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Old 30th January 2013, 05:50 PM   #1761
MWL168 is offline MWL168  United States
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I just finished the construction of the Bal-Bal version with dual power supplies. With the input floating (not connected), one channel measured 0mv DC while the other on measured 2mv. These values don't appear to change after I left it powered on for 30 minutes. Is this normal?

Thanks!
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Old 30th January 2013, 06:08 PM   #1762
qusp is offline qusp  Australia
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is that one channel to ground? or + to - ? also do remember that may be 2mv into the huge impedance of your meter, across a headphone impedance it will be MUCH smaller

but yes, measurements need to have the inputs shorted (both to signal/input ground), short them it should be zero at the output. with the input floating you are doing pretty well at 2mv

Last edited by qusp; 30th January 2013 at 06:27 PM.
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Old 30th January 2013, 06:33 PM   #1763
MWL168 is offline MWL168  United States
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Thanks gusp!

The 2mv DC is measured + to -. I will test it again with the input shorted and report back. I was just surprised that one channel actually measured 0mv with the input floating.
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Old 30th January 2013, 06:39 PM   #1764
qusp is offline qusp  Australia
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well with the input floating, you are essentially measuring the difference between the rails (not literally) plus maybe a bit of DC leakage and RFI, so I guess you matched the PSU rails well on that side.

essentially you could short them to each other, to ground, or even to a DC voltage, as long as its the same thing each is connected to and not floating.

Last edited by qusp; 30th January 2013 at 06:48 PM.
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Old 1st February 2013, 01:51 PM   #1765
MWL168 is offline MWL168  United States
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Hi gusp:

I hooked up the WIRE to my Twisted Pear Audio Beffalo II/IVY set up and discovered some problems. I have both channel PSU for the WIRE set at +/-14.5V. The left channel appears to be working fine but the right channel sounds intermittent with cracking noise and significatly lower in level.

I double checked and the IVY is working properly. The output DC on both channels of the WIRE when connected with the IVY are around 4 mv which I believe came from the IVY. (My IVY always has 4mv or so DC on the outputs.)

Not able to visually identify any suspect, I measured the input DC, the beoken channel has 155mv DC measured between +/- and ground while the working channel has close to 0 DC. If I unhook the inputs from the IVY, the broken channel DC measured some 13.5V while the working channel only measures 34 mv!!!

The weird thing is that I then measured the PSU output voltage and now I am getting 0L (out of range) reading from my Fluke multimeter.

I am totally baffled and do not know where to look next. Any ideas what I chould check or do next?

I had to apply lots of heat when soldering the heatsink. could I have damaged the buffers?

Sorry I do not have pictures - was 1am when I quit but can take some and post them if that'll help!

Any thoughts you or anyone has are greatly appreciated!
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Old 1st February 2013, 02:00 PM   #1766
qusp is offline qusp  Australia
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actually those PSUs are tricky little suckers, harder than the amp board.

there could be all manner of suspects here, need pics, as close as you can. the heatsink is live, where its soldered to and where the buffer tab is soldered to is the negative rail and you could possibly have shorted the rail to ground with the heatsink. the opamps are pretty bulletproof, but the regs may have been damaged with a short like that.

need pics
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Old 1st February 2013, 02:15 PM   #1767
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I would guess it's a solder short somewhere on the input side of things. Pay close attention to the cap soldered to the VOCM pin.

There are some vias that are close to pads, but cannot be bridged to those pads. Double check all the joint on the bad board and make sure they are alright. Reflowing all the joints is probably not a bad idea.

It seems unlikely that heat would have damaged the parts, and they're usually quite tolerant to shorts and even reversed supplies.

Like qusp said, post some good macro pictures of the board itself, as we'll see if we can spot anything out of place.

Regards,
Owen
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Old 1st February 2013, 03:25 PM   #1768
MWL168 is offline MWL168  United States
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Thanks guys! Will follow your recommendations to see I can identify and fix the problem.

Any ideas that the PSU output voltage would become out of range? This sympton actually showed up twice. When I first experience the right channel problem, I immediately measured the PSU and got the same out-of-range reading. I then di-soldered the wires connecting the PSU to the main board and verified the PSUs were still measured +/-14.5V without load.

I then wired the PSU and main board back together again and at first I was able to measure +/- 14.5V supply but a few minutes later it was out-of-range again. This sympton applies to both PSUs. Very odd!
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Old 2nd February 2013, 04:18 PM   #1769
MWL168 is offline MWL168  United States
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Hi Owen and gusp:

I visually inspected the non-working board for possible shorts and re-flowed all the solder points except those of the heat sinks and checked to make sure the heat sinks are not accidentally shorted where they should not. The problem persists after all this.

Both PSU's appear to be working fine - outputting +/- 14.5V as intended. The issue I reported in my previous post may just be an user error.

I am seeing the same symptoms: ~13.7DC on the input when measured from + to ground and - to ground while the working channel measured ~34mv. No DC on the output end (~1.8mv).

I have posted a couple of pictures of the board hoping you can help detect any suspicious culprits.

Any help you can offer is much appreciated!

WIRE-Top.JPG

WIRE-Bot.JPG
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Old 2nd February 2013, 04:40 PM   #1770
opc is offline opc  Canada
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Hi MWL168,

Everything looks pretty good in the pictures, which points towards a damaged OPA op-amp.

Before replacing it though, there are a few things you can double check to be sure.

1. The VOCM pin via is very close to the supply pin via going to the bypass cap. Make sure there is no solder short between the two. Check for continuity with a meter to be sure.

2. Measure both supply nodes (+/-) to GND to make sure there are no direct shorts on the amp board.

If the above two items check out OK, then my best guess would be that the front end op-amp somehow got damaged (over heating or over voltage) and failed.

See the attached picture for the node referenced in (1).

Cheers,
Owen
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File Type: jpg WIRE-Top.JPG (192.7 KB, 386 views)
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