Fishman "Powerchip" - Need A Schematic

Paul Marossy · July 06, 2006, 01:03:15 PM

Anyone ever reverse engineer this circuit? I need a schematic for a repair, but I can't get one from Fishman...

http://www.fishman.com/uploads/products/documents/42.pdf

Paul Marossy · July 07, 2006, 10:45:07 AM

I thought it was a long shot, but of any place to try, this would be it!

BTW, Fishman won't give out schematics, so that is why I put up this post.

Paul Marossy · February 06, 2011, 10:27:56 PM

Just bumping this up to the top in the hopes that maybe someone has done this in the recent past...

Simon Owen · February 07, 2011, 03:38:11 AM

Just read the datasheet:
9V input voltage, 15V pk-pk output voltage. - Voltage doubler on the 9+
from the photo it has a SMT CD4066 bilateral switch - Used to select phase ?
I would think the DIL8 in the photo is the voltage doubler.

Paul Marossy · February 07, 2011, 09:46:50 AM

Quote from: Simon Owen on February 07, 2011, 03:38:11 AM
Just read the datasheet:
9V input voltage, 15V pk-pk output voltage. - Voltage doubler on the 9+
from the photo it has a SMT CD4066 bilateral switch - Used to select phase ?
I would think the DIL8 in the photo is the voltage doubler.

Thanks for the reply. Where did you find this datasheet?!

Anyway, yes, it does indeed use a voltage doubler chip. What I am really wondering about is if it inverts the phase of one type of pickup (the piezos I would assume).

G. Hoffman · February 07, 2011, 08:15:53 PM

If it's under warranty, they will send out a free replacement, and their shipping times are pretty quick. (They were a little slow last week, since they were closed for at least two days because of all the snow! But normally they are great.) Either way, I'm pretty sure they are a pretty basic op amp circuit. All their other preamps are.

Gabriel

Paul Marossy · February 07, 2011, 10:31:31 PM

Mostly I am trying to find out about the design of it. I think I might have to generate a schematic for it to answer my own questions about it.

PRR · February 08, 2011, 02:15:10 AM

> Where did you find this datasheet?!

Page 11 of the link you posted in 2006.

> if it inverts the phase of one type of pickup

Page 5 of the link you posted in 2006: Move the Phase jumper on the backside of the Powerchip to eliminate phase cancellation between the Piezo and Magnetic pickups.

> CD4066 bilateral switch - Used to select phase ?

Their jumper scheme could flop polarity without added complexity. The CMOS apparently senses mono/stereo plug stuck into guitar and routes signals accordingly. Page 2 of that manual: A "smart" electronic switch in the Powerchip circuit automatically routes Piezo and Magnetic pickup signals to either stereo or mono signal paths.

I'm shocked Gabriel didn't comment on drilling into a Strat with a 3/4" spade-bit.

Gurner · February 08, 2011, 07:02:33 AM

Wondering how the phase aligment works - if I understand it correctly piezos pickups will always be phase shifted vs a magnetic pickup sensing the same string - flipping the phase 180 deg won't help?

Paul Marossy · February 08, 2011, 09:44:10 AM

Quote from: Gurner on February 08, 2011, 07:02:33 AM
Wondering how the phase aligment works - if I understand it correctly piezos pickups will always be phase shifted vs a magnetic pickup sensing the same string - flipping the phase 180 deg won't help?

I am currently having a discussion about this at the Parker Guitars forum. I am trying to figure out if the Fishman preamp intentionally inverts the phase on one type of pickup or not. Someone posted pictures of a waveform from their guitar, which shows a "DC offset" on the piezo pickups when compared to the magnetic pickups, but no phase inversion. That was in summing mode where the piezo and magnetic pickups are combined into a mono signal. I am guessing that the "smart switch" probably is what the CD4066 is for. I guess the jumper is to manually overide the smart switch if the pickups should happen to be out of phase with eachother when combined in mono mode, which I would expect provision to be made for this since the PowerChip could end up in any guitar with any number of pickup combinations.

What I am curious about is why there is a "DC offset" and also why the waveform is asymmetrical. Piezo signal is on the left, magnetic is on the right:

Gurner · February 08, 2011, 10:22:11 AM

WRT your posted scope screenshot pics - alas, it's not possible to tell with the phase is inverted from those scope traces ...the timebase is too compressed - you need to able to see the individual cycle/waveforms (something like a 1m/s per div trace) *and* then compare side by side in real time as the string is being plucked.

What you're actually seeing there in those scope traces is the overall signal 'envelope'...and of course the envelope will look broadly similar whether piezo or mag.

As it goes, I have a test guitar with a piezo bridge (hex saddles) and a mag pickup on it too - I've been meaning to do such scope traces to satisfy my own curiousity....will post the results.

re the DC offset - that depends where the person scoped the signal

Paul Marossy · February 08, 2011, 10:29:05 AM

It's not a picture I made, they were using some program to get that, not sure if it was an oscilloscope program or not. I doubt that it was. I think I am going to have to put my own Parker on the scope and look at what is happening...

Gurner · February 08, 2011, 10:32:03 AM

Quote from: Paul Marossy on February 08, 2011, 10:29:05 AM
It's not a picture I made, they were using some program to get that, not sure if it was an oscilloscope program or not. I doubt that it was. I think I am going to have to put my own Parker on the scope and look at what is happening...

i know they weren't, my post was in response to your 'interpretation' of the pictures...

Quote from: Paul Marossy on February 08, 2011, 10:29:05 AMSomeone posted pictures of a waveform from their guitar, which shows a "DC offset" on the piezo pickups when compared to the magnetic pickups, but no phase inversion

like I say, it's impossible to say if the phase is being inverted or not in those pictures

Paul Marossy · February 08, 2011, 10:36:37 AM

No worries. I also noticed that you couldn't tell if the phase was inverted or not from that picture.

PRR · February 08, 2011, 04:30:58 PM

Maybe I'm reading a different Installation Guide?

The "Smart Switch" senses plug ring to know to run preamp in mono or stereo. On a mono plug the "ring" and sleeve are the same thing. A stereo plug has a separate ring.

> piezos pickups will always be phase shifted vs a magnetic pickup

Yeah, a point. Ideally the velocity (magnetic) leads the displacement (piezo) by 90 degrees. But our magnetic pickups are far from exact velocity sensors. Also -- assume the gitar is laying flat, on table or lap. The mag PU senses "sideways". I can't be sure, but the piezo may sense "vertical". A plucked string starts sideways but is soon vibrating vertical as well. The sideways and vertical shakes are of course 90 degrees apart and may be any relationship.

So there is no "in phase" for all conditions, only "best phase" which may be a judgement call.

Fishman knows which way their piezos are polarized, but can NOT know which way the mag pickup is polarized, or how the axe's action and player-style shift the phases. Hence a setting, apparently "set it and forget it".

Paul Marossy · February 09, 2011, 03:48:00 PM

I think I've figured it out based on what someone has said:

"A piezo works by producing a very low level positive signal. It produces no negative signal. It doesn't modulate a DC current but actually produces one from the pressure of the string and waves."

I have not heard this before. But it would explain a few things! Since it's not an electro-magnetic device like a conventional passive electric guitar pickup but works on an entirely different principle, that seems very plausible. A piezo pickup is traditionally made from material that reacts to vibration, and is made either of hard ceramic or certain polymers. An electrical impulse is created proportional to the amount of pressure applied which the preamp then amplifies enough for a guitar amp to work with it. It doesn't create an alternating current (in the sense that I am used to seeing it) because there is no transducer coils to generate that. So I guess that explains those weird waveforms?

Johan · February 09, 2011, 03:59:12 PM

Quote from: Paul Marossy on February 08, 2011, 09:44:10 AM

What I am curious about is why there is a "DC offset" and also why the waveform is asymmetrical. Piezo signal is on the left, magnetic is on the right:

possibly because the signal is decoupled with a cap and the RMS value of the signal is symmetric allthough the signal appears not to be..wich would just be an other way of describing what you just wrote in the post just before this one.

J

Gurner · February 09, 2011, 04:23:19 PM

Quote from: Paul Marossy on February 09, 2011, 03:48:00 PM
I think I've figured it out based on what someone has said:

"A piezo works by producing a very low level positive signal. It produces no negative signal. It doesn't modulate a DC current but actually produces one from the pressure of the string and waves."

That sounds suspect to me - a piezo translates pressure into charge.... it's not doped like say a diode, so will yield an output directly related to positive & negative pressure ...it's got to have a negative swing else it would sound terrible!

Paul Marossy · February 09, 2011, 04:26:16 PM

Quote from: Gurner on February 09, 2011, 04:23:19 PM
Quote from: Paul Marossy on February 09, 2011, 03:48:00 PM
I think I've figured it out based on what someone has said:

"A piezo works by producing a very low level positive signal. It produces no negative signal. It doesn't modulate a DC current but actually produces one from the pressure of the string and waves."

That sounds suspect to me - a piezo translates pressure into charge.... it's not doped like say a diode, so will yield an output directly related to positive & negative pressure ...it's got to have a negative swing else it would sound terrible!

Well, it does have a negative swing in that picture, but it's very small in comparison to the positive half of the cycle. Man I need to put my own Parker on a scope. This weekend I will do that when I have some free time.

G. Hoffman · February 09, 2011, 06:29:42 PM

Quote from: PRR on February 08, 2011, 02:15:10 AM

I'm shocked Gabriel didn't comment on drilling into a Strat with a 3/4" spade-bit.

Well, a Forstner bit would be better.

I've gotta say, though, that I've used a whole bunch of different electric/acoustic preamps, and the Powerchip is one of my least favorite (the Ghost one is pretty mediocre too). It is VERY convenient to install, but it doesn't sound like much. The LR Baggs preamp is better, and my favorite one is the Bartolini - though you'll never trace that circuit, since it is encased in a chunk of epoxy - though there was also a really nice one from D-Tar that I never really got a handle on. It seemed pretty nice, but it wasn't very practical because it was just too big for the inside of an electric guitar (a circuit board about 1"X2.5"). But then, I would expect something nice out of Rick Turner's mind!

Gabriel

News:

Fishman "Powerchip" - Need A Schematic