I finished the premium components amplifier today but left the pot out. DC on one of the channels is 120mV without a resistor on the input, and down to about 70mV with ~14k on the input.
What makes the DC offset on one channel different to the other when all components are the same between channels?
The DC measurement is the same regardless of whether or not there is a load on the output. Am I measuring it correctly?
What makes the DC offset on one channel different to the other when all components are the same between channels?
The DC measurement is the same regardless of whether or not there is a load on the output. Am I measuring it correctly?
Actually there is a way to do that. If you have an unassembled Brian's board, solder all the parts in except for the chip. The mounting holes (for a chip) are pretty tight and they can work as kind of socket. Just insert the chip in, make sure the pins are tight against holes and measure it. 
carlosfm said:
And then measure DC-offset again.
Hi Carlos, was this not mentioned just to law fake the 10k linear pot? I've removed the pot from both amps so now there is only 22k from input to ground.
Also, I've used 16R from channel ground on the PCB to chassis ground. I was unsure whether this was sufficient?
I've just measured the basic amp again this time removing the extra resistors I had on the input and now the DC offset measures 78mV and 100mV.
Carlos, the closest resistor I have at the moment is 2k4r which if I add between the signal and ground pins on the board reduces the offset to 35.26mV and 12.5mV
Carlos, the closest resistor I have at the moment is 2k4r which if I add between the signal and ground pins on the board reduces the offset to 35.26mV and 12.5mV
Vikash said:the closest resistor I have at the moment is 2k4r which if I add between the signal and ground pins on the board reduces the offset to 35.26mV and 12.5mV
In minimized GC circuit the offset depends on input resistance (from +IN to ground). In case of 22k feedback resistor, I wouldn't go for more than 22k input shunt value. In some case I was having as much as 300mV of offset with input resistance of 50K.
When you choose lower values of input resistance (less than 22k), the offset value will go lower as well. On some chips it crosses 0 V reading and goes into positive (or negative ) offset voltage figure.
The replacement pot although better than the previous (error is only half now), is stilll way out as you saidUrSv said:
Peter, are you saying that its ok to leave the caddock 22k on the board and add another smaller resistor (perhaps 1k5r) between signal in and signal ground?
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Nuuk said:Could you please elaborate Carlos?
Yes.
In NI, with the "standard" schematic, these chips produce more DC-offset.
It really depends on the input impedance (including the pot).
Don't forget, the pot is in parallel with the 22k resistor (from +IN to Ground).
DC will even change with the pot position. An op-amp as an input buffer would avoid this and you could tune input impedance for lower DC.
And, an unpredictable issue, there are no two equal chips.
With higher values for Rf and Ri you could use higher values of input impedance.
To avoid high values of DC-offset, sometimes very different from channel to channel, you need to use the Ci caps (Look at Fig. 1 on the LM3875 datasheet).
Good electrolythics bypassed with quality poly 0.1uf caps will bring DC-offset to a couple of mv, without affecting the sound.
Vikash said:
The replacement pot although better than the previous (error is only half now), is stilll way out as you said
Peter, are you saying that its ok to leave the caddock 22k on the board and add another smaller resistor (perhaps 1k5r) between signal in and signal ground?
And I've heard some comments before that linear pots have better matching
I would actually recommend leaving 22k Caddock (on the board) when adding smaller resistor. It should influence the sonics in more positive way. I noticed that having Caddock in shunt position somehow increases the impression of 3-dimentionality, much better than Vishay, for instance.
Peter Daniel said:
And I've heard some comments before that linear pots have better matching
I would actually recommend leaving 22k Caddock (on the board) when adding smaller resistor. It should influence the sonics in more positive way. I noticed that having Caddock in shunt position somehow increases the impression of 3-dimentionality, much better than Vishay, for instance.
True. It's thanks to the linear pot that he was only 87% off with the first one...
If the cost of a worthwhile switch alone wasn't about 150%-200% of a Potentiometer, I'm sure more people would be making Attenuators.
Personally with my first GC I will probably use a Pot, and then when I get better at understanding and building amps I will give it to a friend and build another one with a buffer, attenuator etc.
By that time I'd hope to have a dedicated pre and DAC anyways.
Someone could make ALOT of money if they started making switches at the quality of say Elma but at half the cost.
Personally with my first GC I will probably use a Pot, and then when I get better at understanding and building amps I will give it to a friend and build another one with a buffer, attenuator etc.
By that time I'd hope to have a dedicated pre and DAC anyways.
Someone could make ALOT of money if they started making switches at the quality of say Elma but at half the cost.
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