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LM3875 amp - output issue
LM3875 amp - output issue
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Old 20th September 2018, 10:01 PM   #11
FauxFrench is online now FauxFrench  France
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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.
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Old 20th September 2018, 10:20 PM   #12
Pure_Brew is offline Pure_Brew  United States
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Thanks folks. Looks like that R3 connection was the issue.

I thought perhaps a bad solder joint, but it was a bit worse than that.
Looks like a trace/through-hole was damaged.

I soldered in a little jumper, fired it up and the volume was rockin'

Cheers!
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Old 20th September 2018, 10:22 PM   #13
FauxFrench is online now FauxFrench  France
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Well done! Fault-finding is all about repeated logic and patience.

You may solder that leg of R3 on both sides. That replaces the through-hole.

Last edited by FauxFrench; 20th September 2018 at 10:25 PM.
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Old 20th September 2018, 10:33 PM   #14
steveu is offline steveu  United States
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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.
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Old 20th September 2018, 11:13 PM   #15
Pure_Brew is offline Pure_Brew  United States
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Quote:
Originally Posted by steveu View Post
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!
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Old 24th September 2018, 01:00 PM   #16
Pure_Brew is offline Pure_Brew  United States
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Quote:
Originally Posted by FauxFrench View Post
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!
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Old 24th September 2018, 08:14 PM   #17
FauxFrench is online now FauxFrench  France
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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.

Last edited by FauxFrench; 24th September 2018 at 08:18 PM.
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Old 26th September 2018, 04:41 AM   #18
Pure_Brew is offline Pure_Brew  United States
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Quote:
Originally Posted by FauxFrench View Post
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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