Gainclone with DC-offset problems

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Hi,

I have just finished my gainclone. I have build it around briangt's layout but on a veroboard. My plan is to use it with a preamp based on the AD825.

I have encountered some problems. When I measure the gainclone without the preamp and any sources I get a stable DC at 5mv and 30 mv. The problem is when I connect a source either directly to the gainclone or through the preamp I get alot of DC. The DC goes between 200mv to above 1V and it does not seatle down. Also if I connect the source through my preamp then the Volume control is influencing the DC-ofset.

BTW the preamp has a DC-ofset at 0-1mv.

Could it be either the missing input cap or the missing zobel?

Please, if someone can help me figure out what the problem is then I will be very happy.

Regards
Seve
 
Hi peranders

I have used the schematic that briangt has based his pcb on. Also I have made the layout like the pcb but on a veroboard and then made the connections underneath the veroboard. Most places I have just used the resistors "legs" to connect.I have almost wire it like the pcb traces from the briangt pcb. I have keept the signal and power ground separate and then connected them.

Sadly I don't have an oscilloscope!

I will see if I can find a Camera to take some pics of it

Seve
 
peranders said:
Do you have DC current path from the input (bias current) through the pot? If yes, not good if the pot has a high value. Less than a uA times 50kohms = 50 mV => 30X => 1.5 V out..... [/B]

I forgot to answer this. Sorry this is a little too technical for me. I don't have a pot in my gainclone but only in my preamp and the pot is a 10K

Seve
 
OK, but it helps if you have a schematic. I advice also to check the datasheet because there you have all basic information.

My idea about your truobles is the "input bias currents" which is currents for the input transistors. Those can vary rather much an aren't particulary either.

I missed to read some parts of your text. Brian uses 220 k as feedback and the non-inverting (+) input i grounded. I suspect that you IC has rather high !input offset voltage" and maybe also high "input bioas current". If that is the case you must trim the offset. Haven't got the time to tell you how but someone else here knows.
 
peranders said:
OK, but it helps if you have a schematic.

I have found a schematic of the version I have used. I have borrowed it from the thread about Briangt's non-inverted pcb I also think Briangt is the one who has made it.
 

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I just tried placing a pot (10K) at the input of the gainclone that actually helped a lot. Now I have DC-offset varying from 20-50mv, but it is still influenced by my pot meaning when I turn up the volume on the pot the DC also increases!

Though still strange that I cannot use it with my preamp or with out volume control just connecting the cd-player and using the internal volume control on the cd-player!

seve
 
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Without the pot there is already an unbalance in the input resistance to grnd, but it is apparently not too bad. When you add a pot, the input R goes down (pot + 220// 22k), that upsets the offset. The fact that it moves if you turn the pot gives it away: the varying pot varies the R to gnd through which the input bias current goes (in // with the 22k of course). so the offset varies with the pot. You can completely solve it with a cap between the pot wiper and the input (or between the input and the preamp/CD connection).

For a 10k pot which has a lowest equivalent R of 2.5 k, the cap should be for 10Hz lower 3dB point, about 1/(2*pie*2.5*10e3*10), let's say 6uF or make it 10uF. Should be non-polarized.

Jan Didden
 
Hi Jan,

Thanks for the help. I will try to get a non-polar cap to the input.

Though my preamp already has a output cap I thought that was enough, but probably not...

Still is it normal that there is such a big difference between using the gainclone with a passive volume where I get DC 20-50mv and using a preamp and getting up to DC 1V both places using a 10K pot?

Seve
 
Hi Seve,

If I understand your post correctly you have stable and acceptable levels of dc offset with the gainclone by itself. When you connect your preamp and source you get large and varying levels of dc offset.

When you connect the preamp and source, are you playing music when you measure the dc offset? If so, try measuring the offset with source and preamp connected but no music playing and see what you get.

Check out this thread here:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=23368&highlight=

I'm certainly no expert, and someone please correct me if I'm wrong, but I don't think you should measure the dc offset with music playing as it modulates the dc offset and produces erroneous measurements.

Good Luck!
 
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