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Old 11th August 2012, 08:10 AM   #11
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Hi AndrewT

In your post you stated "Cin is in the wrong location. It must be after the pot."

Please explain what are the benefits, why it must be after the pot?

Thanks
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Old 11th August 2012, 08:16 AM   #12
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Hi Marc Vi.

Another question, in your post "The pot must be placed before Cin".

Please explain why is better.

Thanks
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Old 11th August 2012, 08:34 AM   #13
ddd is offline ddd  Bulgaria
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Dear cvpho1, When you place the C after the pot, you avoid the change of R through which the bias current flows. Otherwise it will flow via lower part of the pot and this will cause additional offset problems. A resistor +in to earth is a must of course.
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Old 11th August 2012, 09:54 AM   #14
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Thaks ddd,

What is an acceptable output dc offset?
1mV...10mV....50mV....?


Quote:
Originally Posted by ddd View Post
Dear cvpho1, When you place the C after the pot, you avoid the change of R through which the bias current flows. Otherwise it will flow via lower part of the pot and this will cause additional offset problems. A resistor +in to earth is a must of course.
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Old 11th August 2012, 10:33 AM   #15
AndrewT is offline AndrewT  Scotland
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perfect = 0.0mVdc
excellent < 5mVdc
good < 20mVdc
acceptable < 100mVdc
not acceptable >= 100mVdc.
potentially damaging for some susceptible drivers > 1Vdc.
damaging > 10Vdc
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Old 11th August 2012, 12:08 PM   #16
ddd is offline ddd  Bulgaria
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Good table!
I could suggest to estimate the offset expectations in advance, using data sheets and circuitry. If you achieve very different results in practice, you've certainly made some mistakes.
You should look for explanation of every offset you got. Not just to say "It's admissible"
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Old 11th August 2012, 12:28 PM   #17
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I'm just finishing a protoboard based on my first diagram (the one with Cin before pot). I'll do some measurements and post them here.
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Old 11th August 2012, 12:55 PM   #18
AndrewT is offline AndrewT  Scotland
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Measure the change in output offset as you change volume setting.
Measure the change in output offset variation (for a fixed setting of volume) as the amplifier warms up from cold to operational temperature.
Repeat this at extremes of volume pot settings. I suggest Top (-0dB), half volume (-6dB), Bottom (-infinity dB) and at the volume setting that matches +IN resistance, to -IN resistance (this maybe around -30dB).
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Old 13th August 2012, 07:54 AM   #19
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First measurements, running idle

Using 100k log ALPS pot and 2 x LM3886 (just bought two new LMs, not paired or tested before) here are the first results:

COLD
L R
3mV 2mV min 0%
1mV 0mV med 50%
-2mV -4mV med 75%
-9mV -10mV max 100%

WARM after 30 min
L R
3mV 2mV min 0%
1mV 0mV med 50%
-2mV -4mV med 75%
-8mV -10mV max 100%

WARM after 60 min
L R
3mV 2mV min 0%
1mV 0mV med 50%
-2mV -4mV med 75%
-8mV -10mV max 100%

Note 1: the "warm" measurement is done after a period of running without any input signal and no load connected (temp went from 29C to 38C).

Note 2: above is mentioned (in %) the position of the pot, not the value of its resistance



Quote:
Originally Posted by AndrewT View Post
Measure the change in output offset as you change volume setting.
Measure the change in output offset variation (for a fixed setting of volume) as the amplifier warms up from cold to operational temperature.
Repeat this at extremes of volume pot settings. I suggest Top (-0dB), half volume (-6dB), Bottom (-infinity dB) and at the volume setting that matches +IN resistance, to -IN resistance (this maybe around -30dB).
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Old 13th August 2012, 10:57 AM   #20
AndrewT is offline AndrewT  Scotland
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<= +-1mV cold to warmed up
<= +-12mV for change in volume control.

I would class these results as very good. Bordering on excellent (not down to the <5mVdc offset).
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