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T-bridge (piezo)


mikhailgtrski

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I got my LR Baggs T-Bridge hooked up - it works great... except when I roll its volume all the way down I get hum.

I have the magnetics wired to the tip, T-Bridge on the ring, an LP-style toggle for mag-both-piezo, and the cavity is fully shielded and star grounded (per the Guitarnuts tutorial, except no "safety capacitor"). I'm using the piezo coax shields as my string ground, and the summed coax shield is connected to the ground lug on the volume pot, which then connects to the star ground (not the pot shell).

Everything else works perfectly - and quietly.

Thanks for your input. :D

Mike

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Check if there is continuity from the saddle to ground. Some of the piezo saddles are not conductive and your strings will not be grounded unless you use a different method of grounding.

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Check if there is continuity from the saddle to ground. Some of the piezo saddles are not conductive and your strings will not be grounded unless you use a different method of grounding.

Thanks. Yes, I checked before I wired it up - there is continuity from the saddles to ground, and it is quiet as long as the volume is up on the piezos, whether using mags/both/piezos.

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sorry for not helping, but i must ask a question. Does it sound nice? Like does it have a nice "acoustic" tone?

Thanks :D

Well, so far I've only played it through my studio monitor (not through a PA yet) but it does seem to have a decent "acoustic" tone. I'm running it through a tube mic/line preamp into my Alesis Q20 for a little reverb, then out to the house. I haven't messed around with any EQ yet, but it doesn't seem like it'll need much tweaking. Now, it's not the same as a good "real" acoustic with a good pickup, but it sounds better than a lot of piezo saddle-equipped acoustic guitars I've heard.

I'll have a more informed review for you after it gets some "real-world" use this weekend. :D

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i know this sounds dumb but have you tired a new battery? active pre-amps can get noisy even when only a small amount of charge is lost from the battery.

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i know this sounds dumb but have you tired a new battery? active pre-amps can get noisy even when only a small amount of charge is lost from the battery.

Thanks, but it's passive - no battery. I'm using an outboard rack-mounted preamp.

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Hmmm... I think I figured out what I did wrong. :D

I didn't fully re-read the Guitarnuts shielding/grounding tutorial (it's been about 7 years since I did the last one) and I mixed my "signal" grounds with my "shield" grounds. It still works, but I probably built in a ground loop. (slaps forehead) :D

Time to fire up the soldering iron again.

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OK, so I separated the shields from the signal ground. Now the cavity shield connects to ground at only one point. Sounds fine, very quiet when I'm playing the magnetics alone - even the single-coil modes are relatively quiet.

But I seem to be getting some crosstalk between the tip and ring... I have the stereo cable run into a little splitter box (TRS jack split out to 2 mono jacks) with a regular mono cable from there into the amp. Sounds fine with the mags-only, but if I switch it over to piezo-only I get a piezo signal coming through on the tip - even though the mags/piezo are -theoretically- separate. Could the crosstalk be due to inductance in the stereo cable?

I think I've got some ground loop issues in the amp rack too. This is becoming a real head-scratcher. :D :D

edit: I may have figured out the crosstalk problem... I was using an LP-type toggle to switch the hot leads for each signal, but that leaves the "off" signal floating, not shorted to ground. According to a schematic on the Fishman site, you use an SPDT on-off-on switch which shorts either the tip or the ring to ground, or neither in the center position.

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That took care of it, that plus ditching the tube preamp and going straight into the Q20 (f/x processor). I was able to disassemble the LP toggle and modify it to work as a center-off switch, so I'm happy I didn't have to rearrange my control layout. :D

The T-Bridge sounds pretty good with a slight boost at 240hz, a little cut at 1Khz, and a little boost above 7.5Khz.

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Those switchcraft type large toggle switches (ala LP) do have crosstalk, however the cheaper box type do not. Just something to add.

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Those switchcraft type large toggle switches (ala LP) do have crosstalk, however the cheaper box type do not.

Hmmm... I modded the switch by replacing the tip that actually operates the contact leaves with a larger one that keeps both contacts apart in the middle position. I am getting a tiny bit of crosstalk in the middle position... I was thinking it's something downstream, as I'm still chasing a bit of a ground loop. So, you think it could be the LP switch? I may end up moving the controls around after all, since I use the mag/both/piezo a lot more than the coil split.

Hughes - I used the T-Bridge live yesterday, and a friend came up afterward and was wondering where the acoustic guitar player was hiding. :D It did sound quite good.

Mike

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