| Adam M. |
After getting my input jacks isolated from the chassis, and verified by checking the resistance with a DMM (OL, which is greater than 10MOhm) I put the amp together in the chassis and got an odd problem... No matter what I do, I get -vin on the output. I have checked to make sure there are no wires crossed, and I dind't see any. I reheated the solder joints to make sure they are good. Resistance from +in to -in is ~21K, which I would expect, rail voltages and ground seem to be correct.
I have looked at the circuit a few too many times at this point, which is why I'm taking a break and posting to see if anyone has made this mistake before. Chip is an LM3886, built to National's circuit. Any thoughts where to look since my eyes are fried? |
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| Mad_K |
| -Vin? Do you have balanced input? |
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| JasonL |
| does any one have a balanced version of the gain clone schematic ? id like to build a pair for a pair of studio ./ computer speakers : O ) |
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| Optical |
| triple check your wiring, ive had this problem several times when ive connected the wrong pins up.. |
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| peranders |
| Adam, do you have any resistor from the MUTE pin? If not, add 10-33 kohms. |
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| Nuuk |
Probably an obvious question but have you isolated your speaker terminals?
Check that you have the correct voltages at the pins of the chip. |
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| janneman |
This -V on the output, against what is that measured? What exactly is '-vin'? Neg supply?
Also, if you isolated the input, is there a ground connection from the input to supply/speaker ground and amp ground?
Jan Didden |
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| carlosfm |
The LM3886 has a ground pin to connect to power ground.
Did you? |
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| Adam M. |
After taking the whole amp apart, I found the problem. I forgot to put a resistor from input to ground, and all is good now. I get the same effect if I just plug something into the input, but I was testing in steps.
For those who asked above, I was talking about the negative rail voltage showing up on the output.
As a theory question, why did not having a resistor from input to ground drive the output to -35Vdc?
Thanks. |
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| Mad_K |
| When you put the -in to gnd resistor you are refering the input to gnd. When a signal is applied to the +input it comes out amplified at the output, and a small (determined by the ratio of this and the feedbackresistor) portion of the outputsignal is fed back to the -input and hereby reducing distortion and setting the gain of the amp. |
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| janneman |
| quote: | Originally posted by Adam M.
After taking the whole amp apart, I found the problem. I forgot to put a resistor from input to ground, and all is good now. I get the same effect if I just plug something into the input, but I was testing in steps.
For those who asked above, I was talking about the negative rail voltage showing up on the output.
As a theory question, why did not having a resistor from input to ground drive the output to -35Vdc?
Thanks. |
There is DC current flowing into or out of the input (depending whether the input stage is PNP/NPN or equivalent FET). This current needs to go somewhere, either to the source (if it is DC coupled) or to ground via the resistor you didn't have. If the current can not go anywhere, the input drifts up or down (look at it as a cap that is being charged because the current has no way to go). Depending on the input stage it drifts to the pos or neg supply rail. Since the amp tries to replicate the input level at the output, your output also drifts to one of the supplies.
Jan Didden |
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