DC buildup in Linkwitz Transform - diyAudio
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Old 15th April 2012, 02:58 PM   #1
petarkb is offline petarkb  Bulgaria
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Question DC buildup in Linkwitz Transform

Newbie here. So, I have 'successfully' built a Linkwitz Transform, with a +-10V power supply. I'm using it to extend the frequency response of my home-made desktop speakers.

But I have a problem:
Channel A behaves correctly, but Channel B develops a DC offset at the output, slowly climbing from 0 to 9 volts, and then the sound gets distorted.

It doesn't seem to be caused by a faulty op-amp. They are TL071s, so I swapped them around. The problem remains. It's also not a mishappen contact between the IC pins, as resistance between all adjacent IC pins in channel B is the same as channel A.

What can cause DC buildup in such a simple circuit?

Last edited by petarkb; 15th April 2012 at 03:11 PM.
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Old 15th April 2012, 03:28 PM   #2
DF96 is offline DF96  England
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A wiring error?
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Old 15th April 2012, 03:43 PM   #3
petarkb is offline petarkb  Bulgaria
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Quote:
Originally Posted by DF96 View Post
A wiring error?
Do you mean, doing a crappy solder job, or connecting something to the wrong place?

If you mean the later, the power supply is only connected to the circuit by ground, and the opamps' Vcc+ and Vcc- pins. There isn't much to miswire.

If you mean a crappy solder job, I've nudged all the components around, but it doesn't affect the output, so they must be soldered correctly, this time.
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Old 15th April 2012, 06:08 PM   #4
DF96 is offline DF96  England
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What about other bits of the circuit? There is more to a circuit than the power wiring. If you are confident that all is correct, then I am confident that nothing is going wrong. You are more confident in your soldering tests than I am, but then I only have 40 years experience so what do I know? Wiggling can find a bad joint, but it can't confirm a good joint.
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Old 15th April 2012, 06:44 PM   #5
sreten is offline sreten  United Kingdom
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Hi,

One thing for sure is one channel is different to the other, and I don't think
it would take DF96 very long to find out what that difference is. So many
people come here saying it doesn't work but they haven't done anything
wrong. 2 out of 3 people here know that is very unlikely to be the case.

, sreten.
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Old 15th April 2012, 06:47 PM   #6
petarkb is offline petarkb  Bulgaria
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Quote:
Originally Posted by DF96 View Post
What about other bits of the circuit? There is more to a circuit than the power wiring. If you are confident that all is correct, then I am confident that nothing is going wrong. You are more confident in your soldering tests than I am, but then I only have 40 years experience so what do I know? Wiggling can find a bad joint, but it can't confirm a good joint.
Sorry for being a newbie, but how do you find the rest of the bad joints?

Topology looked correct the last 25 times I checked, considering re-doing the whole thing...

And what causes tension build-up anyway? Is it some kind of weird opamp thing? Where should I look for it?
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Old 15th April 2012, 06:51 PM   #7
petarkb is offline petarkb  Bulgaria
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Quote:
Originally Posted by sreten View Post
Hi,

One thing for sure is one channel is different to the other, and I don't think
it would take DF96 very long to find out what that difference is. So many
people come here saying it doesn't work but they haven't done anything
wrong. 2 out of 3 people here know that is very unlikely to be the case.

, sreten.
Hey, I never said I did nothing wrong. I'm smarter than that, and my soldering sucks. Furthermore, the circuit resets (tension buildup disappears) when I twist the circuit board, so I'm pretty sure it's my bad.
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Old 15th April 2012, 07:11 PM   #8
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I would look for a bad DC return path at one of the inputs.

Mike
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Old 15th April 2012, 07:12 PM   #9
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What kind of solder are you using Peter?

Using traditional Solder the joints should be smooth, they should not look caked on.

if you are not using lead free solder look for dull (non shiny) joints.
Check these comparisons:
Leads Direct - The Quest for the Perfect Solder Joint

tips:
How to fix a bad solder joint with a solder sucker and solder iron - YouTube
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Old 15th April 2012, 07:32 PM   #10
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by petarkb View Post
Hey, I never said I did nothing wrong. I'm smarter than that, and my soldering
sucks. Furthermore, the circuit resets (tension buildup disappears) when I twist
the circuit board, so I'm pretty sure it's my bad.
Hi,

That is hardly what you said before implied :

"but it doesn't affect the output, so they must be soldered correctly, this time."

Flexing the board making a difference implies a bad / dry joint.

rgds, sreten.
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