P88 Preamp Issue

[Bengali]

Hi,

I have built up the P88 preamp.

http://sound.westhost.com/project88.htm

I'm using a 100K Ladder type Step Attenuator.

The issue I'm having is when I turn the step attentuator
to -33dB(which is 100K and 2.2K), the output goes into oscillation.
It does this for the next few steps and then goes back to normal.
This is with a 1VP-P sine wave on the input.

The other levels are just fine on the step attenuator until I reach the 100K/2.2K. I have double checked and measured the step attentuator, all values are correct.

In the P88 notes, it mentions if you set the P88 2nd stage to 0dB, it may oscillate. I have the switch set to 1 On, 3-4OFF, which should be 4.44dB for both channels.

Should the Op amp be slightly warm to the touch?

Any ideas?

Thanks.

[Nordic]

which chips are you useing?

[Bengali]

OPA2134's

[Nordic]

They should be stable at unity gain.... why not add a tiny bit of gain on the second stage....?

[AndrewT]

Hi,
how/where have you coupled the attenuator?
Before the amp or after the amp?
Or is there a way to tap into the PCB between the two active stages?

[nigelwright7557]

Originally posted by Bengali
Hi,

I have built up the P88 preamp.

http://sound.westhost.com/project88.htm

I'm using a 100K Ladder type Step Attenuator.

The issue I'm having is when I turn the step attentuator
to -33dB(which is 100K and 2.2K), the output goes into oscillation.
It does this for the next few steps and then goes back to normal.
This is with a 1VP-P sine wave on the input.

The other levels are just fine on the step attenuator until I reach the 100K/2.2K. I have double checked and measured the step attentuator, all values are correct.

In the P88 notes, it mentions if you set the P88 2nd stage to 0dB, it may oscillate. I have the switch set to 1 On, 3-4OFF, which should be 4.44dB for both channels.

Should the Op amp be slightly warm to the touch?

Any ideas?

Thanks.

Any idea why the op-amp stages have DC gain as well as AC gain ?

Any audio ccts I have seen usually have a cap between the inverting input feedback resistor and ground.

[Bengali]

I will increase the 2nd stage gain and see if it goes away.

The step attenuator is between the stages. The pic shown is how I have it wired. It is cap coupled.

Thanks :-)

[Bengali]

this is what I get at the 100K/2.2K setting.

top is input, bottom is output

[nigelwright7557]

Originally posted by nigelwright7557


Any idea why the op-amp stages have DC gain as well as AC gain ?

Any audio ccts I have seen usually have a cap between the inverting input feedback resistor and ground.

Any DC offset from the first stage will be amplified and you could end up with a serious offset.

[AndrewT]

Hi,
are two of the switches 1 to 4 always closed?
Each active stage becomes an opamp follower (gain 0dB) if the feedback switch is open.
I don't like that. What happens to your oscillation if you change the switch positions?

I'm guessing that R5 & R6 are remnants from the linear pots ESP used.
If you have a logarithmic attenuator then R6 can be open circuit or 100k. 100k pot feeding Zin=15k is not usually recommended.
R5 could be much lower, maybe 100r like the second stage.

There appears to be no RF filtering at any of the inputs.