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.
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.
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. . . a lot easier to just install in R0 then to change it. . .
This only works when the amplifier is in monophonic mode (parallel mode).
However, if you have the amplifier in Stereo mode, then Both R0 and R4 can be paralleled by an additional 22k resistor.
You can locate then conviently (one for right and another for left) between Signal IN and Signal Ground.
Another possible location for 22k additional load is directly + to - upon the rca jack
I usually suggest transformers: http://www.diyaudio.com/forums/audi...kit-building-instructions-22.html#post1712724
Just share my experience in bulid a gainclone, I have High DC offset, not in mV but in V .. , .. icheck again, 1. like the advice in top, i change the R, if i use out of spec. 1k instead 470R , then I double 1k again, have 500R.
2. Clean Up the paste of solder (lotfett) in back of pcb with gasoline using a brush , this job good too after soldering a pcb,clean the dirt of solder...
DC offset goes to minimum to 10mv ...hope this help...just share...( sorry for bad english)
, .. icheck again, 1. like the advice in top, i change the R, if i use out of spec. 1k instead 470R , then I double 1k again, have 500R. 2. Clean Up the paste of solder (lotfett) in back of pcb with gasoline using a brush , this job good too after soldering a pcb,clean the dirt of solder...
DC offset goes to minimum to 10mv ...hope this help...just share...( sorry for bad english)
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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.
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.
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
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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: http://www.diyaudio.com/forums/audi...-kit-building-instructions-5.html#post1524877 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.
Check the average offsets for LM3875 as presented here: http://www.diyaudio.com/forums/audi...-kit-building-instructions-5.html#post1524877 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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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?
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?
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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.
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.
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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