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Old 14th June 2008, 01:15 PM   #1
reiver is offline reiver  United States
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Default How to create a DC offset trimpot?

My recent posts have me very interested in trying to lower my amplifier's DC offset as much possible, but my amp didn't come with a trimpot for that purpose.

Based on the attached circuit, is adding a DC offset trimpot as simple as putting one in a particular spot between two transistors? Is it really that easy, or do you need to create a whole new circuit?

-Bryan
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Old 14th June 2008, 01:31 PM   #2
AndrewT is online now AndrewT  Scotland
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Hi,
P2 would normally adjust the output offset.
But this LTP has a current mirror and P2 becomes a VAS bias adjust only (maybe?). Try changing P2 and tell us what happens.
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Old 14th June 2008, 02:41 PM   #3
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This is one way of doing it:

http://headwize.com/ubb/showpost.php...=42615&fpage=2

A pot instead of (or in parallel with) R3/R4 can also tune the DC-offset.
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Old 14th June 2008, 03:01 PM   #4
AndrewT is online now AndrewT  Scotland
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I don't recommend unbalancing the LTP.
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Old 14th June 2008, 03:42 PM   #5
reiver is offline reiver  United States
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Default re:

In my manual the P2 pot is called a "Balance control"and its purpose is as "a distortion control that requires specialized test equipment to adjust".

Not knowing exactly what P2 does, I hate to change the factory setting as the pot is fixed with a dab of paint.

I like the idea of substituting a pot for R3/R4, I'll let you know what the outcome is...

-Bryan
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Old 14th June 2008, 07:59 PM   #6
reiver is offline reiver  United States
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Default 1 or 2 pots?

I need some help deciding how to place the trimpots mentioned above........Which would make more sense?

- Replace both R3 and R4 with separate trimpots (I'm thinking 100ohm to give me some range).

- Or replace just one of the resistors with a trimpot, and dial it up/down to in to zero out the DC?


I would think it tricky to keep separate pots in 'synch'. Thanks in advance for advice.......Bryan
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Old 14th June 2008, 08:26 PM   #7
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Just how much is your DC offset now?

Would it make any sense to match a new pair of diff transistors and see if that helped?

While changing R3 or R4 will lower the DC offset isn't that going to be counter productive because the pair would no longer be matched?

Would it be possible to add a DC servo to the circuit to lower the DC offset?
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Old 14th June 2008, 09:22 PM   #8
reiver is offline reiver  United States
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It's not bad now (13mv, 20mv) but when I bridge the channels it goes to 80mv. My other amp a Musical Concepts Xl280 w/PA3 with 1mv, so I'd like this amp above to be equally low.

I was just sorta hoping there was an easy method to add a DC offset adjustment to a Hafler 1200/9130 that doesn't have one.

In terms of re-matching the input transistors, I would love to do that. Too bad noone online offers a trustworthy service of matching transistors for audiophiles(!). Or do they?
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Old 14th June 2008, 09:34 PM   #9
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I think your safest bet is to follow the instructions in the link above, not adding a pot instead of R3/R4. It was just an example of how it can be done. A trimpot/resistors from rails to negative input will do the same job (manually) as a DC servo. Though it's not cheaper than a servo circuit, it's more straightforward.

Your P2 is to set the bias current to the VAS transistors and should have nothing to do with offset.
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Old 14th June 2008, 11:10 PM   #10
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Quote:
Originally posted by reiver


In terms of re-matching the input transistors, I would love to do that. Too bad noone online offers a trustworthy service of matching transistors for audiophiles(!). Or do they?

I have a cheap multimeter that measures the gain of transistors.
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