ADCOM GFA-555 Output DC Offset Voltage Measurements - diyAudio
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Old 30th August 2010, 02:44 AM   #1
PB2 is offline PB2  United States
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Default ADCOM GFA-555 Output DC Offset Voltage Measurements

A friend here recently rebuilt his 555 and found a large difference in the DC output offset between the two channels and he asked me to measure my working, original amps for comparison. I will also explain the source of this error as time permits.

Here is my data:
Input open circuited which is the right way to do it for this amp, taken cold:
....... No Load 4 ohm Load
Right 23mV 23mV
Left 15mV 14mV
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Last edited by PB2; 6th January 2014 at 10:21 PM.
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Old 30th August 2010, 12:40 PM   #2
boholm is offline boholm  Denmark
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IMHO: These measurements are not in any way alarming.

It is however also a good idea to measure having the inputs shorted.
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Old 30th August 2010, 12:50 PM   #3
PB2 is offline PB2  United States
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I agree, they are not alarming, he asked for them just as a reference.

Please look at the schematic for this amp and see if you agree
that the test should be done with the input open circuited.
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Old 30th August 2010, 02:49 PM   #4
boholm is offline boholm  Denmark
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It shold be tested with the inputs shorted. In this way you are closest to an operating situation, since most outputs are low ohmish - 10 to 100 ohms.
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Old 30th August 2010, 04:58 PM   #5
wahab is offline wahab  Algeria
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If it was only about DC output value, this would
be great.
There s design issues that are more urgent to treat
in this disaster amp....
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Old 30th August 2010, 05:30 PM   #6
PB2 is offline PB2  United States
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wahab, this is just about DC offset please do not post here about your claims that this amp has problems. Start your own thread.
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Old 30th August 2010, 05:32 PM   #7
PB2 is offline PB2  United States
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Quote:
Originally Posted by boholm View Post
It shold be tested with the inputs shorted. In this way you are closest to an operating situation, since most outputs are low ohmish - 10 to 100 ohms.
Did you look at the schematic?

Do you realize that you are not making any sense?

Most outputs have a low Z that is true, however they are usually
AC coupled with a cap and therefore have no bearing on the DC offset.

Shorting the inputs to the ADCOM is wrong for this measurement unless your
pre is DC coupled.

Look at the schematics and stop repeating yourself here.
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Old 30th August 2010, 05:34 PM   #8
PB2 is offline PB2  United States
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Quote:
Originally Posted by wahab View Post
If it was only about DC output value, this would
be great.
There s design issues that are more urgent to treat
in this disaster amp....
No scales on the curve again.

Mods would you please delete his posts and keep him out of here.

He is thread crapping again.
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Old 30th August 2010, 05:42 PM   #9
THD+N is offline THD+N  United States
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Quote:
Originally Posted by wahab View Post
If it was only about DC output value, this would
be great.
There s design issues that are more urgent to treat
in this disaster amp....
Didn't your mother teach you that if you do not have anything "nice" to say, then to say nothing at all?

We all know your opinion of the GFA-555 amp...

PB2 - I recently acquired a GFA-555 (early production 64XX serial number) and I have 1.4mV / Right and 22.1mV / Left (no load). I measured mine with the input "open".

Last edited by THD+N; 30th August 2010 at 05:46 PM.
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Old 30th August 2010, 05:45 PM   #10
PB2 is offline PB2  United States
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Quote:
Originally Posted by THD+N View Post
Didn't your mother teach you that if you do not have anything "nice" to say, then to say nothing at all?

We all know your opinion of the GFA-555 amp...
LOL, I was thinking the same thing!!!!

Thanks for your data THD!
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Last edited by PB2; 30th August 2010 at 05:52 PM.
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