Problems with HP GC, urgent

[Cradle22]

Hi!

I've built me (for my ghetto blaster project) a high-pass GC, using a circuit from the nationals on-line filter calculator, like this:



With the following values:

R1 3200 K
R2 240 K
R3 2.2 K
R4 0.1 K
C1 2.2 nF
C2 2.2 nF

It should give a a xo-frequency of about 75 - 80 Hz with a Q of 0.6


Supply voltage is +- 35 V (25 - 0 - 25 toroid), with only GC-like 2 * 1000 uF at the pins of the IC, which is an OPA458.

When I turn on the amp, with no signal and no load connected, nothing happens (OK), supply voltages are OK, but at the output pin I have nearly the full positive output rail!

Has anyone some suggestions? This is the first time I am trying to use those filters... And I have to be finished until friday evening, since I want to use the blaster in my skiing holidays...


Thanks,

Arndt

[millwood]

could it be tha the wiring is faulty?

[Cradle22]

Hi!

Originally posted by millwood
could it be tha the wiring is faulty?

If the circuit is right, the wiring should be, as well. Checked both channels more than twice, both channels carry DC offset of approx. 34 V...

I will try my low pass (using OPA 541) next...

Bye,

Arndt

[Matttcattt]

i asume you mean OPA548 not OPA458.

are you sure you have the inputs the correct way round? i switched them on my amp (OPA548, then OPA541) and i got rail voltage at the output.

i mean inverting and non-inverting inputs on the chip, by the way.

[Cradle22]

Hi!

Originally posted by Matttcattt
i asume you mean OPA548 not OPA458.

are you sure you have the inputs the correct way round? i switched them on my amp (OPA548, then OPA541) and i got rail voltage at the output.

i mean inverting and non-inverting inputs on the chip, by the way.

Yeah, I meant the OPA 548...

Actually I found a mistake by back-engineering (drawing a diagramm by the connections I made)...

The feedback resistor (R3) is not from output to negative input , but to positive input...

On all of my four ICs...

Can not change it right now, maybe later...

Thanks anyway,


Arndt

[Cradle22]

Yep, that was it...

Now it is 300 mV DC offset on both HP channels, little bit high ... haven't tested the LP channels yet...

Tomorrow...

Bye,

Arndt

[Matttcattt]

good to hear you got it working.

[Cradle22]

Hi!

Is it somehow possible to lower the DC offset? Problem is, whenever I raise the resistance from negative input to ground, the gain decreases...

Is there a way to avoid this change in gain and filter characteristics, but still dropping DC offset (I want to achieve at least <100 mV)?

Thanks,

Arndt

[moamps]

Originally posted by Cradle22
[B]Hi!

I've built me (for my ghetto blaster project) a high-pass GC, using a circuit from the nationals on-line filter calculator, like this:
With the following values:
R1 3200 K
R2 240 K
R3 2.2 K
R4 0.1 K
C1 2.2 nF
C2 2.2 nF
It should give a a xo-frequency of about 75 - 80 Hz with a Q of 0.6

Hi,

You use IMHO wrong R and C combinations (too high impedance network). For first decade filters recomended caps value are 100nF to 470nF.
Also, increase feedback resistors ten times and use large elco in series with R4 for avoiding DC offset problems.

Regards

[Cradle22]

Hi!

Originally posted by moamps


Hi,

You use IMHO wrong R and C combinations (too high impedance network). For first decade filters recomended caps value are 100nF to 470nF.
Also, increase feedback resistors ten times and use large elco in series with R4 for avoiding DC offset problems.

Regards

I have used small feedback resistor values because is was adviced to do so in this thread:


active XO GC

Can I use "normal" feedback values without risking instability?

Only two days unti this has to work...

Bye,

Arndt