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Old 14th February 2015, 08:08 AM   #1
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Join Date: Mar 2009
Default Buffer output for PCM1794A

Hello, first thread here

I before had LT1028+BUF634 after the I/V converter of PCM1794A.
It worked very well, not in term of quality though

Today I built a discrete diamond buffer to replace the BUF634, like in the picture.
And, at the start it sounded much better. But then the output level was getting gradually smaller,
while the background noise gradually become larger, after playing for a minute.

By the wonder of too hot, tried increasing the resistances to lower the current but still the output level got smaller gradually.

Any reasons for that?
Will using a current source instead of a resistor solve the problem?
Or maybe the DC of the buffer feedbacked to the I/V converter from the buffer output to negetive of LT1028 then I/V?
But BUF634 gives 0.5V DC output when standalone too.

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Old 14th February 2015, 09:35 AM   #2
AndrewT is offline AndrewT  Scotland
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Join Date: Jul 2004
Location: Scottish Borders
The bias currents of the output transistors is not fixed.
Cold operation will be different from warm and different again from hot.

You need as a minimum to add an emitter resistor to each output device.

You can also add an emitter resistor to each of the inputs.

Have a look at W.Jung's site.
I'm sure he explains.
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Old 14th February 2015, 01:47 PM   #3
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Thanks so much for the reply.

So by using a current source part of the problem can be solved,
as bias current can be more constant.

Then resistors to emmiters.
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