The multi-turn pots
Yes, there are differences in resistors, caps, even the chips on both channels aren't matched.
Anyway, even with 1% resistors I always measure them and pick two (one for each channel) with the closest values.
Differences in DC offset between the two channel you will always have.
The solution for that is using the 50k multi-turn pot, as I use.
You can easily tune the two cannels for 0.0 DC offset.
Before putting the pots, I had 10mv on one channel and 10.5mv on the other, which is not bad at all.
But now I sleep in peace.
Minimum DC offset should always be a goal when designing an amp.
The speaker drivers will work in a more linear way, resulting in less distortion and better sound.
I'm not saying you would notice, if you have low values of DC offset already.
mikelm said:
one is about 10mV the other about 30mV
mike
Yes, there are differences in resistors, caps, even the chips on both channels aren't matched.
Anyway, even with 1% resistors I always measure them and pick two (one for each channel) with the closest values.
Differences in DC offset between the two channel you will always have.
The solution for that is using the 50k multi-turn pot, as I use.
You can easily tune the two cannels for 0.0 DC offset.
Before putting the pots, I had 10mv on one channel and 10.5mv on the other, which is not bad at all.
But now I sleep in peace.
Minimum DC offset should always be a goal when designing an amp.
The speaker drivers will work in a more linear way, resulting in less distortion and better sound.
I'm not saying you would notice, if you have low values of DC offset already.
fedde said:
It's funny isn't it, when I took the resistors out the sound became bigger, smoother, more effortless more low level detail and sense of recorded room acoustic, all in all quite wonderful
however I am using a .22ohms at the o/p, with my current speaker leads I think there is too much pF and the amp was unhappy without the resistor.
cheers
mike
mikelm said:
It's funny isn't it, when I took the resistors out the sound became bigger, smoother, more effortless more low level detail and sense of recorded room acoustic, all in all quite wonderful
however I am using a .22ohms at the o/p, with my current speaker leads I think there is too much pF and the amp was unhappy without the resistor.
The reason I mentioned the .22 resistor is that I get the feeling that what we may be talking about here is loss of quality because of onset of slight instability.
probably to get good results with no o/p res & no non inverting i/p res the speaker leads would have to be very low capacitance ( and all other stability issues will need to have been addressed. )
cheers
mike
I built a lot of this amps and 28mV offset is a standard value. After building few of Nelson Pass designs, I'm not at all concerned with few milivolts of offset.
I checked it before and I also preffered non inverting input directly to ground. I will check it once more and see how it compares now.
I checked it before and I also preffered non inverting input directly to ground. I will check it once more and see how it compares now.
I agree. Even though I use mainly active crossovers, or no crossovers, 28mV hasn't yet done any damage to the speakers.
Within that 0-28mV 'envelope' what is more important is sound quality. My 'trouble' at present is doing some rational testing as I have so many variables like different voltage PSU's, 0.22R on output, trimpot on inverting input, different value attenuator and my new speakers! Is this a hobby or a bl**dy life sentance?
Nuuk said:
My 'trouble' at present is doing some rational testing as I have so many variables like different voltage PSU's, 0.22R on output, trimpot on inverting input, different value attenuator and my new speakers! Is this a hobby or a bl**dy life sentance?
You have my deepest sympathy
having done a few mods & comparisons a couple of weeks back I'm currently in a phase when I'm mostly just listening to music !
you should definitely try it sometime
mike
Carlos, I have just tried 20K trimmers in my GC monoblocks and got a variation in DC offset between 28mV (0r) and 24mV (20K)but nowhere near zero.
I wonder if, when you measured that 4K it could have been 46K?
I've stopped for lunch now but this afternoon, I will put a 30K resistor in series with the trimmer and see what happens.
(I should be out on my bike but the Portuguese have taken all the sun again
)Nuuk said:
Carlos, I have just tried 20K trimmers in my GC monoblocks and got a variation in DC offset between 28mV (0r) and 24mV (20K)but nowhere near zero.
I wonder if, when you measured that 4K it could have been 46K?
I've stopped for lunch now but this afternoon, I will put a 30K resistor in series with the trimmer and see what happens.
(I should be out on my bike but the Portuguese have taken all the sun again
Hey Nuuk,
No, it was around 4K what I measured.
And before mounting the pot I measured it was about the middle of its resistance (25k), and I mounted it in a way it decreases resistance as I turn the pot leftwise.
After mounting it, I turnd left, and left, and left...
When it hit 0.0~0.1 mv I stoped and measured the pot.
It was around 4k.
I don't know what's happening in your case, but the pots are cheap and you could buy two of them and test.
Oh, and it's a sunny day here.
If you bring your bike into my house we could go and take a big ride.
And then I'll show you my GC and the ultra-high-end preamp
I'm making for myself.It's playing, it doesn't have a box yet, but it really delivers.
Reports soon.
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