh, no.

Connect meter to an unstressed piezo. Bend one way, the meter reads positive, bend the other way, it reads negative.

Look fast! The long-term DC of a piezo is DEAD_ZERO!! You only get spurts one way or the other, which bleed-off through the load (or meter). Good for audio.

The output must be AC, with equal area under each half (no net DC).

> why the waveform is asymmetrical

Guitar, most music, is asymmetrical.

However that photo shows different asymmetry between two source on the same(??) signal.

There could be mechanical bias. A piezo under a string may not bend down as much as up. That would compress the down-swings, but also fatten them to maintain charge equality.

Paul Marossy · February 10, 2011, 09:31:36 AM

Quote from: PRR on February 10, 2011, 01:34:10 AM
> "A piezo works by producing a very low level positive signal. It produces no negative signal."

Uh, no.

Connect meter to an unstressed piezo. Bend one way, the meter reads positive, bend the other way, it reads negative.

Look fast! The long-term DC of a piezo is DEAD_ZERO!! You only get spurts one way or the other, which bleed-off through the load (or meter). Good for audio.

The output must be AC, with equal area under each half (no net DC).

Maybe it would be more correct if it were said that it produces a much weaker negative signal? That picture seems to support that. I dunno, I don't know much about the physics of piezo pickups - that's why I'm asking all the gurus here!

PRR · February 10, 2011, 07:06:45 PM

The piezo output should be symmetric..... not sure what that image is showing.

This would be typical:

http://www.analog.com/library/analogdialogue/archives/43-02/mems_microphones.html

Paul Marossy · February 10, 2011, 09:21:01 PM

That looks more like what I would expect to see. That image was from some program that an individual was using to look at the waveform of his guitar, not sure which one it was, though. I question its accuracy...

AgtSlck · February 23, 2021, 01:54:03 PM

I'm looking into using my Parker P-36 with a Boss WL-20T Wireless, and when the transmitter is inserted, the PowerChip does goes into "Stereo" mode, which means that the Piezo Output is on the Ring of the Output Jack, which on the WL-20 is used for charging, and therefore the Piezo doesn't come out at all, I have the version that has the smart switch (a friend of mine has a version that has a physical switch for Mono/Stereo operation).

I want to see if anyone here knows how does the Smart Switch "senses" the type of plug being inserted...

I know this is a freakishly old thread, but I see a lot of technical knowledge... and I did not see a resolution... so hope someone did find the Schematic or some more info on the Smart Switch... and yes I understand my topic has nothing to do with the polarization/phase...

I know the CD4066 IC is the Switch, but the "smart" part is what operates it... and that is where I would like any info any of you may have...

I know I can use an adapter to get the WL-20T to work, Fishman did tell me that... but that defeats the purpose of the Wireless Form Factor... I also know that shorting the Sleeve & Ring is not something I can use, as Fishman said it would damage the PowerChip... which means the sensing is done some other way...

Thanks in advance for any light you could shed on this...

iainpunk · February 23, 2021, 04:33:57 PM

get yourself a mono male to mono female cable, or make one. it need not be long as short wires as possible is allright, its just to make sure the bass is in mono mode.

cheers

AgtSlck · February 23, 2021, 06:59:33 PM

I know I can use the Mono male to female solution... it is what I have been using, but having that cable dangling is not very nice, so I duct taped the female end to the strap... I need to make sure to unplug the male end from the guitar (so not to drain the battery) and need to carefully insert the WL-20T...

If I could have a more "elegant" solution, like add little toggle switch or a small switch on the back of the guitar that forces the PowerChip into Mono mode... it'd be way easier to switch guitars going from song to song... and I know... 1st world troubles... but if it is possible... why not... besides... I have a lot of time to kill in this pandemic times...

iainpunk · February 24, 2021, 05:38:02 PM

in that case u could try taking out the guitar's jack and just soldering a tiny wire across the tip and ring lugs. a stereo cable helps u identifying which lug is which. u can also go for a simple toggle switch for that, but that might require drilling in your guitar, or you leave the switch loos inside the cavity, where you can switch it when you open up the guitar.

cheers, Iain

AgtSlck · February 27, 2021, 02:36:48 AM

Quote from: iainpunk on February 24, 2021, 05:38:02 PM
in that case u could try taking out the guitar's jack and just soldering a tiny wire across the tip and ring lugs. a stereo cable helps u identifying which lug is which. u can also go for a simple toggle switch for that, but that might require drilling in your guitar, or you leave the switch loos inside the cavity, where you can switch it when you open up the guitar.

cheers, Iain

Fishman has confirmed that shorting the Sleeve & Ring would damage the PowerChip... so I still need to find out how the "sensing" is actually done... not sure if shorting the Ring & Tip would do the trick... and also doubt that Fishman would tell me... the person responded twice, but has dodged the main question...

Cheers... AGT SLCK

ztan · March 23, 2021, 10:30:02 AM

Quote from: AgtSlck on February 27, 2021, 02:36:48 AM
Fishman has confirmed that shorting the Sleeve & Ring would damage the PowerChip... so I still need to find out how the "sensing" is actually done... not sure if shorting the Ring & Tip would do the trick... and also doubt that Fishman would tell me... the person responded twice, but has dodged the main question...

Cheers... AGT SLCK

I've also been thinking about this. I think it likely the piezo ring output runs through the CD4066 switch into a capacitor before attaching to the ring. The ring is then attached to a large value pull up resistor which supplies logic level to the switch and gives the voltage offest on the piezo output. Operation like so:

Mono plug - ring pulled down to ground and this sends logic low to the switch and disconnects the piezo output.
Stereo plug - ring pulled up to a logic high voltage, piezo output connected via switch and capacitor.

Reading the CD4066 datasheet, the logic level threshold is determined by the supply voltage - anyone know how big the voltage offset on the ring output is?

ztan · March 23, 2021, 10:37:32 AM

Quote from: AgtSlck on February 23, 2021, 06:59:33 PM
If I could have a more "elegant" solution, like add little toggle switch or a small switch on the back of the guitar that forces the PowerChip into Mono mode... it'd be way easier to switch guitars going from song to song... and I know... 1st world troubles... but if it is possible... why not... besides... I have a lot of time to kill in this pandemic times...

If you can find the pull up resistor attached to the ring, you could insert a switch into that part of the circuit. You may need an operating microscope.

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Fishman "Powerchip" - Need A Schematic