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Old 6th June 2003, 09:36 AM   #61
Cobra2 is offline Cobra2  Norway
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Default Re: Re: ....Bridge/paralell ...


Have tried trim-pots for nulling DC-offset, but something does not add up....
I did de-solder the output-resistor, and null each chip (some variation here), checked and fine-tuned after 30min.
Then re-soldered, and now I have ~20mV ?
It seems that there is a difference when adjusting towards ground and between chips. What gives?
As mentioned...the production-spread among these chips are larger than life...

Arne K
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Old 6th June 2003, 11:06 AM   #62
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Default Re: Re: Re: ....Bridge/paralell ...

Koinichiwa,

Quote:
Originally posted by Cobra2

Have tried trim-pots for nulling DC-offset, but something does not add up....
I did de-solder the output-resistor, and null each chip (some variation here), checked and fine-tuned after 30min.
Then re-soldered, and now I have ~20mV ?
It seems that there is a difference when adjusting towards ground and between chips. What gives?
As mentioned...the production-spread among these chips are larger than life...

Arne K
First, Jeff Rowland uses 0.5R Output Resistors, worth going up to anyway.

Secondly, I suspect there are some interactions between the chips. Try nulling offsets with the chips connected normally and repeat a few times while the Amp settles it's temperature. It is clear that JR does not employ Servos (he seems to share my dislike for their sound) and if he can so shall we DIY'ers.

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Old 7th June 2003, 01:27 AM   #63
popes is offline popes  South Korea
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Default am I right?

offset null..
am i right?
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Old 7th June 2003, 01:47 AM   #64
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Default Re: am I right?

Koinichiwa,

Quote:
Originally posted by popes
offset null..
am i right?
Wrong. In your case use a 470K (18 turn) Pot. in all seriousness, for your schematic a dead short ot 22k will make little difference.

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Old 7th June 2003, 04:01 AM   #65
popes is offline popes  South Korea
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Default thanks, Kuei Yang Wang

what do you think about my project?

opa548 3pararel bridge(use 548 6ea for one chanel )
i will use input R 6.2K ohm
feedback 100K ohm
offset null 470K ohm (multi turn)
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Old 7th June 2003, 10:10 AM   #66
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Default Re: thanks, Kuei Yang Wang

Koinichiwa,

Quote:
Originally posted by popes
what do you think about my project?

opa548 3pararel bridge(use 548 6ea for one chanel )
If you like the sound of th OPA's /

Quote:
Originally posted by popes
i will use input R 6.2K ohm
feedback 100K ohm
offset null 470K ohm (multi turn)
YOU will need around a 10K multiturn offset trimmer, as you do not employ a DC blocking capacitor in the input to each chip.

Sayonara
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Old 7th June 2003, 10:30 AM   #67
popes is offline popes  South Korea
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Default dc blocking capacitors

i will use input transformer lundhal 1544A
if use transformer , is the DC block capacitor necessary?

thanks Kuei Yang Wang
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Old 7th June 2003, 10:32 AM   #68
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Default Re: dc blocking capacitors

Koinichiwa,

Quote:
Originally posted by popes
i will use input transformer lundhal 1544A
if use transformer , is the DC block capacitor necessary?
No, but leaving it out means the value of your offset trim resistor changes, that's all.

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Old 7th June 2003, 10:38 AM   #69
popes is offline popes  South Korea
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Default another question..

???

how much value need for me??

thank you very much
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Old 7th June 2003, 10:46 AM   #70
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Default Re: another question..

Koinichiwa,

Quote:
Originally posted by popes
???

how much value need for me??

Hey, I forgot, the OPA's are FET input I think, so you cannot null the offset the same way as on the LM3875/3886. Simply use 0.47/0.5R output resistors, ground the non-inverting input and hope for the best.

If they are not FET Input your Pot needs to be around twice the value of all the resistors that are not DC blocked on the inverting input in paralell, so in your case 6k2//220k*2 = 12k - NPV is 10k.

Sayonara
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