Help me make an op amp perform worse.

[bluebird]

I'm wet/dry mixing a direct (through one opamp buffer) signal with a signal thats going through an input transformer, output transformer and tubes. The tube/transformer signal is about 35 degrees phase shifted from the dry signal at 20Khz. Its a lot better at about 15K. But when I mix the wet and dry I'm getting a roll off from about 15K to 20K. Its about 1db down at 20K nothing I can't live with but it would be nice to be able to phase shift the dry signal up top to match the tube/transformer signal.

Any tricks I can do to the buffer op amp to shift the phase up top a little?

I noticed a TL074 worked a little better than a Mc33079 because its a little slower of an opamp. do they make a 741 in a quad? Maybe that will be sh*tty enough to match the transformer... Wow never thought I'd be looking for a crappy op amp!

Oh and the mixer is just two buffers into a virtual earth op amp via resistors.

 

[CJ]

this might work,

page 15 of this link.

https://labcit.ligo.caltech.edu/~vsanni/ph5/pdf/BasicOpAmpApplications.pdf

 

[scott2000]

do they make a 741 in a quad? Maybe that will be sh*tty enough to match the transformer..

RC4136??? 

Man you guys are insane! Feels like following you by hitching a ride on my skateboard, then you turn down a dirt road and lose me.....lol

 

[buildafriend]

looks like you gotta go Bob Widlar.

IIRC 741 doesn't work well above 4k.. I don't think the slew rate is what you're after. Active filtering is my first guess

 

[JohnRoberts]

I'm wet/dry mixing a direct (through one opamp buffer) signal with a signal thats going through an input transformer, output transformer and tubes. The tube/transformer signal is about 35 degrees phase shifted from the dry signal at 20Khz. Its a lot better at about 15K. But when I mix the wet and dry I'm getting a roll off from about 15K to 20K. Its about 1db down at 20K nothing I can't live with but it would be nice to be able to phase shift the dry signal up top to match the tube/transformer signal.

Any tricks I can do to the buffer op amp to shift the phase up top a little?

You could try overcompensating an op amp with external compensation pins (like a 5534). Instead of 22pf, maybe try 100 pf or even more.

I noticed a TL074 worked a little better than a Mc33079 because its a little slower of an opamp. do they make a 741 in a quad? Maybe that will be sh*tty enough to match the transformer... Wow never thought I'd be looking for a crappy op amp!

Oh and the mixer is just two buffers into a virtual earth op amp via resistors.

I would advise caution about assuming the differences are this simple.

JR

 

[bluebird]

Thanks for the info guys.  Phase shifter looks interesting. Thought about the compensation cap on a NE5534, but this is a quad op amp in a pre existing circuit. No option for a compensation cap.  Maybe there's a way to just boost the high end a bit when the cross fader is in the middle part of the mix.  Back to the drawing board.

 

[user 37518]

this might work,

page 15 of this link.

https://labcit.ligo.caltech.edu/~vsanni/ph5/pdf/BasicOpAmpApplications.pdf

+1 on the all pass filter

 

[JohnRoberts]

Thanks for the info guys.  Phase shifter looks interesting. Thought about the compensation cap on a NE5534, but this is a quad op amp in a pre existing circuit. No option for a compensation cap.  Maybe there's a way to just boost the high end a bit when the cross fader is in the middle part of the mix.  Back to the drawing board.

With a simple null test (subtract one stem from the other and see what's left) you can identify exactly what the difference is and even work on making the null deeper with tweaks.

or not.

JR

 

[bluebird]

I'm looking at it with an Audio Precision so I can see exactly whats going on, and what the phase shift is at what frequency for both signals and the mix.  I'll try the all pass, hopefully I can tag it on it on the the existing buffer amp.

 

[scott2000]

abbey mentioned this in another thread...

"the current-FB would result in much higher BW/lower phase-shift at HF"


He was Speaking of current feed back type op amps and how this is one of their special features.....  AD811 was mentioned as one family....

I'm not even sure how to tell the difference if it's not mentioned in the data somewhere but it was interesting to hear the certain types .....

edit////I can't tell if it's the transformer or the amp being discussed so.......

edit//// ok ....I guess you're saying it's the tubes/transformer;;;and title...????? ,,,doh

 

[bluebird]

Just to let people know, I was able to match the phase with the circuit CJ posted. Thanks bro!

Crappy picture of my AP below. The tube transformer path is in blue and the phase shifted op amp path is in red. One is frequency resp. and the other is phase. You can see the (180) - 0 - (-180) degree scale on the right. dbu level scale is on the left.

The phase curve for red and blue are pretty right on. Who needs tubes and transformers?!! just joking

 

[scott2000]

Woah!

 

[buildafriend]

You timed out @ 30Hz! gotta run all the sweeps again! jk

glad it works and thanks for sharing