LM3875 amp - output issue

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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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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?
 
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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.
 
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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

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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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.
 
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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....
 
Now we are getting to results we can use.
From pin3 to the connection between R3 & C3, I get nothing....
Likewise from pin8 to the connection between R3 & C3 I get nothing....

Obviously pin 8 to junction R3/C3 should give the value of R3. It doesn't. You seem to have a bad connection. Likewise from pin 3.
It seems your "bad" amplifier has been operating as a voltage follower due to this bad connection (amplification only 1 time).

Now, measure pin 8 to the leg of R3 towards pin 8. Connection? If yes, measure on the two pins of R3 - 680 Ohm? If yes, then measure pin 8 to the leg of the 22uF capacitor towards R3.
There must be a bad connection in that string.
 
The symptoms sound like a failure of the feedback decoupling capacitor ~C3. This is also the most likely failure of audio amplifiers due to aging, aside perhaps from semiconductor failure due to abuse. I would also expect an input coupling cap but I don't see one. Perhaps on another PCB? The larger supply caps are also prone to drying out with age and certain Asian capacitors are notorious for early failure. If you want to keep this amp for a time then you may want to replace all the electrolytic capacitors. Note that the failure of C3 can cause an amplifier to become unstable and overheat with oscillations. Such capacitors also occasionally become intermittent, and appear to fix themselves, albeit not for long, so you just want to replace any suspect caps. Be sure to get the polarity right.
 
The symptoms sound like a failure of the feedback decoupling capacitor ~C3. This is also the most likely failure of audio amplifiers due to aging, aside perhaps from semiconductor failure due to abuse. I would also expect an input coupling cap but I don't see one. Perhaps on another PCB? The larger supply caps are also prone to drying out with age and certain Asian capacitors are notorious for early failure. If you want to keep this amp for a time then you may want to replace all the electrolytic capacitors. Note that the failure of C3 can cause an amplifier to become unstable and overheat with oscillations. Such capacitors also occasionally become intermittent, and appear to fix themselves, albeit not for long, so you just want to replace any suspect caps. Be sure to get the polarity right.

Thanks for the input. The failure in C3 is that it wasn't connected! That's because the resistor behind it R3, had been completely disconnected electrically.

No other caps on the board, just those 3. Any others would be in the separate PSU.

Agree on changing out old electrolytics- always a good call!
 
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.

So I've been testing these amps by listening to a variety of sources, source material and a couple pairs of different speakers. So far, so good.

But I am wondering about a few things previously mentioned here. C2 does follow C1, as C2+ is connected to C1 -. So given that pin 4 is connected to V- , then I suppose this is where they "meet"?

Also, R1 is labeled R1 on the board, labeled "220R" and measured 220 ohms.
I did not perform a capacitance test. (Going to be harder to check now after the reassembly). Certainly "looks" like a cap.
It may be interesting to note that the input seems very sensitive. I have a Bellari phono preamp with a gain-pot that I usually have to crank way up to get anywhere close to matching other sources when used with a preamp. I am going direct to these amps so far for testing, without a preamp in between, and I noticed how little gain it takes with the Bellari directly connected to hit high volume. I've plugged the Bellari directly into other amplifiers directly and did not notice this.

I wonder if this has something to do with the fact that these were marketed and designed for use with the company's passive preamp. Would the topology I have attempted to trace out lead to this behavior?

Finally, concerning R3, any idea what moving the value up and down would change either audibly or show under test?

Thank you!
 
But I am wondering about a few things previously mentioned here. C2 does follow C1, as C2+ is connected to C1 -. So given that pin 4 is connected to V- , then I suppose this is where they "meet"?

C2- is no doubt connected to the negative supply rail and pin 4 on the LM3875. Where C2+ is connected to C1-, it should be ground.

Also, R1 is labeled R1 on the board, labeled "220R" and measured 220 ohms.
I did not perform a capacitance test. (Going to be harder to check now after the reassembly). Certainly "looks" like a cap.
It may be interesting to note that the input seems very sensitive. I have a Bellari phono preamp with a gain-pot that I usually have to crank way up to get anywhere close to matching other sources when used with a preamp. I am going direct to these amps so far for testing, without a preamp in between, and I noticed how little gain it takes with the Bellari directly connected to hit high volume. I've plugged the Bellari directly into other amplifiers directly and did not notice this.


Though it looks strange, it is very likely 220 Ohm. It would be unlikely to measure such a low impedance "in-circuit" if it was a capacitor.

I wonder if this has something to do with the fact that these were marketed and designed for use with the company's passive preamp. Would the topology I have attempted to trace out lead to this behavior?

Possible adaptation to a passive preamp. The gain is 22K/680+1=33 times. Quite a normal value.

Finally, concerning R3, any idea what moving the value up and down would change either audibly or show under test?

If you reduce the value of R3, you increase the gain. If you increase the value of R3, you reduce the gain. You cannot go above 2K2 (10 times amplification) because of stability issues.

You may increase R1 if you want lower gain.
 
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But I am wondering about a few things previously mentioned here. C2 does follow C1, as C2+ is connected to C1 -. So given that pin 4 is connected to V- , then I suppose this is where they "meet"?

C2- is no doubt connected to the negative supply rail and pin 4 on the LM3875. Where C2+ is connected to C1-, it should be ground.

Also, R1 is labeled R1 on the board, labeled "220R" and measured 220 ohms.
I did not perform a capacitance test. (Going to be harder to check now after the reassembly). Certainly "looks" like a cap.
It may be interesting to note that the input seems very sensitive. I have a Bellari phono preamp with a gain-pot that I usually have to crank way up to get anywhere close to matching other sources when used with a preamp. I am going direct to these amps so far for testing, without a preamp in between, and I noticed how little gain it takes with the Bellari directly connected to hit high volume. I've plugged the Bellari directly into other amplifiers directly and did not notice this.


Though it looks strange, it is very likely 220 Ohm. It would be unlikely to measure such a low impedance "in-circuit" if it was a capacitor.

I wonder if this has something to do with the fact that these were marketed and designed for use with the company's passive preamp. Would the topology I have attempted to trace out lead to this behavior?

Possible adaptation to a passive preamp. The gain is 22K/680+1=33 times. Quite a normal value.

Finally, concerning R3, any idea what moving the value up and down would change either audibly or show under test?

If you reduce the value of R3, you increase the gain. If you increase the value of R3, you reduce the gain. You cannot go above 2K2 (10 times amplification) because of stability issues.

You may increase R1 if you want lower gain.

Very nice - thank you for the reply!
 
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