Help with small DC offset on output

Fixed! See last post

I've just got hold of a lovely Integra DRX-7 AVR (9.2-channel, class D) for almost nothing, as it was stuck in protect mode. After disassembly (not a trivial task with this beast!) I found one channel had blown its mosfets (IRF6775MPBF, DirectFET Isometric smd package), driver IC (IRS20957STRPBF), burned a couple of resistors and had a suspect input op-amp (TLC071), presumably caused by a speaker short or similar.

I've replaced the faulty components and everything else on this channel measures OK on my DMM as compared with the schematic and a working channel. Because electronic components are very hard and costly to source in New Zealand, so the parts have had to come from Aliexpress.

It now powers up with sound on all channels (yay!), but the faulty channel has a faint audible hiss with a DC offset of around 230mV - not enough to trigger protection but enough to cause annoying pops when changing settings. The poweramp board isn't accessible without complete disassembly, so can't actually test anything on the board while its running.

I've attached the schematic for the power amp section. Can anyone suggest where to start looking for the issue or what/how to test? Maybe Leaky diode or transistor? Or even fake parts?

TIA, Jon
 

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It could be the modulator chip or the integrator opamp / feedback circuitry since the DC offset is (ideally) supposed to be compensated by the negative feedback.

Are you sure that the DC offset is not a result of an offset at the input of the particular channel ?
 
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Are you sure that the DC offset is not a result of an offset at the input of the particular channel ?

Thanks, yes, there's no DC offset at the input.

I've partially disassembled so I can apply power directly to the main transformer and thence to the amp board, and measured a few voltages where I can get at things safely, comparing them with an OK channel. With no input, pin 6 (Out) on the opamp measures around 3.7V on the bad channel, but around 5.9V on good ones, so something not quite right there. Other pins on the opamp compare OK.

Bit scared to start probing the driver as it would be very easy to accidentally short pins. But I'll do a few more measurements when my hands feel steady enough, to see if I can find anything else suspicious.
 
The gate-driver is probably alright. The DC offset appears to be an error introduced by the system itself, for example, by the small signal / preamplifier sections within the unit.

If probing the gate-driver, take care to ensure that the ground return is the same for both scope channels. Alternatively use a differential scope.
 
Well the probing hasn't revealed much of use to me. And in probing the driver IC, the probe slipped and momentarily bridged a couple of pins, with a not unexpected result. Ouch. So I've just had to replace (again) both mosfets, the driver IC, two SMD resistors and one of the 0.22ohm 5W resistors. Took the opportunity of putting a different TLC071 in while I was at it to see if that makes a difference. Everything working again now, but still around 176mV DC and hiss on that channel.

The DC offset appears to be an error introduced by the system itself, for example, by the small signal / preamplifier sections within the unit.

Yes, I think you're right. The hiss gets louder as I turn the volume up, but the DC offset remains constant (and is less than it was, with a different TLC071). Maybe a noisy/leaky (fake?) TLC071?

By the way, the output phase on these Height-1 channels is reversed (SPKR+ is actually GND), so the offset is actually -0.176mV.

Not sure where to go from here, but wondering if adding a 10uF DC blocking capacitor between the output of the opamp and the input of the driver IC (before R6622) might help - the reference design for the IRS2092 IRAUDAMP7D has this on the driver input. Thoughts?
 
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Looks like there's something else at play, something related to the oscillation
Yes, good thinking. By a process of elimination, I've swapped the opamp with another channel's original one, and the DC offset and hiss have moved to that channel instead. So definitely the opamp at fault - looks like I need to find an affordable source in NZ for an original TI TLC071ID rather than Chinese fakes from Aliexpress.
 
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Out of interest I set up one of the Chinese TLC071s on the breadboard as a unity-gain amplifier with +/- 6V supply. The offset between input and output measured as 18.1mV That would certainly account for a larger offset at the amplifier output!

I replaced the chip on the testbed with a genuine TI TL071CP, and the offset between input and output didn't even register on the mV scale, so probably below 10uV.

Teach me to buy cheap (fake) Chinese chips! Anyway I've ordered an original TLC071ID now so once it arrives, I can hopefully finish this project successfully.

Thanks for the help.
 
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All Fixed!

New opamp arrived today so I carefully removed the bad one and put in the new one. All fixed now, no DC offset, no hiss, working beautifully on all nine channels :)

Learnt a lot too - e.g. how to remove and replace DirectFET Isometric SMD mosfets (flux, solder paste, hot air, a steady hand and lots of patience), how fantastic Chip Quik low temperature solder is for removing SMD chips (wish I'd discovered this a long time ago!), and not to trust cheap Chinese chips (although, surprisingly, the Infineon smd mosfets and driver from Aliexpress all work fine).

Thanks for the help, Jon.
 
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