Biamp 3875 problem

sauuuuuce · 22nd April 2005, 10:47 PM

Here's the deal.....running a biamp.......active crossover works fine. The amps were fine individually. When pumping music through them they both distorted badly. Also if you let that mess go for a couple of minutes the HF amp shorts from V+ to V-. Aby ideas? Need more info? I have a 600VA toroid feeding a full bridege then to 2200uF caps for PS.

ingrast · 22nd April 2005, 10:53 PM

Quote:

Originally posted by sauuuuuce
... Aby ideas? Need more info? ....

Yes, please more info on your setup.

Rodolfo

pinkmouse · 22nd April 2005, 10:58 PM

sauuuuuce, please don't swear, I get really annoyed if I have to manually edit posts.

sauuuuuce · 22nd April 2005, 11:08 PM

sorry about the swearing

I'll get a scematic up shortly

sauuuuuce · 28th April 2005, 06:11 PM

LF side is same just switch the HPF for a LPF

carlosfm · 28th April 2005, 06:24 PM

You have a gain of 2 on the LM3875.

sauuuuuce · 28th April 2005, 07:15 PM

The feed back resistor is supposed to be 20k

ingrast · 28th April 2005, 07:31 PM

Quote:

Originally posted by sauuuuuce
LF side is same just switch the HPF for a LPF

There are some observations:

First. Do you need such low gain (2x)? I have not tested this type of monolithic amplifiers at gains much lower than recommended (I've got down to 10x with no problems, not below). There exists the possibility of instability, and I suggest to check changing R14 from 1K to at least 10K, or 18K - 22K as recommended and see what happens.

Related to the previous point and with respect to the HF amplifier which is the one depicted, you have a 1 nF DC decoupling capacitor (C5). Not going into details, this converts the amplifier to an (unintended) voltage follower at low frequencies exacerbating the potential instabilities.

C5 (and the corresponding one for the LF side) should be fairly large (about 100 uF), and since they must be electrolytic to achieve this value, should be polarized. A handy alternative is to return the (-) side to the -V rail, but that will couple and amplify any ripple there, so it is better to devise an auxiliary simple zener regulator for biasing. (unless you have the -V rail already regulated and clean).

Lastly, don't forget to bypass the + and - rails to ground with 100nF and at least 10 uF on each side as close to the chips as practical. You may also want to try Carlos's snubberizing.

Hope some of this helps.

Rodolfo

Edit:

Quote:

..The feed back resistor is supposed to be 20k..

Sorry I posted before your correction. Except for the gain issue, other points hold.

Rodolfo

Fossil · 29th April 2005, 02:28 AM

sauuuuuce,

when u said u test individually, do u mean with or without the active xover?

sauuuuuce · 29th April 2005, 03:06 AM

Tested without active xover. so just the LM3875 circuit. I am figuring the problem, or at least part of it, roots in joining the circuits. just can't pin point it....I could be wrong

22nd April 2005, 10:47 PM	#1
sauuuuuce diyAudio Member Join Date: Jul 2003 Location: Indiana	Biamp 3875 problem Here's the deal.....running a biamp.......active crossover works fine. The amps were fine individually. When pumping music through them they both distorted badly. Also if you let that mess go for a couple of minutes the HF amp shorts from V+ to V-. Aby ideas? Need more info? I have a 600VA toroid feeding a full bridege then to 2200uF caps for PS. __________________ It wouldn't be what it is, if it wasn't what it was

22nd April 2005, 10:58 PM	#3
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	sauuuuuce, please don't swear, I get really annoyed if I have to manually edit posts. __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

22nd April 2005, 11:08 PM	#4
sauuuuuce diyAudio Member Join Date: Jul 2003 Location: Indiana	sorry about the swearing I'll get a scematic up shortly __________________ It wouldn't be what it is, if it wasn't what it was

28th April 2005, 07:15 PM	#7
sauuuuuce diyAudio Member Join Date: Jul 2003 Location: Indiana	oops The feed back resistor is supposed to be 20k __________________ It wouldn't be what it is, if it wasn't what it was

29th April 2005, 03:06 AM	#10
sauuuuuce diyAudio Member Join Date: Jul 2003 Location: Indiana	Tested without active xover. so just the LM3875 circuit. I am figuring the problem, or at least part of it, roots in joining the circuits. just can't pin point it....I could be wrong __________________ It wouldn't be what it is, if it wasn't what it was


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28th April 2005, 06:24 PM	#6
carlosfm Banned Join Date: Mar 2003 Location: Lisbon, Portugal	You have a gain of 2 on the LM3875.

29th April 2005, 02:28 AM	#9
Fossil diyAudio Member Join Date: Sep 2001 Location: SG	sauuuuuce, when u said u test individually, do u mean with or without the active xover?

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