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Old 28th July 2003, 09:27 AM   #1
Nuuk is offline Nuuk  United Kingdom
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Default DC offset variation.

I have just done quite a long search but have not turned up an answer as to why DC offset can vary so much.

I have just built another IGC and the offset on one channel is 32mV and on the other 52mV!

I have used a wire fro non-inverting input to ground and no resistor on inverting input to ground.

I have checked the circuit and everything seems perfect so could it be a variation in the chip!

BTW I have 254K for the feedback resistor and 10K for the input. Rails are 36VDC.

Any thoughts?
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Old 28th July 2003, 12:36 PM   #2
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Default Re: DC offset variation.

Quote:
Originally posted by Nuuk
I have just done quite a long search but have not turned up an answer as to why DC offset can vary so much.

I have just built another IGC and the offset on one channel is 32mV and on the other 52mV!

I have used a wire fro non-inverting input to ground and no resistor on inverting input to ground.

I have checked the circuit and everything seems perfect so could it be a variation in the chip!

BTW I have 254K for the feedback resistor and 10K for the input. Rails are 36VDC.

Any thoughts?

I just took a look at the data sheet of the LM3886 chip (I don't know what chip you are using, so I just picked this one as an example.) shows that the input offset voltage is nominally 1mV and can be as much as 10mV. If the first chip you measured had an input offset of 1mV that would lead to ~12-20mV offset due to the gain of the circuit. If the second one ws 2mV, that would lead to 24-40mV offset due to the gain of the circuit. 10mV input offset would lead to 120-200mV of offset.

In actuality this amount of offset is more than acceptable. 200mV offset leads to 55mW of dissapation in your speakers.

Scott
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Old 28th July 2003, 12:56 PM   #3
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Quote:
In actuality this amount of offset is more than acceptable. 200mV offset leads to 55mW of dissapation in your speakers.
Thanks Scott, I am not too worried about 52mV but was curious as to what caused the difference between the two channels when they are seemingly identical.

I can always add a 254K from non-inverting to ground if I want to bring both offesets closer to zero but I'll have a listen first!
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Old 28th July 2003, 01:18 PM   #4
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Quote:
Originally posted by Nuuk


Thanks Scott, I am not too worried about 52mV but was curious as to what caused the difference between the two channels when they are seemingly identical.

I can always add a 254K from non-inverting to ground if I want to bring both offesets closer to zero but I'll have a listen first!

Well I just ran some more numbers. Let's say you put a 52mV RMS signal into an average 89dB senstive speaker at 1kHz. At 1M you'd have a signal level of about 52dB. Not loud, but audible in a quiet room.

At DC you would have a physical woofer offset of approximately .075mm. At 200mV DC offset you'd have a woofer offset of approximately .25mm. Now that's pretty interesting. Let's say that woofer had a linear excursion of 3mm one way. You'd be using 8% of your woofer excursion for DC offset.

Hmmm....I'll have to think more about that one. Maybe I'll work up the reduction in dB of headroom that you'd see because of this...where did I put my excursion to output volume spreadsheet...


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Old 28th July 2003, 01:19 PM   #5
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Thats what i did Nuuk. I tried a 500k multi turn pot on the non inverted and it driffs slightly( up to .005v). So i just buy reisitors in 100 quan lots and sort thru them and find matching resistors.Even then the offset varies about .002 v.Close enough for me.There is very little difference in the sound.
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Old 28th July 2003, 05:08 PM   #6
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Well, here are some observations!

Despite the offset, there is absolutely no sound from the speakers when powering up, and no sign of any cone movement.

Without anything playing, there is dead silence from the speakers even with my ear a few inches away. This is in comparison to my GC monoblocks that have zero DC offset (with the equivalent of the feedback resistor from non-inverting to ground) but a slight thump when powering up and a very slight hum when listening a near the speakers.

Sound quality is fine. I don't know if I was immediately impressed as much as with my first GCs but of course I didn't know what to expect then! Also, the new GC is using new input caps where the previous ones had used caps.

The sound had improved slightly over the last couple of hours and no doubt will do so more as everything burns in.

BTW - I am using the LM3875s. The circuit is the same one shown on my DD Gainclone page except that I have 75K from input to ground, 6uF blocking caps and the feedback value is 254K..
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Old 28th July 2003, 05:49 PM   #7
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thats basically what setup I am using NUUK except I am only using a 1.5uf mkp cap because thats what I had on hand


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Old 28th July 2003, 06:00 PM   #8
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Quote:
Originally posted by Nuuk
Well, here are some observations!

Despite the offset, there is absolutely no sound from the speakers when powering up, and no sign of any cone movement.

Without anything playing, there is dead silence from the speakers even with my ear a few inches away. This is in comparison to my GC monoblocks that have zero DC offset (with the equivalent of the feedback resistor from non-inverting to ground) but a slight thump when powering up and a very slight hum when listening a near the speakers.
Once the amp is powered up, try disconnecting the speaker and reconnecting it real fast a couple of times. If there is even a small DC offset you'll hear it. Again, the DC offset you mentioned shouldn't be an issue, as you've discovered.

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Old 28th July 2003, 06:04 PM   #9
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Do yall think you get better sound with the equal feedback resistor (to pin 7) or without it? I do notice a slight thump when i power on and a slight humm.
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Old 28th July 2003, 08:44 PM   #10
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Quote:
Do yall think you get better sound with the equal feedback resistor (to pin 7) or without it? I do notice a slight thump when i power on and a slight humm.
If I am totally honest, I don't think that I can hear a difference. I can hear a difference when using the 18K/22K combination but between the wire or resistor on non-inverting to ground, it is too close too call.

So, you either go for zero offset but with a thump and hum, or
some DC offset without them!
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