High DC offset with no source (LM4780) - Page 6 - diyAudio
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Old 2nd August 2010, 08:51 AM   #51
AndrewT is offline AndrewT  Scotland
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DC offset is the DC output voltage that does not correlate with a DC input.
Set the input voltage to 0Vdc and the output should be close to 0Vdc. The error is the output offset.

This requires the input to have Zero Volts DC and the easy way to do that is to short the signal input to the signal ground.

But that is not the way we operate our music systems.
We plug in a source. We must check that the source does not result in an excessive output voltage. Both checks are necessary.
If either results in excessive output voltage then the "system" must be altered.
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Old 29th December 2011, 01:56 PM   #52
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Quote:
Originally Posted by Peter Daniel View Post
You don't even need to change resistor value, just install another 22k resistor in place of R0 (input shunt resistance becomes 11k)

Typical offset values for parallel LM4780 kit, without potentiometer or source connected, are 70-120mV. It's about double what I'm getting with LM3875 kit and that's the price you pay for more power.

With source connected or potentiometer adjusted to typical settings, the actual offset will be much lower.
Peter,

I measured the offset values of my parallel LM4780 kit are over 70-120mv without any source connected, that are 178~180mv. Is anything I must check what happened? If source connected, the offset values are 0.384v. I don't know if it's normal. PS. without Cz and Rz installed

The kit with good sound I like it.
Brant

Last edited by brantyen; 29th December 2011 at 02:03 PM.
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Old 29th December 2011, 03:20 PM   #53
AndrewT is offline AndrewT  Scotland
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These higher output offsets are typical for a DC coupled amplifier.
I do not recommend a DC coupled amplifier for any beginner or new to DIY building.
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Old 29th December 2011, 03:50 PM   #54
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It's not a matter of DC coupled or not, the LM4780 has natively higher offset values for this particular configuration.

Check the average offsets for LM3875 as presented here: Commercial Gainclone kit- building instructions On average, LM4780 will show double that.

You may decrease the offsets by reducing input shunt impedance (from 22k to 10K).

As to seeing 380mV with source connected, it's definitely not right and it seems like your source is adding some offsets of its own. You should install coupling cap at the amp input (2uF or bigger).

Cz and Rz have no effect on offset values.
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Old 29th December 2011, 05:59 PM   #55
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My connected source's offset value is 11.2mv. Is it too high for input?

Brant
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Old 29th December 2011, 06:32 PM   #56
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The offset from a source will be amplified by the amp, so with 11mV from source you can expect 300mV at amp's output. You do need a coupling cap.
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Old 30th December 2011, 02:00 AM   #57
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Quote:
Originally Posted by Peter Daniel View Post
The offset from a source will be amplified by the amp, so with 11mV from source you can expect 300mV at amp's output. You do need a coupling cap.
Got it!

thanks Peter
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Old 30th December 2011, 02:45 AM   #58
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I just have 2 pieces of Mundorf silver/oil 2.2uf for decreasing input dc offset(prepare for GG preamp output caps). After setup and listening, I feel they decrease few high frequency but more round, denseness, lively and clearly. (offset value is down from 380mv to 270mv)

Just thinking that whether change 22k to 10k(R4 right?) or not.
Does it lower sound quality by 10k?
Over 170mv without protection, will it damage speaker in future day?

Last edited by brantyen; 30th December 2011 at 02:48 AM.
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Old 30th December 2011, 09:15 AM   #59
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Quote:
Originally Posted by brantyen View Post
I just have 2 pieces of Mundorf silver/oil 2.2uf for decreasing input dc offset(prepare for GG preamp output caps). After setup and listening, I feel they decrease few high frequency but more round, denseness, lively and clearly. (offset value is down from 380mv to 270mv)
You should not use an input coupling cap without offset control. Offset control means to either use an NFB blocking cap or a DC servo.

Quote:
Originally Posted by brantyen View Post
Just thinking that whether change 22k to 10k(R4 right?) or not.
Does it lower sound quality by 10k?
It won't affect sound quality, but DC offset. For low DC offset, you want both amp inputs to see the same DC impedance. Usually that means to use the same value for input and feedback resistor, but in a DC coupled amp you need to take into consideration that at the input all resistors are in parallel, i.e. resistor at the source's output to ground, input resistor, volume potentiometer, etc. and you have to calculate the effect of that change on the entire impedance.

Quote:
Originally Posted by brantyen View Post
Over 170mv without protection, will it damage speaker in future day?
The trouble is not so much in damaging the speaker. Although several 100 mV of DC are not desirable, most speakers will stand them.

The issue is that in a passive multi-way system the woofer will be held off centre by the offset voltage, while the DC component is filtered out for the other drivers by the crossosver.

The effect is, as if you mounted the woofer slightly recessed or protruded in relation to the midrange speaker and/or tweeter. Some people do that on purpose to align the points of sound emanation, but they do that before designing the crossover accordingly. If the crossover is not adapted to that situation, the speaker's frequency response will become more rippled around the crossing frequency between the woofer and the next driver. Sound quality will be affected, because parts of the music will become either exaggerated or swallowed.
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Old 31st December 2011, 09:43 AM   #60
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Quote:
Originally Posted by Peter Daniel View Post
It's not a matter of DC coupled or not, the LM4780 has natively higher offset values for this particular configuration.

Check the average offsets for LM3875 as presented here: Commercial Gainclone kit- building instructions On average, LM4780 will show double that.

You may decrease the offsets by reducing input shunt impedance (from 22k to 10K).

As to seeing 380mV with source connected, it's definitely not right and it seems like your source is adding some offsets of its own. You should install coupling cap at the amp input (2uF or bigger).

Cz and Rz have no effect on offset values.
Thanks Peter and Pacificblue.

I just install another 22k in kit to R0 that decreases shut impedance to 11k(jumper connected). I got about -110mv offset value with no source connected.
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