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Old 13th February 2006, 05:17 PM   #11
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Default Wiring Diagram/Further Error?

Ok - here's the wiring diagram of the input of the right channel.

After reading another thread, I'm a bit worried that I may have put in my input cap backwards. Is this the case? Might this be the problem?

Also, I've measured the DC offset. In the cutting-out channel, it starts at about 60mV, and starts rising slowly... after a few minutes (didn't time it) it was at 400mV, and I shut it off. I checked the other channel, and this one started at about -60mV, and exhibited the same slow rising... hmm - yeah... this doesn't look good - I guess neither channel is working properly - as soon as the one channel has cut out, I turned off the (common) power supply right away. I would imagine the other channel would cut out as well if I would leave it on long enough.

Does this set off any warning bells? Could this be caused by a backwards cap? I only hope I haven't damaged my speakers too much...

Jeremy
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Old 13th February 2006, 06:16 PM   #12
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[7) on the +ive input, I put a 27Kohm resistor, and then a 2.2uF cap - this went to the pad of R2.]

You seem to be getting your channels mixed up.. R2 is the left ..

Also is the resistor or the cap' connected to the board? They should be connected as in your dia'..

The direction of the cap is dependent on the source voltage, over 2.5 V, + to source, under 2.5V, - to source.. I think?

Here is the original cct of the Sonic T..
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Old 13th February 2006, 06:45 PM   #13
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audio1st - you're right - I was confused about the channels. The pad of R1 is where I've put the input.
And it is indeed wired as I've drawn it - input RCA (with 50K to ground)->2.2uF->27K->R1 pad.

I've just measured my source - it puts out -6mV? Am I doing this correctly? I plug an RCA cable into the analog output of my DAC, and touch the positive lead to the centre pin, and the negative lead to the outside bit, right? Does the DAC need to be loaded or something before it puts out a higher voltage? Anyway, the manufacturer claims the DAC puts out "2.17V RMS at 1KHz Nominal (for 0dB digital level)", which seems about right. Which also means I've probably put the input cap backwards... Could this cause a drifiting DC offset?

Sorry for all of the newbie questions... :-(

thanks,
Jeremy
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Old 13th February 2006, 09:09 PM   #14
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The rising DC offset is a new one to me, never seem that before. But I guess it could be caused by a reversed electro cap.

If you are using a polarized cap, then the + side goes toward the amp (R1) and neg does toward the input (DAC). So have a look at that cap.

Your diagram looks correct. Don't worry about any DC offset on the DAC, it will be blocked by C3.
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Old 13th February 2006, 11:48 PM   #15
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Ok, my cap was backwards. I've swapped it's polarity, and now we have something just as strange, but probably better. :-)

The channel that was previously not working, starts at a DC offset of 60mV, and drifts downwards, seemingly settling at -4mV after about an hour of operation.

The channel that was previously deemed working starts at DC offset -110mV, and quickly difts to a fairly stable -113mV.

The only crazy guess I can come up with is that the cap throws some sort of field and this is scrambling the little brain of the poor 2024... Due to sloppy thinking and implementation of the first amp, the caps are oriented quite differently on each channel, but they sit more or less right below the 2024.

Two questions left: Is -113mV DC offset harmful to speakers (4ohm)? If so, is there any way of reducing it?

thanks,
Jeremy
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Old 14th February 2006, 02:43 AM   #16
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Hi Jeremy,
Does this mean you have both channels working now?

Tom Jones sings - "It's not Unusual" to see 110mV offset.
But that is about the limit of what the specs say. If all chanels are now working, I say pick the sides with the lowest offset and go with that.
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Old 15th February 2006, 11:34 PM   #17
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Default I think they're working!

Well, I ran the amps for a few hours yesterday, and they didn't heat up much, and neither one cut out, so I'd say they're working now. :-)

Unfortunately I wasn't able to choose which channel I was to use - it had to be the right channel. I'll post some pictures eventually.

Thanks for all the help!

Jeremy
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