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Old 5th September 2014, 03:24 AM   #1
ammel68 is offline ammel68  United States
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Default How can this work?

I was just looking at the following preamp with tone controls on eBay.

It uses two AD827 op amps and has supposedly a gain of 10.

Because of the AD827's high input bias current, wouldn't this design have a large amount of DC offset at its outputs or do the green NP elctrolytics filter out the DC offset?

Also, wouldn't this circuit have the ole' "swishing" sound when rotating the volume pot because of DC on the wiper?

I personally like the AD827, but I've yet to find a design where someone has placed a volume pot in front of one.
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Old 5th September 2014, 04:21 AM   #2
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If the DC resistance is balanced on the +in an the -in of an opamp, then the DC offset is determined by the input offset current. That is the difference between what the +in is drawing and what the -in is drawing in current. (Opamps are designed to amplify differences between the inputs.) As long as you block DC from passing through the pot with a capacitor, there should be no rustling.

It's hard to guess what's what by that picture alone.
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Old 5th September 2014, 04:22 PM   #3
ammel68 is offline ammel68  United States
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How would you balance the inverting input offset current if the noninverting input current changes because of a volume pot in front of it?

I'm not questioning your reply in any way. I'm trying to understand how you can use a higher input bias current with a volume pot in front of the noninverting input.
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Old 5th September 2014, 05:52 PM   #4
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Well a schematic would certainly help to stop groping in the dark.

Jan
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Old 5th September 2014, 06:00 PM   #5
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Oops I forgot the half of the point I was trying to make. If a capacitor is between the pot and the opamp, the opamp can't draw DC through the cap, so for the purpose of offset, it's like the pot isn't there! Win-win: it fixes rustle and offset issues. (If you plug something with DC problems directly into the pot, you would still get rustle. It's a compromise.)
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Old 5th September 2014, 07:03 PM   #6
DF96 is offline DF96  England
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Quote:
Originally Posted by ammel68
How would you balance the inverting input offset current if the noninverting input current changes because of a volume pot in front of it?
Input offset current is the difference between the two input currents. A volume pot is unlikely to change the input currents and hence the input offset current, but it may change the effective input offset voltage due to Ohm's law.

A circuit would be nice. We are good, but we are not that good!
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Old 6th September 2014, 01:13 AM   #7
ammel68 is offline ammel68  United States
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Quote:
Originally Posted by DF96 View Post
A circuit would be nice. We are good, but we are not that good!
Yeah...it would be nice but it isn't posted, so I think we can forget a schematic for that circuit.

Getting back to what I'm trying to learn here...how would you modify the AD827 circuit below in order to minimize DC offset at the outputs? I put a capacitor in right after the volume pot, as barefootwhistler suggested, but not sure what value to use. I typically see smaller values like 2.2uF used.
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Old 6th September 2014, 01:27 AM   #8
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The value of the electrolytic depends on the resistor values on the op-amp circuit.
On most of the circuits I have seen a 10uf/63V is common.
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Old 6th September 2014, 01:43 AM   #9
sreten is online now sreten  United Kingdom
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Hi,

Bog standard practise is to add a cap between
the bottom feedback cap and ground to reduce
DC gain to unity so the offset is not amplified.

rgds, sreten.
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Old 6th September 2014, 03:04 AM   #10
ammel68 is offline ammel68  United States
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Add a cap between RG and ground like drawn here?
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