Problems with HP GC, urgent

[Cradle22]

Hi!

Putting a 22uF polarized cap between R4 and ground reduced offset to 12 mV, but also introduced massive distortions!

Still not the way to go...

[moamps]

Originally posted by Cradle22
Putting a 22uF polarized cap between R4 and ground reduced offset to 12 mV, but also introduced massive distortions!

Hi,

f(-3) for 22uF/100ohm is ca 70Hz, what is too high. Try 220uF, maybe.

Regards

[Cradle22]

Actually, 220 uF was what I tried out first, but DC was only reduced from 335 mV without cap to 270 mV with 220 uF cap...

Maybe it would be better to increase R3 / R4, but it still bothers me that Kuei said that such values should not be used...

Thanks so far,

Arndt

[Nappylady]

Originally posted by Cradle22
Is it somehow possible to lower the DC offset? Problem is, whenever I raise the resistance from negative input to ground, the gain decreases...
[/B]

The gain is determined by a *ratio* of the feedback resistor to the resistor from the inverting input to ground. So, if you want to increase the value of the inverting-ground resistor, you simply increase the feedback resistor by the same factor!

It seems to me, however, that you will want to reduce the values of those two resistors in order to reduce your DC offset.

Good luck!

[Cradle22]

Hi!

Originally posted by Nappylady


The gain is determined by a *ratio* of the feedback resistor to the resistor from the inverting input to ground. So, if you want to increase the value of the inverting-ground resistor, you simply increase the feedback resistor by the same factor!

It seems to me, however, that you will want to reduce the values of those two resistors in order to reduce your DC offset.

Good luck!

I know how to calculate the feedback resistors. And I would say that it is easier to reach lower DC offset by increasing the values, but still there was this statement of someone who said that the feedback resistors should be small in a sallen-key topology, which should lead to a better rolloff at the XO frequency.

Don't know if I have the time to do some more work on this today, and tomorrow I'm off for Saas Fee (which is what I've built this thing for in the first run)...

Bye,

Arndt

[Cradle22]

Hi!

Well, looking at the thomson application notes for the TDA2030A, where they implement something like I want to do, I realize that they also added polarized caps in series to the output, which is also a (not preferred) way to get rid of DC offset...

But since sound quality is not of highest priority on my wish list, I will also try that (it should still be better than the average boombox out there).

Bye,

Arndt

[moamps]

Hi,

this schematic works well (checked) with LM3875.
(without C3 DC offset is about 70mV)

Regards

[Cradle22]

Hi!

I've built "your" circuit exactly as you described it, but without C3, and got -2.4 V DC offset...

I think I will leave this idea alone, get me some active XO boards from ESP, and use them...

No use for my upcoming holiday, but the next one is in February...

Thanks anyway,

Arndt