Dead Pre-amp?

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Dead Pre-amp?

Postby Ryan Mazzocco » Tue Dec 18, 2018 8:30 pm

Hey guys. I have a customer that brought in an Ibanez Acoustic/Electric 12-string that is putting out very little signal. When we hook it up to an amplifier and turn everything all the way up to 11 there is barely a little bit of sound coming out. When I plug it directly into a PA with the button selected to MIC it sounds great. When I select LINE it's dead. I can't for the life of me figure out what's going on here. The only thing I can figure is the pre-amp is dead which is why the line input can't detect it. A Mic signal is so low to begin with that must be why the powered mixer is able to pick it up, right? What else could be wrong that I could check? Is there a way to test the pre-amp?

Thanks!
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Re: Dead Pre-amp?

Postby Bob Gramann » Tue Dec 18, 2018 9:23 pm

Does the guitar require a battery? Is the battery good? Is the battery making contact inside its case? I have seen some battery powered guitars where the battery box design is incompetent and the battery doesn’t make a good connection. Those are the easy questions. Past those, you’ll have to get a probe and search for signal from the pickup through the electronics to the jack.
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Re: Dead Pre-amp?

Postby Mark Swanson » Wed Dec 19, 2018 1:26 pm

If it is a pickup system with a mic AND a piezo, and the mic is working, I'd suggest that the problem is the piezo pickup itself, or maybe it is disconnected from the preamp or the wire broke. If the mic output works but the piezo does not it's probably not the preamp. Could still be, but probably not.
    Mark Swanson, guitarist, MIMForum Staff
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Re: Dead Pre-amp?

Postby David King » Wed Dec 19, 2018 3:29 pm

Where is this button to select between mic and line, on the PA or on the guitar?
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Re: Dead Pre-amp?

Postby Ryan Mazzocco » Wed Dec 19, 2018 8:19 pm

David King wrote:Where is this button to select between mic and line, on the PA or on the guitar?


On the PA

Bob Gramann wrote:Does the guitar require a battery? Is the battery good? Is the battery making contact inside its case? I have seen some battery powered guitars where the battery box design is incompetent and the battery doesn’t make a good connection. Those are the easy questions. Past those, you’ll have to get a probe and search for signal from the pickup through the electronics to the jack.


Honestly the battery compartment seems suspect, but it has a built in tuner which seems to be getting power. As far as I can tell it's making the connection. I tested the batteries and they are good.

Mark Swanson wrote:If it is a pickup system with a mic AND a piezo, and the mic is working, I'd suggest that the problem is the piezo pickup itself, or maybe it is disconnected from the preamp or the wire broke. If the mic output works but the piezo does not it's probably not the preamp. Could still be, but probably not.


It's just under saddle piezo, no mic.
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Re: Dead Pre-amp?

Postby Joshua Levin-Epstein » Thu Dec 20, 2018 3:51 pm

If the preamp were dead, there would be no signal going through it. So there would be nothing for the "MIC" setting to amplify. I would hypothesize that there is no or little gain from the preamp. Because I don't know my brass from an oboe, I would start at the pick up and work my way out (as was previously suggested). Unplug the pick up and alligator clip it to an amplifier input. I suspect you'll get some sound. Now pull out the preamp, open it up and look for broken or cold solder joints, and/or broken components.While its out of the guitar, pull the pick up and plug the whole thing together on your bench. Look for suspect connections and jiggle everything to see if you get a jump in sound.

Is the preamp housed in the control box or is it built onto the output jack? If its on the jack, I might be tempted to replace it with something like a Fishman powerjack.
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Re: Dead Pre-amp?

Postby Brian Evans » Fri Dec 21, 2018 4:29 pm

A piezo pickup will typically have an output level of 50 - 100 mV maximum, often 10 - 20 mV. This corresponds well with MIC level inputs to a typical PA system. A magnetic guitar pickup will have an output level of 100 - 200 mV, up to 5 times that (1V) for some very high output active pickups. These are maximums, so played delicately a pickup might be putting out only a few millivolts. A pre-amp is used to raise this up to line level, which is a defined level of around 1 - 2 V. A guitar amp has a pre-amp stage, to go from a few millivolts to line level, and a power amp stage to take the line level and add power (current) to it. So it's very reasonable to expect a line level input to a PA system to pretty much ignore a pickup - magnetic or piezo - and require a pre-amplified signal. It's also very reasonable to expect a MIC level input to work well with a Piezo pickup output. Where the question arises here for me, is that it's not working well with a guitar amplifier. Piezo's are known for wanting a pre-amp to get them up to where a typical guitar amps works well, and for wanting an acoustic guitar amp rather than a magnetic guitar amp (different frequency response requirements). But in general, any guitar amp should at least produce a sound. You did change the battery in the on-board pre-amp, did you not? Is there an output jack on-off switch that could have gotten boogered?
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Re: Dead Pre-amp?

Postby David King » Mon Dec 24, 2018 1:42 pm

I like what Brian wrote, I'd say that if the tuner is getting sufficient signal from pickup then everything is fine with the guitar.
The definition of "line-level" is not a hard and fast thing but in "pro audio" it's +4dBu. In home audio it's a much more sensitive -20dBu. Confusion reigns in the realm of "in between audio" which is where a lot of PAs reside.
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Re: Dead Pre-amp?

Postby Ryan Mazzocco » Wed Dec 26, 2018 2:06 pm

Hey everyone. I want to thank you for all your help and advice. I took everything you all said under consideration as i attempted to tackle this. Long story short, I fixed it. I don't know how, but it's working.
I basically just followed the direction I got here, took everything apart, tested everything I could, ultimately finding nothing along the way. I put it all back together and what do you know, it's working fine. So, obviously something was not right, but whatever it was must have been corrected when I connected it all back together. Se even though I don't know exactly what was wrong or how I fixed it I still learned a lot from the responses and digging into this thing. Thanks to all.

Ryan
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Re: Dead Pre-amp?

Postby Daryl Kosinski » Thu Jan 03, 2019 1:45 pm

Ryan

Those things happen.

I serviced and repaired industrial electronics for 39 years. There have been times when pulling the circuit boards and connections one at a time cleaning and reinstalling fixed the issue. A little invisible corrosion can cause lots of issues.
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Re: Dead Pre-amp?

Postby Peter Wilcox » Thu Jan 03, 2019 2:21 pm

Many's the time I've taken a broken thing apart, haven't found the problem, put it back together, and it worked again. I just figure I have the magic touch. :D
Maybe I can't fix it, but I can fix it so no one can fix it
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