Hey everyone, I have a weird issue with a mic preamp. The XLR input works just fine, but when the 1/4" input is used, there's a weird crackling sound. It is a 5 pin phono connector, which seems kind of strange to me, but here is what each pin is:
1: Tip
2: ? (shorted to sleeve)
3: Sleeve
4: Shunt pin (shorted to ground/sleeve when no input)
5: Ring (shorted to sleeve - mono operation)
Now, what seems weird to me is that the 1/4" output jack (which works properly) has no connection for pin 4 in the circuit, but for the input there is a trace that runs off to the main circuit board and I'm measuring 10k between that point and ground. Following the trace, I can see it leads to some resistors, caps and diodes. Is this just pop prevention?
Can anyone shed some light on what is going on here? I had it working without the crackling when I first tested it outside of the enclosure, but now it crackles in or out of the enclosure.
1: Tip
2: ? (shorted to sleeve)
3: Sleeve
4: Shunt pin (shorted to ground/sleeve when no input)
5: Ring (shorted to sleeve - mono operation)
Now, what seems weird to me is that the 1/4" output jack (which works properly) has no connection for pin 4 in the circuit, but for the input there is a trace that runs off to the main circuit board and I'm measuring 10k between that point and ground. Following the trace, I can see it leads to some resistors, caps and diodes. Is this just pop prevention?
Can anyone shed some light on what is going on here? I had it working without the crackling when I first tested it outside of the enclosure, but now it crackles in or out of the enclosure.
Crackling is typically either a scratchy pot or a bad solder joint. Reflow the solder on all pcb mounted jacks, switches, etc.
Absoltely nothing weird about NOT shorting the output of a solid state circuit, especially if there is another output such as xlr.
Absoltely nothing weird about NOT shorting the output of a solid state circuit, especially if there is another output such as xlr.
SOOOO much more helpful to post the schematic or at least the model name of the product. We can only guess at the circuit from here. Many amplifiers use extra input jack contacts for various purposes. SOme control muting, so the circuit won;t amplify unless something is plugged into the jack.
Oh, yes, THAT one.
I was afraid it was one of the Million amp models out there.
The guy wearing tennis shoes has the answer, just ask him:
Click on the image to enlarge it and find him.
I was afraid it was one of the Million amp models out there.
The guy wearing tennis shoes has the answer, just ask him:
Click on the image to enlarge it and find him.
Thanks for the cynicism, much appreciated.
It's an ART Tube MP Studio. Best I could find is this, which is a little small for reading component values, but it does show the topology.
But really, leadbelly's advice is what I needed to hear. I'm not as concerned with understanding everything in the circuit, but I needed some direction on generally what to search for with crackling on a single input.
It's an ART Tube MP Studio. Best I could find is this, which is a little small for reading component values, but it does show the topology.
But really, leadbelly's advice is what I needed to hear. I'm not as concerned with understanding everything in the circuit, but I needed some direction on generally what to search for with crackling on a single input.
It shows one thing helpful, and that is that the 1/4" jack feeds an op amp that in turn feeds the same circuit as the XLR, so since the XLR input works OK, that pretty much localizes the problem to the 1/4 jack and the op amp.
Certainly DO take LB's advice and look at the solderwork.
The schematic also shows your mystery trace from the jack. It is the signal lead going into the circuit. The diodes are clamping diodes to prevent large signal excursions from driving the input circuits past their power rails. The caps provide isolation. remember, the XLR circuits potentially have +48v on them. SO caps are needed.
Certainly DO take LB's advice and look at the solderwork.
The schematic also shows your mystery trace from the jack. It is the signal lead going into the circuit. The diodes are clamping diodes to prevent large signal excursions from driving the input circuits past their power rails. The caps provide isolation. remember, the XLR circuits potentially have +48v on them. SO caps are needed.
You say it has a 5 pin phono connector, but on the schematic there is a xlr and a 1/4 jack.
Does it crackle with phantom power On ? That could be the problem. Although phantom is usually only applied to the xlr connector, something can be wrong.
With a phone plug inserted check if you have phantom voltage on the 1/4 jack. If the caps are bad they can also lead to problems.
Look also for shorts between adjacent solder blobs.
Does it crackle with phantom power On ? That could be the problem. Although phantom is usually only applied to the xlr connector, something can be wrong.
With a phone plug inserted check if you have phantom voltage on the 1/4 jack. If the caps are bad they can also lead to problems.
Look also for shorts between adjacent solder blobs.
I think he meant phone rather than phono. The 1/4" jack has the tip ring and sleeve, plus cutout contacts on tip and ring - total five.
Phono means RCA jack to me.
Phono means RCA jack to me.
Yes I meant phone. I'm talking about a 1/4" jack with 5 pins.
It crackles with phantom power off and it seems to get louder when phantom power is on. There is a small fluctuating voltage on one of those "extra" pins (the one not connected to ground of course).
Sound like we have a bad capacitor somewhere?
It crackles with phantom power off and it seems to get louder when phantom power is on. There is a small fluctuating voltage on one of those "extra" pins (the one not connected to ground of course).
Sound like we have a bad capacitor somewhere?
Care to elaborate on that thought? I can't see how that would put a voltage on the input.
+1 on a probable bad Op Amp, specifically U1 (TL072).
BEWARE THERE IS A SCHEMATIC ERROR.
It shows U1 + input (U1-3) and - input (U1-2) shorted which is wrong.
Carefully desolder U1 , use a good pump and if necessary finish with solder wick, and put a new one there.
There are other possibilities but this is the first in the list.
For the next time, I suggest you post the direct link to the schematic (a PDF) stored in Elektrotanya, what you posted here is just a preview, that´s why it´s so small.
ART 127V4001-10X TUBE MP V3 REV.B SCH Service Manual free download, schematics, eeprom, repair info for electronics
Please post results.
BEWARE THERE IS A SCHEMATIC ERROR.
It shows U1 + input (U1-3) and - input (U1-2) shorted which is wrong.
Carefully desolder U1 , use a good pump and if necessary finish with solder wick, and put a new one there.
There are other possibilities but this is the first in the list.
For the next time, I suggest you post the direct link to the schematic (a PDF) stored in Elektrotanya, what you posted here is just a preview, that´s why it´s so small.
ART 127V4001-10X TUBE MP V3 REV.B SCH Service Manual free download, schematics, eeprom, repair info for electronics
Please post results.
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