wiring from preamp to jack

GaryBass

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Hi - a friend brought me his ukulele to fix - there is no amplification. I pulled out the jack and circuit board and found the white lead broken off at the jack. I solder it back on to where it looked like it belonged and still no sound. The battery is fresh and the white connector is firmly attached to the circuit board. My question is where does the white wire goes? There is a red wire soldered to the short lead on the jack and the shield is soldered to the strain relief clamp. I soldered the white wire to the second longer lead on the jack. Does any one have a wiring schematic or picture. I am also wondering why 3 wires would connect to this type of jack if the guitar cable only wires to a hot and shield on the guitar plug.

Thanks,
Gary
 
The jack plug may have three connectors one being earth. Was there any trace of solder on an empty tag? That is usually the clue as to the correct one when wires come adrift. Are you sure you did a good soldering job. Dry joints cause such problems.
It would be helpful to know make of pickup. There are circuit diagrams online for most.
 
Hi Gary,
I can't get a clear mental picture of your setup, so I might be wrong, but if your jack is a stereo or switching type, the third wire may be a power switching wire for the amp.

Plug inserted switches the amp on.
Plug removed switches the amp off.

This is an automatic battery power saving mechanism.

If this is the case, the wire that connects to the tip terminal is the signal wire, the bare braided/shielded wire is the base/ground wire and the other will be the power switching wire. This usually functions by providing an amp power ground (switching) point through the jack when the plug is inserted.
Some sophisticated amps can run a stereo setup, but I think that is unlikely here.
I am assuming that you have an onboard battery powered amp wired to a separate guitar type ¼ inch output jack. If this is not the case, disregard my post. Good luck with it.
 
The jack plug may have three connectors one being earth. Was there any trace of solder on an empty tag? That is usually the clue as to the correct one when wires come adrift. Are you sure you did a good soldering job. Dry joints cause such problems.
It would be helpful to know make of pickup. There are circuit diagrams online for most.
Hey thanks. I can try to re-solder if I get another whack at it. The empty tag had solder on it but I don't recall if I saw traces on the wire that broke off. Anyhow that's where I soldered disconnected white wire. Thanks again I really appreciate your feedback.
 
Hi Bazuku - thanks for your reply. You may have explained it. To me the jack looks like a stereo jack so the third connection may just be that, the "power on/off" switch.
 
If there are no electronics in a cutout on the side of the instrument and no visible place to hold either a 9v or CR2032 battery, but there is a circuit board attached to the endpin jack on the inside, this sounds like a Mi-Si preamp.

They need a special cable or special wall adapter that fits the standard 1/4" audio plug in the endpin jack in order to recharge the preamp, there is no visible battery and it uses 'super capacitors'.

If it is a Mi-Si, and the cables are intact but still no sound, it could be that the power has run out and the preamp needs to be re-charged.

Many threads here on UU talk about how great the Mi-Si pickup and preamp system is and how it works.

Go here and look for "Acoustic Trio" and see if that matches the unit you have in front of you:

http://mi-si.com/products/]

EDIT: see photo here:

acoustic-trio.jpg
 
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Hi Bazuku - thanks for your reply. You may have explained it. To me the jack looks like a stereo jack so the third connection may just be that, the "power on/off" switch.

Gary,

If this is the case, a VOM should show a small voltage at the 'mystery' wire/lug. This may be only mV depending on the amp design and/or the state of the battery. The test should be conducted UNPLUGGED. I can't predict what that voltage should be, as I just build them, I don't design them. :)
 
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Gary,

If this is the case, a VOM should show a small voltage at the 'mystery' wire/lug. This may be only mV depending on the amp design and/or the state of the battery. The test should be conducted UNPLUGGED. I can't predict what that voltage should be, as I just build them, I don't design them. :)

MiSi has directions on how to charge them using a stereo jack and a 9 V battery. The markings on the capacitor show 5.5 V.

I have one that the pick up is destroyed on, and I'm not quite sure what to do with it .
 
Hi aremick

I should clarify my comments. I was assuming that the OP has a conventional pickup/preamp, not the Mi-Si, so my suggestions will be inappropriate/confusing if applied to the Mi-Si.

I have no experience with the Mi-Si, but understand that it uses the L.R.Baggs soft-style pickup, and so, can see no reason that an experienced technician would not be able to replace that for you. A generic same style pickup may also work, but some one with Mi-Si experience should guide you on this.
 
Hi aremick

I have no experience with the Mi-Si, but understand that it uses the L.R.Baggs soft-style pickup, and so, can see no reason that an experienced technician would not be able to replace that for you. A generic same style pickup may also work, but some one with Mi-Si experience should guide you on this.

I wired a Jack to it and tried it with my Risa, which has a shadow pick up in it, and a couple of other ukes with soundboard transducers and all I got was massive distortion.
 
I wired a Jack to it and tried it with my Risa....
.

Do you mean that you are by-passing the Mi-Si preamp in these tests?
If so, the problem with running passive piezo pickups straight to an amp is that of gross impedance mismatch. An impedance matching (active) devise is needed to overcome this problem. The Mi-Si preamp does this job very nicely.
With no impedance matching buffer, the sound will be harsh and thin with a narrowed frequency spread. Could this be the cause of the distortion that you are experiencing or am I not correctly understanding what you are doing?
 
No.

The pickup was destroyed, which us why I was given it. I wired an instrument jack to the pickup lead, then plugged my instruments in and plugged the MiSi output into an amp...
 
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A-ha.... sorry, I misread it completely.... ignore my comments and hope that some one else will have some Mi-Si specific knowledge to help.
I'll leave my previous posts unedited in case any other readers are wondering why their passive piezo pickups sound woeful when connected directly to (not all, but some) guitar amps.:)
 
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Yep, that's good information.

I wonder if maybe the output of the pickups that I've tried isn't just " too hot" for the misi preamp. The pick up in the Risa is a hard rod peizo, and of course the soundboard pickups are large ceramic peizos also. I have read somewhere that the soft piezos' output is a lot lower then the hard ones.
 
My experience with piezo pickups has been limited to disk and hard-shaft 'home brew' set ups. I always feed these into a 'home brew' source-follower type preamp.
Usually these combinations provide adequate impedance coupling but very little gain, so I can't comment on the possibility of a piezo causing very severe clipping/distortion.... not saying it's not possible, just that I don't know and have not experienced it. I have always likened a piezo to a capacitive devise that generates a useful signal voltage with an extremely high natural impedance, …. this is in contrast to an inductive (magnetic) pickup that has a much lower natural impedance. I would encourage you to follow up any of my suggestions/ideas with a bit of reading as I don't have any formal electronic theory qualifications, I only know what I have learned through half a lifetime of hobbyist circuit hacking and project building.
 
I dusted off the soldering iron, and finished a couple of things I was playing with - a Tillman JFET buffer and a Kemo M040N preamp. Both seemed to work pretty well when fed with either the Risa or the USB transducer uke.

I originally tried the Kemo straight and got a LOT of distortion, but added the 10k pot on the input and got a nice clean signal at lower input levels.

I'm wondering if wiring a similar pot to the input of the MiSI would do the same thing...

BTW, each would benefit with some form of tone control, but I can do that with the amp so that is not a high priority.
 
Aremick,

It's good to learn that you are a fellow DIYer.
Years ago, when I was making classical guitars, I made up a batch of PCBs for Tillman type preamps and did a bit of experimentation. I used the 2N5457 and J201 and did not notice any perceivable difference. I also substituted pots for R2 and R3 to try different biasing but it got very confusing with questionable advantage, so I reverted to the (DT) suggested values. I also tried similar circuits with capacitor isolated inputs, but saw no advantage there either, possibly because my value selections were less than optimal.

The Kemo may be more prone to distortion with an unchecked input because its gain is around 70 times higher than the Tillman (40dB compared to 3dB, if my maths holds up). I think of the Tillman as an impedance matching buffer rather than a preamp, but that is just a personal opinion.

I understand that, on request, Mi-Si will provide instructions for fitting tone and volume pots to their preamps... this may help your distortion problems(see their FAQs page).

I tend not to do as much experimentation/fiddling these days, …. just go with what the experts suggest.:)
 
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