Chasis + GC complete. First listening tests...

[Vikash]

Originally posted by UrSv


Not faulty. Normal.

Hi UrSv, 87% out is normal and not faulty when its rated at 10%? I've recieved a replacement from RS and will see if that is as far out...

[Peter Daniel]

Originally posted by Vikash
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.

[UrSv]

Originally posted by Vikash


Hi UrSv, 87% out is normal and not faulty when its rated at 10%? I've recieved a replacement from RS and will see if that is as far out...

IMHO, Yes. Desirable?. No. The first part where the pot starts using the resistive layer usually with certain pots will be way off right when it starts. As soon as you're just a tiny bit into the surface things get better.

[Nuuk]

And then measure DC-offset again.

Could you please elaborate Carlos?

[Vikash]

Originally posted by UrSv

IMHO, Yes. Desirable?. No. The first part where the pot starts using the resistive layer usually with certain pots will be way off right when it starts. As soon as you're just a tiny bit into the surface things get better.

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?

[carlosfm]

Originally posted by Nuuk
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.

[Peter Daniel]

Originally posted by Vikash

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.

[UrSv]

Originally posted by Peter Daniel


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...

[carlosfm]

With cheap pots you have to buy a bunch of them.
Log or linear, the problem is the same.
Measure them all, and select the one(s) that have lower impedance mismatch between the two tracks(channels).

Then... it's not cheap.

[TaaJ]

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.