Gainclone DC on output

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Hi Andrew,

Attached is my layout schematic and the actual schematic I am trying to copy. As far as I can tell, I have the same circuit (I am using just one half of the parallel layout).
The numbers in orange circles do not denote the pins (they were for comparison from my earlier layout).

Also, my Pin 10 was -2.7mV, hence higher than pin 9 (-3.4mV), hence my DC offset was positive.

Could you please tell me what I am missing?

Thanks,
Mohit
 

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+IN sees 1k0 + 33k
0.2uA (typical) should read 0.2*10^-6 * 34*10^3 ~ 6.8mVdc @ test point 8
and a maximum <34mVdc

-IN sees 30k + 0r1 + ~6r ~ 30006r
typical voltage at test point 3 should be 6mVdc and a maximum <30mVdc

You are not getting ~6.8mVdc, nor ~6mVdc, so something does not match between build and paper sch.
In addition your diff is only 0.7mVdc, so we know that the DIFFERENCE between the calculations should also equal 0.7mVdc. That confirms the build is nearly the same as the paper sch.
 
Hmm, I don't understand how my pin 10 ans pin 9 voltages are negative. Also, I just checked the resistance between audio + and audio -, and it came out to be 0.2K ohms. I unplugged the jack, put it in another source. Resistance came out to be ~40K. I put the jack back in the original source and it was ~30K. Would you know why this is? Also, While trying multiple things today, I tried this second source too, still did not have an output. I was so depressed all day today that slept for 3 hrs in the afternoon as a result of which am still awake at 2 :(

Hope I am able to solve this using your expertise.

Thanks again,
Mohit
 
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The diff matches.
The voltages levels are too low, they don't match.

It could be that your Ib is MUCH lower than typical, but that would be very unusual.

remember that 72nA? Did you calculate the other Ib?
The datasheet shows typical as 0.2uA = 200nA

Is your Ib ~1/3rd of typical?
 
I did not measure Ib, which I think from the diag is the current entering pin 10, right? I will check that tomorrow morn. Also, because all day I had not received any replies, I have unsoldered the chip from the board (it is so painful).

What would you recommend, should I put the same chip back or should I put in a new one?
I checked the resistances between many pins, and they seem to match with those of the new pin.

Please let me know, and I will measure the current accordingly.

Thanks,
Mohit
 
I had removed the 3886 last night, and put it back on this morning. Rechecked Pin 10, it is now at -1.9mV. Also, checked the input audio signal, it is at -1.9mV DC as well, and the voltage across Rb as a result is 0. So current draw is almost 0. What could be the reason for this?

Thanks much,
Mohit
 
Hi All,

Apologies for the delayed update, was a little busy at work. One of my chips got damaged as you saw in the last post, I replaced it, and now all is well. I have one chip per channel, and they work well together. There is still some of the DC offset thing becoming audible at high volumes, but otherwise all is well. For the offset, I will further change some of the connectors and wire layout, and that should take care of it I think. Again, surprisingly, I never had such issues before.

Also, I don't know why, I am not liking the sound of the amp anymore. They are lacking a bit in bass for some reason, even though I have a pair of floorstandings with dual 8" woofers hooked up to them (in my bedroom, ~16'X12', polarity is correct, have checked it, and reversing it kills the bass substantially. My toroid is ~250VA and capacitance is 60KuF). Also, the sound is not as defined as it used to be. There is no audible hum or any feedback from the speakers, but the sound is a bit lackluster. My layouts for both the channels is pretty much what I has posted some days back - point to point on a stripboard. I will play around more and provide an update. For now, no more touching these chips, they are somewhat sensitive :(

It was good learning from all of you (my soldering iron must have clocked an additional 50 hrs during all the troubleshooting ;), now I am off to building some discrete amps...hope you don't see me anytime soon ;)

Thanks again,
Mohit
 
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