-vin at output

Adam M. · 31st August 2004, 02:06 AM

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?

Mad_K · 31st August 2004, 06:49 AM

-Vin? Do you have balanced input?

jleaman · 31st August 2004, 06:53 AM

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 )

Optical · 31st August 2004, 09:10 AM

triple check your wiring, ive had this problem several times when ive connected the wrong pins up..

peranders · 31st August 2004, 09:11 AM

Adam, do you have any resistor from the MUTE pin? If not, add 10-33 kohms.

Nuuk · 31st August 2004, 09:36 AM

Probably an obvious question but have you isolated your speaker terminals?

Check that you have the correct voltages at the pins of the chip.

janneman · 31st August 2004, 11:45 AM

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

carlosfm · 31st August 2004, 12:36 PM

The LM3886 has a ground pin to connect to power ground.
Did you?

Adam M. · 1st September 2004, 03:25 AM

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.

Mad_K · 1st September 2004, 06:52 AM

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.

31st August 2004, 02:06 AM	#1
Adam M. diyAudio Member Join Date: Feb 2004 Location: Pittsburgh	-vin at output 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?

31st August 2004, 06:49 AM	#2
Mad_K diyAudio Member Join Date: Sep 2002 Location: Norway	-Vin? Do you have balanced input? __________________ Mads K

31st August 2004, 06:53 AM	#3
jleaman diyAudio Member Join Date: Apr 2002 Location: Nanaimo	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 ) __________________ Baby Danika, Born January 20th 1:54am.. http://itsparks.dyndns.orgMix a LittleUntangle&Opendns.org Perfect..

31st August 2004, 09:11 AM	#5
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	Adam, do you have any resistor from the MUTE pin? If not, add 10-33 kohms. __________________ /Per-Anders (my first name) or P-A as my friends call me

31st August 2004, 09:36 AM	#6
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	Probably an obvious question but have you isolated your speaker terminals? Check that you have the correct voltages at the pins of the chip. __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.


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31st August 2004, 09:10 AM	#4
Optical diyAudio Member Join Date: Nov 2002 Location: Auckland, New Zealand	triple check your wiring, ive had this problem several times when ive connected the wrong pins up..

31st August 2004, 11:45 AM	#7
janneman diyAudio Member Join Date: May 2002 Location: Where Germany, The Netherlands and Belgium meet Blog Entries: 6	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 __________________ /New Linear Audio publication: Baxandall & Self on Audio Power!

31st August 2004, 12:36 PM	#8
carlosfm Banned Join Date: Mar 2003 Location: Lisbon, Portugal	The LM3886 has a ground pin to connect to power ground. Did you?

1st September 2004, 03:25 AM	#9
Adam M. diyAudio Member Join Date: Feb 2004 Location: Pittsburgh	Doh! 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.

1st September 2004, 06:52 AM	#10
Mad_K diyAudio Member Join Date: Sep 2002 Location: Norway	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. __________________ Mads K

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