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LM3875 amp - output issue
LM3875 amp - output issue
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Old 19th September 2018, 11:43 PM   #1
Pure_Brew is offline Pure_Brew  United States
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Default LM3875 amp - output issue

Hi folks,
I picked a pair of these Musical Laboratory "Bosangwha" mono amplifiers the other day, with the original PSU.

They are LM3875TF* based. I have found no info such as schematics or manuals at this point.

They've been who knows where and through who knows what.

One amp would appear to be in original condition, while the other looks like it had the chip replaced, as it has a different look/batch# and the soldering job looks - a little messy.

So the original works well and has plenty of volume. But the other has really low volume.

I also noticed the power led on the original is orange, while the problem amp appears more of a red color- kinda make me think of some sort of mute feature.

I can swap the PSU outputs, but it definitely follows this amp.

Not sure where to begin exactly- but I wonder if a poor soldering job of the LM3875TF would cause this. Otherwise it works just very quiet. At first I thought it had no output as the other is so much louder. Have to crank preamp gain all the way up to get about 60db out of it.

Thanks in advance
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Last edited by Pure_Brew; 20th September 2018 at 01:21 PM. Reason: * Fixed typo - these are indeed both LM3875TF
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Old 20th September 2018, 12:33 PM   #2
FauxFrench is offline FauxFrench  France
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Just to understand: You have two mono amplifiers that originally were LM3876 based. In one amplifier the LM3876 has been replaced with an LM3875 (photo). The LM3876 amplifier plays very well but the LM3875 has some sound but not much. Correct?

A first important observation from the datasheets is that LM3876 and LM3875 have different pin-layouts. You cannot just replace an LM3876 with an LM3875. If this has been done it is no-doubt the source of your problems.

LM3875: HTTP 301 This page has been moved

LM3876: http://www.farnell.com/datasheets/2001459.pdf

Actually, in an article on the Internet it is stated that it is the LM3875 that is the original IC in the Bosangwha: Mono and Stereo High-End Audio Magazine: Interview with Musical Laboratory*

Do you have an LM3876 in one amplifier and an LM3875 in the other?

Please clarify the situation?


NB: Do you have an Ohm-meter?

Last edited by FauxFrench; 20th September 2018 at 12:50 PM.
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Old 20th September 2018, 12:41 PM   #3
Michael Bean is online now Michael Bean  United States
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Those chips have totally different pin-outs...the LM3876 has the same pin-out as the LM3886, the LM3875 won't work here.

Mike
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Old 20th September 2018, 01:26 PM   #4
Pure_Brew is offline Pure_Brew  United States
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I'm so sorry about the typo! (Fixed)

These are Both LM3875TF in each mono amp.

I'm using a multimeter now to map things out. Checking continuity & resistances.
Nothing obvious so far...
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Old 20th September 2018, 01:58 PM   #5
FauxFrench is offline FauxFrench  France
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OK. You have two amplifiers that should be alike. Without power on any of them, you measure the resistance values of the (luckily) few resistors that decide the gain. Do you measure the same values for the two?

You can to a large extent rely on the circuit diagrams in the datasheet.

Last edited by FauxFrench; 20th September 2018 at 02:06 PM.
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Old 20th September 2018, 02:11 PM   #6
Michael Bean is online now Michael Bean  United States
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Yeah, now I see the silk screen shows a 3875.
Is there any chance that those big, flat, crazy wire substitutes are uninsulated and possibly shorting to the ground pours on the PCB?

Mike
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Old 20th September 2018, 04:26 PM   #7
Pure_Brew is offline Pure_Brew  United States
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Quote:
Originally Posted by Michael Bean View Post
Yeah, now I see the silk screen shows a 3875.
Is there any chance that those big, flat, crazy wire substitutes are uninsulated and possibly shorting to the ground pours on the PCB?

Mike
I thought of that but there was no evidence of it- unless someone shorted something before I got my hands on it.

So here is what I have mapped out so far.(see photo) I haven't pulled any parts for measuring at this point.

The only noticeable part variation between the two mono amps is R3. The banded value of each is a bit different (6.8Kon the good amp, 7.5k on the bad). I didn't pull them. The measured value (in-circuit)of each is roughly 1/10 of the banded value.
That doesn't seem odd to me as they respond to measurement similarly, but I'm wondering if the different values are based on customization, depending on the chip measurements.

The only other part in that box is the LED light between V- and V+
Also, I cannot quite read the value of C3. Small electrolytic maybe 22uf.
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Old 20th September 2018, 05:36 PM   #8
Mark Whitney is offline Mark Whitney  Netherlands
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680 to 1k is what would normally be used for R3.
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Old 20th September 2018, 06:14 PM   #9
FauxFrench is offline FauxFrench  France
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For C1 and C2 it is most likely so that one is connected to the positive supply rail (pin 1) and the other to the negative supply rail (pin 4). The point where they meet is ground.

As Mark also has noticed, the values indicated do not allow much gain. LM3875 should not even be stable at such closed loop gain (minimum 10 times for stability).

My experience with in-circuit measurements, without supply voltage, is that I arrive at values much closer to the real values than 1/10th.

Can I persuade you to do the following two comparative DC-impedance measurements on both the bad and good amplifier?
* input to ground,
* output (pin 3, without speaker) to the connection point between R3 and C3.

One more request, could you please measure the resistance of R1 (yellow) without input connected? It looks like a 220pF capacitor but is assumed to be a resistor.

22uF is very likely. This value is also used for TDA7294 and LM1875.

Last edited by FauxFrench; 20th September 2018 at 06:36 PM.
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Old 20th September 2018, 07:01 PM   #10
Pure_Brew is offline Pure_Brew  United States
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Mark- interestingly, the measurement for R3 is indeed 680 ohms. But the resistor bands are for 6.8k unless I'm color blind to the 4 bands.

On those C1/C2 caps, I'll have to look further.

Input to ground measures 22.17k ohms. (R1 measures 221ohms, R2 21.95k)

From pin3 to the connection between R3 & C3, I get nothing....
Likewise from pin8 to the connection between R3 & C3 I get nothing....
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